Field of the Invention

The present invention relates to a tunable waveguide e.g. for incorporation in optoelectronic devices to tune the output frequency of the devices. In particular, but not exclusively, the present invention provides a tunable waveguide structure for dynamically tuning, or switching, the output frequency of a plasmonic waveguide, for example provided in a semiconductor laser such as a quantum cascade lasers (QCLs).

Background of the Invention

Optoelectronic devices, typically semiconductor devices, are used widely to generate electromagnetic radiation in many applications. For example, semiconductor lasers are widely used as sources of coherent radiation. In recent years semiconductor lasers have been able to provide coherent radiation at relatively high output powers and have been tunable between well defined (principal) output frequencies.

Optoelectronic devices, and in particular semiconductor lasers, typically have a gain region in which the generated radiation propagates prior to emission. Indeed, semiconductor lasers typically provide an optical cavity about the gain region. For example, at the respective ends of a laser stripe (at the end facets), partially transmitting mirrors are provided to form a Fabry-Perot cavity (or resonator). This arrangement is used in both semiconductor laser diodes and in semiconductor quantum cascade lasers.

However, this arrangement typically results in a number of competing lasing modes existing in the devices, which can lead to instability in the amplitude (intensity) of the emitted radiation and in the spectral profile of the emitted radiation. In other words, the (central) wavelength of the emitted radiation may vary significantly over time. This is problematic because many applications require both a substantially invariant output intensity (or at least a reliably controllable output intensity) and an accurately and precisely defined output wavelength.

Accordingly, distributed feedback (DFB) lasers have been proposed in the prior art. In DFB lasers a periodic grating is formed close to the active region of the laser. This grating has a static filter response and effectively acts as an optical filter causing a single wavelength to be fed back to the active region, or gain region, and causes lasing. Often, since the grating provides the feedback that is required for lasing, reflection from the end facets is not required.

DFB lasers are able to provide a stable output wavelength that is in essence defined during manufacture of the laser by defining the pitch of the grating. Nevertheless, the output may be tuned slightly by adjusting the drive current or temperature of the device, which in turn alters the characteristics of the active, or gain, region of the laser and thus can result in a different output wavelength.

The present inventor has previously proposed a novel modification to DFB lasers, in which the grating is formed to be aperiodic, thereby providing an aperiodic distributed feedback (ADFB) laser. This concept is described in detail in GB2493733A, the entire content of which is incorporated herein by reference.

Such ADFB lasers can provide stable outputs at a plurality of particular, and typically well- defined, output frequencies, e.g. lasing modes. An ADFB grating provides a static filter response which effectively provides an optical filter with the ability to feedback multiple wavelengths to the active, or gain, region of the laser. Indeed, ADFB lasers have recently been shown by the present inventor to provide broadband spectral control and discrete emission frequency tuning. The tuning between the discrete emission frequencies (defined by the filter response of the ADFB grating) is achieved via variations in the QCL drive currents.

Whilst the use of ADFB gratings is a highly promising way to tune the output of lasers, they rely on subtle laser gain region tuning to achieve mode switching between the resonances defined by the filter response of the ADFB grating. As gain movement is small in such lasers, the result spectral range and flexibility of the device is limited.

Thus, the present inventor proposes the present invention with the knowledge that there is a need for an optoelectronic device, particularly a laser and preferably a semiconductor laser, which is dynamically tunable between two or more (principal) output wavelengths (e.g. lasing modes) in a controllable and reproducible manner; preferably without the need for active region gain tuning.

Summary of the invention

Accordingly, in an aspect the present invention provides a waveguide including: a grating defining a plurality of grating elements, arranged to provide scattering sites for radiation propagating within the waveguide; and a graphene film, provided for at least one of the grating elements, configured to alter the scattering effect of the at least one of the grating elements.

The grating elements collectively function to define a filter response of the grating, thereby (at least partly) defining the output (e.g. the emission spectrum) of the waveguide. Altering the scattering effect of the grating element(s) therefore alters the filter response of the grating and consequently alters the output (e.g. the emission spectrum) of the waveguide. Preferably, the graphene film, provided for at least one of the grating elements, is configured to enhance the scattering effect of the at least one of the grating elements. The grating is typically arranged such that the scattering effect of the respective grating elements provides an overall filtering effect on the radiation propagating in the waveguide. Thus, enhancing the scattering effect of the at least one of the grating elements enhances the filtering effect provided by the grating. Thus, in embodiments, the present invention provides a waveguide with a grating having an improved, or enhanced, filter response.

The filter response of the grating is changeable by suitable control of the Fermi energy of the graphene film. Varying the Fermi energy of the graphene film correspondingly varies the conductivity of the graphene. For example, increasing the Fermi energy correspondingly increases the conductivity of the graphene film; and vice versa. Thus, the graphene film is arranged such that modification of the Fermi energy of the graphene film varies (alters) the scattering effect of the at least one of the grating elements, thereby changing the overall filer response of the grating.

In other words, the graphene film, provided for at least one of the grating elements, is preferably (dynamically) controllable to vary (alter) the scattering effect of the at least one of the grating elements; and is therefore preferably (dynamically) controllable to vary (alter) the overall filter response of the grating, thereby providing (dynamic) control of the filter response of the grating.

The graphene film, provided for at least one of the grating elements, is preferably

(dynamically) controllable to enhance the scattering effect of the at least one of the grating elements; and is therefore preferably (dynamically) controllable to enhance the overall filter response of the grating, thereby providing (dynamic) control of the filter response of the grating.

According to the present invention, the Fermi energy of the graphene may be configured to enhance the scattering effect of the at least one of the grating elements. Preferably, the Fermi energy of the graphene is controllable, electrically, to enhance the scattering effect of the at least one of the grating elements. In this way, the Fermi energy of the graphene is controllable, and thus the enhancement of the filter response of the grating is controllable.

The graphene film, provided for at least one of the grating elements, is preferably electrically biasable to alter the scattering effect of the at least one of the grating elements. Thus, a waveguide according to the present invention is able to be dynamically tuned during use to control the output of the waveguide by modifying the electrical bias applied to the graphene film.

The graphene film may be biased by electrochemical gating, for example to modify the Fermi energy of the film. However, preferably, the Fermi energy of the graphene film may be modified by the application of a bias voltage to the graphene film.

The graphene film may be provided for each grating element. The grating may be formed on a semiconductor material. The grating may be formed of a metal.

Optionally, the grating elements are defined in the grating to be an aperiodic series.

However, they may be defined in the grating to be a periodic series.

Where the grating is provided as a distributed feedback grating, the present invention provides an improved, or enhanced, distributed feedback grating.

The waveguide is preferably a plasmon or plasmonic waveguide. Even more preferably, the waveguide is a surface plasmon waveguide.

The graphene film may overlay at least a portion of the grating body (at least partly defining the grating elements) in addition to being located at the at least one of the grating elements. For example, the graphene film may overlay a portion of the metallic body of the grating in addition to being located at the at least one of the grating elements.

The graphene film may itself be overlaid with a dielectric layer, or an electrolytic layer. The dielectric or electrolytic layer may itself be overlaid with a bias pad (electrical contact) for applying a voltage bias across the dielectric or electrolytic layer between the graphene film and the bias pad in order to provide a means for dynamic electrical control of the graphene film. Hence, dynamic control of the Fermi energy of the graphene film may be provided.

In embodiments, the electrolytic layer may include a synthetic polymer, for example PVA, and an ionic salt. The ionic salt maybe a lithium salt, for example. The lithium salt may be LiCI04, for example.

