Transducer structure for an acoustic wave device The invention relates to acoustic wave devices, in particular to transducer structures for an acoustic wave device. In recent years, surface acoustic wave (SAW) devices have been employed in an increasing number of practical applications, such as filters, sensors and delay lines. In particular, SAW filters are interesting for mobile phone applications due to their ability to form low loss high order bandpass filters without employing complex electrical circuits with unprecedented compactness. Therefore, SAW filters provide significant advantages in performance and size over other filter technologies. In a typical surface acoustic wave device, one or more inter-digitated transducers (IDTs) are formed over a surface propagating substrate and are used to convert acoustic waves to electrical signals and vice versa by exploiting the piezoelectric effect of the substrate. An inter- digitated transducer (IDT) comprises opposing "electrode combs" with inter-digitated metal fingers disposed on a piezoelectric substrate. A Rayleigh surface acoustic wave develops on the substrate by electrically exciting the fingers. The other wave types, shear and longitudinally polarized wave, travel in the volume and get absorbed, thus requiring optimized metal grating thicknesses to be used for filter applications. Conversely, an electrical signal can be induced across the fingers by a surface acoustic wave propagating in the piezoelectric substrate material beneath the transducer. SAW devices commonly use wafers made from a monolithic Quartz, LiNbO ₃ or LiTaO ₃ crystals as piezoelectric materials. However, the use of piezoelectric substrates leads to either high sensibility to temperature in the case of LiNbO ₃ or LiTaO ₃ or weak electromechanical coupling in the case of Quartz depending on the piezoelectric material used. Furthermore, elastic wave velocities are generally limited by the single crystal material properties, particularly considering phase velocity which remains between 3000 and 4000m/s most of the time. Indeed, in the case of Quartz, Rayleigh surface acoustic waves are the most used modes and their phase velocity ranges from 3000 to 3500 m.s ^-1 . The use of shear waves allows for phase velocity up to 5100 m.s ^-1 . In Quartz, coupling is limited to 0,5%. In the case of Lithium Tantalate, Rayleigh waves exhibit phase velocity in the range 3000-3500 m.s ^-1 but the mode coupling may hardly reach 2%. Rayleigh waves on Lithium Niobate reach phase velocities up to 3900m.s ^-1 with a coupling factor of 5,6%, potentially achieving 8% with using a SiO ₂ passivation layer above the IDTs. Shear waves also called pseudo modes, on LiTaO ₃ and LiNbO ₃ are exhibiting radiation leakage, so called leaky modes. In that case, the surface is partially guiding the waves. Therefore, the electrode grating plays a major role in trapping the energy close to the surface. The phase velocity is in the range of 4000-4500 m.s ^-1 for both materials. Finally, compressional modes can also be excited on LiTaO ₃ and LiNbO ₃ substrates along certain crystal cuts but also here, the modes are leaky by nature, thus requiring specific electrode thicknesses vs frequency to minimize leakage effects due to wave radiation into the bulk. One approach to overcome the leakage effects has led to the use of composite substrates. A composite substrate comprises a piezoelectric layer formed over a base substrate. A composite substrate gives a large choice of materials for the base substrate and base substrate materials with a high acoustic wave propagation velocity can be chosen, such as Diamond, Sapphire, silicon carbide or Silicon. Similar to optics, the use of such a base substrate leads to the guidance of the mode. Composite substrates can combine strong electro mechanical coupling, i.e. an electromechanical coefficient k _s ² larger than 1%, and temperature stability, i.e. a temperature coefficient of frequency (TCF) smaller than 20ppm/K, and can improve the performance of the SAW devices and provide flexibility of design. However, acoustic wave devices are limited to operating frequencies from about 1 to 3 GHz, as for the given phase velocities, the electrode pitch or mechanical period p of the comb electrodes determines the wavelength l of the acoustic wave given by the relation p=l/n, with n ≧ 2, generally equal to 2. Operation at frequency above 2 GHz requires metal dimension and thickness of the order or below 100nm, which present stability problems of the structure. Thus, in practice, it is difficult to further miniaturize comb electrodes when higher operating frequencies are required. This is on the one hand due to the need to use higher resolution lithography technology compared to the I-line lithography used today in SAW industry, and on the other hand due to electric losses occurring in the structure. Therefore, to create SAW devices above 3 GHz, a strong technological effort is required. The object of the invention is thus to overcome the drawbacks cited previously by providing an inter-digitated transducer structure for an acoustic wave device with improved parameters and to provide an acoustic wave device that can function for frequencies exceeding 3 GHz but that can still be manufactured using standard I-line lithography. The object of the invention is achieved by a transducer structure for an acoustic wave device comprising: a piezoelectric layer, a pair of inter-digitated comb electrodes, comprising a plurality of electrode means with a pitch p characterized in that the electrode means of the inter-digitated comb electrodes are embedded in the piezoelectric layer and the acoustic impedance of the electrode means is less than the acoustic impedance of the piezoelectric layer. According to a variant the entire inter-digitated comb electrodes can be embedded. In the inventive device, because the electrode means are embedded in the piezoelectric layer and with the acoustic impedance of the electrode means being smaller than the one of the piezoelectric layer a shear wave like propagating mode (electrode mode) essentially limited to the volume of the electrode means, can be excited. Actually, the boundary conditions are such that is becomes possible to excite that shear mode within the electrode means. Due to the acoustic impedance contrast, the reflection at the lateral edges of the electrode means is large enough to essentially confine the energy within the electrode. Due to the grating configuration by the interdigitated comb electrodes and in the presence of an alternating electrical polarity, some vibration is nevertheless induced within the piezoelectric layer that leads to a coherence between the vibration of the electrodes, which leads to the resonance phenomenon with phase vibration from one electrode the next neighbouring electrode being opposite. The operating acoustic wavelength l of the transducer is linked to the resonant frequency fr of the transducer structure as f _r = v / 2p = v / l, v being the acoustic wave propagation velocity in the acoustic wave propagating substrate. Thus, in the given geometry much higher equivalent phase velocities, e.g. even of the order of 10.000m.s ^-1 can be observed than mentioned above. Since the bulk acoustic wave inside the electrode means has a resonance at higher frequency compared to the guided wave in the piezoelectric layer in prior art constellations as described above, this mode allows the transducer structure to function at higher frequencies above 3 GHz, without being at the limit of the fabrication technology of I-line lithography compared to state of the art transducer structure. In particular, the ratio between acoustic impedance of the electrode means to the piezoelectric layer is preferably less than 0.5. Material combinations yielding an electro mechanical coupling of more than 2%, preferably more than 3%, favour the establishment of the mode inside the electrodes and a coherent coupling between the electrodes. According to a variant, the pitch p satisfies the Bragg condition given by p= l/2, l being the operating acoustic wavelength of said transducer. In this condition, the electrode mode is more efficiently excited. According to a variant of the invention, the metallization ratio a/p of the electrode means, with “a” the width and “p” the pitch of the electrode means, can be comprised between 0,3 and 0,75, in particular between 0,4 and 0,65. Using an a/p ratio in this range favours the formation of the excited bulk acoustic wave in the electrode means, and reduces or suppresses the contribution of the acoustic surface mode of the piezoelectric layer. According to a variant of the invention, the piezoelectric layer can be provided over a base substrate. Using a base substrate favours the confinement of energy near the surface and in particular, inside the electrode means. According to a variant, the transducer structure can further comprise an attachment layer, in particular silicon dioxide (SiO ₂ ) between the piezoelectric layer and the base substrate. Via an attachment layer, various materials can be used as piezoelectric layer and underlying base substrate to optimize the transducer structure. silicon dioxide can be used to improve the temperature coefficient of frequency (TCF). According to a variant, the transducer structure can further comprise a high velocity layer between the piezoelectric layer and the base substrate, wherein the high velocity layer is made of a material allowing a higher phase velocity of a shear wave than the material and crystal orientation of the piezoelectric layer. This will allow for accelerating the fundamental shear mode, the phase velocity of which can be larger than the slow shear bulk wave velocity also called surface skimming bulk wave (SSBW) of the base substrate, in which case the fundamental shear mode cannot be guided in the piezoelectric layer but will dissipate in the base substrate. According to a variant, the high velocity layer can be positioned between the attachment layer and the base substrate. By positioning the attachment layer between the piezoelectric layer and the high velocity layer, one can take advantage of the acceleration properties and at the same time the attachment process of the piezoelectric layer to the underlying structure does not have to be changed so that high quality piezoelectric layers can be obtained independently of the material choice of the high velocity layer. According to a variant, the transducer structure can further comprise a trap rich layer, in particular a polysilicon trap rich layer, between the piezoelectric layer and the base substrate. With the trap rich layer leak currents can be suppressed. According to a variant, the trap-rich layer can be positioned between the high velocity layer and the base substrate. In this order, the individual advantageous of the various layers can be kept in the overall structure. According to a variant, the transducer structure can further comprise a covering layer on top of the embedded electrode means and the piezoelectric layer. This further improves guiding of the shear mode in the electrodes and further reduces the possibility of electro migration. According to a variant, the covering layer can be made of a material and/or having a crystal orientation allowing a higher phase velocity of a shear wave than the material and/or crystal orientation of the piezoelectric layer. The material of the covering layer can thus be chosen such that the velocity of the fundamental shear mode becomes accelerated compared to its velocity in the piezoelectric layer. This promotes dissipation into the volume of the base substrate so that essentially only the electrode mode remains in the guiding domain of the piezoelectric layer. According to a variant of the invention, the transducer structure can further comprise a Bragg mirror underneath the piezoelectric layer and/or underneath the