ELECTROMAGNETIC CAVE SUPPORTING TAMM MODI-CAPABLE

Description

Domaine technique :

[0001] [The present invention relates to an electromagnetic cavity with a Tamm structure having at least one resonance frequency (Terahertz) THz. Technique antérieure :

[0002] The following documents disclose prior art electromagnetic wave processing techniques: XP055655106, US2017 / 254751A1, XP055655076, XP055655118, XP055698852, CN103728275A, XP055655072 and XP036898993.

[0003] Hereafter, a THz cavity will be defined as a resonant cavity having at least one resonant frequency in the typical range of 0.1 THz to 10 THz.

[0004] THz cavities are used in the development of numerous passive and active components such as THz radiation sources, detectors, sensors, modulators, and filters. Furthermore, these cavities are essential for THz devices designed to achieve strong radiation-matter coupling. THz cavities also improve the sensitivity of spectroscopic systems by significantly increasing the interaction length between the absorbing medium and the radiation. Current THz cavities fall into two main categories: optical and electronic resonators.

[0005] Electronic resonators are based on metallic structures that allow for very sub-wavelength confinement of radiation because propagation effects are negligible and the modal volume is not directly limited by wavelength. Examples of such metallic structures include patch resonator antennas and split ring resonators. However, these electronic resonators have a low quality factor (typically 10 at room temperature), which represents a significant limitation for the fabrication of many THz components.

[0006] In contrast, optical resonators such as Fabry-Pérot cavities possess a very high quality factor, which is a very attractive characteristic for the development of many THz components. It is notably known to fabricate THz cavities adapted to achieve strong light-matter coupling between cavity photons and 2D electron gases (Zhang, Qi, et al. "Collective non-perturbative coupling of 2D electrons with high-quality-factor terahertz cavity photons." Nature Physics 12.11 (2016): 1005).

[0007] However, the modes of these cavities are based on standing waves with dominant propagation effects. Consequently, the modal volume of optical resonators is large; indeed, one dimension of the resonator must be commensurate with the resonance wavelength (with λeff / 2 being half the wavelength within the active layer). This diffraction limit is unfavorable for significant interaction of the optical resonator modes with small active or passive elements. Thus, it is very difficult to efficiently couple elements with a characteristic size of a few tens of microns with the modes of typical optical resonators at THz frequencies. Furthermore, the resonance frequency of these optical cavities can only be adjusted by the effective length of the resonator, and the mode profile is difficult to control or modify.

[0008] There are currently no THz resonators that combine a high quality factor with modes confined in a relatively small volume and that also allow control of the properties of optical modes.

[0009] The invention aims to resolve the aforementioned limitations of the prior art and proposes a new type of THz resonator, based on a Tamm mode cavity. Résumé de l'invention :

[0010] To this end, an object of the invention is a Tamm-mode electromagnetic cavity having a resonance frequency in the THz range, comprising: an interference mirror reflecting in the THz range consisting of a stack of dielectric layers comprising an alternation, along a z direction, of two different layers, a so-called high refractive index layer and a so-called low refractive index layer, the index of the low refractive index layer being lower than that of the high refractive index layer, manufactured by mechanical stacking of layers or by assembling dielectric layers one on top of the other; a metallic top layer deposited or transferred onto a dielectric top layer of said interference mirror so as to form a structure supporting at least one Tamm mode in the THz range, the metallic top layer being structured so as to form an antenna having a resonance frequency equal to that of the electromagnetic cavity.

[0011] According to particular modes of the invention: at least one of the low refractive index layers of the interference mirror is formed by a spacer separating two high refractive index layers so as to obtain an air layer between two high refractive index layers; the top metallic layer is continuous; the top metallic layer is structured so as to control the transverse mode and the polarization of the Tamm mode(s); the structured top metallic layer forms a bowtie, patch, dipole or split-ring resonator antenna; the dielectric layers are made of high resistivity silicon, semi-insulating GaAs or quartz; the dielectric layers are made of a polymer film such as Kapton; the cavity includes a layer called the mirror layer above the top metallic layer and separated by a dielectric layer, said mirror layer being made up of a bottom metallic layer and a top support layer;The cavity comprises an active element with a characteristic size of between 1 and 100 microns, preferably between 10 and 40 microns, placed in the cavity so as to be able to be coupled to the Tamm mode excited in said cavity; the active element is placed within an air layer of the interference mirror located below the upper dielectric layer; the active element is made of graphene; the upper metallic layer is a superconducting layer.