In embodiments, the waveguide is provided in an optoelectronic device; in other words the waveguide may be incorporated into an optoelectronic device. The grating may be arranged to provide distributed feedback within the optoelectronic device, thereby controlling the spectral output of the device.

Thus, by affecting the scattering effect of the at least one grating element, the graphene film may be controllable, e.g. electrically biasable, to alter the emission spectrum of the radiation output by the optoelectronic device.

The optoelectronic device may include an active region of semiconductor material, and the grating elements may be arranged to affect (i.e. may be arranged to provide a scattering effect on) the radiation propagating in the active region.

The graphene film at the at least one of the grating elements may be arranged to form an interface with at least a portion of the active region.

The optoelectronic device is preferably a solid state laser, for example a semiconductor laser.

The optoelectronic device is preferably a quantum cascade laser.

The grating elements are preferably defined in the grating to be an aperiodic series such that:

the laser is tunable to emit coherent radiation at any selected one of at least two principal output frequencies; or

the laser is tunable to emit coherent radiation at any selected one of at least three principal output frequencies.

For example, the laser may be tunable between respective output frequencies (lasing modes) by variation of the drive current supplied to the active region, or gain region, of the waveguide. Thus, suitable control of the drive current and of the Fermi energy of the graphene film preferably allows for fine tuning of the emission spectrum of the radiation output by the waveguide.

The laser is preferably arranged to emit coherent radiation (i.e. to exhibit lasing modes) between 1 and 10THz.

The graphene film optionally comprises a stack of no more than 10, 9, 8, 7, 6, 5, 4 or 3 monolayers of graphene. For example, the graphene film may comprise a stack of no more than 2 monolayers of graphene. In a preferred embodiment, the graphene layer is optionally a monolayer of graphene. For example, a respective monolayer may be determined to be present if more than 90% of the area of the graphene layer includes the stated number of monolayers, more preferably 95% of the area, most preferably 99% of the area.

The present inventor also proposes that by controlling the Fermi energy of the graphene film provided for each of the respective grating elements, or for respective groups or sets of the grating elements, independently of one another, the overall filter response of the grating provided in the waveguide can be tuned dynamically; for example, to switch the output of the waveguide between predetermined emission spectra. In particular, by suitable control of the filter response of the grating, the spectral output of the waveguide is controllable so as to enhance or suppress the emission of predetermined frequencies of radiation in the waveguide. In embodiments, the independent control of the respective grating elements, or groups thereof, allows a laser incorporating the present invention to be switched between lasing modes.

For example, the present invention provides a tunable waveguide comprising: a grating defining a plurality of grating elements, arranged to provide scattering sites for radiation propagating within the waveguide; and a first graphene film located provided for each grating element of a first set of said grating elements, said first set of grating elements including at least one of the plurality of grating elements; and a second graphene film provided for a second set of said grating elements, said second set of grating elements including another at least one of the plurality of grating elements; wherein at least one of the first and second graphene films is independently controllable to alter the collective (e.g. overall or combined) scattering effect (on the radiation propagating in the waveguide) of the first and second sets of grating elements, thereby modifying the output of the waveguide. Preferably, the waveguide is incorporated into an optoelectronic device such as a laser, to provide a tunable laser.

The respective sets of grating elements collectively function to define a filter response of the grating, thereby (at least partly) defining the output (spectrum) of the waveguide. Altering the scattering effect of the grating element(s) of a set of grating elements therefore alters the (overall) filter response of the grating and consequently alters the output (spectrum) of the waveguide.

Each of the first and second graphene films may be independently controllable to alter respectively the scattering effect (on the radiation propagating in the waveguide) of the first set of grating elements and the second set of grating elements, thereby modifying the output of the waveguide.

Suitable independent control of the first and/or second graphene films therefore allows the output of the waveguide to be tuned (by changing the scattering effects provided by the first and/or second sets of grating elements to alter the overall filter response provided by the grating). In particular, the spectral output of the waveguide can be tuned by suitable control of the graphene films. For example, the spectral output of the waveguide can be tuned between at least two principal frequency components.

The grating is typically arranged such that the scattering effect of a set of grating elements provides a filtering effect on the radiation propagating in the waveguide.

Preferably, the graphene film, provided for a set of grating elements is controllable to enhance (increase) the scattering effect of the set of grating elements. Enhancing the scattering effect of a set of grating elements enhances (increases) the filtering effect provided by the set of grating elements.

Preferably, the graphene film, provided for a set of grating elements is controllable to diminish (reduce) the scattering effect of the set of grating elements. Diminishing the scattering effect of a set of grating elements diminishes (reduces) the filtering effect provided by the set of grating elements.

Preferably, the graphene film, provided for a set of grating elements is controllable to alter the phase of the scattering effect of the set of grating elements. Alteration of the phase of the scattering effect of a set of grating elements alters the filtering effect provided by the set of grating elements.

Due to the way in which each respective set of grating elements contributes to the overall filter response of the grating, altering the scattering effect (scattering strength or phase) of each set provides the ability to alter a specific filter band strength in frequency space or to alter a specific filter band location in frequency space..

In other words, by (independent) control of the scattering effect provided by the respective sets of grating elements, the overall filtering effect (filter response) provided by the plurality of grating elements in the grating is controllable to tune the output of the waveguide; in particular, the spectral output of the waveguide can be tuned, or switched, by suitable control of the scattering effect provided by the respective sets of grating elements. As explained above, the scattering effect of a grating element, and therefore of a set of grating elements, is controllable (both by enhancement and by diminishment) by adjusting the Fermi energy of the graphene film provided in the grating element, or set of grating elements.

Accordingly, the present invention provides a waveguide which is able to be dynamically controlled to tune the output of the waveguide. In particular, the spectral output of the waveguide is dynamically controllable, by respectively altering the Fermi energy of the first graphene film and/or the second graphene film suitably.

For example, the first and second graphene films are preferably independently controllable to alter respectively the scattering effect of the first set of grating elements and the second set of grating elements on the radiation propagating in the waveguide so as to alter the spectral output of the waveguide. Therefore, the principal frequency components of the output of the waveguide can be altered by suitable control of the graphene films, in particular by suitable control of the Fermi energy of the graphene films. Thus, the spectral output of the waveguide can be tuned to provide emitted light with a desired emission spectrum, or spectral profile.

The first and second graphene films are preferably independently electrically biasable to alter respectively the scattering effect of the first set of grating elements and the second set of grating elements (on the radiation propagating in the waveguide) so as to alter the spectral output of the waveguide.

Therefore, the present invention provides a convenient way to alter, or dynamically tune, e.g. in real time, the output of the waveguide; in particular, to alter, or dynamically tune, the spectral output (or emission spectrum) of the waveguide, e.g. in real time. The waveguide is preferably a plasmon waveguide. Even more preferably, the waveguide is a surface plasmon waveguide.

When the waveguide of the present invention is employed in a solid state laser, e.g. a semiconductor laser, the first and second graphene films are preferably independently controllable to alter respectively the scattering effect of the first set of grating elements and the second set of grating elements on the radiation propagating in the waveguide so as to tune the waveguide between at least a first lasing mode and a second lasing mode, the lasing modes being centred on respectively different (output) frequencies.

The first set of grating elements preferably includes a plurality of grating elements. The second set of grating elements preferably includes a plurality of grating elements.

The first set of grating elements is preferably arranged to cause the waveguide to output radiation centred on at least a first frequency. The second set of grating elements is preferably arranged to cause the waveguide to output radiation centred on at least a second frequency, being a different frequency to the first frequency. In other words, the first and second set of grating elements are arranged to provide respective filtering effects or filter responses.