embedded electrode means. A Bragg mirror reduces energy loss towards the base substrate, and can add mechanical stability to the device structure. According to a variant of the invention, the thickness of the embedded electrode means can be less or equal the thickness of the piezoelectric layer. Thus, thicker electrodes compared to state of the art transducer structure are used and thereby, besides allowing the establishment of the shear mode in the electrodes, the stability of the electrodes can be increased and Ohmic losses reduced. According to a variant of the invention, the thickness te of the electrode mean l > t _e > 0,1*l. In this thickness range, it becomes possible to concentrate the electromechanical field near the surface, in a region of thickness comparable to that of the electrode for single crystal or of that of the piezoelectric layer plus electrode for composite wafers. This will result in an improved electromechanical coupling and spectral purity allowing for single or at least limited number of, mode contribution to the device response. According to a variant of the invention, the acoustic impedance of the base substrate of the composite substrate is of the order of the acoustic impedance of the piezoelectric layer, in particular within a range of plus or minus 25% of the acoustic impedance of the piezoelectric layer, more in particular within a range of plus or minus 15% of the piezoelectric layer. In particular, in case the electrodes have the same thickness as the piezoelectric layer, the impedance matching allows the confinement of the shear mode in the electrodes. According to a variant, the embedded electrodes can be filled into grooves in the piezoelectric layer. The grooves can have across-section with a pyramidal shape or a trapezoidal shape or a V-shape or a U-shape and/or wherein the side walls and/or bottom of the grooves have a convex or concave or scalloped shape. In particular, the pyramidal or trapezoidal shapes with the shorter of the parallel sides at the surface of the transducer structure, lead to improved quality factors compared to grooves with vertical walls. According to a variant of the invention, a dielectric layer can be provided on the bottom of the grooves. According to a variant of the invention, the side walls and bottom walls of the grooves can be covered with a conductive material and the remainder of the grooves can be filled with a dielectric material. According to a variant of the invention, the grooves can extend through the piezoelectric layer and the sidewalls of the grooves can be covered by a conductive material and the remainder of the grooves can be filled with a dielectric material. According to a variant of the invention, only the sidewalls towards the piezoelectric layer can be covered by the conductive material. According to a variant of the invention, the dielectric material can be a material with a higher shear wave phase velocity than the conductive material. As already explained this will allow accelerating the phase velocity of the fundamental shear wave so that it can be above the SSBW velocity and therefore allow for its dissipation in the base substrate. According to a variant of the invention, the dielectric material can have a temperature coefficient frequency with a sign opposite to the temperature coefficient frequency of the conductive material. Thus, the device can be used over a wider temperature range. According to a variant of the invention, the dielectric material of the covering layer and the dielectric material filled in the grooves can be the same. Thus, in one process step both advantageous features can be realized. According to a variant, the material of the electrode means can be made of a material that is lighter than Manganese, in particular aluminium or an aluminium alloy comprising Cu, Si or Ti. In particular, the combination of Aluminium with Lithium Tantalate yields at coupling factors of more than 3% while the fundamental shear mode in the piezoelectric layer is absent. According to a variant, the piezoelectric layer can be lithium tantalate (LiTaO ₃ ) or lithium niobate (LiNbO ₃ ). For both materials, composite substrates, in particular so called Piezo on Insulator substrates (POI), can be obtained in an industrial scale. According to a variant, the base substrate can be silica, silicon dioxide or glass. quartz or fused quartz or glass or LiTaO ₃ or LiNbO ₃ or silicon, in particular Si(111). Using these substrates, the shear mode inside the electrode means remains, whereas the fundamental shear mode in the piezoelectric layer can be suppressed. In Si(111) the SSBW velocity is particularly low compared to Si(100). At the same time, a piezoelectric layer on glass or SiO ₂ substrates can be obtained in an industrial scale, e.g. by using the piezoelectric-on-insulator (POI) substrates. According to a variant, the high velocity layer is one of AlN, Al ₂ O ₃ , Si ₃ N ₄ , SiC or carbon based, in particular single crystal diamond, amorphous cabide, nano-gran polycrystal diamond. According to a variant, the covering layer can be one of AlN, Al ₂ O ₃ , Si ₃ N ₄ , SiC or carbon based, in particular single crystal diamond, amorphous carbide, nano-grain polycrystal diamond. According to a variant, the dielectric material can be carbon based, in particular single crystal diamond, amorphous carbide, nano-grain polycrystal diamond, or AlN or SiO2. These materials allow reducing the fundamental shear mode contributions. The use of SiO ₂ can improve the TCF properties. According to a variant, said pair of inter-digitated comb electrodes can comprise one region or more regions in which two or more neighbouring electrode means belong to the same comb electrode while having the same distance to each other as the neighbouring electrode means belonging to different comb electrodes. Here the distance is an edge to edge distance which corresponds to the pitch p defined above. According to a variant, the two or more neighbouring electrode means belonging to the same comb electrode have the same geometry as the neighbouring electrode means belonging to different comb electrodes. By having neighbouring electrode means linked to the same potential, but still having the same mechanical periodicity, acoustic sources are removed from the system so that the electro mechanical coupling is reduced. This can be used to tailor the electro mechanical coupling and thereby, when forming filters, adjusting the transfer function of a filter, namely by fine-tuning the width of the band pass. According to a variant regions with two or more, in particular at least three neighbouring electrode means can belong to the same comb electrode are not periodically distributed, in particular are randomly distributed. They are in particular characterized in that neighbouring regions have differing distances with respect to each other over the extend of the transducer structure. By reducing the symmetry of the system, spurious contributions of higher order periodicities can be reduced or even suppressed. According to a variant, regions with two or more neighbouring electrode means belonging to the same comb electrode can have different numbers of neighbouring electrode means belonging to the same comb electrode. By having regions with different numbers of neighbouring electrode means connected to the same potential spurious contributions can be further reduced. According to a variant, the electrode means of the transducer structure can have dimensions that are realizable by I-line lithography, in particular having a width larger than 350nm. Thus cheaper lithography means compared to lithography tools using a 248nm or 193nm or even shorter wavelength can be used to fabricate devices that can be used with frequencies above 3 GHz. The object of the invention is also achieved with an acoustic wave device comprising at least one transducer structure as described previously, in particular, an acoustic wave resonator, and/or an acoustic wave filter and/or an acoustic wave sensor. The acoustic wave device, which can be operated at frequencies of more than 3 GHz can be manufactured using i-line lithography technology without using further advanced and therefore more expensive lithography tools. Thus, compared to a Rayleigh surface wave using SAW device, which has its frequency limited to at most 2 GHz, using the electrode shear wave mode allows extending the frequency range without having to change the patterning technology. When using composite substrates a reduced first order temperature coefficient of frequency (TCF) smaller than 20ppm/K can be achieved, resulting in device performances being stable with temperature. Using the invention, acoustic wave bandpass filter with relative bandwidths larger than 5% or even 10% and up to 15% can be achieved. According to a variant, the acoustic device can comprise a radio frequency (RF) supply means configured to drive the transducer structure with a RF signal above 3 GHZ. Thus using I-line lithography devices working above 3 GHz can be realized. According to a variant, the acoustic wave device can comprises input and output transducer structures as described previously. The object of the invention is also achieved with a method of using a transducer structure as described above, comprising a step of applying an alternating potential to the two interdigitated electrodes to excite a shear mode predominantly or exclusively occurring within the electrode means compared to the piezoelectric layer and having an equivalent velocity higher than the fundamental shear wave mode of the piezoelectric layer. At least, the vibration amplitude in the electrode means is larger than in the piezoelectric layer. The use of the higher frequency of the shear wave propagating in the electrode means of the transducer structure as described previously, in particular for a ladder filter and/or impedance filter and/or coupling filter, leads to devices that function above 3 GHz, more in particular above 3,5 GHz. At the same time, I line lithography can be used to form the device. The object of the invention is also achieved with a method of using a transducer structure like described above, comprising a step of applying an alternating potential to the two interdigitated electrodes to excite a shear mode in the electrode means with a pair number of neutral lines without exhibiting shear movement inside the electrode and having an equivalent velocity higher than the fundamental shear wave mode of the piezoelectric layer. Exploiting this mode allows to take advantage of the higher resonance frequency so that devices like a ladder filter and/or impedance filter and/or coupling filter can be realized that function above 3 GHz, in particular above 3,5 GHz, while at the same time I-line lithography is used to form the pattern of the device. The invention may be understood by reference to the following description taken in conjunction with the accompanying figures, in which reference numerals identify features of the invention. Fig.1 illustrates an inter-digitated transducer structure on a composite substrate for acoustic wave devices according to a first embodiment of the invention. Fig. 2 illustrates schematically the mode obtained with the transducer structure illustrated in Fig.1. Fig.3a shows the simulated wide band harmonic admittance of an excited mode according to an example of the first embodiment of the invention. Fig.3b shows a mesh view illustrating the vibration of the mode observed. Fig.3c shows a mesh view illustrating the vibration of the mode at phase opposition. Fig.3d shows a zoom of the simulated wide band harmonic admittance of the excited mode as shown in Fig.3a. Fig.3e shows the conductance and resistance of the excited mode. Fig.3f shows the dispersion properties of the excited mode. Fig 4a to 4c illustrates three different electrode shapes within a piezoelectric layer, Fig. 4a according to a second