Field of the invention

The invention relates to a device and a method for mixing electromagnetic waves with frequencies up to the terahertz range.

The field of the invention is, but not limited to, electronic devices for THz applications.

Background of the invention

Historically known as the "far-infrared" domain, the terahertz (THz) region is a still under-used region of the electromagnetic spectrum. It extends typically from 100 GHz to 10 THz and corresponds to wavelengths between 3 mm and 30 microns.

The THz frequency domain corresponds in particular to characteristic resonances of many chemicals and biological agents. Thus, THz spectroscopy, which is mainly based on the analysis of vibration modes of complex molecules, is a very promising way of detecting and identifying biological and chemical compounds.

So far, the lack of good quality sources able to deliver enough spectral power is an important limitation to the use of THz waves. However, recent advances in the field of THz illumination sources such as quantum cascade lasers or external mixers, and in the field of coherent detection, open the way to the realization of sensing devices and remote identification using THz spectroscopy. In particular, thanks to the availability of sources generating THz power of about 10 mW around 1 THz, and to the development of efficient heterodyne-type detections, the fabrication of compact and sensitive detection devices for THz becomes possible.

A heterodyne receiver, as used in heterodyne-type detection, is a receiver which is designed on the principle of frequency mixing, or heterodyning. A received signal (RF) is mixed in a heterodyne mixer with the signal of a local oscillator (LO) to be converted into an intermediate frequency signal (IF), according to a down-conversion process. The frequency of the IF signal is lower than the frequency of the RF signal, so the IF signal is easier to process. Reversely, heterodyne mixers can be also used in up conversion for THz wave generation . In this config uration, an IF signal is mixed with a LO signal in order to generate a RF signal lying in the THz domain . The RF signal can be rad iated by an antenna in the surrounding med ia or propagated into a waveg uide.

To achieve the freq uency mixing, the heterodyne mixer must have a non-linear current-voltage response at the frequencies of operation (of RF, IF and LO) with q uad ratic terms. So the combination of two sig nals with freq uency fi and freq uency f ₂ , respectively, on the mixer leads to the generation of sig nals with freq uency terms (fi + f ₂ ) and (fi - f ₂ ), respectively. Some of these freq uency terms may then be filtered out, depend ing on whether up conversion or down conversion is sought.

So, heterodyne receivers are key systems for hig h-resol ution spectroscopy.

For achieving heterodyne detection in the THz freq uency range, it is known to use local oscillators such as, for instance :

- Quantum cascade lasers (QCL), but with the dou ble l imitation of a relatively limited tunability and of limited operation possibil ities at cryogenic temperatures;

- THz waves obtained by the photomixing of the frequencies of two infrared or near-infrared lasers beams in a semicond uctor material such as LT-GaAs. This approach has the advantage of al lowing hig h intrinsic tunabil ity and operation at room temperatu re, but with the d isadvantage of a particularly low terahertz power available, of typically less than a microwatt;

- mill imeter-wave subharmonical ly-pumped mixers based on planar GaAs

Schottky-d iode technology.

For achieving heterodyne detection in the THz freq uency range, it is also known to use mixers such as, for instance :

- Schottky diode based mixers. With this kind of mixers, a terahertz local oscil lator delivering a few mil liwatts of power is req uired . So Schottky d iode based mixers are usual ly combined with low freq uency QCLs. This approach can be extended between 2 and 3 THz, but as the performances of the mixers tend to deteriorate at higher freq uencies, the local oscil lator power must be increased accordingly and final ly the overal l sensitivity of the detection system decreases; - Hot electron bolometers (HEB) mixers. Due to the low level of power usually supplied by THz sources used as local oscillators, high-performance mixers such as the HEBs, which are derived from the space industry, are often required. Tunable terahertz sources such as photomixers may be used. But again, the required local oscillator power tends to increase as the frequency increases. In the frequency range of 2 to 3 THz, the required power becomes difficult to obtain by photomixing and it is better to use QCLs.

All these detection schemes currently require the implementation of cryogenic systems, as they use components which must be operated at very low temperatures.

So, the known heterodyne detection schemes in the THz frequency range still have important limitations:

- available THz mixers such as bolometers or Schottky diodes have a low sensitivity and require the use of local oscillators with a high power to achieve heterodyne detection schemes with high sensitivity. So quantum cascade lasers are usually needed;

- the tunability of these heterodyne detection schemes is in general limited, and so is the possibility of achieving broadband detection;

- as most of these systems operate at low temperatures, they turn out to be quite bulky and very expensive.