By suitable control of the first and second graphene films, the overall filtering effect (filter response) of the first and second sets of grating elements can be modified and controlled so as to allow the output of the waveguide, in particular the spectral output, to be tuned suitably. The grating elements may be defined in the grating to be a periodic series. The first and second set of grating elements may be interspersed such that each adjacent pair of grating elements of the first set of grating elements is separated by a grating element of the second set of grating elements.

The grating elements in the first set of grating elements are preferably distributed in a first plurality of sections, the spacing between adjacent grating elements in each of the sections of the first plurality of sections corresponds to a grating period L. The grating elements in the second set of grating elements are preferably distributed in a second plurality of sections, the spacing between adjacent grating elements in each of the sections of the second plurality of sections corresponds to the grating period of L. Each adjacent pair of the first plurality of sections is preferably separated by at least one respective section of the second plurality of sections.

The grating elements may be a periodic series, an aperiodic series, a quasi-periodic series, a chirped series, arranged in a superstructure or concatenated gratings with an aperiodic basis.

The grating elements are preferably defined in the grating to be an aperiodic series. The grating elements in the first set of grating elements are preferably distributed in a first plurality of sections, the spacing between adjacent grating elements in each of the sections of the first plurality of sections corresponds to a grating period L. The grating elements in the second set of grating elements are preferably distributed in a second plurality of sections, the spacing between adjacent grating elements in each of the sections of the , second plurality of sections corresponds to a respective odd integer multiple of L/2. Each adjacent pair of the first plurality of sections is preferably separated by at least one respective section of the second plurality of sections.

Therefore, combining an aperiodic grating such as that described in GB2493733 with a semiconductor laser including a waveguide according to the present invention allows the effect of the aperiodic grating to be dynamically altered so that undesirable lasing modes can effectively be suppressed or "switched off', and/or, so that desirable lasing modes can be enhanced, or "switched on".

For example, by suitable control of the first graphene film, the radiation scattering effect of the first set of grating elements can be reduced so that the first set of grating elements does not contribute to defining a lasing mode of the laser. On other hand, by suitable control of the second graphene film, the scattering effect of the second set of grating elements can be maintained or enhanced so that the second set of grating elements does contribute to defining a lasing mode of the laser.

Consequently, the output of the laser is able to be switched dynamically between lasing modes by suitable control of the graphene films. Advantageously, no change in laser driving current is necessary. Therefore, the thermal stability of the laser is improved even though the laser is controllable to switch its output between at least two lasing modes. However, in embodiments, the output of the waveguide be tuned by suitable control of the graphene films in accordance to the present and by suitable laser gain region tuning, for example by varying the drive current as in the prior art. Thus, a waveguide according to the present invention may include one or more electrical contacts arranged to provide a driving electrical current for generating radiation within the waveguide for emission therefrom. Thus, the spectral output of the waveguide may also be tuned by varying the driving current, as for example in a similar way to that described in GB2493733.

In general, the scattering effect of the grating elements of the respective sets of grating elements can be reduced or enhanced as desired, by suitable control of the graphene films, so as to alter the output of the waveguide.

The first and second graphene layers are preferably independently controllable respective portions of the same graphene layer overlaid on the grating.

In embodiments, the graphene layer is itself overlaid with an electrolytic layer. The electrolytic layer is itself preferably overlaid with a bias pad for applying a voltage bias across the electrolytic layer between the graphene layer and the bias pad.

A respective bias pad may be provided for each graphene film, to facilitate electrical biasing thereof.

The waveguide may be provided in an optoelectronic device. The grating may be arranged to provide distributed feedback within the optoelectronic device to control the spectral output of the device.

The first and second graphene films are preferably independently electrically biasable to alter the spectral output of the radiation emitted by the optoelectronic device. The optoelectronic device may include an active region of semiconductor material, and the grating elements may be arranged to affect the radiation propagating in the active region.

The first graphene film located in at least one of the grating elements may be arranged to form an interface with at least a portion of the active region.

The second graphene film located in at least one of the grating elements may be arranged to form an interface with at least a portion of the active region.

In embodiments, the optoelectronic device is a semiconductor laser. The optoelectronic device is preferably a quantum cascade laser. The laser is arranged to emit coherent radiation between 1 and 10THz.

Each graphene film optionally comprises a stack of no more than 10, 8, 7, 6, 5, 4 or 3 monolayers of graphene. For example, each graphene film optionally may comprise a stack of no more than 2 monolayers of graphene. In a preferred embodiment, each graphene film is optionally a monolayer of graphene. For example, each optional number of monolayers is determined to be present if more than 90% of the area of the respective graphene film includes the stated number of monolayers, more preferably 95% of the area, most preferably 99% of the area.

In any aspect or embodiment, the waveguide may further include a dielectric, or

semiconductor, body on which the grating is formed. The grating elements are optionally formed in the grating. The grating elements optionally extend into the underlying dielectric, or semiconductor, body.

In all embodiments and aspects, the grating may be considered to be a filter. In all embodiments and aspects, the grating elements may be considered to be filter elements.

In all embodiments and aspects, the grating elements may be voids. The respective graphene film may be provided in the void. The respective graphene film may be provided to span the void.

In all embodiments and aspects, the grating may provide distributed feedback. The grating may be a distributed feedback grating.

Thus, the present invention may provide an optoelectronic device having a waveguide including: a grating defining a plurality of grating elements, arranged to provide a distributed feedback effect within the waveguide; and a graphene film, provided for at least one of the grating elements, configured to alter the distributed feedback effect provided by the grating; for example configured to be controllable to alter or vary the distributed feedback effect provided by the grating.

The present invention may provide a tunable optoelectronic device having a waveguide comprising: a grating defining a plurality of grating elements, arranged to provide distributed feedback within the waveguide; and a first graphene film located provided for each grating element of a first set of said grating elements, said first set of grating elements including at least one of the plurality of grating elements; and a second graphene film provided for a second set of said grating elements, said second set of grating elements including another at least one of the plurality of grating elements; wherein at least one of the first and second graphene films is independently controllable to alter the collective (e.g. overall or combined) distributed feedback effect (within the waveguide) provided by the grating, thereby modifying the output of the waveguide. Preferably, the waveguide is incorporated into an

optoelectronic device such as a laser, to provide a tunable laser.

Any feature disclosed herein may be incorporated into any aspect or embodiment described herein unless the incorporation is expressly stated to be undesirable, or it is understood by a skilled person to be technically impossible.

Brief Description of the Drawings

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 shows an example of a semiconductor quantum cascade laser incorporating a waveguide according to the present invention;

Figure 2 shows an example of a semiconductor quantum cascade laser incorporating a tunable waveguide according to the present invention;

Figure 3 shows a closeup of a schematic cross-section of a slit in the grating of the example shown in figure 2, in which the grating microstructure was coated in a thin layer of polymer electrolyte (PVA + LiCI04), which could be electrically biased to form Debye layers at each electrode;

Figure 4 (a) shows the emission spectra from QCL A prior to FIB milling to form the distributed feedback grating, the dips around 2.97 and 3.02 THz are due to strong atmospheric water absorption lines;

Figure 4 (b) shows the calculated spectral reflectivity response p(f) (the filter response) of the ADFB grating, assuming |Δη| = 0.1;

Figure 4 (c) shows the emission spectra from QCL A after FIB milling to form the aperiodic distributed feedback (ADFB) grating;

Figure 4 (d) shows the modified emission spectrum after graphene deposition, displaying a decreased filtering effect and thereby an increased favourability for the original FP cavity modes in Figure 4 (a).