embodiment, Fig. 4b according to the first embodiment and Fig. 4c according to a third embodiment. Fig 4d illustrates the harmonic susceptance corresponding to the three different electrode geometries. Fig 4e illustrates the harmonic conductance corresponding to the three different electrode geometries. Fig.4f to 4h illustrate further variants of electrode shapes with concave, convexe or scalloped side walls. Fig. 4i and 4j illustrate two further variants of electrode shapes, with a dielectric layer in the groove. Fig. 5 illustrates the effect of the thickness of the electrodes on the occurrence of the shear mode confined within the electrodes. Fig.6 illustrates an inter-digitated transducer structure for acoustic wave devices according to a fourth embodiment of the invention. Fig.7a illustrates an inter-digitated transducer structure for acoustic wave devices according to a fifth embodiment of the invention. Fig.7b shows the simulated wide band harmonic admittance of an excited mode according to the fifth embodiment of the invention. Fig.7c shows the simulated wide band harmonic admittance of an excited mode according to a variant of the fifth embodiment of the invention. Fig.7d shows the simulated wide band harmonic admittance of an excited mode according to a variant of the fifth embodiment of the invention. Fig.8 shows a sixth embodiment of the transducer structure according to any one of the first to fifth embodiments of the invention. Fig.9a illustrates a seventh embodiment of the invention and relates to a filter at 5GHz. Fig 9b illustrates the transfer function of the filter according to Fig.9a. Fig.10a illustrates a higher order mode according to the invention. Fig 10b illustrates the harmonic conductance and resistance of this mode for LiTaO ₃ and SiO ₂ substrates. Fig 10c illustrates the harmonic conductance and resistance of this mode for Al2O ₃ and Si substrates. Fig.11 illustrates an eighth embodiment of the invention with suppressed acoustic sources. Fig.12a to 12c illustrate three variants of a ninth embodiment of the invention. Fig.13a to 13d illustrate the process to obtain the shape of the electrodes as illustrated in Fig. 12a. Fig.14a and 14b illustrate the admittance and impedance of the transducer structure according to the ninth embodiment when using diamond like carbon as dielectric. Fig 14c illustrates a mesh view illustrating the vibration of the mode observed in the transducer structure according to the ninth embodiment. Fig.15a and 15b illustrate the admittance and impedance of the transducer structure according to the ninth embodiment when using aluminium nitride as dielectric. Fig.16a illustrates the conductance G and resistance R, Fig. 16b a zoom on the resonance and Fig.16c a zoom on the anti-resonance of the transducer structure according to the ninth embodiment when using silicon dioxide as dielectric. Fig.16d illustrates a fourth variant of the transducer according to the ninth embodiment. Fig. 17a and 17b illustrate two variants of a transducer structure according to a tenth embodiment. Fig. 18a and 18b show two images taken by an electron microscope showing a practical example of the transducer according to the invention. Fig. 18c illustrates the finite element mesh used to simulate the behaviour of the practical example shown in Fig.18a and 18b. Fig. 19a illustrates the experimental measurements of susceptance and resistance of the practical example. Fig.19b illustrates the results of susceptance and resistance obtained by numerical simulation of the structure as shown in Fig.18c. The invention will now be described in more detail using advantageous embodiments in an exemplary manner and with reference to the drawings. The described embodiments are merely possible configurations according to the invention and it should be borne in mind that the individual characteristics as described above can be provided independently of one another or in combination to achieve further embodiments according to the invention. Fig.1 illustrates an inter-digitated transducer structure for acoustic wave devices according to a first embodiment of the invention. The transducer structure 100 comprises an acoustic wave propagating substrate 102. The acoustic wave propagating substrate can be a composite substrate 102, comprising a piezoelectric layer 104 formed on top of a base substrate 106. The composite substrate can be a so-called piezoelectric-on-insulator substrate or POI substrate. In other embodiments, the piezoelectric layer can be so thick to form a bulk material. The piezoelectric layer 104 herein described by way of example may be Lithium Niobate (LiNbO ₃ ) or Lithium Tantalate (LiTaO ₃ ). The piezoelectric material layer 104 can be attached to the base substrate 106 by direct bonding e.g. using a SmartCut ^TM layer transfer technology. The thickness of the piezoelectric layer 104 formed on the base substrate 106 is of the order of one wavelength l or smaller, in particular is of about 2 µm or less, in particular less than 0,5l, even more in particular less than 0,4l. The piezoelectric layer herein described by way of example may be Lithium Niobate (LiNbO ₃ ), in particular LiNbO ₃ with a crystal orientations defined according to standard IEEE 1949 Std- 176 as (YXl)/ q with 36°< q<52° or 60°< q<68° or 120°< q<140° and as (YXt)/ Y with 85°< Y < 95° and as (YXwlt)/F qY with F =90°, -30°< q<+45° and 0°< Y <45° or Lithium Tantalate (LiTaO ₃ ), in particular LiTaO ₃ with a crystal orientation defined according to standard IEEE 1949 Std-176 as (YXl)/ q with 36°< q<52°, more in particular LiTaO ₃ with a 42° Y-cut, X- propagation, defined as (YXl)/42° cut according to standard IEEE 1949 Std-176. As already mentioned, according to a variant, the thickness of the piezoelectric layer 104 can be superior to the wavelength l, so that a piezoelectric layer thickness equivalent to a bulk piezoelectric material can also be used. The base substrate 106 used in the first embodiment of the invention is a silica substrate. silica, quartz, fused quartz or glass. For this kind of substrate the slow shear bulk wave velocity (SSBW) is inferior to the one of the fundamental acoustic bulk shear mode in the piezoelectric layer so that the bulk shear mode of the piezoelectric layer is suppressed by radiation from the surface to the bulk and diffusion. Other substrates with a high acoustic wave propagation velocity equal or larger than 4500m/s, like silicon, diamond, sapphire, silicon carbide, silicon nitride or aluminium nitride may be used but in that case the fundamental guided shear mode, corresponding to the one excited when electrodes are positioned on top of the piezoelectric layer, can exist. In this embodiment, the thickness of the base substrate 106 is larger than the thickness of the piezoelectric layer 104. A preferred situation corresponds to a base substrate thickness that is at least ten times larger than the thickness of the piezoelectric layer 104, in particular 50 to 100 times larger. Furthermore, the acoustic impedance of the base substrate 106 of the composite substrate 102 is of the order of the acoustic impedance of the piezoelectric layer, in particular is within a range of plus/minus 25%, more in particular within a range of plus/minus 15%. According to a variant, they are the same. In a variant of the invention, the base substrate 106 may further comprise a trap-rich layer close to the top layer of piezoelectric material. Such a trap-rich layer can improve the isolation performance of the base substrate 106 and may be formed by at least one of polycrystalline, amorphous, or porous materials such as for instance polycrystalline Silicon, amorphous Silicon, or porous Silicon. Under the term “trap-rich” one understands a layer that can absorb electrical charges without, however, forming a conductive layer. The composite substrate can be obtained by a layer transfer method whereby a piezoelectric layer is transferred to a base substrate. Methods like bonding and thinning or a layer transfer method like Smart-Cut ^TM allowing subwavelength layer transfer can be used. Such composite substrate may comprise an additional layer or layer stacks as for instance bonding or attachment layers, in particular SiO ₂ , or other functional layers, e.g. trap-rich, Bragg mirror, low velocity/high velocity stack. In a variant, the base substrate 106 may be a semiconductor on insulator (SOI) substrate. A Silicon on Insulator substrate (SOI) is obtained by molecular adhesion for instance using an intermediate SiO ₂ (bonding) layer and a mean to transfer a Silicon layer such as the said Smart-Cut ^TM process. The transducer structure 100 further comprises a pair of opposing inter-digitated comb electrodes 108 and 110, each of which has a plurality of electrode means, 112_i and 114_j (here 1 <= I,j <=4) respectively, extending from their respective conducting portion 116 and 118 and being inter-digitated with each other. The comb electrodes 108 and 110 and in particular the electrode means 112_i, 114_j are formed of any suitable conductive metal as long as the acoustic impedance is lower than in the piezoelectric layer 104, for example pure Aluminium or alloy such Al doped with Cu, Si or Ti. In general, electrode materials that are lighter than Manganese, thus starting from Chromium or lighter are suited. According to this embodiment, the aspect ratio a/p, with a the width and p the pitch of the electrode means 112_i, 114_j is comprised between 0,3 and 0,75, in particular between 0,4 and 0,65. The metallization or aspect ratio a/p and the thickness t _e of the electrode means 112_i, 114_j are the parameter to control radiation losses and electromechanical coupling in the device. An electrical load 120 is illustrated as being coupled across the comb electrodes 108, 110. However, it will be understood that a source potential 120 may also be coupled across the electrodes 108, 110, depending upon whether the transducer 100 is utilized to excite acoustic waves in the substrate 102 or to convert received acoustic waves to electrical signals, or both. The electrode means, for example 112_1 to 112_4 and 114_1 - 114_4 are inter-digitated and are connected to alternating potentials via their respective comb electrodes 108 and 110. The alternating potential can be +V and -V as illustrated or a mass and a load/source potential. In this embodiment, the electrode means 112_i, 114_j all have the same length l, width a, and thickness te. According to a variant of the invention, the electrode means 112_i, 114_j can also have different length l and/or width a and/or thickness te. The electrode means 112_i and 114_j and their respective comb electrode 108 and 110 are provided in the same plane. According to a variant, the electrode means 112_i ad 114_j are embedded into the piezoelectric layer 104 and the comb electrodes 108, 110 providing the electrical connection between the electrode means could be placed above the piezoelectric layer 104. Furthermore, an electrode pitch p, defined as l/2, l being the operating wavelength of the acoustic wave, is used for the transducer structure 100, corresponding to the Bragg condition. The electrode pitch p corresponds to the distance between two neighbouring electrode means from opposite comb electrodes 108 and 110, e.g. between 112_3 and 114_3. In this case, the wavelength l corresponds to the distance between two neighbouring electrode means from the same comb electrodes 108 or 110, e.g. between 112_3 and 112_4. In such Bragg condition, the transducer is said to be working in a synchronous mode at the operating frequency f _r , where all the excited acoustic waves in the transducer structure are coherent and in phase. Thus, the electrode pitch p defines the frequency of utilisation of the transducer structure. The operating frequency fr is fixed by the condition of phase agreement given by v/2p, v being the effective phase velocity of the acoustic wave