[0012] Another object of the invention is a Tamm-mode electromagnetic cavity having a resonant frequency in the THz range, comprising: an interference mirror reflecting in the THz range consisting of a stack of dielectric layers comprising an alternation, along a z direction, of two different layers, a so-called high refractive index layer and a so-called low refractive index layer, the index of the low refractive index layer being lower than that of the high refractive index layer, manufactured by mechanical stacking of layers or by assembling dielectric layers one on top of the other; a metallic top layer deposited or transferred on a dielectric top layer of said interference mirror so as to form a structure supporting at least one Tamm mode in the THz range, the metallic top layer being structured so as to form a network of metallic strips of width s, period p, separated by a distance a and of filling factor ff=s / p with p=s+a.

[0013] According to particular modes of the invention: the filling factor ff of the metallic strip network varies along the x direction so as to allow the coupling of a plurality of different THz frequencies of incident radiation with said electromagnetic cavity.

[0014] Another object of the invention is a method of using an electromagnetic cavity according to any one of the preceding claims, comprising: the illumination of said cavity by an incident radiation propagating along said direction z at a frequency THz equal to the resonance frequency of said cavity; the excitation of a Tamm mode at a resonance frequency of the cavity.

[0015] According to specific modes of this usage process: the angle of incidence of the incident radiation on said cavity is non-zero, preferably greater than 25°; it includes the transmission of THz radiation. Brève description des figures :

[0016] Other features, details and advantages of the invention will become apparent from the description provided with reference to the accompanying drawings given by way of example, which represent, respectively: [ Fig. 1 ], an electromagnetic THz cavity with Tamm modes according to a first embodiment of the invention; [ Fig. 2 ], the reflectivity of the electromagnetic cavity of the first embodiment of the invention as a function of the wavelength of the incident radiation; [ Fig. 3A ], an electromagnetic THz cavity with Tamm modes according to a second embodiment of the invention, tunable in resonance frequency; [ Fig. 3B ], the electromagnetic field profile within the Tamm-mode electromagnetic THz cavity according to the second embodiment of the invention. [ Fig. 4A ] And [ Fig. 4B ], the evolution, as a function of the filling factor, the resonance frequency and the quality factor of the Tamm-mode electromagnetic THz cavity of the second embodiment; [ Fig. 5A ] And [ Fig. 5B ], respectively an electromagnetic THz cavity with Tamm modes according to a third embodiment of the invention and the evolution, as a function of the filling factor, of the quality factor of said cavity; [ Fig. 6 ], an electromagnetic THz cavity with Tamm modes according to a fourth embodiment of the invention; [ Fig. 7 ], an electromagnetic THz cavity with Tamm modes according to a fifth embodiment of the invention. Description détaillée :

[0017] Optical resonators combining low losses and reduced modal volume are known in the near-infrared spectral range. These are based on Tamm mode cavities, which combine a metallic mirror and a Distributed Bragg Reflector (DBR) type interference mirror, where the DBR is made by epitaxy of successive layers of dielectrics or semiconductors (Guillaume Lheureux's PhD thesis. Study of the laser effect in Tamm plasmon structures. Plasma Physics [physics.plasm-ph]. Claude Bernard University - Lyon I, 2015. French).<NNT : 2015LYO10341> . <tel-01327966>These Tamm mode cavities allow for good mode confinement at the interface between the DBR and the metallic mirror deposited directly onto the DBR. Furthermore, losses due to the metal are lower because the mode is distributed predominantly within the DBR.