Figure 5 shows the measured emission spectra of QCL A, with graphene applied, at three driving currents (I) just above the lasing threshold, at Vgate = 0 V (upper row of plots) and 1 V (lower row of plots), the application of Vgate = 1 V improves the filter strength in the presence of graphene (i.e. fewer lasing modes are observed);

Figure 6 shows the measured emission spectra of QCL A, with no graphene applied, at three driving currents (I) just above the lasing threshold, at Vgate = 0 V (upper row of plots) and 1 V (lower row of plots), the application of Vgate = 1 V has no effect in the absence of graphene;

Figure 7 (a) shows the measured laser emission spectrum collected from QCL A before ADFB patterning, just above the lasing threshold driving current; Figure 7 (b) shows the calculated grating filter response p(f);

Figure 7 (c) shows the measured laser emission spectrum collected from QCL A after FIB milling;

Figure 7 (d) shows the measured laser emission spectrum collected from QCL A after graphene deposition;

Figure 7 (e) shows that the presence of graphene dramatically increases the number of lasing modes due to a reduction in p(f) strength;

Figure 7 (f) shows the output power and electrical characteristics of QCL A at various stages of fabrication;

Figure 8 (a) shows a polymer electrolyte deposited over the laser ridge and biased independently (V _ga te), similarly to Fig. 2;

Figure 8 (b) shows that under the influence of the V _ga t _e a Debye layer forms, altering the graphene surface doping and therefore the Fermi energy EF;

Figure 8 (c) and (d) show the emission spectra collected just above lasing threshold for Vgate = 0 and 1 V respectively, the latter showing improved filtering;

Figure 8 (e) shows the calculated filter response p(f}

Figure 8 (f) shows that a reduced N (\.e. improved filtering) is recorded for a wide range of QCL operating currents - the inset window shows an equivalent plot for QCL A where an electrolyte is deposited alone without graphene, and showing that N is largely insensitive to Vgate;

Figure 9 (a) shows the fundamental TM eigenmode of a typical THz QCL waveguide;

Figure 9 (b) shows a modelled, short waveguide section containing a 1 pm wide slit in the metal;

Figure 9 (c) shows a cross-section of the simulated longitudinal electric field profile of a time- varying standing wave;

Figure 9 (d) shows that the waveguide mode is strongly coupled to the slit;

Figure 9 (e)-(g) shows left: absolute electric field magnitude in the slit region, centre: time- varying E _y field component, right: illustrative corresponding oscillating charge distribution, for (e) an empty slit; (f) a low-doped graphene (EF = 50 meV) layer supporting a confined dipole plasmon, which acts to reduce THz scattering; and (g) a high-doped graphene

(EF = 300 meV) layer in which a multipole plasmon arises and scattering increases once again; and

Fig. 10 (a) shows time domain modelling of ADFB grating lasers, and demonstrates that when p(f) is embedded within a FP laser cavity, N (here normalised to N in a simple FP cavity) falls with increasing \An\, a measure of the grating scattering strength;

Fig. 1 0 (b) shows the dependence of N on the ADFB grating coupling phase Δπ _φ ; Fig. 1 1 (a) shows a representative plot of the gain in an optoelectronic device, such as a laser, with two active regions;

Fig. 11 (b) shows a representative plot of the change in filter response (strength) of an ADFB grating as a function of applied bias (0V, 1 V and 5V);

Fig. 1 1 (c) shows emission spectra for a dual gain device, where the plots top to bottom show: the emission spectrum for a Fabry Perot (FP) device, the predicted filter response of the ADFB grating, and 3 pairs of plots for the emission spectrum of the device respectively at 0V, W and 5V;

Fig. 12 shows a pictorial representation of the variation in the filter response (right hand side) when the scattering strength of the grating elements (e.g. voids) is varied (left hand side).

Detailed Description and Further Optional Features of the Invention

The present inventor has realised that because the Fermi energy in graphene, and therefore its conductivity and in particular its plasmon properties, can be adjusted (for example by electrical gating and/or surface doping) it is suitable for affecting the surface plasmons (SP) in surface plasmon waveguides; and thus can be harnessed to create plasmonic

waveguides with dynamic and user-defined properties. More specifically, the present inventor demonstrates that graphene is capable of supporting terahertz (THz) SP modes and changing the properties of such THz SP waveguides. In this way, the emission

characteristics of a waveguide, for example a plasmonic waveguide such as a surface plasmon waveguide, can be controlled.

Surface plasmons are collective oscillations of electrons at a metallic/dielectric interface which can couple with photons to form surface plasmon polariton (SPP) modes. These propagating modes offer a means to guide and control light and can be used to beat the diffraction limit, focusing electromagnetic energy down to scales well below a single wavelength.

The present invention has particular usefulness in the control of THz quantum cascade lasers (QCL) for example, and THz QCLs will be used below to explain the invention.

Nevertheless, the present invention should not be considered to be limited to THz QCLs. In particular, it should be noted that graphene is used to modify the scattering effect of the DFB or ADFB) grating element, e.g. the slits, so as to modify the filter response of the grating. This principle is more widely applicable than THz QCLs.

In the example discussed below, the THz QCLs have integrated aperiodic distributed feedback (ADFB) gratings, providing a multi-band filtering 'signature'. In other words, the ADFB gratings result in easily identifiable discrete lasing modes in the output of the QCL. It is shown that the filtering strength and characteristics (i.e. the filter response) of the grating can be altered directly by exerting control over the Fermi energy of the graphene film, and in exerting control over the Fermi energy of the graphene film, the graphene influences the SP mode.

Use of the ADFB, and thus the multi-band signature, makes the change in filtering strength readily discernible, and this is one reason for demonstrating the effect of graphene using the ADFB QCLs. However, the present invention is not limited to ADFB QCLs. For example, the present invention is applicable to a wider range of optical devices, such as optoelectronic devices, including semiconductor optoelectronic devices including semiconductor lasers such as diode lasers, distributed feedback (DFB and ADFB) QCLs and other QCLs.

Device fabrication, characterisation and results

THz QCLs, based upon semi-insulating SP waveguides (180 pm wide, 6 mm long) as shown in Fig. 1 , were fabricated from a GaAs/Alo.i5Gao.85As bound-to-continuum active region (AR), then characterised in pulsed operation (10 kHz pulse rate, 1 ps pulse width) at≤ 10 K.

In the present case, the THz QCL was fabricated to include an ADFB grating.

The ADFB grating, with a multi-band filter response p(f), was subsequently introduced into the upper layers of the QCL waveguides by focussed ion beam (FIB) milling.

The grating elements (voids) are narrow and shallow slits which penetrate the metallic uppermost waveguide layers, interrupting the guided SP mode and creating a discontinuity in its complex modal refractive index. The slits are typically < 1 pm in width across slit. The slit width should be sub-wavelength, with respect to the radiation in the waveguide. So, for THz waveguides, the wavelength inside material is -30 micron (equivalent to ~100 micron free space wavelength). Thus, slit widths of 1-5 micron work for THz QCLs.

For other devices emitting radiation at different wavelengths, such as diode lasers and other lower wavelength devices, the slit widths should be scaled accordingly. For example, for a 1.55 micron laser (wavelength ~ 500nm inside material) the slit width should be less than 100 nm.

For a given ADFB filter design, individual p(f) band strengths are proportional to the modal refractive index contrast |Δη| between the milled and un-milled waveguide regions, and influence the response of the compound Fabry-Perot (FP) plus p(f) system.

With sufficient |Δη| the laser emission exhibits a clear spectral signature of p(f).