propagating in the transducer structure 100 and p the electrode pitch of the transducer structure 100. The electrode means 112_i, 114_j are embedded in the piezoelectric layer 104, preferably fully embedded in the piezoelectric layer 104, so that their thickness t _e is equal to or less than thickness t of the piezoelectric layer 104. The thickness of the electrode means 112_i to 114_j should satisfy the following relationship with respect to the wavelength l: 0,1 < t _e / l< 1. Therefore, the electrode means 112_i to 114_j are thicker than the electrodes of state of the art inter-digitated transducer structures with the interdigitated electrodes formed over the piezoelectric substrate. They therefore offer a better stability and reduced electric losses. Furthermore, power handling is improved as embedded electrodes limit the possibility of acousto-migration and electro-migration due to power effects. The metal is located in a groove, which prevents direct metal contact due to surface diffusion and metal migration. The conducting portions 116, 118 of the comb electrodes 108, 110 can be provided on the piezoelectric layer 104 and/or the embedded electrode means 112_i, 114_j. The charge distribution in the electrodes 108 and 110 of the transducer structure 100 excites an acoustic wave in the electrical field direction, meaning perpendicularly to the extension direction z of the electrode means 112_i, 114_j of the electrodes 108, 110, as shown by the arrow E in Fig.1. In SAW devices of the art different modes can exist, e.g. Rayleigh surface acoustic waves, Lamb waves or shear waves. In contrast thereto, the inventive transducer structure 100 enables a new mode which leads to a shear like mode that is mainly concentrated inside the electrodes, which will be called electrode mode in the following. This is because the electrode means are embedded in the piezoelectric layer and in that the acoustic impedance of the electrode means being smaller than the one of the piezoelectric layer. Actually, the boundary conditions are such that it becomes possible to excite that shear mode within the electrode means. The maximum of the vibration occurs in the centre of the electrodes and are changing signs from one electrode means to the next neighbouring electrode means. Due to the acoustic impedance contrast, the reflection at the lateral edges of the electrode means is large enough to essentially confine the energy within the electrode. Due to the grating configuration by the interdigitated comb electrodes and in the presence of an alternating electrical polarity, some vibration is nevertheless induced within the piezoelectric layer that leads to a coherence between the vibrations of the electrodes, which leads to the resonance phenomenon but still guidance of the mode can be obtained. The operating acoustic wavelength l of the transducer is linked to the resonant frequency fr of the transducer structure as fr = v / 2p = v / l, v being the acoustic wave propagation velocity in the acoustic wave propagating substrate. The mode is schematically illustrated in Fig.2, which is a top view onto the transducer structure 100a and the two neighboring electrode means 112_3 and 114_3. Numerical simulations were realized, see S. Ballandras et al, Finite-element analysis of periodic piezoelectric transducers Journal of Applied Physics 93, 702 (2003); https://doi.org/10.1063/1.1524711 and have evidenced the excitation of this mode in which shear movement is concentrated within the electrode means. Actually, shear motion is also present within the piezoelectric layer 104, the larger vibration magnitude occurs, however, in the electrodes with alternate vibration direction from one electrode to the other compared to the vibration magnitude in the piezoelectric layer 104. Fig.3a shows the simulated wide band harmonic admittance of an excited mode according to a first example of the first embodiment of the invention. For this embodiment, the transducer structure comprises Al-Cu electrodes embedded in a LiTaO ₃ piezoelectric layer over a silica substrate. The wavelength l of the transducer structure is equal to 2,8 ^m, thus p=1.4µm and the aspect ratio a/p of the electrodes is equal to 0,43. In this embodiment, like illustrated in Fig. 1, the electrodes 112_i, 114_j are embedded in the piezoelectric layer 104 and represent a continuous material from a geometrical point of view although physical properties of the resulting layer are periodically distributed. As can be seen from the results illustrated below, the signature of the mode is similar to a leaky-SAW propagating at an equivalent phase velocity about 9850m.s ^-1 , thus higher than the surface skimming bulk wave (SSBW) in silicon dioxide, being typically around 3750 m.s ^-1 . Indeed, considering the electrical period of the structure to be 2.8µm with a metallization ratio a/p of around 0,43 and the resonance frequency of 3,45 GHz, an equivalent velocity of about 9850m.s ^-1 is achieved. The velocity of the mode is obtained by the product of the resonance frequency 3,45 GHz times the electrical period 2,8µm. The velocity of the mode is computed as the sum of frequencies at the beginning and at the end of the stopband times the mechanical period, see further below. Fig.3a shows the simulation data obtained for a configuration in which the shear mode of the electrodes, as already illustrated in Fig.2, is excited. The graph represents the conductance G in S units on the left Y axis and the susceptance B in S units on the right Y axis, as a function of the frequency in MHz on the X axis, for both G harmonic and B harmonic. As can be seen, a wide band harmonic admittance is observed, with a shear resonance at 3,45 GHz. The shear resonance at 3,45 GHz is the predominant excited mode. Furthermore, two much smaller contributions can be seen around 1,5 GHz and around 7 GHz. Fig.3b and 3c illustrate deformations inside the structure of the transducer. The movement is concentrated to the area of the interdigitated electrodes 108, 110 only. The vibrations in the piezoelectric layer 104 were almost not visible in the simulation. According to the invention, the above-mentioned parameters were such that a shear bulk acoustic mode is excited predominantly within the metal electrode means 112_i, 114_j, polarized in the y direction and yielding a displacement in the z direction. The useful E field extends along the x direction. Thus, the vibration is mainly located within the electrodes with a deformation close to the fundamental shear bulk wave of the electrode itself. Although shear motion is also observed within the piezoelectric layer 104, the largest vibration magnitude occurs in the electrodes 112_i, 114_j with alternate vibration direction. The shear displacement direction is alternated from one electrode to another when the transducer 100 is excited by a +V/-V electrical polarization structure. This opposition in phase of the electrodes increases the accumulation of charges at the edge of the electrodes, thus increasing the excitation of the bulk mode in the electrodes 112_i, 114_j. The piezoelectric layer 104 is confining stresses on the electrode edges and the coherence of the vibration occurs when satisfying boundary conditions along the grating since the transducer structure operates at the Bragg conditions. Although shear motion is also present within the piezoelectric layer 104, the largest vibration magnitude occurs in the electrodes with alternate vibration direction from one electrode to the other. The predominant acoustic wave propagating in the transducer structure is thus a bulk shear wave that is essentially confined within the electrodes 112_i, 114_j. The resonant frequency fr of the transducer structure 100 is given by fr = v / 2p = v / l, v being the acoustic wave propagation velocity in the acoustic wave propagating substrate and ^l the operating acoustic wavelength of the transducer. Since the bulk acoustic wave in the electrode means has a resonance at higher frequency than the guided wave of the fundamental shear mode in the piezoelectric layer, this transducer structure can work at higher frequencies, in particular above 3 GHz. Thus, higher frequencies than with prior art devices can be exploited, without the need to use more sophisticated lithography tools to move to smaller features than the ones currently used in SAW industry, particularly I-line lithography stepper. A main aspect of the invention is thus that this type of structure enables the excitation of a shear mode mainly located in the electrode, resembling a shear bulk mode confined within the electrode means, as explained above. The electrode mode can be qualified as a pseudo or leaky mode, that is confined with a leakage which is reduced but not totally suppressed which leads to the synchronization between electrodes. This mode exhibits an equivalent velocity much larger than what is achievable with state of the art inter-digitated transducer structures on composite substrates exploiting the shear mode in the piezoelectric layer. The other modes, are much lower in intensity or even suppressed which is mainly due to the use of the silica substrates with the low SSBW velocity therefore allowing the standard shear mode to dissipate. The inventive configuration can be understood as an individual resonator network or grating, wherein the resonators are coupled via the piezoelectric layer, therefore even though the phase velocity is higher than compared to the fundamental shear mode, dissipation into the substrate is prevented, at least to a large extent, comparable thus to leaky modes. Fig.3d shows a zoom of the simulated wide band harmonic admittance of the excited mode shown in Fig.3a, according to the first embodiment of the invention. The graph shown in Fig. 3d represents the conductance in S unit on the left Y axis and the susceptance in S unit on the right Y axis, for a frequency range between 3400 to 3600 MHz on the X axis, for both G harmonic and B harmonic. A resonance around 3,475 GHz and an anti-resonance around 3,525 GHz can be seen. The resonance and anti-resonance are well separated and give a coupling factor of about 3%. The resonance also takes place at the beginning of the stop-band. In this particular case, a reflection coefficient of about 9%, a quality factor Q of the resonance of about 500, and for anti-resonance of about 1000 are achieved. The coupling factor can be improved by modifying the characteristics of the transducer structure, such as the aspect ratio a/p, the thickness of the electrodes and the materials used. In particular, the aspect ratio a/p and the thickness of the electrodes enable to control the velocity, the electromechanical coupling, the quality factor or radiation losses and the reflection coefficients. Fig. 3e shows the conductance and resistance of the excited mode for a variant of the first embodiment obtained for the same material choice but an aspect ratio a/p of 0,57. Also in this configuration, the shear mode concentrated inside the electrodes is excited. The graph represents the Conductance in S unit on the left Y axis and the Susceptance in S unit on the right Y axis, for a frequency range between 3450 to 3750 MHz on the X axis, for both G harmonic and R harmonic. A resonance around 3550 GHz and an anti-resonance around 3700 GHZ is obtained. In this case, increasing the aspect ratio a/p from 0,43 to 0,57 leads to an increase in equivalent phase velocity of the propagating mode to more than 10 km.s ^-1 and to an increase in coupling factor to more than 10%. However, the reflection coefficient is now less than 5%. At resonance, the Q factor is still equal to 500 but at the anti-resonance, the Q factor is now equal to 350. Fig. 3f illustrates the dispersion properties of the excited mode as illustrated in Fig. 3a, according to the first embodiment of the invention. In that case, the admittance is calculated for various normalized