In addition, coupling THz radiations waves in components such as Schottky diodes is not easy. Schottky diodes are made of successions of layers, and the THz waves must be coupled on the side of the component. So the practical implementation may be complex, as it requires a tight focusing of the THz waves, with the risk of a poor coupling efficiency.

It is an object of the invention to provide a heterodyne mixer for heterodyne detection up to the THz frequency range, which overcomes the limitations of the prior art.

It is a further object of the invention to provide a heterodyne mixer for heterodyne detection up to the THz frequency range, which allows broadband operation, high sensitivity, and operation at room temperature.

It is still a further object of the invention to provide a heterodyne mixer for heterodyne detection up to the THz frequency range, which allows an easy implementation.

Summary of the invention Such objects are accomplished with a mixing device for mixing electromagnetic signals, comprising at least one mixer and coupling means for coupling electromagnetic signals to said mixer(s) so as to generate mixed signals with frequency content corresponding to sum and/or difference of frequencies of said coupled electromagnetic signals,

characterized in that said mixer(s) comprise(s) a self-switching device (SSD) with a conductive substrate and insulating features defining in said conductive substrate an elongated channel, said elongated channel providing a charge carrier flow path in said conductive substrate between first and second areas, and being dimensioned and arranged such that said charge carrier flow path is limited by depletion regions the size of which is dependent on a voltage difference between said first and second areas.

According to some modes of realization, the mixing device of the invention may be used for generating mixed signals:

- with frequency content corresponding to sum of frequencies of coupled electromagnetic signals, which corresponds to an up-conversion process;

- with frequency content corresponding to a difference of frequencies of coupled electromagnetic signals, which corresponds to a down-conversion process.

According to some modes of realization, the mixing device of the invention may comprise first coupling means for coupling to the self-switching device and/or for transmitting high-frequency signals incident in the form of electromagnetic waves, and second coupling means for coupling to the self- switching device and/or for transmitting intermediate-frequency signals.

For instance:

- at least a high-frequency signal and an intermediate-frequency signal may be coupled to the device for generating a mixed high-frequency signal (up-conversion);

- two or more high-frequency signals may be coupled to the device for generating a mixed intermediate-frequency signal (down-conversion).

The second coupling means may comprise wired connections with the first and second areas of the self switching device(s).

The first coupling means may comprise at least one planar antenna with a pair of conductive patches connected respectively to the first and second areas of the self switching device(s). The mixing device of the invention may comprise biconical or bowtie antenna(s), with the self switching device(s) inserted between the patches of said antenna(s).

The antenna(s) and the self-switching device(s) may be comprised on the same surface. They may be for instance part of a same surface of a component.

According to some modes of realization, the mixing device of the invention may comprise:

- any kind of antenna, adapted to the signals to be coupled;

- antennas located on the side of the component opposing the surface with the self-switching device(s);

- narrow band antennas specifically matched to the frequency of a local oscillator (LO) signal.

According to some modes of realization, the mixing device of the invention may comprise:

- several self-switching devices connected in parallel to the patches of the antenna(s);

- a plurality of antennas oriented according to a similar direction;

- a plurality of antennas connected in series and/or in parallel by conductive links between their patches, so as to form an array of antennas and mixers.

According to some modes of realization, the mixing device of the invention may be able to process electromagnetic signals with frequencies higher than 0.2 THz.

According to another aspect of the invention, it is proposed a method for mixing electromagnetic signals, comprising steps of:

- coupling electromagnetic signals to a mixing device of the invention,

- collecting mixed signals with frequency content corresponding to sum and/or difference or frequencies of coupled electromagnetic signals.

According to some modes of realization, the method of the invention may further comprise steps of irradiating the mixing device, implementing at least one antenna, with high-frequency signals in the form of electromagnetic waves.

It may further comprise steps of: - irradiating the mixing device with a first high-frequency signal incident on one side of the antenna(s), and with a second high-frequency signal incident on the opposite side of said antenna(s);

- irradiating the mixing device with linearly polarized high-frequency signals having an axis of polarization substantially parallel to the orientation of at least some of the antenna(s);

- irradiating several antennas of an array of antenna and mixers with high-frequency signals, and collecting mixed intermediate-frequency signals on said array of antennas and mixers using wired connections.