Figure 7(a) shows the typical emission spectra from a FP QCL (QCL A), prior to the FIB milling which introduces the ADFB grating structure. As can be seen, the emission spectrum is not limited well to any particular frequencies.

For each of Figs. 4(a) to 4(d), the left hand plots are for a relatively low V(QCL), i.e. a relatively low voltage applied across the stack of quantum wells making up the QCL structure; whilst the right hand plots are for a relatively high V(QCL).

The intended p(f) of the ADFB filter introduced to this laser is presented in Fig. 4(b) and was calculated using the approximate Fourier transform relationship with the real space relative permittivity of the grating, using |Δη| = 0.1. This is explained in the inventor's published article in Applied Physics Letters 101 , 121103 (2012), the entire contents of which are hereby incorporated by reference. In reality, |Δη| is unknown for QCL A, but it is expected to be lower than 0.1 due to the observed lack of switchable, single mode selectivity, see Fig. 4(c). After initial characterisation of the QCL device, a large area, high quality, monolayer (up to 99% by area) of graphene was introduced as an overlayer to the ADFB grating as shown shown in Fig. 1 for example.

Use of chemical vapour deposition (CVD) allowed the creation of extensive graphene sheets, large enough to cover the entire millimetre-scale grating area.

Since the graphene monolayer thickness is -0.3 nm (over two orders of magnitude smaller than the slit widths and depths), it only fills < 0.01 % of the slit volume. Therefore, the present inventor has realized that changes in the slit refractive indices (and hence |Δη|) are attributable to the existence of intra-slit graphene-supported SPs.

Fig. 4(d) for example shows the graphene-modified emission spectra of QCL A. In this particular arrangement a reduced filtering effect is seen, with FP mode suppression less pronounced than Fig. 4(c). Consequently, there is no longer a one-to-one relationship between the dominant lasing modes and the p(f) bands at lower bias (VQCL); whereas at high V(QCL) the emission becomes almost indistinguishable from the original FP QCL emission. Thus, the present inventor has established that graphene films located in the grating slits can successfully interact with (have an effect on) the THz optical modes in a QCL. In essence, the present inventor has established that graphene films located in the grating slits can successfully interact with (have an effect on) the optical modes in a waveguide, for example in a plasmonic waveguide such as a surface plasmon waveguide. The influence of the graphene on the filter response p(f) was then varied by electrochemical gating of the graphene. An electrolytic film, consisting of a synthetic polymer (PVA) and a lithium salt (LiCI04), was deposited over the graphene-covered microstructure as shown in Fig. 2.

The graphene surface doping (and hence conductivity) is changed by applying a bias (Vgate) across the electrolyte, as shown in Fig. 3, influencing the supported plasmon modes (and consequently |Δη|) and altering the p(f) band strengths. In other words, the Fermi energy of the graphene is changed by applying the bias. The resulting change in emission from the waveguide is most clearly discernible around the onset of lasing in the example QCL A. Fig.

5 shows such emission spectra, with V _ga t _e off (= 0 V, top row of plots) and on (= 1 V, bottom row of plots). Under electrolyte biasing (i.e. at V _ga te = 1 V) a stronger filtering effect is seen, the multi-moded (FP) nature of the spectra are drastically reduced and only the most highly favoured ADFB-dictated modes reach lasing threshold.

By way of contrast, similar measurements were performed on QCL A with the electrolyte present but with no graphene present. Fig. 6 shows the resulting emission spectra, with Vgate off (= 0 V, top row of plots) and on (= 1 V, bottom row of plots).

Unsurprisingly, the resultant spectra shown in Fig. 6 differ from Fig. 5, because the devices producing the respective spectra shown in Figs. 5 and 6 each have a respectively different |Δη|. However, importantly, it is seen in Fig. 6 that the weak spectral filtering remains unchanged by V _9a te- Thus, it can be said that a graphene layer or film, incorporated into an active plasmonic waveguide, can interact with a guided optical mode therein. In essence, the introduction of the graphene layer provides control over THz QCL emission by providing a means to dynamically adjust |Δη|, thereby controlling the p(f) signature (filter response) of the (ADFB or DFB) grating as a whole.

In other words, the filtering effect provided by the grating shown in Fig. 2 can be enhanced or diminished by suitable adjustment of the Fermi energy of the graphene film by varying the Vgate applied to the graphene film.

Further discussion

As will be appreciated from the discussion herein, the ability to control and tune the emission spectrum of a waveguide such as a plasmonic waveguide, e.g. a surface plasmon waveguide, is highly desirable.

Indeed, as will be appreciated from the discussion herein, enhanced spectral flexibility in semiconductor lasers such as terahertz quantum cascade lasers (THz QCLs) would spur the transition of these promising devices from the research laboratory to real world applications. However, the combination of hitherto low temperature operation, long wavelengths and small device sizes prohibit the use of many existing spectral manipulation techniques. As has been discussed, distributed feedback (DFB) structures, with an effective refractive index perturbation |Δπ|, can be introduced to laser waveguides to define specific emission frequencies. But a fixed \An\ is not ideal for subsequent spectral control. Thus, the present inventor proposes and demonstrates an important alternative - the use of tunable graphene plasmons to dramatically alter |Δη| within a patterned plasmonic waveguide, and hence the spectral response of the plasmonic waveguide.

In particular, THz QCLs are used to demonstrate the tunability of graphene plasmons to dramatically alter |Δπ| within the patterned THz QCL waveguide, and hence the spectral response of the THz QCL waveguide

Traditionally, plasmonic devices are fabricated using noble metals, most commonly gold, in which the Fermi energy EF (and therefore its conductivity and ability to support SP modes) is not readily adjustable.

In contrast, the F in graphene may be altered by a variety of methods, including electrical gating and surface doping. Consequently, the present inventor has realized that not only can graphene be used in the construction of SP-based structures, it can also be used to modify their behaviour in situ, opening the door to active plasmonic applications.

To date, plasmonic resonances have been reported in graphene at infra-red down to THz frequencies in simple, optically passive structures. Here, the present inventor demonstrates the controllable influence of graphene on an optically active device, in particular a

(plasmonic) waveguide in a THz semiconductor laser.

To demonstrate the advantageous effects of the invention, electrochemical doping is used to alter the Fermi energy EF within a graphene-modified aperiodically patterned SP-based laser waveguide so that the multiband spectral filtering in the patterned waveguide is affected by the presence of confined plasmon standing wave in graphene, which in turn controls the laser emission. The spectral manipulation is believed to be dependent upon EF. And, in an aspect, the present invention provides an electrical control mechanism (not possible in traditional photonic structures) which is highly appealing for a variety of applications such as in situ modulation and laser mode tuning or switching.

In a particularly preferred embodiment, a THz QCL is employed as the optoelectronic device to which the present invention is applied. As has been explained, THz QCLs are

semiconductor heterostructures containing a large number of quantum wells which provide THz gain from carefully designed inter-subband transitions within the conduction band.

As mentioned above, conventional refractive index waveguiding is incompatible with QCL heterostructure growth techniques and would introduce unacceptable optical losses at such long wavelengths, hence THz QCL waveguides rely upon surface waves at a metal-dielectric interface, typically realised in a simple ridge configuration. Periodic distributed feedback (DFB) or aperiodic distributed feedback (ADFB) gratings are typically introduced to such waveguides by patterning sub-wavelength slits directly into targeted areas of the laser ridge. As explained above, this can be achieved by FIB milling for example.

Such ADFB multiband active filters are used in THz QCLs to define and control multi-colour laser emission.