wavelengths near a value of 0,5 corresponding to the edge of the said first Brillouin zone which is also identified as the Bragg condition. For normalized wavelengths smaller than 0,5, the mode is less efficiently excited as one can see on the curve and at lower frequency. Considering this and by plotting the evolution of the maximum admittance modules versus frequency and normalized wavelength, the projected 2D representation is obtained recalling the classical dispersion curve of any wave propagation in a periodic lattice. As can be seen, the resonance occurs at the beginning of the stop-band. The choice of the substrate is important when other modes than the shear mode concentrated in the electrodes have to be suppressed or at least are only weak compared to the desired mode. As mentioned above, this condition is obtained when SSBW velocity in the substrate is lower than velocity of the fundamental shear mode in the piezoelectric layer. In this case, the fundamental shear mode will penetrate the substrate and its energy becomes dissipated. Furthermore, the acoustic impedance should be close to the one of the piezoelectric layer to favour the mode. Other substrates with a high acoustic wave propagation velocity equal or larger than 4500m.s ^-1 , like silicon, diamond, sapphire, silicon carbide or aluminium nitride may still be used but in that case the bulk shear mode can exist besides the mode of interest concentrated in the electrodes. That is also why it is not mandatory to use a composite substrate. Even bulk piezoelectric substrates of Lithium Tantalate or Lithium Niobate might be used with electrodes embedded in the surface area. Even in this case the electrode mode can be observed. Fig. 3g and 3h illustrate deformations inside another example according to the first embodiment. Here the shear movement is less concentrated to the area of the interdigitated electrodes 108, 110, but still predominant only. The vibrations in the piezoelectric layer 104 were almost not visible in the simulation. For this example of the embodiment, the transducer structure comprises Al-Cu (2% Cu) electrodes embedded in a LiTaO ₃ (YXI)/42° piezoelectric layer over a silica substrate. The wavelength l of the transducer structure is equal to 2,8 ^m, thus p=1,4µm and the aspect ratio a/p of the electrodes is equal to 0,5, with a groove depth of t _e /l=20%. In this embodiment, like for the first example illustrated in Fig. 1, the electrodes 112_i, 114_j are embedded in the piezoelectric layer 104. Fig. 3h is a view of the mode in the xz-plane illustrating that the electrode mode meets the above mentioned synchronism condition f = v/2p as indicated by reference numeral 130. Fig. 3h shows that the shear movement inside the electrodes 108, 110 is characterized by two neutral vibration points 132 in each electrode 108, 110. Furthermore the edges 134a, 134b, 136a, 136b of each electrode 108, 110 move synchronously. This indicates that within one electrode 108 or 110, electrical charges of the same sign are present at both interfaces. Fig. 4a to Fig. 4c illustrate three variants of such a realisation with three different electrode shapes considered for the excitation of the electrode-resonance mode. The figures illustrate a cut view in the x-y plane of Fig. 1 through one electrode means 112-i or 114-i. In all three figures, one mechanical period is represented. The embedded electrodes are filled into grooves in the piezoelectric layer. For simulation purposes (the results are illustrated in Figures 4d and 4f), the grating is assumed infinitely long and excited by a harmonic +V/-V excitation. The electrodes radiate energy into the bulk substrate from their bottom interface with the substrate. The Al electrode 201a, 201b, 201c is respectively represented by horizontal crosshatch, the piezoelectric layer 203, here LiTaO _3, is in inclined crosshatch and the bottom layer 205 in vertical crosshatch which can be either SiO ₂ or another interface material or the same material as the piezoelectric layer 203. Fig.4a illustrates a second embodiment of the invention. Here the groove in the piezoelectric layer 203 has a pyramidal or trapezoidal cross section. The electrode 201a filled into the groove there, according to a second embodiment of the invention, has a pyramidal or trapezoidal type shape in its cross-section. In this embodiment, the shorter side of the parallel sides of the trapeze being aligned with the upper surface of the piezoelectric layer 203. The electrode 201b, according to the first embodiment as illustrated in Fig.1, is of a vertical type illustrated in Fig.4b. The electrode 201c, according to a third embodiment of the invention, is filled into a groove having a trapezoidal shape but in contrast to the second embodiment, the short side of the parallel side is arranged inside the piezoelectric layer 203 and not at its surface. Thus, the electrode 201c has of a truncated V-shaped type. Thus, the electrodes fill grooves of varying shape inside the piezoelectric layer 203, which depending on its thickness behaves like a bulk material. The Figs. 4d and 4e illustrate the results of the simulation and provides a comparison of excitation efficiency of the electrode-resonance mode for the three electrode shapes: Fig.4d illustrates the harmonic susceptance and Fig.4e illustrates the harmonic conductance. The simulations clearly indicated that the pyramidal shape is of higher interest than the two others with, however, a lower coupling efficiency. Further design options are illustrated in Figs.4f to 4h. The variants are based on the design as illustrated in Fig.4b but can be adapted to the other embodiments as well. Fig.4f, illustrates an electrode 201d inside a groove in the piezoelectric layer 203 with sidewalls 207a that have a concave shape. Fig.4g illustrates an electrode 201e inside a groove in the piezoelectric layer 203 with sidewalls 207b that have a convex shape. Fig. 4h illustrates an electrode 201f inside a groove in the piezoelectric layer 203 with sidewalls 207c that have a scalloped shape. By increasing the surface of the sidewalls, more electrical charges of the same sign can be present at the interface, thereby improving the charge distribution contributing to the mode signature and therefore the strength of the mode excitation. It can also be noted that the Q factor of the mode, e.g. the trapping efficiency, depends on the shape of the electrode. Therefore, it is preferred to optimize both active surface and aspect ratio to improve the operation conditions for the electrode mode. In addition or as an alternative, the bottom of the grooves could also have convex or concave or scalloped shape. Fig.4i illustrates a further variant, based on the first embodiment as illustrated in Fig.4b which can also be adapted to the other embodiments and variants. A dielectric layer 211, e.g. Si ₃ N ₄ , is provided on the bottom 215 of the groove in the piezoelectric layer 203. Then the conductive layer 213 of the electrode is provided to fill up the groove. Due to the presence of the dielectric layer 211 no electric charges are present at the bottom of the electrode. Using a dielectric with a higher shear velocity than the conductive material can accelerate the fundamental shear mode so that its phase velocity is beyond the SSBW velocity of the bottom layer so that it becomes better absorbed in the bottom layer 205. This phenomenon will be described in more detail further below with respect to Figs.7 and 12. Fig. 4j illustrates a further variant, based on the previous variant. Here the conductive layer 217 is interfacing the bottom layer 205. The dielectric layer 211 is again screening the conductive layer 217 towards the piezoelectric layer 203 so that the conductive layer 215 interfaces the piezoelectric layer only via the side walls 219 and 221. It has been found that in case the conductive material of the electrode has no contact with the piezoelectric material above and below, like illustrated in Figs.4i and 4j, parameters like phase velocity, resonance Q factor, reflection coefficient and coupling factor k ² can be optimized compared to comparative structures without such features. All variants described above with respect to the Figs.4a to 4h can be realized according to the variant shown in Fig.4j with the electrode interfacing the bottom layer 205. Fig.5 indicates the effect of the thickness of the electrode means 112_i and 114_j on the shear mode within the electrodes. Here the harmonic analysis for Aluminium electrodes embedded in a (YXl)/42° Lithium Tantalate piezoelectric layer on a 325 nm thick silicon dioxide layer on Si(100) are illustrated for a 1 µm pitch and a metal/piezoelectric a/p ratio set to 0,35 is presented. Fig.5 shows the admittance for a relative metal layer height t _e /l from 5% to 15%. One can identify that only starting at a t _e /l ratio larger than 0,1 a resonance can be observed. Compared to the other examples illustrated above, the resonance occurs at a higher frequency around 5,15 GHz, which is due to the smaller pitch. Fig.6 illustrates an inter-digitated transducer structure for acoustic wave devices according to a fourth embodiment of the invention. The transducer structure 300 comprises a different acoustic wave propagating substrate 302 in comparison with the substrate 102 of the transducer structure 100 of the first embodiment, which is the only difference with respect to the first embodiment. All other features are the same and will therefore not be described in detail again but reference is made to their description above. The transducer structure 300 comprises a composite substrate 302 comprising, like the composite substrate 102, a piezoelectric layer 104, formed over the base substrate 306, but furthermore comprises an acoustic mirror 304, also called Bragg mirror, formed above the base substrate 306 and below the piezoelectric layer 104. The Bragg mirror 304 comprises a plurality of stacked layers 306 to 309, the layers with an even reference numeral 306, 308 being of a first material and the layers with an odd reference numeral 307, 309 being of a second material. The first and second materials have different acoustic impedances, so that the Bragg mirror 304 comprises a stacking of alternating high and low impedance layers. The Bragg mirror 304 has a periodical repetition of a pair of layers with a thickness of about a quarter of a wavelength with alternating high/low impedances, to ensure the reflection. The first and second materials can be chosen amongst Tungsten, Molybdenum, LiTaO ₃ , Al2O ₃ , AIN, LiNbO ₃ , Si ₃ N ₄ and any combination of SiO ₂ and Si ₃ N ₄ (known as silicon oxy-nitride and noted SiOxNy with x and y controlling the amount of each element in the compound) and ZnO, Aluminium or SiO ₂ . In a variant, the first material and the second material can be exchanged so that the first material has a low impedance and the second material has a high impedance. In this embodiment, the Bragg mirror 304 is represented as having four layers 306-309 forming the stack of alternating high and low impedance layers. However, in another variant, the Bragg mirror 304 can also have more or less than four layers of alternating high and low impedance forming the stack. Increasing the number of pairs in a Bragg mirror 304 increases the mirror reflectivity and increasing the impedance ratio between the materials in the Bragg pairs increases both the reflectivity and the bandwidth. A common choice of materials for the stack is for example Titanium dioxide and silica. According to the invention, the piezoelectric layer 104 and the Bragg mirror 304 are arranged such as to reduce the contributions of additional modes present in the structure, to promote a unique mode within the transducer structure 100 to ensure the spectral purity of an acoustic wave device based on such transducer structure 100, preventing spectral pollution. One approach is to