According to still another aspect of the invention, it is proposed a system for mixing electromagnetic signals, comprising :

- a mixing device of the invention with an array of antennas and mixers,

- means for directing, respectively, a first high-frequency signal in the form of an electromagnetic wave on one side of the antenna(s), and a second high-frequency signal in the form of an electromagnetic wave on the opposite side of said antenna(s),

- wired connection means for collecting mixed intermediate-frequency signals on said array of antennas and mixers.

Description of the drawings

The method according to embodiments of the present invention may be better understood with reference to the drawings, which are given for illustrative purposes only and are not meant to be limiting. Other aspects, goals and advantages of the invention shall be apparent from the descriptions given hereunder.

- Fig . la illustrates a front-view of a self-switching device (SSD), while

Fig. lb, Fig . lc and Fig. Id illustrates the modes of operation of the device,

- Fig. 2 illustrates a current-voltage characteristic of a SSD,

- Fig . 3 illustrates a front-view of a mixing device of the invention,

- Fig. 4 illustrates a detailed view of an antenna of the mixing device of the invention,

- Fig. 5 illustrates a side view of a mixing device of the invention showing the coupling of electromagnetic wave signals,

- Fig . 6 shows measured power of the IF signals versus its frequency for different geometries of devices.

Detailed description of the invention With reference to Fig . la, a mixing device of the invention comprises one or several components usually referred to as "Self-Switching Device", or SSD 1. These SSDs 1 are used as mixers for mixing the signals, as explained later.

Examples of SSDs 1 and modes of realization of such components applicable in the context of the invention are described in the document WO 02/086973.

A SSD 1 comprises a conductive layer 2. This conductive layer 2 is made for instance in an INGaAs/InP substrate containing a conductive layer with mobile electrons, located about 50 nm below the surface.

Other material arrangements may be used for the manufacturing of the

SSDs 1 :

- high mobility materials such as InAs or InSb may be used in order to reach much higher frequencies;

- even if heterojunctions provide the best performances, a material structure comprising a doped layer on top of an isolating substrate, such as

SOI (Silicon On Insulator), may be used for manufacturing the SSDs 1. In that case, the results in terms of cutoff frequencies may be not the best, but the use of a silicon-based technology may be advantageous for the integration .

Insulating trenches 3 are made in that conductive layer 2, using a wet chemical etching technique, or a radiative ion etching (RIE) technique. These insulating trenches 3 are for instance about 100 nm deep, so as to be deeper than the conductive layer 2, and to form insulating lines 3.

The insulating lines 3 comprise first insulating lines 8 which separate the conductive layer 2 into a first area 10 and a second area 11, except for a channel 6.

The insulating lines 3 comprise also second insulating lines 9 which extend from the first insulating lines 8 into the second area 11. These second insulating lines 9 define thus an elongated channel 6 in the conductive layer 2, which provides an electron flow path for mobile electrons to travel between the first area 10 and the second areas 11. This elongated channel 6 has a typically a length in the order of 1 to 2 micrometers and a width in the order of 50 nm to 100 nm .

The first area 10 and the second area 11 of the conductive layer 2 are connected to electrical terminals 4, 5, respectively. With reference to Fig . lb, when no voltage is applied between terminals 4, 5, depletion regions 7 appear in the elongated channel 6 along the insulating line 9, so that only a narrow area of the flow path is available for electron conduction . These depletion regions 7 are due to surface states and Fermi levels at the etched surface.

With reference to Fig . lc, when a positive voltage is applied to terminal 5 and a negative voltage is applied to terminal 4 (or at least when a higher voltage is applied to terminal 5 than to terminal 4), a positive voltage exists on the outer side of insulating lines 9, which contributes to decrease the size of the depletion regions 7 by electrostatically lowering the potential in the channel 6. So the electron conduction and therefore the current flowing in the channel 6 are larger.

With reference to Fig . Id, when now a negative voltage is applied to terminal 5 and a positive voltage is applied to terminal 4 (or at least when a lower voltage is applied to terminal 5 than to terminal 4), a negative voltage exists on the outer side of insulating lines 9, which contributes to increase the size of the depletion regions 7 by electrostatically increasing the potential in the channel 6. This creates a narrower channel or even a pitched-off channel . Thus only little or even no current may flow in the channel 6.