A large area, high quality graphene film (grown by chemical vapour deposition) can then be stacked onto the ADFB QCLs as an overlayer on the grating microstructure. At each stage of modification - unperturbed, ADFB patterned and graphene covered laser ridge - the devices can be characterized, and the results are discussed herein.

Several lasers were tested and they demonstrated similar behaviour, with minor differences arising from the properties of the original ADFB grating. Here, for brevity, we concentrate the discussion on QCL A which is shown in Fig. 7 (and Fig. 1 ).

With an unperturbed waveguide, i.e. with no ADFB or DFB grating elements patterned into the waveguide, QCL A lased on numerous longitudinal Fabry-Perot (FP) cavity modes as shown in Fig. 7 (a). Subsequent introduction of the ADFB grating (according to the general teaching of GB2493733 by the present inventor) gave rise to multiband spectral filtering. The grating spectral reflectivity response (filtering response) p(f) in the region of interest is shown in Fig. 7(b) and its influence was to suppress many of the original FP lasing modes, as seen in Fig. 7(c). In Figs 7(a), (c) and (d) the plots on the right hand side are magnified plots (relative to the y-axis) of the left hand side plots to provide a better guide for the eye when observing the filtering effect provided by the ADFB grating. As can be seen from Fig. 7(c) the principle modes in the waveguide correspond much more strongly to the filter response shown in Fig. 7(b) provided by the ADFB grating than the modes in the Fig.7(a) prior to the FIB milling which generates the ADFB grating.

As is shown in Fig. 7(d), deposition of graphene over the grating region led to emission spectra closer to the original unperturbed FP cavity (i.e .closer to that shown in Fig. 7(a)), indicating a reduction in the filtering effect of the ADFB grating as a result of weakened scattering by the individual grating elements due to the presence of the ungated (undoped) graphene at the grating elements. Changes in filtering signature were observed across a wide range of laser operating currents (/) and can be evaluated here in terms of N, the observed number of lasing modes. Figure 7(e) shows the N-l behaviour of the FP, ADFB and graphene-modified QCL A.

Moving from a FP to an ADFB waveguide a significant drop in N is observed. However, after graphene deposition N returns to the pre-grating values. Indeed, it should be noted that some degree of spectral filtering does still occur; in Fig. 7(d) we see evidence of the multiband signature in the relative intensities of the lasing modes, but not complete mode suppression. For reference, the electrical and output power characteristics of QCL A are presented in Fig. 7(f).

A principal feature of preferred embodiments of the present invention is the electrical modulation of the filtering strength, or filtering response, of the grating via suitable control of the Fermi energy of the graphene layer. This can be achieved electrically by biasing the graphene layer, or electrochemically by doping of the graphene layer.

One way to access the high doping regime is achieved by depositing a polymer electrolyte (e.g. LiCI0 ₄ with e.g. PEO) over the graphene modified grating region of QCL A, as shown in Fig. 8(a) and 8(b) for example. This provides a similar effect to that of the device shown in Fig. 2 and as expanded in Fig. 3, for example. Accordingly, the graphene can be highly electrochemically doped, or electrically biased, by applying a bias (V _gat e) between the top of the polymer film and the graphene film itself.

The application of the bias voltage leads to an increase or enhancement in the filter strength (filter response) of the grating, the effect of which is readily observed in the laser emission spectrum.

For example, figures 8(c) and 8(d) show representative spectra with un-biased polymer (Vgate = 0 V) and with a biased polymer V _gate = 1 V respectively, collected just above the lasing threshold current. Application of the non-zero V _ga t _e voltage produces a redistribution of spectral power, with pure single-mode emission from each filter band, and many low power lasing modes (which are seen in the un-biased laser) being suppressed entirely. The remaining lasing frequencies closely follow p( ), see Fig. 8(e) for example. On the right hand side of the plots, the x-axis is expanded whilst the y-axis contracted to provide an alternate view of the plots on the left hand side.

As will appreciated from Figs. 8(c) to (e), by increasing V _ga t _e e.g. from 0V to 1 V, the filter response of the grating is enhanced. This is seen when comparing the frequency spectrum of Fig. 8(c) and 8(d) and noting that the well defined peaks in the spectrum of Fig. 8(d) correspond more closely with, and are more well defined relative to, the filter response shown in Fig.8(e).

The reduction of N (due to increased filtering strength) occurs over approximately 25% of the operational current range of QCL A, see Fig. 8(f) for example. For comparison, the inset in Fig. 8(f) shows the result when the electrolyte layer was applied to QCL A without the graphene layer being present. In this case N is largely insensitive to V _ga t _e .

These measurements show that control of the Fermi energy in graphene can be used to control (or tune) the optical response of an active system, in this case discriminating between different laser modes, despite a huge scale mismatch with the THz radiation; at < 0.5 nm thick, monolayer graphene is about five orders of magnitude thinner than the ~100 μιτι free space wavelength.

To help with understanding the principles underpinning the present invention, the present inventor has proposed mechanisms by which graphene influences the lasing in QCLs, based on performing electromagnetic modelling of the novel devices using a range of software packages; e.g. MATLAB, HFSS, COMSOL, FIMMPROP (photon design). Although the proposals from the inventor focus on QCLs, the invention should not be considered to be limited thereto.

Figure 9(a) shows the typical QCL ridge waveguide structure (operating around 2.8 THz) used in the calculations and corresponding to the studied experimental devices. The calculated fundamental transverse magnetic (TM) eigenmode is most intense at the metal- semiconductor boundary and prior 2D simulations (FIMMPROP by photon design) have shown that removal or substitution of the metal can strongly influence the eigenmode properties. However, these limited eigenmode simulations do not accurately represent the behaviour of the sub-wavelength slits in our ADFB gratings, for which a more rigorous three- dimensional (3D) model is required.

Figure 9(b) shows a short waveguide section containing a single 1 pm wide chamfered slit, penetrating the gold overlayer across the entire ridge width. A cross-section through a full 3D electric field FDTD simulation (finite difference time domain simulation, performed using HFSS) of this structure is given in Fig. 9(c), with further detail of the slit region in Fig. 9(d).

Figures 9(e)-9(g) show the simulated electric field magnitudes (left), it's time-varying vertical electric field component E _y (middle) and illustrative representations of the possible associated charge density distributions (right), for an empty slit and for slits containing graphene with £ _F = 50 meV and 300 meV respectively. As expected, the slit (as for any aperture within a waveguide) acts as a scattering site for propagating THz radiation.

Classical electrodynamics allows one to treat extremely sub-wavelength apertures as simple dipole scattering sites, with an associated dipole moment which can be deduced from the electric field intensity across the surface of the aperture. In Fig. 9(e) we see that in the absence of graphene, simulations predict large intra-slit electric fields and hence strong radiative scattering of the intense propagating THz surface wave. An examination of E _y (and E _z ) reveals that the slit without graphene is an effective scatterer due to the dipole charges produced by the metallic slit edges.

An introduction of planar graphene within the slit strongly affects the overall charge distribution and therefore the THz scattering strength. At EF = 50 meV (low-doped case, equivalent to the experimentally deposited monolayer without gating), the graphene electron plasma introduces a second dipole field distribution (localised graphene plasmon), located within the slit and oriented counter to the existing field, see Fig. 9(f). The result is a reduction of the total effective dipole strength which makes slits containing low-doped graphene poor scattering sites for THz radiation, reducing the effectiveness of ADFB-graphene filter.