[0018] However, this solution cannot be transposed to the THz range using conventional manufacturing methods due to the wavelengths, which are on the order of hundreds of microns. For DBR, this implies dielectric or semiconductor layer thicknesses of several tens of microns. Indeed, DBR is generally fabricated by epitaxial growth of the different layers, which is not feasible for such significant thicknesses.

[0019] The invention is based on a Tamm mode resonator adapted to the THz spectral range. figure 1 Figure 1 represents a schematic profile view of a THz 10 electromagnetic cavity with Tamm modes according to a first embodiment of the invention. In this embodiment, the cavity is formed of a periodic stack 7 and a metallic layer 5. The periodic stack 7 comprises an alternation, along a z-direction, of dielectric or semiconducting layers with high and low refractive indices forming an interference mirror or DBR (distributed Bragg reflector), and having a top layer with a high refractive index. By low and high refractive indices, it is understood that the so-called high refractive index is greater than the so-called low refractive index. It reflects a certain range of wavelengths (called the band gap) in the spectral range of the incident THz radiation 1 propagating along the z-direction.The thickness of each layer is approximately an odd multiple of λ / 4n (where n is the refractive index of the layer), and λ is the central wavelength of the band gap of this periodic stack, a band gap that contains the resonance wavelength of the Tamm mode cavity. This ability to use dielectric or semiconductor layers thicker than λ / 4n is very important because it allows, in particular, the use of thicker layers when the same layers in thinner layers are technologically difficult to obtain, as is the case with silicon.

[0020] The fabrication of this stack 7 has the advantage of being carried out without microfabrication processes such as semiconductor epitaxy used in the infrared-visible range. Commercially available dielectric layers (examples of which will be presented below) are simply mechanically stacked one on top of the other along the z-direction.

[0021] In a non-limiting example, the stack 7 consists of alternating layers of high-resistivity silicon 2 (high refractive index) and air 4 (low refractive index) along a z-direction. In this example, the air and high-resistivity silicon layers have thicknesses eair = 75 µm and eSi = 70 µm, respectively. The band gap of the interference mirror formed by the stack 7 is centered on λB = 300 µm (equivalent to 1 THz); the layers having eair = λB / 4nair and eSi = 3λB / 4nSi, respectively (where nair is the refractive index of air in the THz and nSi is the refractive index of silicon in the THz). The air layers are formed between the silicon layers using a spacer 3. According to the embodiment of the figure 1 The cavity 10 comprises a stack 7 of two pairs of layers and a top silicon layer. In another embodiment, the cavity comprises a periodic stack of a plurality of layer pairs separated by respective spacers. In yet another embodiment, the dielectric layers of the stack 7 are flexible polymer films that can exhibit very low losses in the THz range, such as Kapton, or are semi-insulating GaAs or quartz.

[0022] Using the transfer matrix method, well known to those skilled in the art, it is possible to calculate the reflectivity of the Bragg mirror 7 of the cavity 10 as a function of the frequency of an incident radiation 1. The reflectivity as a function of frequency is represented by curve 21 of the figure 2 and highlights a certain range of frequencies where reflectivity is high, before decreasing outside this region. This is the band gap region of the Bragg mirror (DBR), with center frequency f B = c / λ B .

[0023] The stacking of cavity 10 of the figure 1 is covered by a continuous metallic top layer 5. By metallic, we mean a material with metallic electrical conductivity. In the embodiment of the figure 1 The top metallic layer is a 200 nm thick layer of gold. In this embodiment, the top metallic layer is continuous and does not completely cover the top dielectric layer of the stack 7. In another embodiment, the top metallic layer completely and seamlessly covers the top dielectric layer. It is deposited on top of the last dielectric layer of the stack 7 using techniques known to those skilled in the art, for example, vacuum deposition. In another embodiment, the metallic layer is replaced by a superconducting layer, such as a niobate-type superconductor. Alternatively, the top metallic layer can be made of doped graphene. Alternatively, this layer can be applied, i.e., bonded to the stack.