Fig . 2 shows a typical current-voltage characteristic 15 of SSD 1. Current

I is plotted in function of voltage V.

It can be seen that the behavior of such component is quite similar to that of a semiconductor diode :

- when a negative voltage is applied on the terminals 4, 5, no current flows through the SSD 1 ;

- when a positive voltage, higher that a threshold, is applied on these terminals, a current flows through the SSD 1 ;

- the current varies in a non-linear way for some ranges of positive voltages.

The current-voltage characteristics 15 may further be modified by modifying structural parameters of the component such as dimensions and shape of the channel 6, but also by applying a DC bias voltage on the terminals 4, 5.

For instance : - when the width of the channel 6 is increased , the shape of the current- voltage curve becomes more similar to that of a non symmetric resistor (with a saturation for hig h bias) ;

- red ucing the length of the channel 6 allows increasing the cutoff frequencies;

- a geometrical asymmetry, such as for instance a V-shaped geometry in which the channel 6 has a width that varies from one extremity to the other like a "V", increases the non-l inearity of the current-voltage curve. That non- linearity comprises a q uad ratic term that provides the necessary mixing behavior, and that may also give rise to hig her harmonics usable when using the SSD 1 as a freq uency multiplier.

So, a SSD 1 behave, at least in some extends, like a semiconductor d iode even if works on a completely different principle.

The document WO 02/086973 report possible uses of SSDs as rectifiers for detecting average power of high-frequency signals.

In this kind of applications, a hig h-freq uency voltage is appl ied on terminals 4, 5 of a SSD and the average current flowing throug h the component is measured at low freq uencies. As the current is blocked on one d irection, its average has a non-zero value which is representative of the average voltage of the high-frequency signal .

The use of diodes for mixing sig nals is of cou rse known, but it is not their abil ity to block the current flow in one d irection which is of interest then, but rather their non-l inear current-voltage characteristic with a q uad ratic term .

More precisely, voltage sig nals Ei and E ₂ with respective frequencies fi and f ₂ are superposed (added) on the diode's terminal . The resulting current Iq flowing throug h the d iode has then a term which corresponds to a sq uared sum of these voltage sig nals Ei and E ₂ , and which prod uces sig nals at sum and d ifferences of frequencies (fi + f ₂ ) and (fi - f ₂ ), respectively.

For instance, with Ei = cos(2nfit) and E ₂ = cos(2nf ₂ t),

Iq ~ [cos(2nfit) + cos(2nf ₂ t)] ²

~ cos[2n(fi - f ₂ )t] + cos[2n(fi + f ₂ )t] ( Eq . 1 )

Depending on whether up-conversion or down-conversion is soug ht, one of the terms is then filtered out.

So, a necessary cond ition to achieve a mixing effect is that the current- voltage characteristic of the component in use has a q uad ratic part. And this condition must be fulfilled at the frequency of the mixed signals, that is for instance at the intermediate frequency IF (fi - f ₂ ) for down-conversion.

It is an advantage of the invention to have recognized that SSDs 1 may have a current-voltage characteristic 15 comprising a quadratic part usable for mixing applications within a bandwidth extending up to the THz range. In other words, the voltage-current characteristic 15 of the SSD 1 as shown in Fig. 2 may be preserved at all useful frequencies within the range.

Simulation results have shown that a stable current-voltage characteristic 15 with a quadratic part may be obtained in a frequency range extending from DC to 450 GHz, or even to 2 THz with some configurations of SSDs 1, using for instance high mobility materials and channels 6 with reduced length as previously explained. In other words the current bandwidth may be extended up to 2 THz at least.

This means that intermediate frequencies IF (fi - f ₂ ) up to 2 THz may be obtained, with very broad modulation bandwidth, thanks to the bandwidth of the current.

In addition, these performances may be obtained at room temperature, without cooling.

With reference to Fig. 3, Fig. 4 and Fig. 5, we will now describe a mixing device 20 using SSD mixers 1 for mixing signals 40, 41 in the THz range to generate a signal at a lower intermediate frequency.

A typical application of such device is to mix a modulated signal at RF frequency fi with a reference signal from a local oscillator at OSC frequency f ₂ , so as to generate a signal at lower intermediate frequency IF (fi - f ₂ ) still containing the modulation information.