On the other hand, at EF = 300 meV (heavily doped case), the plasmon wavelength in graphene becomes large and the electron plasma moves coherently inside the slit resulting in large simulated field intensities across the entire slit width and efficient scattering of THz radiation Fig. 9(g). We therefore qualitatively expect a higher efficiency of the ADFB- graphene filter for heavy doped case than for the low-doped one. Note that at & = 300 meV compression of the THz excitation wavelength should lead to a propagating plasmon wavelength of ~6 μηι, much larger than the 1 pm slit width.

Therefore, the electromagnetic simulations suggest that graphene plasmon standing waves (localised graphene plasmons) are generated by geometrical constraints (the slit boundaries) and provide the active control of filter operation. The grating filter response p( ) is given by the compound effect of radiative coupling to many slits, selectively enhancing or suppressing cavity modes.

Therefore, by applying the EF-dependent scattering seen in Fig. 9 to a grating it becomes possible to weaken or reinforce p( ) accordingly.

Time domain modelling (TDM) - a powerful tool allowing all aspects of the laser to be considered, from the laser gain (as dictated by the full laser rate equations) to the ADFB grating structure - was used to study this behaviour.

Within the TDM, the slit scattering strength is quantified by the effective complex modal refractive index contrast |Δπ|, a common approach for DFB laser systems.

As expected, TDM laser emission is highly sensitive to scattering strength. Figure 10(a) reveals an exponentially decreasing N as |Δη| is increased from zero (equivalent to an unperturbed FP cavity). In fact, this behaviour has recently been verified experimentally by varying slit depths in a range of lasers. It is this sensitivity of the optical filtering to scattering strength that is accessed via graphene doping. In Fig.8 we see that for low-doped graphene N is close to that of the FP cavity, but falls by a factor of three (averaged over I < 1.6 A) with the application of V _gate . Therefore from Fig. 10(a) we estimate that via application of gate control we achieve a strong electrical modulation of \An\ of approximately 80% or more.

According to the inventor's model, both the magnitude and phase of An (|Δη| and ΔΑ? _Φ respectively) can be altered independently, each giving rise to a variety of changes in the resultant calculated laser spectra. For ease of quantitative comparison between numerical and experimental results a single measureable aspect of the laser spectra was required. The number of Iasing modes (N) was chosen as the clearest metric of the ADFB spectral filtering of laser emission. Lasing modes were defined as any spectral peak lying above the statistical noise floor. Multiple simulations were run for each An and the calculated N values subsequently averaged, thereby improving their statistical significance. Note that in addition to the observed exponential dependence of Λ/ οη \An\ (the scattering strength) - an intuitive result - there is a clear dependence on Αη _φ .

In this case the relationship is more complex, with N values depending on constructive interference between neighbouring ADFB sites; N maxima occurring for Δη _φ values equal to integer multiples of ττ, as seen in Fig. 10(b).

Thus, it is postulated that introduction of graphene to the ADFB QCLs may alter Δ ι _φ (in addition to |Δπ|), influencing the observed N.

However, unlike \An\ which can reduce N from its maximum value to effectively unity (single laser mode operation), only a ~60% reduction from the (large) N maxima is possible via Δη _φ . The use of electrically controllable graphene plasmons to modify active photonic systems opens the door to new device possibilities. In principle, each grating slit (or small group of slits) can be purely electrically gated on an independent basis, allowing individual tailoring of scattering strengths.

In combination with the highly flexible aperiodically patterned multiband filter approach this would provide a higher level of spectral control.

An operator would be free to electronically rewrite the spectral response of a waveguide e.g. in a laser at will, simply by addressing the appropriate graphene-containing grating elements (slits) to generate an arbitrary response function p(f).

For example, each grating element containing graphene, or a set of such grating elements, could be independently controllable to generate the desired response function p(/) such that each grating element, or set thereof, could be considered to be a pixel, whereby modifying the pixel between a first state and a second state results in the grating element (or set of grating elements) results in the grating element having an effect on the spectral output of the waveguide.

While re-writable graphene plasmonic structures are particularly appealing for incorporation into THz lasers where spectral control is traditionally difficult, they can also be scaled to shorter wavelength optoelectronic systems, greatly expanding their potential technological impact.

Thus, the present inventor has provided a means to utilise gate-tunable graphene plasmons to offer a powerful alternative control mechanism for THz QCLs and other short and long wavelength optoelectronic devices. Spectral filtering is achieved in THz QCLs using periodic or aperiodic distributed feedback gratings - patterns of waveguide slits designed to filter multiple frequency bands. The localised scattering strength of each slit, and hence the optical filter strength, is modified by the introduction of graphene plasmons.

Electrochemical doping is proposed as a mechanism for changing the Fermi energy of the graphene, thereby altering the plasmon properties to give an estimated change in the modal effective refractive index perturbation of ~80%. This enormous, electrically controlled switch, in optical filtering paves the way toward highly spectrally flexible graphene-modified THz lasers.

Further example

To help demonstrate the wide applicability of the present invention, the present invention has been applied to yet another plasmonic waveguide structure incorporating a different gain material in the active region of a QCL, and the emission spectra compared with more simulated results.

A THz QCL using semi-insulating SP-waveguides (160um wide, 6mm long) was fabricated from a GaAs/Alo.-isGao ssAs heterostructure with two bound-to-continuum active regions providing simultaneous emission at -2.65 and 2.9THz. This is shown schematically in Fig. 11(a) for example, in which there is shown the gain spectrum for such a material. Again, the ADFB grating was introduced into the top layers of the laser waveguide using focussed ion beam milling (FIB). The device was then characterised in pulsed operation (10kHz pulses at 1 % duty cycle) at 10K using a FTIR spectrometer and Bolometer detector.

By adjusting the VQCL across the QCL structure, the emission spectrum of the QCL can be controlled, i.e. tuned, as described in Applied Physics Letters, 102, 181 106 (2013), the entire disclosure of which is incorporated herein by reference.

After initial device characterisation, a large area high quality, monolayer (up to 99% by area) CVD graphene was transferred as an overlayer to the ADFB grating. The general structure therefore resembles the device shown in Fig. 1 for example.

Then, in order to be able to control the Fermi energy of the graphene layer an

electrochemical top gate was overlaid on the graphene layer as described above. The general structure therefore resembles the device shown in Fig. 2 for example. A bias can then be applied to the graphene across the electrolyte, to be varied from 0V to 5V, past the breakdown point of the electrolyte.

Results for the device are presented in Figure 11(c). The top Fabry Perot plot shows the emission spectrum from the device prior to FIB patterning to introduce the ADFB grating. The next plot, labelled p( ) shows the predicted filter response of the patterned ADFB grating.

Then there are three pairs of plots, respectively showing the emission spectrum of the ADFB grating waveguide with graphene applied, and with a gate bias of 0V, 1 V and 5V. Each pair of plots shows both the emission spectrum and a x50 closeup of the emission spectrum.

As can be seen increasing the gate bias (from 0V to 1 V to 5V) on the graphene results in the laser emission becoming more multimode. As will be appreciated the lasing modes are located exactly on each of the filter bands present in the grating.

It is therefore suggested that in this device the application of the gate bias to the grating significantly increases in strength the filter response of the grating. This is shown figuratively in Fig. 11(b).

Fig. 12 provides a brief figurative explanation of the way in which the present invention can be used to tune the spectral output of a suitable waveguide.

The "real space grating" on the left hand side is representative of a real space arrangement of grating slits, or voids. As is known, the grating slits provide scattering sites for the radiation propagating in the waveguide.