optimize the thicknesses of the stack of the Bragg mirror 304 to promote the unique mode within the transducer structure, and to realize an efficient reflection coefficient for this mode. The Bragg mirror 304 thus acoustically isolates vibrations generated within the electrode means 112, 114 from the base substrate 106. Fig.7a illustrates an inter-digitated transducer structure for acoustic wave devices according to a fifth embodiment of the invention. The transducer structure 400 comprises a covering layer 402 on top of the transducer structure 100 of the first embodiment, which is the only difference with respect to the first embodiment. All other features are the same and will therefore not be described in detail again but reference is made to their description above. The transducer structure 400 comprises a composite substrate 102 comprising a piezoelectric layer 104 formed over the base substrate 106. In this embodiment of the invention, a layer 402 is present on top of the embedded electrodes 108, 110 and of the piezoelectric layer 104. The layer 402 can be a passivation layer or a guiding substrate, comprising a high velocity low loss material such as Silicon, Sapphire Al2O ₃ , Garnets i.e. Yttrium-based materials, aluminium nitride AlN, silicon carbide SiC, silicon nitride Si ₃ N ₄ . According to a further variant, layer 402 could also be a carbon based layer, e.g. single-crystal diamond, an amorphous carbide layer, nano-grain polycrystal diamond (NCD) or an all diamond like carbon layer that could push compressional wave velocity above 15 km.s ^-1 and shear wave velocity above 7 km-s ^-1 . In yet another variant, a layer of SiO2 can be used as layer 402. SiO2 can play the role as a TCF corrector, to improve the TCF value of the overall structure. The covering layer 402 can also be made of glass, e.g. as TCF corrector, and in general Silicon based-substrates. Using a high velocity low loss material accelerates the phase velocity of the fundamental shear wave mode above or further above the SSBW velocity of the base substrate so that the unwanted mode can be suppressed by dissipation into the base substrate 106. In the embodiment shown in fig.7a, the covering layer 402 is made of the same material as the base substrate 106 of the composite substrate 102. However, the covering layer 402 can be different from the base substrate of the composite substrate. According to a further variant, the layer 402 can be present only on top of the electrode means 108, 110, or only on top of the piezoelectric layer 104. Fig. 7b to Fig. 7d show the simulated wide band harmonic admittance of an excited mode according to the fifth embodiment of the invention for an acoustic wavelength of 2,8 µm, an a/p ratio of 0,5 with Aluminium electrodes and (YXl/42°) Lithium Tantalate as the piezoelectric layer 104. For Fig.7b, the covering layer 402 and the base substrate 106 is silicon dioxide. The excited mode occurs at 3,5 GHz. For Fig.7c, the covering layer 402, the base substrate 106 and the piezoelectric layer 104 is (YXl/42°) Lithium Tantalate. The excited mode occurs as well at around 3,5 GHz. There is a further contribution visible at about 6,5 GHz, which can be attributed to a third harmonic. It is, however, essentially present in the conductance, the corresponding susceptance has no sign change, indicating low coupling. For Fig. 7d, the covering layer 402 and the base substrate 106 is a Silicon substrate. The excited mode occurs at 3,5 GHz, but here the fundamental shear mode in the piezoelectric layer is also excited and visible at lower frequency, namely at about 1,8 GHz. The invention also relates to an acoustic wave device comprising two transducer structures each one according to any one of the first to fifth embodiment of the invention. In an alternative, only one of the two transducer structures can be a surface acoustic device according to the invention whereas the other one can be according to the state of the art. The acoustic wave device can be an acoustic wave resonator and/or an acoustic wave filter and/or an acoustic wave sensor and/or a high frequency source. The acoustic device can comprise a radio frequency (RF) supply means configured to drive the transducer structure with a RF signal above 3 GHZ. Fig.8 shows a sixth embodiment of the transducer structure. The transducer structure 500 of Fig.8 differs from the transducer structure of Fig.1 in that the interdigitated electrodes 512, 514 only partially fill the grooves 510 in the piezoelectric regions 504. According to the manufacturing process, the thickness of the metallic layer is not constant over the entire removed region 510. Due to surface energy properties, the thickness of the metallic layer at the side-wall 508 is superior to the thickness within the centre part. The transducer structure 500 functions in the same way as the transducer structures described above. The use of the frequency of the bulk wave propagating in the electrode means of a transducer structure as described above in an acoustic wave device, in particular for a ladder filter and/or impedance filter and/ coupling filter, allows to generate contributions at high frequency, in particular above 3 GHz, more in particular above 3,5 GHz. Using such embedded electrode means for the transducer structure, the performance of acoustic wave devices as well as their application ranges can be improved compared to bulk piezoelectric substrates without having to change the manufacturing tools, namely I line lithography. Fig.9a and Fig.9b illustrate as seventh embodiment a practical example of a filter illustrating an effective implementation of the invention according to a typical market demand, e.g. for 5G sub-6-GHz (C-band) filtering. The practical example is based on Al electrodes embedded in LiTaO ₃ on silica with the following parameters: pitch p=1µm yielding a wavelength l of 2 µm thus providing a resonance near 5 GHz, like illustrated in Figure 5 and a thickness of 700nm for the electrodes and the piezoelectric layer, thus a ratio of te/l of 0,35. According the usual approach of SAW ladder filter design, the resonance of the series branch occurs at anti- resonance of the parallel branch. This example according to the invention can be manufactured in a single batch using standard SAW manufacturing technology, e.g. using I line lithography and a single metal layer deposition. A fine-tuning of the resonance frequencies using the grating pitch p and/or the aspect ratio a/p and/or the t _e /l ratio can be realized. A passivation layer or Bragg mirrors like illustrated in Fig.7a and Fig 6 might be used as well to improve the characteristics. The filter of the practical example is based on the basic transducer structure as illustrated in Fig.1. Based on that approach, resonators are formed and combined in series and parallel to form cells that can be arranged as cascades or as ladder-filter structures like known in the art. In the practical example, two gratings of 35% with a respective material ratio a/p = 0,6 and 0,65 were used. In both cases, harmonic admittance and impedance were computed and are illustrated in Fig.9a. The resonance of the a/p=0,65 grating is close to the anti-resonance of the a/p=0.6 grating, which is the prerequisite for the design of a ladder filter as mentioned above. By combining the responses the transfer function of a 4 p-cell filter as known in the art, can be computed, the result of which is illustrated in Fig.9b. The transfer function illustrates that an ultra-compact filter operating at more than 5 GHz with electrodes as thick as 700 nm allows improved power handling as the electrodes are embedded according to the invention. In this configuration physical migration is absent. The inventive filter provides a coupling factor ks² in excess of 10% for LiTaO ₃ . This could be improved by using LiNbO ₃ taking into account the scaling between the two materials. The proposed design shows a bandwidth of 300 MHz. By tuning the resonance-anti-resonance condition an improved match can lead to high band width of the order of 400 MHz as well. Like mentioned above tuning can consist in adjusting the pitch and/or the ratio a/p. Fig. 10a illustrates that also higher harmonic modes of the electrode shear mode of the electrodes can be excited. Fig.10a actually illustrates the shear movement of a third harmonic. In this mode, four neutral vibration points or lines are observed in each electrode. The electrode shear mode as shown Fig.10a is obtained for a transducer structure with a 700 nm thick Al vertical electrodes, metal ratio a/p is 0,5, pitch is 1,4µm and the piezoelectric layer LiTaO3 (YXl)/42°. Contrarily to fig.7b, here the 3rd harmonic yields significant vibration at the edge of the electrode thus generating a signature of the mode for which a coupling can be characterized. In Figure 10b, comparison is achieved for LTO, Si, SiO ₂ and Sapphire substrate considering a pyramid shape of the electrode, like illustrated in Fig.4a, yielding better Q than the other types of electrodes. The other parameters are the same as for Fig.10a. Fig.10b and Fig.10c illustrate the corresponding harmonic conductance G and susceptance B for various base substrates, LiTaO ₃ , SiO ₂ , Si and sapphire. These modes are of interest as due to the high resonant frequency around 8,75 GHz they show a high equivalent velocity. Using a transducer structure according to the invention to excite the corresponding mode can advantageously be used in the development of high frequency sources. Fig 10c, illustrating a sign change for the susceptance, indicates that coupling of the mode is effective. Fig. 11 illustrates an eighth embodiment of the invention. It illustrates an inter-digitated transducer structure 200 for a surface acoustic wave device according to a first embodiment of the invention. The inter-digitated transducer structure 200 comprises a pair of inter-digitated comb electrodes 202 and 204 each comprising a plurality of electrode means 1206 and 208 that are embedded into the piezoelectric layer 212. Like in the first embodiment, the electrode means 206 and 208 have the shape of fingers 206, 208. In a variant of the embodiment, the electrode means could also have spilt fingers 206, 208 comprising each two or more directly adjacent electrode fingers belonging to the same comb electrode. The piezoelectric layer 212 is part of a composite substrate 210 further comprising a base substrate 214. The piezoelectric layer is of the same material and has the same properties with respect to thickness as described in the other embodiments. The thickness of the base substrate 214 can be larger than the thickness of the piezoelectric layer 212 to impose its thermal expansion to the piezoelectric layer 212 and to reduce the transducer’s sensitivity to temperature changes. A preferred situation corresponds to a base substrate thickness that is at least ten times larger than the thickness of the piezoelectric layer 212. The base substrate 214 is of the same material as in the first embodiments. By using a different material for the base substrate 214, the flexibility in the design can be enhanced. The pair of inter-digitated comb electrodes 202 and 204 comprises a plurality of electrode fingers 206 and 208. The electrode fingers, for example 206_1,208_1 to 206_4, 208_4 respectively and 208_5, 206_7 to 208_8, 206_10 are inter-digitated and are connected to alternating potentials via their comb electrodes 202 and 204 and embedded in the piezoelectric layer 212. The alternating potential can be +V and -V as illustrated or in a variant a mass and a load/source potential. The electrode fingers are metallic and all have the same length l, width w, and thickness t. Furthermore, also here an electrode pitch p, defined as l/2, is used for the transducer structure 200. The number of electrode fingers is not fixed and the device can comprise more or less than illustrated in Fig.11. According