The mixing device 20 of the invention comprises several SSD mixers 1. These mixers 1 are connected through their terminals 4, 5 to pairs of patches 30 of a conical shape forming bow-ties antennas 21 (or other types of antennas). Several SSDs 1 are connected in parallel to a pair of patches 30 of an antenna 21. These SSDs 1 may be done as distinct components, or they may share the same conductive surface 2 just comprising several channels 6 in parallel.

The antennas 21 are connected in parallel by conductive links 22, so as to form columns of antennas 21. Several columns of antennas 21 are in turn connected in series so as to form an array of antennas 21 and mixers 1. Al l the antennas 21 are oriented along the same d irection .

The antennas 21 and the SSD mixers 1 are al l done on a same su rface of a component 20 using classical techniq ues from the semicond uctor ind ustry. Antenna patches 30 may for instance be made of pieces or metal vacuum- deposited on the surface holding the already-done SSD . Components bound ing may be done by using gold micro-wires. Components bou nding may also be done by d irect integ ration of components and/or connection tracks d uring the fabrication .

So the mixing device 20 of the invention is particularly easy to real ize. The planar structure allows also an easy and efficient coupl ing of hig h- freq uency sig nals. The mod ulated sig nal and the reference sig nal are broug ht in the form of beams 40, 41 or electromagnetic waves wh ich are focused onto the surfaces of the mixing device 20. A first of these beams 40 is focused on the surface hold ing the antennas 21. A second of these beams 40 is focused on the surface opposite. Both beams 40, 41 are coupled with the antennas 21 , d irectly or throug h the d ielectric layers supporting the components surface.

The beams 40, 41 are linearly polarized, with their axis of polarization aligned with the axis of the antennas 21.

The electric field intercepted by each of the antennas 21 ind uces a voltage d ifference on the terminals 4, 5 of the SSD mixers 1 which are connected to the patches 30. If two h ig h-freq uency sig nals 40, 41 are coupled to the antennas 21 simultaneously, the voltage d ifferences sum up on the SSD terminals 4, 5. As explained before, a resulting current with freq uency content at intermed iate freq uency appears on the SSD terminals 4, 5.

As the SSDs 1 of the device 20 are connected in series and in parallel, the resulting currents combine and may be collected at the terminals 23 of the mixing device, and eventually converted in voltage.

A large part of the surface of the mixing device is occupied by the antennas 21 , so the structure allows very efficient coupling of the power of the hig h-freq uency sig nals. The sig nals generated at intermed iate frequency by al l SSDs 1 are also combined in power to be efficiently collected at the component's terminals 23.

It is important to notice that this simple and efficient architecture is made possible by the fact that SSDs 1 have a planar structure req uiring only one conductive layer, and that they allow power coupl ing by the face. This is an important advantage of the device of the invention compared for instance to mixers based on Schottky d iodes whose manufacturing req uires several layers and in which the coupling of hig h-frequency power must be done by the side of these layers.

Fig . 6 shows examples of measured power of intermediate freq uency IF signals, in function of the IF freq uency, for d ifferent geometries of SSD devices 1 made on Ga N heterostructures :

- A2 : sq uare geometry wherein the channel 6 has a constant width, and a length of about 1 micrometer;

- El : sq uare geometry wherein the channel 6 has a constant width, and a length of about 2 micrometers;

- D2 : V-shaped geometry, wherein the channel 6 has a width which varies from one extremity to the other (like a "V"), and a length of about 2 micrometer.

These measurements have been done using RF and OSC sig nals frequencies lying in the freq uency ranges of, respectively, 0.28 to 0.36 THz, and 0.22 to 0.32 THz. The RF and OSC freq uency spacing was adjusted so as to generate intermed iate freq uencies IF between 3 and 40 GHz.

These measurements show the abil ity of the SSDs 1 devices to be used as non-linear mixers within a bandwidth rang ing up to the THz range . They show also that the current-voltage characteristic 15 which was only described at low-frequencies in the prior art works is still val id up to the THz range so that a SSD can be used to do a particularly efficient hig h-freq uency mixer as described in the invention .

While this invention has been described in conjunction with a number of embodiments, it is evident that many alternatives, modifications and variations wou ld be or are apparent to those of ord inary skil l in the appl icable arts. Accord ing ly, it is intended to embrace al l such alternatives, modifications, eq uivalents and variations that are within the spirit and scope of this invention .