[0024] The addition of a metallic top layer 5 confines the electromagnetic field at the interface between the DBR and the metallic layer 5 for incident radiation 1 with a wave vector perpendicular to the interface, propagating along the z-direction. This mode exists for both electric transverse (TE) and magnetic transverse (TM) polarization. Sub-wavelength confinement of the modes on the metal side is achieved due to the very large negative dielectric constant of the metal. Conversely, on the dielectric side, confinement is achieved due to the presence of the DBR band gap. In the embodiment of the figure 1 , the phase matching between the metallic mirror 5 and the stack 7 induces a resonance frequency of the Tamm modes at the center frequency of the DBR.

[0025] The DBR 7 of the embodiment of the figure 1 Having a reflectivity centered at λB = 300 µm, the fundamental Tamm mode frequency is approximately fTamm = ωTamm / 2π = 0.91 THz. Many other Tamm modes with higher frequencies exist in the cavity. This frequency of the Tamm mode will subsequently be called the "cavity resonance frequency." Thus, incident radiation at this frequency, propagating along the z-direction with TM or TE polarization, will be coupled in the cavity and excite the Tamm mode at the interface between the upper metallic layer and the upper dielectric layer of the Bragg mirror. To allow excitation of the Tamm mode, the difference between the frequency of the incident radiation and the cavity resonance frequency is less than the spectral width of the resonant mode. The cavity reflectivity of the figure 1 as a function of the wavelength of the incident radiation 1 is presented in curve 22 of the figure 2 obtained by the transfer matrix method. Compared to the reflectivity of DBR 7 alone (curve 21) we observe an absorption peak of cavity 10 corresponding to the Tamm mode, in the band gap of the DBR.

[0026] This Tamm mode exhibits a longitudinal extension (along the z-direction) of λeff / 2 within the DBR 7. Furthermore, in the transverse direction (along the x and y directions), the electric field of the Tamm mode is confined to the region of the DBR located below the upper metallic layer. The transverse spatial extension along the x-direction of the Tamm mode is therefore essentially limited by the transverse dimension L(m,x) along the x-direction of the upper metallic layer 5, thus enabling transverse confinement of the Tamm mode. Similarly, the transverse spatial extension along the y-direction of the Tamm mode is essentially limited by the transverse dimension along the y-direction of the upper metallic layer 5.

[0027] The quality factor Q of cavity 10 of the embodiment of the figure 1 is 100.

[0028] In another embodiment, the upper metallic layer is a conductive material with lower reflectivity, such as graphene, and the cavity 10 operates in transmission mode. In yet another embodiment, the incident radiation 1 and reflected radiation 6 are oblique in incidence. Oblique here means that the angle formed by the direction of propagation of the incident radiation with the z-direction of the stack is non-zero, preferably greater than 25°.

[0029] [ Fig. 3A Figure 20 illustrates a schematic profile view of a THz 20 electromagnetic cavity with Tamm modes according to a second embodiment of the invention. In this embodiment, the DBR is identical to that of cavity 10, but the upper metallic layer 5 is discontinuous. It is structured to form a network of metallic strips of width s, separated by a distance a, and with a filling factor ff = s / p, where p = s + a. It is known that the resonant frequency of cavity 20 and the quality factor of cavity 20 decrease as the filling factor decreases.

[0030] [ Fig. 4A ] And [ Fig. 4B Figures 20 and 5 respectively show the evolution of the resonance frequency and quality factor as a function of the filling factor of the Tamm cavity, in the case where the metallic top layer is structured so that the filling factor is constant along the x-direction. Here, the periodicity p of the Tamm cavity is fixed at 75 µm, while the width of the metallic bands s varies with ff. It is observed that the resonance frequency and quality factor increase with the filling factor. It is therefore possible to tune the resonance frequency of the cavity by varying the filling factor ff for different regions of the metallic top layer.

[0031] Thus, in the method of implementation of the figure 3A The upper metallic layer 5 is structured to have a plurality n of different metallic band lattice filling factors, varying along the x-direction, so as to allow the coupling of a plurality of different THz frequencies of the incident radiation with the cavity 20 and the excitation of Tamm modes of different frequencies. Specifically, the structured upper metallic layer has several different regions, each with a different filling factor, giving the combination of this region and the DBR below a resonant frequency different from the others.

