Photonic substrate and use of such a substrate in an optical modulation device

Abstract

The invention relates to a photonic substrate (1) comprising a support (5) having a first face and a second face opposite the first, an electrical charge trapping layer (4) disposed on the first face of the support (5), a dielectric layer (3;3a,3b) on and in contact with the electrical charge trapping layer, and an electro-optical layer (2) formed of a single-crystal material of optical quality, the electro-optical layer (2) being disposed on and in contact with the dielectric layer (3;3a,3b). The invention also relates to the use of such a photonic substrate to form an optical modulation device and the resulting optical modulation device. Figure 1

Description

Title of the invention: Photonic substrate and use of such a substrate in an optical modulation device. FIELD OF THE INVENTION

[0001] The present invention relates to a photonic substrate, that is to say, a substrate that can be used to manufacture photonic components and to receive these components. This substrate finds a particular application in forming an optical modulation device. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Optical transmitters are devices that convert digital bit streams from the electrical domain to the optical domain, at rates ranging from a few tens of gigabits per second (Gbps) to hundreds of Gbps, typically between 200 Gbps and 400 Gbps for high-performance transmitters. Such a transmitter usually comprises a continuous-wave laser source and an optical modulator that encodes the binary stream into a light beam whose phase or intensity is modulated.

[0003] It is known in the prior art, for preparing such an optical modulator, to use a thin-film photonic substrate, consisting of a lithium niobate layer transferred onto a support via a dielectric layer. These modulators are sometimes referred to as thin-film modulators. Optical waveguides and electrodes are formed on and within the lithium niobate layer in order to guide the light radiation and modulate it by exploiting the Pockels effect.

[0004] For an introduction to thin-film photonic substrates, reference may be made to the document by Martin F. Volk et al, "Low loss ridge waveguides in lithium niobate thin films by optical grade diamond blade dicing," Opt. Express 24, 1386-1391 (2016).

[0005] For most of the applications envisaged, optical transmitters have strict performance requirements. The modulators must operate at very high speeds (typically an electro-optical bandwidth >67 GHz), be linear, and energy-efficient. This is recalled in the document by Prashanta Kharel, et al., "Breaking voltage-bandwidth limits in integrated lithium niobate modulators using micro-structured electrodes," Optica 8, 357-363 (2021).

[0006] These performance requirements impose strict characteristics on the photonic substrate used to form the modulators, both optically (low propagation loss, low device variability) and electrically (losses of transmission over the targeted bandwidth). Transmission losses must be as low and frequency equalized as possible for the modulator to be fast, linear and energy efficient.

[0007] The document by Gengxin Chen, et al., “High performance thin-film lithium niobate modulator on a Silicon substrate using periodic capacitively loaded traveling-wave electrode”, APL Photonics, 1 February 2022, proposes to reduce transmission losses in the thin-film photonic substrate by creating cavities in the silicon support, under the dielectric layer and the lithium niobate thin film. SUBJECT OF THE INVENTION

[0008] One object of the invention is to provide an alternative to prior art solutions. More specifically, one object of the invention is to provide a photonic substrate for an optical modulation device, capable of limiting transmission losses in this device. Another object of the invention is to provide a thin-film optical modulator formed on the photonic substrate of the invention and taking advantage of its characteristics. BRIEF DESCRIPTION OF THE INVENTION

[0009] With a view to achieving one of these goals, the object of the invention proposes a photonic substrate comprising: a. a support having a first face and a second face, opposite to the first; b. an electrical charge trapping layer disposed on the first face of the support; c. a dielectric layer on and in contact with the electrical charge trapping layer; d. an electro-optical layer formed of a single-crystal material of optical quality, the electro-optical layer being disposed on and in contact with the dielectric layer.

[0010] According to other advantageous and non-limiting features of the invention, taken alone or in any technically feasible combination: - the electro-optical layer has an absorption coefficient of less than 10A-3 in the spectral band between 0.7 micrometer and 1.6 micrometer, preferably less than 10A-2; - the electro-optical layer is made of lithium niobate, and it can exhibit a crystalline X section; - the electro-optical layer has a thickness between 300nm and 600nm; - the electro-optical layer has a free face whose roughness is less than 0.5 nm in root mean square value measured over a field of 10 micrometers by 10 micrometers; - the electro-optical layer has a unit of thickness variability of less than 50 nm over its entire extent; - the dielectric layer comprises or is made up of a first dielectric layer of silicon oxide having an absorption coefficient of less than 10A-3 in the spectral band between 0.7 micron and 1.6 micron. - the dielectric layer includes a second dielectric layer, arranged between the first dielectric layer and the electrical charge trapping layer, the second dielectric layer comprising nitrogen; - the dielectric layer has a thickness between 2 microns and 10 microns, preferably between 2 microns and 5 microns. - the electrical charge trapping layer is made of polycrystalline silicon; - the photonic substrate includes a complementary polycrystalline layer arranged on and in contact with the second face of the support; - the electrical charge trapping layer consists of a silicon-rich oxide having an atomic concentration of silicon between 50% and 99.9%, preferably in an atomic concentration of silicon between 70% and 90% and oxygen in an atomic concentration between 10% and 30%. - the electrical charge trapping layer also includes nitrogen in an atomic concentration of between 8% and 45%; - the electrical charge trapping layer has a thickness between 10 nm and 30 micrometers, preferably between 50 nm and 5 micrometers; - the support (5) is made of monocrystalline silicon; - the support has an electrical resistivity greater than 750 Ohm.cm, preferably greater than 2000 Ohm.cm; - the photonic substrate includes a compensation layer arranged on the side of the second face of the support; - the photonic substrate is in the form of a circular plate with a diameter between 100 mm and 200 mm, for example 150 mm.