to a variant of the invention, the electrode fingers 206, 208 can also have different length l, width w and thickness t _e . Like in the first embodiments, the thickness te is equal or less than the thickness of the piezoelectric layer 212. The eighth embodiment has the particularity in that the transducer structure 200 further comprises a region 218, also called second region, where two neighbouring electrode fingers 208_4 and 208_5, also meaning directly neighbouring electrode fingers, are connected to the same potential, here +V, without any electrode fingers 206 from the opposing inter-digitated comb electrode 202 inbetween. The two neighbouring electrode fingers 208_4 and 208_5 can also be connected to –V, or to mass, or to a load/source potential V _IN (not shown). In this context first region or first regions are parts of the transducer structure in which directly neighbouring electrode fingers belong to different comb electrodes. In the variant where the electrode means 206, 208 are represented by split fingers 206, 208 of two or more adjacent fingers at the same potential, two neighbouring electrode means 206, 208 connected to the same potential can refer to all fingers of the split fingers 206, which are connected to the same potential of the split fingers 208. But it could also be that at least one electrode finger of the split fingers 206 is connected to the same potential of the split fingers 208. In Fig.11, the region or second 218 is actually placed in the middle of the transducer structure 200, so that on each side, left and right, of the region 218, eight electrode fingers or four electrode finger pairs are present. In a variant of the embodiment, the region 218 can be placed at a different position in the transducer structure, so that the electrode finger pairs are distributed not evenly on either side of the region 218. The region 218 can also be placed on either extremity end of the transducer structure 200. As mentioned previously, the electrode fingers 206_1, 208_1 to 206_4, 208_4 respectively and 208_5, 206_5 to 208_8, 206_8 are inter-digitated and have alternating potentials. It can actually be seen, that due to the presence of the region 218, on the left side of the region 218, the inter-digitated electrode fingers 206_1, 208_1 to 206_4, 208_4 are at alternating potential -V /+V respectively, while on the right side of the region 218, the inter-digitated electrode fingers 208_5, 206_5 to 208_8, 206_8 are at alternating potential +V / -V respectively. A pair of neighbouring electrode fingers connected at alternating potential defines an electro- acoustic source. For example, here in Fig.11, the neighbouring inter-digitated electrode fingers 206_1 and 208_1 at alternating potential -V / +V define an electro-acoustic source 220. But the neighbouring inter-digitated electrode fingers 208_1 and 206_2 at alternating potential +V / -V define also an electro-acoustic source 222. Thus, the pair of neighbouring inter-digitated electrode fingers 206_2, 208_2 to 206_4, 208_4 each also defines an electro-acoustic source 220 and respectively, the pair of neighbouring inter-digitated electrode fingers 208_2, 206_3 and 208_3, 206_4 each also defines an electro-acoustic source 220. In particular, here, on the left of the region 218, four active electro-acoustic sources 220 and three active electro-acoustic sources 222, with in total eight inter-digitated electrode fingers 206_1, 208_1 to 206_4, 208_4 are present. On the right side of the region 218, a pair of neighbouring inter-digitated electrode fingers, for example 208_5 and 206_5 connected at alternating potential +V / -V defines also an electro- acoustic source 222, and the pair of neighbouring inter-digitated electrode fingers 206_5 and 208_6 at alternating potential -V / +V define an electro-acoustic source 220. On the right side of the region 218, four active electro-acoustic sources 222 and three active electro-acoustic sources 220, with in total eight inter-digitated electrode fingers 208_5, 206_5 to 208_8, 206_8 are present. But here, the electro-acoustic sources 220, 222 on the left side of the region 218 are in opposition of phase with the electro-acoustic sources 222, 220 on the right side of the region 218, in particular of p. However, as the electrode pitch p is defined as l/2, this signifies that the transducer structure 200 is operating in a synchronous mode, at the Bragg condition. Therefore, the plurality of electro-acoustic sources 220, 222 on the left side of the region 218 are all in phase and coherent with each other, while the plurality of electro-acoustic sources 222, 220 on the right side of the region 218 are all in phase and coherent with each other. In the region 218, there is no electro-acoustic source 220 or 222 between the two neighbouring electrode fingers 208_4 and 208_5, as they are both connected to the same potential. According to a variant, the polarity of the potential could be swapped between the first and second inter-digitated comb electrodes 206 and 208, or connected to mass on the one comb electrode and a load/source potential VIN on the other comb electrode. Due to the presence of the second region 218 in the transducer structure 200, the phase of the electro-acoustic sources within the transducer has been inverted by p, as the electro- acoustic sources on the left side of the region 218 are in opposition of phase with the electro- acoustic sources on the right side of the region 218. Thus, destructive interference is created between the electro-acoustic sources in the transducer by combining the energy emitted from each side of the two electrode fingers connected to the same comb electrode toward the transducer, whereas the energy emitted toward the outside of the transducer will actually be launched and reflected by the mirror, positioned on either side of the transducer structure in a SAW device. Therefore the amount of electro-acoustic sources present in the transducer structure 200 being coherent and in phase in the transducer structure 200 is reduced compared to a transducer structure of the same size, where all the electrode fingers are at alternating potentials, as e.g. shown in Fig 1. As a consequence, the electromechanical coupling coefficient k _s ² in the transducer structure has been reduced. Here, in this particular embodiment, the left and right side of the region 218 in the transducer structure 200 have exactly the same number of inter-digitated electrode fingers 206 and 208, namely eight, resulting in seven active electro-acoustic sources, as the region 218 is located in the middle of the transducer structure 200. Here, the electromechanical coupling coefficient k _s ² in the transducer structure 200 is reduced by a factor 2. Again, as destructive interference is created between the electro-acoustic sources in the transducer structure, by combining the energy emitted from each side of the two electrode fingers connected to the same comb electrode toward the transducer, whereas the energy emitted toward the outside of the transducer will actually be launched and reflected by the mirror. The transducer efficiency is therefore reduced by a factor of two. Furthermore, the possibility of phase coherence of the modes being reflected at the interface 216 of the composite substrate 212 is also modified compared to the prior art situation. If the phase is shifted within the transducer structure, there is no chance to detect waves, which do not match the phase matching condition. Therefore, there is a reduction in detection of the reflected acoustic waves from the interface 216, which will in turn lead to a reduction of the parasitic resonances at unwanted frequencies due to these reflections in the filter performance of a SAW device based on a transducer structure 200. Therefore, the generation and/or detection of acoustic waves in the transducer structure 200 according to the invention is controlled by the amount of electro-acoustic sources in phase present in the transducer structure 200. By having two neighbouring electrode fingers connected to the same potential results in a phase change of p within the structure which has a positive influence on the efficiency of the transducer structure to reject parasitic modes. It is not needed to vary the dimensions of the transducer, such as width or length or inter-electrode distance of the electrode fingers, which would have an impact on the fabrication technology of such structures and might significantly reduce the quality of the resonance of a resonator exploiting the above described transducer structures. According to a variant of the eighth embodiment, more than just one region 218 could be present in the transducer structure, therefore increasing the number of suppressed electro- acoustic sources in the transducer structure and thereby decreasing the electromechanical coupling coefficient k _s ² further. This is an efficient way to control the bandwidth of the filter, thus giving more degree of freedom to address various filter bands. According to another variant more than just two neighbouring electrode means 208_4 and 208_5, like three or more, could be linked to the same potential, thereby suppressing further sources. In addition, or according to another variant, more than one region with suppressed sources could be present. In that case, it would be advantageous to distribute them in a random manner over the extension of the transducer structure. In case there are more regions, the number of neighbouring electrode means linked to the same potential is different. Figs. 12a to 12c illustrate three variants of a ninth embodiment of the invention. Unlike the above described cases, as illustrated for example in Figs.1 or 4a to 4c, in which the grooves within the piezoelectric layer are filled exclusively with a conducting material, in particular a metal like Al or an Al alloy, the variants of the transducer structure of the ninth embodiment also comprise a dielectric material within the grooves of the piezoelectric layer. Besides this difference, the variants of the sixth embodiments have the same structural features and properties as the first embodiment and can be combined with any one or a combination of the other embodiments two to eight. Fig.12a illustrates a partial cut view of a transducer structure 600 of a first variant of the ninth embodiment with two neighbouring electrode means 612 and 614 each belonging to a different comb electrodes. The electrode means 612 and 614 are embedded in grooves 616 and 618 in a piezoelectric layer 604 provided on base substrate 606 via an attachment layer 608. The side and bottom walls of the grooves 616 and 618 in this embodiment are covered by a conductive material 620 and 622, e.g. Al or an Al alloy like described above. The remainder of the grooves 616 and 618 is at least partially filled up with a dielectric material 624 and 626, in particular diamond carbon. Fig.12b illustrates a transducer structure 650 according to a variant of the ninth embodiment. Elements with the same reference numeral as used for the first variant 600 will not be described again, but reference is made. In this variant, the grooves 652 and 654 extend through the entire piezoelectric layer until the attachment layer 608. Again the sidewalls of the grooves 652 and 654 and the bottom of the groove, now in contact with the base layer 606 attachment layer 608 are covered by the conductive material 656 and 658. The remainder of the grooves 652 and 654 is at least partially filled up with the dielectric material 660 and 662. The same materials as in the first variant can be used. The interest of this embodiment is to allow for using a material with a phase velocity larger than the one of the metallic material, in particular the Al-based metals. The consequence is that higher frequencies can be reached by this embodiment compared to embodiment 1. Fig.12c illustrates a third variant of a