[0032] Also, in region 21 of the upper metallic layer, the filling factor is ff 1 = s 1 / p 1 so as to couple incident radiation at wavelength λ 1 by exciting a Tamm mode at this wavelength. In region 22 of the upper metallic layer, the filling factor is ff n = sn / pn so as to couple incident radiation at wavelength λ n by exciting a Tamm mode at this wavelength.

[0033] In another embodiment, the top structured metallic layer has a single region and a single filling factor, used to very precisely adjust the resonant frequency of the cavity after stack fabrication (DBR).

[0034] In a non-limiting example, the upper metallic layer 5 of cavity 20 is structured such that the filling factor varies progressively from 10% to 100% along the x-direction in order to go from a resonant frequency f1 = 0.65 THz for region 21 of cavity 20 to a resonant frequency f2 = 1 THz for region 21 of cavity 20, respectively (see figure 4A ).

[0035] As shown by figures 4A And 4B , this structure of the upper layer 5 of the figure 3A This allows for a resonance frequency of approximately 35% of cavity 20, at the expense of the quality factor, which decreases with the resonance factor. Indeed, the quality factor is approximately Q=10 for a filling factor ff=10% and approximately Q=200 for a filling factor ff=90%.

[0036] [ Fig. 3B Figure ] represents the electric field profile of the Tamm mode at λ₁ = c / f₁ in region 21 of cavity 20, for a filling factor ff = 0.5 and p = 75 µm, giving a resonance frequency f₁ = 0.87 THz. Thanks to the confinement of the electric field between the metallic strips above the top dielectric layer of the stack, the electric field is localized in this region, resulting in a higher electric field amplitude (approximately 3.5 V / m). Furthermore, it is observed that below the top dielectric layer of DBR 7, the electric field is more intense (greater than 5 V / m), and even more so in the regions directly below the areas not covered by the top metallic layer.

[0037] This is an additional feature compared to the embodiment of the figure 1 where the electric field is homogeneously confined to the dielectric-air interface. This effect is related to the periodic subwavelength distribution of the metal, which brings about a field enhancement between the bands

[0038] The structuring of the upper metallic layer therefore allows for an enhancement and confinement of the electric field of the Tamm mode.

[0039] The presence of air between the dielectric layers and between the metallic strips makes it easy to place an active element in the regions of cavity 20 where the Tamm mode electric field is strongest. This allows for easy THz light-matter coupling with active elements having characteristic dimensions of 1 to 100 microns, preferably 10 to 40 microns, and a high energy density. The characteristic dimension refers to the longest dimension of an element (for example, the diagonal in the case of a cube).

[0040] In this embodiment, the cavity can be used in transmission geometry because the metallic top layer has a non-zero transmission.