The y-axis of each plot on the left hand side is indicative of the scattering strength of each slit, or void. So, we can say that the top left plot is representative of the scattering strength of an ADFB grating with a graphene layer under uniform bias. Whereas the bottom left plot is representative of an ADFB grating with a graphene layer under non-uniform bias, i.e. the graphene layer in the first, second and sixth slits, or voids, is under a different bias to the graphene layer in the third, fourth and fifth slits, or voids. As explained above, the effect of biasing the graphene differently results in different scattering strengths or effects, and thus will affect the emission spectrum of the waveguide. This is represented on the right hand side of Fig. 12. In the top right plot, the filter response provides a series of uniform peaks in in k-space. Whereas, in the bottom right plot, the filter response provides a series of non-uniform peaks in k-space.

Thus, the filter response for the grating having the non-uniform biased graphene layer(s) will result in a different emission spectrum to that of the uniformly biased graphene layer(s).

Indeed, by biasing the graphene layers in the first, second and sixth voids suitably, the scattering strength of those voids could be "turned off completely, resulting in a very different filter response.

Thus, according to the present invention by suitably biasing the graphene layers respectively provided in the voids of a grating having a plurality of voids, the filter response of the grating is fully controllable to tune the spectral output (emission spectrum) of the waveguide.

In essence, the present inventor has demonstrated electrically gated graphene control of the filter response of the grating, provided on the waveguide, sufficient to significantly affect the emission spectrum of the waveguide.

It is to be noted that the graphene layers respectively provided in the voids may be portions of a single graphene film, e.g. overlaid on the grating and arranged to span the respective voids.

The independent control of the graphene layers in the voids may be achieved by effectively pixelating the gate contact overlaid on the graphene (and/or indeed pixelating the graphene itself). In other words, respective bias gate pads may be provided for sets of graphene layers respectively provided in the voids (slits) of the grating. Each set of graphene layers may comprise one or more graphene layers.

By the application of suitable bias voltages to the respective bias gate pads, the respective graphene layers provided in the voids (slits) can be controlled to modify the filter response of the grating to tune the spectral output (emission spectrum) of the waveguide.

The present invention has been explained with reference to THz QCLs. Nevertheless, the present invention should not be considered to be limited to THz QCLs. For example, other devices which would benefit from a waveguide according to one or more aspects of the present invention include any optoelectronic device in which the (radiation) scattering effect of one or more scattering sites is modifiable by the application of graphene to the scattering site.

Graphene

Graphene is typically understood to be a monolayer of hexagonally packed carbon atoms. Nevertheless in the present invention, the graphene film (or graphene layer) need not be a monolayer. For example, the present invention is operable if the graphene film located in the at least one grating elements is up to 10 monolayers thick, up to 9 monolayers thick, up to 8 monolayers thick, up to 7 monolayers thick, up to 6 monolayers thick, up to 5

monolayers thick, up to 4 monolayers thick, up to 3 monolayers thick or up to 2 monolayers thick. Although a graphene monolayer is preferred. Graphene is an atomically thick, two dimensional sheet composed of sp2 carbons in a honeycomb structure. It can be viewed as the building block for all the other graphitic carbon allotropes. Graphite (3-D) is made by stacking several layers on top of each other, with an interlayer spacing of ~3.4 A and carbon nanotubes (1-D) are a graphene tube. Graphane is hydrogenated graphene, the carbons of the C-H groups being sp3 carbons.

Single-layer graphene is one of the strongest materials ever measured, with a tensile strength of -130 GPa and possesses a modulus of ~1 TPa. Graphene's theoretical surface area is ~ 2630 m2/g and the layers are gas impermeable. It has very high thermal (5000+ W/mK) and electrical conductivities (up to 6000 S/cm).

Graphene was first reported in 2004, following its isolation by Professor Geim's group.

Graphene research since then has increased rapidly. Much of the "graphene" literature is not on true monolayer graphene but rather two closely related structures:

(i) "few layer graphene", which is typically 2 to 10 graphene layers thick. The unique properties of graphene are lost as more layers are added to the monolayer and at 10 layers the material becomes effectively bulk graphite; and

(ii) Graphene oxide (GO), which is a graphene layer which has been heavily oxidised in the exfoliation process used to make it and has typically 30at% oxygen content. This material has inferior mechanical properties, poor electrical conductivity and is hydrophilic (hence a poor water barrier).

There are a variety of methods to produce graphene [Nature Nanotechnology, 2009, DOI: 10.1038/nnano.2009.58]. Novoselov et al. produced their first flakes by the mechanical exfoliation of graphite by using an adhesive tape to isolate individual layers [Science, 2004, 5296, pp 666-669]. It has been shown subsequently that graphite can also be exfoliated by using ultrasonic energy to separate the layers when in an appropriate solvent, such as NMP (N-methyl pyrrolidone) [Nat. Nanotechnol., 2008, 3, 563; J. Am. Chem. Soc, 2009, 131 , 361 1].

Graphite is an allotrope of carbon, the structure of which consists of graphene layers stacked along the c-axis in a staggered array usually denoted as ABAB. The layers are held together by weak van der Waals forces so that the separation between layers is 0.335 nm. Graphite is a cheap and abundant natural material, which makes it an excellent raw material for inexpensive production of graphene.

As noted above, graphite has been used to make graphene via exfoliation, wherein the stacked layers of graphite are separated to produce graphene. This has been achieved by using ultrasound (ultrasonic exfoliation, USE) and also by intercalating compounds into the graphite interlayer structure so as to weaken the interlayer bonding and promote layer separation.

There are two routes that have been reported to intercalate compounds into graphite structure: chemical and electrochemical. The chemical method is based on the direct reaction of solid graphite materials with the intercalation species (usually in liquid or vapour phase). This process is kinetically slow and usually assisted by sonication or heating. The second route, the electrochemical approach, involves generating the intercalated species through an electrochemical reaction on a graphite cathode or on a graphite anode.

The most famous example of the electrochemical approach is based on the lithium ion battery. For decades, graphite was used as negative electrode in lithium ion battery due to its high electrical conductivity and its ability to host lithium between the graphene layers. The lithium-graphite intercalation compounds decompose readily in water giving rise to lithium hydroxide and free standing graphene sheets. Loh et al. mimicked the lithium ion battery principle to intercalate Li into graphite and then applied a sonication step to exfoliate graphite [US 2013/0102084 A1 , and WO 201 1/162727]. This work is also discussed in a related paper [JACS, 2011 , 133, 8888-8891]. However, due to the slow kinetic nature of the intercalation process, the lithium was limited to the areas close to the edges. Upon exfoliation in water, graphite with expanded edges was produced and further intercalation, water decomposition and sonication steeps were needed to achieve exfoliation.

Liu et al. [Adv. Funct. Mater. 2008, 18, pp. 1518-1525] reported the exfoliation of graphite using an ionic liquid-water mixture electrolyte to form "kind of IL-functionalized" graphene nanosheets. Scheme 1 in this paper suggests that the material was produced by the exfoliation of the anode but in their discussion the authors mention the role of the cation. Lu subsequently studied the route in more detail and discussed the possible mechanism involved in the production process [ACS Nano, 2009, 3(8) pp. 2367-2375]. In their paper, they stated "according to the proposed mechanism by Liu, the positively charged imidazolium ion is reduced at the cathode to form the imidazolium free radical which can insert into the bonds of the graphene plane. At the fundamental level, there are several questionable aspects about the radical-insertion mechanism proposed by Liu, especially when the ILs are mixed with water at 1 :1 ratio and where an operational voltage as high as 15 V is applied". Lu et al. showed that the graphene nanosheet production is exclusively at the anode and is due to an interaction of decomposed water species and the anions from the ionic liquid, such as BF ₄ ^~ .