transducer structure 690. The only difference with respect to the second variant is that the dielectric material 692 and 694 extends through the entire thickness of the grooves 696 and 698 to reach the attachment layer 608, so that only the sidewalls of the grooves 696 and 698 are covered by the conductive material 656 and 658. Figs. 13a to 13d illustrate the method to obtain the first variant according to the ninth embodiment. The grooves 616, 618 are etched into the piezoelectric layer 604 as illustrated in Fig. 13a. Then a metal deposition step is carried out as illustrated in Fig.13b to cover the piezoelectric layer 604 and the walls of the grooves 616, 618 with a metallic layer 700. Subsequently, a dielectric layer 702 is deposited onto the metallic layer 700 such that the grooves 616 and 618 are at least partially filled up with the dielectric material. This is illustrated in Fig.13c. Finally, a polishing step is realized, e.g. a CMP polishing step to obtain the transducer structure 600. The second variant of the transducer structure 650 can be obtained by adapting the etching step such that the grooves reach through the piezoelectric layer 604 until the base substrate 606. Fig.14a illustrates numerical simulation results of the conductance G and susceptance B and Fig.14b the conductance G and susceptance B as well as resistance R and reactance X of the structure as shown in Fig.12b. In the simulation, Diamond-Carbon was used as dielectric material and aluminium as the metal within the groove. The piezoelectric layer 604 is LiTaO ₃ (YXl)/42°. The base substrate 606 is also Lithium Tantalate (same crystal cut) bonded to the piezoelectric layer 604 by an attachment layer 608 of silica SiO ₂ . This structure, also called homotype bounding, meaning that the same material is used as substrate and piezoelectric layer is bonded using SiO ₂ . SiO ₂ can be used as etch-stop layer to control the piezoelectric layer thickness and therefore the excitation layer thickness. SiO ₂ can furthermore help reducing the TCF. The part of the structure underneath the silica attachment layer is also called radiation domain, in which unwanted modes dissipate, whereas the desired mode remains in the guiding domain above the attachment layer 608. The mechanical period or electrode pitch p is 1,4 mm and the embedded electrode thickness is 500 nm with a 100 nm thick metal layer 656, 658 and a 400 nm thick AlN/Carbon Diamond as dielectric fill-in material 660, 662., For the simulation, an aspect piezo/electrode ratio close to a/p = 0,5 was used. The mode observed corresponds to a leaky wave, but the quality factors Q _r at resonance and Q _a at anti-resonance are found equal to 400 and 670 respectively for a coupling factor of 2,2% with a phase velocity of 12 km. s ^-1 . Fig.14c illustrates the electrode mode as obtained by numerical simulation, with a shear mode like vibration present in the electrode, namely the metallic part 656 and the dielectric part 660, whereas the piezoelectric layer 604 is less moving. Fig 14c illustrates the finite element mesh of the piezoelectric layer 604, the embedded electrode as well as the attachment layer 608. In the simulation, the boundary conditions take into account the wave behaviour in the radiation domain beneath the attachment layer 608. Fig.15a illustrates the numerical simulation results of the harmonic conductance G and susceptance B and Fig.15b the harmonic conductance G and susceptance B as well as harmonic resistance R and reactance X with aluminium nitride AlN as the dielectric 660, 662 in the structure as shown in Fig.12b. Besides the dielectric material, all other structural parameters were the same. AlN has a phase velocity reduced to 11.3 km.s -1 compared to diamond like carbon. In this variant, a coupling factor of 4,4% and quality factors Qr=1850 and Qa=990 can be observed. Fig. 16a illustrates the numerical simulation results of the harmonic conductance G and resistance R, Fig.16b a zoom on the resonance and Fig.16c a zoom on the anti-resonance. In this variant the dielectric 660, 662 is silicon dioxide SiO ₂ in the structure as shown in Fig.12b. Besides the dielectric material, all other structural parameters were the same as in the first and second variant of the ninth embodiment. The use of SiO ₂ improves the TCF compared to transducer structures with grooves exclusively filled with metallic material of the observed mode. This is due to the fact that the TCF coefficient of SiO ₂ being equal to +80ppm.K ^-1 is opposite to the one of metals. Indeed, here a TCF value of the resonance of -11ppm.K ^-1 and of -14,7 ppm.K ^-1 for the anti- resonance is observed when using SiO ₂ as dielectric 660, 662. The coupling factor is 6,7% and the quality factor of the resonance QR is larger than 5000. QAr is less, about 650, but can be improved by optimising the structural parameters of the design. The observed TCF numbers can be even further improved when providing an additional layer of SiO ₂ above the transducer structure 650 as illustrated in Fig.16d. It illustrates the transducer structure 670 according to a fourth variant of the ninth embodiment. The transducer structure 670 corresponds to the transducer structure 650 of Fig.12b, except for the presence of the additional layer 672 of SiO ₂ . The dielectric material could also extend down through to the attachment layer 608 like illustrated in Fig.12c. Another aspect of providing such layer is the acceleration of the phase velocity of the mode, as already described above in relation to Fig. 7, to accelerate the fundamental shear mode above the SSBW velocity of the substrate 606, if needed to dissipate the fundamental mode. observed. Instead of using the same dielectric material to fill the grooves and to provide the additional layer 672, two different materials can be used to further optimise TCF as well as phase velocity. Figs.17a and 17b illustrate two variants of a tenth embodiment of the invention. Situations can occur in which the electrode mode can be observed at the same time as standard guided shear modes, e.g. the fundamental guided shear mode, as illustrated for instance in Fig.7d. Such a situation can also occur when using a POI composite substrate with a LiTaO ₃ piezoelectric layer on SiO ₂ . A typical example of such a POI substrate is 600nm thick LiTaO ₃ on 500 nm thick SiO ₂ on a 1 mm trap-rich poly Si layer on a base substrate of Si(100). In this configuration, the persistence of the fundamental guided shear mode is observed at a phase velocity between 3800 and 4200 m.s ^-1 and thus yields potential problems when addressing high frequency bands using the electrode mode at frequency about twice and a half higher than the electrode mode. As already mentioned, see description in relation to Figs. 3a to 3f, the choice of the base substrate 106, 205, 306, 406, 506, 606 plays an important role. Indeed, in case the SSBW velocity of the substrate is smaller than the one of the fundamental acoustic bulk shear mode in the piezoelectric layer, the bulk shear mode will enter the base substrate and becomes dissipated therein. The contribution of the fundamental guided shear mode can already be reduced by using a Si(111) base substrate, which presents the advantage of having a SSBW velocity smaller than the one on Si(100), typically 4700 m.s ^-1 or even less, instead of 5650 m.s ^-1 . Si orientation corresponding to (YXw)/45° is particularly interesting in that purpose. However, in that case, we still have a signature of the fundamental shear guided mode which can still be observed, even though, we suppress a couple of higher order modes as already described. To even further reduce the remaining contribution of the fundamental guided shear mode, the transducer structure according to the tenth embodiment comprises at least one additional layer within the stack. The additional layer is chosen to accelerate the fundamental guided shear wave to push its velocity further above the SSBW velocity of the substrate. The transducer structure 700 as illustrated in Fig.17a comprises a LiTaO ₃ piezoelectric layer 704 with embedded electrodes 712, 714, 716, here made of aluminium, on a SiO ₂ layer 706. The structure further comprises a trap-rich poly Si layer 708 on a base substrate 710 of Si (111). The transducer structure 700 further comprises an additional layer 718, also called high velocity low loss layer, sandwiched between the SiO ₂ layer 706 and the trap rich layer 708. The high velocity layer 718 is one of a AlN, Al2O3, Si3N4 or SiC layer. All these materials exhibiting compressive bulk wave velocity values of more than 10 km.s ^-1 and shear bulk wave velocity of more than 5 km.s ^-1 . As an alternative, the high velocity layer 718 can be based on Carbon: single-crystal Diamond, amorphous carbide layer, nano-grain polycrystal diamond (NCD) and all diamond-like carbon layers that could push the compressional wave velocity above 15 km.s ^-1 and shear wave velocity above 7 km.s ^-1 . Fig.17b illustrates a second variant of the tenth embodiment. It is based on the first variant, but comprises in addition a second additional layer 720, also a high velocity low loss layer, which can be of the same or a different material than the first additional layer 718. Both layers 718, 720 will accelerate the velocity of the fundamental guided shear mode.Fig.18a shows an image taken by an electron microscope showing a practical example of the transducer according to the invention. The picture shows a side cut view of a transducer structure 800, corresponding to the structure of the first embodiment as illustrated in Fig. 1 the transducer structure 800 has aluminium electrodes 802 embedded into a LiTaO ₃ (YXl)/42° bulk substrate 804. The layer 806 above the electrodes 802 and the surface of the bulk substrate 804 has been added after measuring the properties of the transducer and serves as a contrast- enhancing layer for imaging purposes. The pitch was p=3.4µm and the height of the electrode 802 h=510nm. The aspect ratio a/p was 0,5. The structure had 50 pairs of electrode means. Fig. 18b is an enlarged view of the region highlighted by rectangle 808 thus illustrating the shape of the electrode 806. The groove in the bulk substrate 804 has a trapezoidal shape with the longer of the parallel sides being aligned with the surface 810 of the bulk substrate 804. The sidewalls 812 and 814 are slightly concave shaped, the bottom surface 814 being convex. Fig. 18c illustrates the finite element mesh 820 simulating the electrode 802 shape of the practical example embedded in the piezoelectric substrate 804 used to simulate the behaviour of the practical example shown in Fig.18a and 18b. Fig. 19a illustrates the experimental measurements of conductance and resistance of the practical example. Fig.19b illustrates the results of conductance and resistance obtained by numerical simulation of the structure as shown in Fig.18c. The fabricated device as shown in Figs.18a and 18b showed a phase velocity of the mode of 10950 m. s ^-1 a coupling factor K _s ² of 1,85% and a quality factor Q _ar =350. The mode was observed at a frequency of about 1,6 GHz thus much higher than the 580 MHz of the fundamental shear mode. The conductance and resistance as measured are illustrated in Fig. 19a. The simulation results using a FEM mesh as illustrated in Fig. 18c and considering an infinitively long transducer structure provided results in conformity with the experimental results for the above described electrode mode. A velocity of 10862 m. s ^-1 , a coupling factor K _s ² of 0,62% and a quality factor Q _ar of the order of 100 can be observed. In addition, the mode occurs at about 1,6 GHz and shows similar behaviour in the conductance and resistance dependency. A number of embodiments of the invention have been described. Nevertheless, it is understood that various modifications and enhancements may be made without departing the following claims.