[0041] In order to obtain a THz cavity tunable in resonance frequency while maintaining a high quality factor, the cavity 30 of the third embodiment of the invention, schematically shown in profile in the figure 5 includes a mirror 31. This mirror 31 is composed of a lower metallic layer 32 and a support layer 33. In the embodiment of the figure 5 The assembly 20 formed by the DBR 7 and the metallic top layer 5 is identical to that of the figure 3A and the lower metallic layer 32 of the mirror 31 is made of gold and the upper support layer 33 of the mirror 31 is made of silicon. The mirror 31 is placed above the upper metallic layer 5 at a distance D=λ m / 4 from the latter in order to maximize the intensity of the electric field at the level of the upper metallic layer and in the region below the upper dielectric layer of the stack 7. The mirror is separated from the layer by a dielectric layer 34. In the example of the figure 5 This layer is an air layer. Here, λm corresponds to the longest resonance wavelength of cavity 30, that of region 21 (λ1 = c / f1). In another embodiment, λm corresponds to the average of the resonance wavelengths of cavity 30. Mirror 30 reflects the transmitted radiation 34 from the assembly 20 formed by the DBR7 and the upper metallic layer 5, thus increasing the reflectivity of the cavity-mirror assembly. This transmitted radiation 34 is due to the structure of the metallic layer 5 and is not present, or is less intense, in the embodiment of the figure 1 where the upper metallic layer 5 is not discontinuous. The mirror 31 can be separated from the upper metallic layer by a spacer. The curve 51 of the figure 5B shows the evolution of the quality factor of cavity 30 (with mirror 31 above) as a function of the fill factor, in the case where the upper metallic layer is structured so that the fill factor is constant along the x-direction. The comparison of its evolution with that of the quality factor of cavity 20 without mirror 31 above (curve 52) figure 5B Figure 30 clearly illustrates the improvement in the quality factor of cavity 30 caused by the addition of the mirror, particularly for fill factors below 80%. For example, for a fill factor ff=40%, the addition of mirror 31 increases the quality factor from Q=20 to Q=120. Cavity 30 of the embodiment of the figure 5 exhibits a polarization dependence. Indeed, it can only function with incident radiation possessing a TE polarization (along the y direction).

[0042] In a fourth embodiment, an electromagnetic cavity THz 60 is similar to the electromagnetic cavity 10 of the embodiment of the figure 1, à 2 with the exception of major differences. First, the metallic top layer is structured to be an antenna with a resonant frequency equal to that of the electromagnetic cavity 10 formed by the DBR 7 and the metallic top layer 5. Second, it includes an additional low-index phase layer 66 between the high-index top layer and the metallic layer. In a non-limiting example, this phase layer is made of a polymer with an intermediate index between air and silicon 2. This embodiment is illustrated in the figure 6 .

[0043] In this fourth embodiment, the upper metallic layer 5 is continuous and has a length Lmx along the x direction so as to be a metallic dipole antenna having a resonance frequency equal to that of cavity 10. This choice of dipole antenna allows an exaltation of the quality factor and a local exaltation of the electric field of the Tamm mode when an incident radiation of polarization TM (along the x direction) propagates along the z direction in the cavity.

[0044] This field enhancement provides access to a significant energy density. It allows, from incident radiation at frequency THz with TM polarization (along the x direction) and propagating along the z direction in the cavity, the realization of light-matter coupling by inserting active elements with characteristic dimensions of a few tens of microns in the region where the electric field of the Tamm mode is enhanced (for example at the corners of antenna 5 in the third embodiment of the invention).

[0045] In another embodiment, the upper metallic layer 5 is structured to be a bowtie antenna, patch antenna, or split-ring resonator with a resonant frequency equal to that of the cavity 10, thus enabling local enhancement of the electric field of the Tamm modes. Bowtie antenna, patch antenna, and split-ring resonator structures, well known to those skilled in the art, allow for local enhancement of the electric field of the Tamm mode by several orders of magnitude (between the two opposing triangles in the case of the bowtie antenna and in the gap region in the case of the split-ring resonator).

[0046] This enhancement provides access to an extremely interesting energy density for achieving light-matter coupling by inserting active elements with characteristic dimensions of a few tens of microns in the regions where the electric field of the Tamm mode is enhanced.

[0047] In another embodiment, the upper metallic layer 5 is structured to optimize its antenna structure, enhancing its interaction with the DBR and allowing control of the transverse profile and polarization of the Tamm mode excited by the incident radiation 1. In a non-limiting example, a bowtie antenna allows localization of the mode, as well as concentration of the electric field, in the gap between the two parts of the antenna. In another non-limiting example, a dipole antenna can be chosen to be 1D (a line thinner along one of its dimensions and having a resonant frequency THz along only one of its dimensions) or 2D (square in shape). It is thus possible to choose to have control over the polarization or, conversely, to be insensitive to it. Indeed, a 1D antenna has a resonance for a well-defined TM or TE polarization and allows the polarization of the excited Tamm mode to be fixed.Conversely, a 2D antenna resonates in the same way for both TM and TE polarizations, allowing a Tamm mode to be excited with these two polarizations.

[0048] [ Fig. 7 ] illustrates a fifth embodiment of the invention in which an electromagnetic cavity 40 with Tamm modes allows the coupling of incident radiation 1 at a frequency THz with an active graphene region 61. In this embodiment, the stacking 7 is identical to that of the embodiment of the figure 3 The upper metallic layer is identical to that of the fourth embodiment and has a resonant frequency equal to that of cavity 10. Cavity 40 further comprises a graphene layer 61 beneath the upper dielectric layer, in contact with its lower face. This graphene layer 61, which may be partially encapsulated, is the active element or active region of cavity 40.

[0049] In the implementation of the figure 7 The graphene layer is continuous and completely covers the underside of the top dielectric layer of the DBR. This graphene layer is deposited by chemical vapor deposition (CVD).

[0050] In another embodiment, the graphene layer is continuous and does not completely cover the underside of the upper dielectric layer of the DBR. It is a sheet, for example a few tens of microns across, produced by exfoliation and placed at the center of the Tamm mode, i.e., at the maximum of the Tamm electric field enhanced by the upper metallic layer. In yet another embodiment, the upper metallic layer 5 is structured to act as an antenna with a resonant frequency equal to that of the cavity 10, thus enabling a local enhancement of the electric field of the Tamm mode. In this embodiment as well, the graphene sheet 61 is placed below the upper dielectric layer of the DBR at the maximum of the electric field of the Tamm mode in order to achieve the strongest possible light-matter coupling.

[0051] The graphene 61 layer can be a single layer, in which case its thickness is that of a carbon atom (approximately 3.5 Å). Alternatively, the graphene 61 layer can be a stacking along the z-direction of a plurality n of single layers, in which case its thickness is equal to n×3.5 Å.

[0052] Furthermore, the cavity 40 includes a metallic layer 63, called the electrode, beneath the graphene layer 61 but not completely covering it. The electrode 63 is a metallic layer in contact with the graphene layer 61 by a metallic electrical contact (not shown in the figure 7 ). In the implementation of the figure 7 This metallic contact is ensured by a gold metallic deposit a few microns across and approximately 200 nm thick. In another embodiment, the contact can be made in any other metal or by using a conductive layer such as doped graphene.

[0053] An electrical circuit 64 is connected to the electrode 63 by a metal line 65 extending from the cavity 40. Preferably, the metal line 65 has a diameter much smaller than the resonant wavelength of the cavity in order to limit the perturbations of the Tamm mode introduced by this metal line. Furthermore, the electrical circuit is also connected to the metal layer 5 or to the upper dielectric layer of the DBR. The electrical circuit 64 is configured to apply a potential difference or gate voltage between the electrode 63 and the metal layer 5.

[0054] Applying a gate voltage via electrode 63, connected to the electrical circuit 64, creates an electric field at the graphene layer, allowing a Fermi level to be imposed or fixed at this layer. Graphene has a band gap of 0 eV and a band structure exhibiting linear dispersion in a Dirac cone pattern. Without this electric field, the Fermi level is poorly controlled; therefore, the electrical circuit dictates the electronic and optical behavior of the graphene. Thus, by choosing a gate voltage such that the Fermi level is at 0 eV, which corresponds to the Dirac delta function in the dispersion relation, the conductivity of the graphene at THz frequencies is governed by interband processes. Conversely, by choosing a gate voltage such that the Fermi level is above a few meV, the absorption of the graphene at THz frequencies is governed by intraband processes.

[0055] Thus, depending on the grid voltage applied by the electrical circuit 64 and chosen by the user, the active region in graphene 61 will behave as an optical gain medium, a photoconductor, a modulator or an absorber for the Tamm mode excited in the cavity by the incident radiation THz 1.

Claims

1. Tamm electromagnetic cavity (10, 20, 30, 40, 60) possessing a resonant frequency in the THz domain, comprising: - an interference mirror that is reflective in the THz domain, this mirror consisting of a stack of dielectric layers (7) comprising an alternation, in a z-direction, of two different layers, a layer referred to as the layer of high refractive index (2) and a layer referred to as the layer of low refractive index (4), the index of the layer of low refraction being lower than that of the layer of high refractive index, being manufactured by stacking layers mechanically or by joining dielectric layers to one another; - an upper metal layer (5) deposited on or added to an upper dielectric layer of said interference mirror so as to form a structure that supports at least one Tamm mode in the THz domain, the upper metal layer (5) being structured so as to form an antenna possessing a resonant frequency equal to that of the electromagnetic cavity.

2. Electromagnetic cavity (40) according to any one of the preceding claims, wherein the upper metal layer is continuous.

3. Electromagnetic cavity according to any one of the preceding claims, wherein the upper metal layer is structured so as to control the transverse mode and the polarization of the one or more Tamm modes.

4. Electromagnetic cavity according to claim 1, wherein the structured upper metal layer forms a bow-tie antenna, a patch antenna, a dipole antenna, or a split-ring resonator.

5. Tamm electromagnetic cavity (10, 20, 30, 40, 60) possessing a resonant frequency in the THz domain, comprising: - an interference mirror that is reflective in the THz domain, this mirror consisting of a stack of dielectric layers (7) comprising an alternation, in a z-direction, of two different layers, a layer referred to as the layer of high refractive index (2) and a layer referred to as the layer of low refractive index (4), the index of the layer of low refraction being lower than that of the layer of high refractive index, and being manufactured by stacking layers mechanically or by joining dielectric layers to one another; - an upper metal layer (5) deposited on or added to an upper dielectric layer of said interference mirror so as to form a structure that supports at least one Tamm mode in the THz domain, the upper metal layer being structured so as to form a grating of metal strips of width s and of period p, separated by a distance a, and of fill factor ff=s / p with p=s+a.

6. Electromagnetic cavity (20, 30) according to the preceding claim, wherein the fill factor ff of the grating of metal strips varies in the x-direction so as to allow a plurality of different THz frequencies of incident radiation to be coupled to said electromagnetic cavity.

7. Electromagnetic cavity according to any one of the preceding claims (40), wherein at least one of the layers of low refractive index of the interference mirror is formed by a spacer (3) separating two layers of high refractive index so as to obtain a layer of air between two layers of high refractive index.

8. Electromagnetic cavity according to any one of the preceding claims, wherein the dielectric layers are made of high-resistivity silicon, of semiinsulating GaAs or of quartz.

9. Electromagnetic cavity according to any one of the preceding claims, wherein the dielectric layers are made of a polymer film such as Kapton.

10. Electromagnetic cavity (30) according to any one of the preceding claims comprising a layer referred to as the mirror layer (31), above the upper metal layer and separated by a dielectric layer, said mirror layer consisting of a lower metal layer (32) and an upper carrier layer (33).

11. Electromagnetic cavity (40) according to any one of the preceding claims, comprising an active element (61) of a characteristic size comprised between 1 and 100 microns, and preferably 10 and 40 microns, placed in the cavity so as to be able to be coupled to the Tamm mode excited in said cavity.

12. Electromagnetic cavity (40) according to the preceding claim, wherein the active element (61) is placed within a layer of air of the interference mirror, said layer being located below the upper dielectric layer.

13. Electromagnetic cavity (40) according to any one of claims 11 to 12, wherein the active element (61) is made of graphene.

14. Electromagnetic cavity (40) according to the preceding claim, comprising a metal layer referred to as the electrode (63) making electrical contact with the graphene active element and connected to an electrical circuit (64) configured to apply a gate voltage to said graphene active element.

15. Electromagnetic cavity according to any one of the preceding claims, wherein the upper metal layer (5) is a layer of superconductor.

16. Method for using an electromagnetic cavity according to any one of the preceding claims, comprising: - illuminating said cavity with incident radiation (1) propagating in said z-direction at a THz frequency equal to the resonant frequency of said cavity; - exciting a Tamm mode at a resonant frequency of the cavity.