[0011] According to another aspect, the invention proposes to use the photonic substrate which has just been presented to form a thin film optical modulation device.

[0012] According to yet another aspect, this thin-film optical modulation device comprises: a. a support having a first face and a second face, opposite to the first; b. an electrical charge trapping layer disposed on the first face of the support; c. a dielectric layer on and in contact with the electrical charge trapping layer; d. at least one waveguide defined in an electro-optical layer formed of a single-crystal material of optical quality, the at least one waveguide being disposed on and in contact with the dielectric layer and being associated with modulation electrodes. Brief description of the drawings

[0013] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the single attached figure in which:

[0014] [Fig.l] [Fig.2] [Fig.3]

[0015] Figures 1, 2 and 3 represent embodiments of a photonic substrate according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] For the sake of simplicity in the following description, the same references are used for identical elements or elements performing the same function in the different modes of implementation of the invention or in the presentation of the prior art.

[0017] Figures 1, 2 and 3 represent several embodiments of a photonic substrate 1 which is the subject of this description.

[0018] The photonic substrate 1 is advantageously in the form of a circular wafer, the diameter of which is standardized to allow handling by standard semiconductor industry equipment. This diameter is typically between 100 mm and 200 mm. However, the shape and dimensions of the substrate 1 are not a limitation, and these shapes and dimensions can be freely chosen.

[0019] Whatever the shape and size adopted by the photonic substrate 1, it generally comprises a support 5, an electric charge trapping layer 4 (more simply referred to as the "trapping layer" in the rest of this description) disposed on a first face of the support 5, a dielectric layer 3 on and in contact with the trapping layer 4 and an electro-optical layer 2 disposed on and in contact with the dielectric layer 3.

[0020] The electro-optical layer 2 is made of a material capable of producing electro-optical effects, that is, of varying at least one of its optical properties when subjected to a changing electrical quantity, for example, a changing electric field. This could be, for example, the Pockels effect. The material constituting the electro-optical layer 2 can be a ferroelectric material, such as lithium niobate or barium titanate, or even lithium tantalate. In all cases, it is made of a single-crystal material of optical quality and can have a thickness between 300 nm and 600 nm to allow for the definition of waveguides, for example, those of an optical modulator.

[0021] For the purposes of this application, the "optical quality" of a layer means that the absorption coefficient of that layer is less than 10A-3 in the spectral band between 0.7 micrometers and 1.6 micrometers. Preferably, this absorption coefficient is less than 10A-2 in this spectral band.

[0022] Thus, when the electro-optical layer is made of single-crystal lithium niobate, it advantageously presents a crystal cut X, although it is not excluded that it may present another crystal cut, for example Z or Y.

[0023] To ensure proper propagation of the optical mode and limit excessive absorption of this mode, the electro-optical layer 2 has a free face (the face not in contact with the dielectric layer 3) with a roughness of less than 0.5 nm in root mean square value measured over a 10-micrometer by 10-micrometer area. It also advantageously has a thickness variability unit of less than 50 nm over its entire extent. Its interface with the dielectric layer 3 must be free, as far as possible, from defects such as particles, contaminants (particularly metals), and local delamination with the dielectric layer (known as "voids" in Anglo-Saxon terminology).

[0024] The dielectric layer 3 is also of optical quality and therefore has an absorption coefficient of less than 10A-3, preferably less than 10A-2, in the spectral band between 0.7 micron and 1.6 micron.

[0025] Advantageously, this is a silicon oxide which has a refractive index between 1.4 and 1.5 in the spectral range considered, which offers a significant contrast with the refractive index of the electro-optical layer 2 (between 2.18 and 2.25 when it is made of lithium niobate) allowing the optical mode to be confined in this layer.

[0026] This oxide can be formed, at least in part, using an LPCVD (Low Pressure Chemical Vapor Deposition) or PECVD (Plasma Enhanced Chemical Vapor Deposition) technique. This deposition can to be completed by annealing in order to improve the absorption properties of this layer and ensure its optical quality, as will be detailed in a later section of this description.

[0027] The dielectric layer 3 has sufficient thickness to distance the electro-optical layer 2 from the support 5, and to limit electromagnetic coupling. This thickness can therefore be between 2 microns and 10 microns, preferably between 2 microns and 5 microns to facilitate the fabrication of the photonic substrate 1.

[0028] In certain embodiments, illustrated in Figures 2 and 3, a compensation layer 3' can be formed on the side of the second face of the support 5. It is known that when a relatively thick dielectric layer is formed on one face of a support, greater, for example, than 3 or 5 microns, this thickness can cause significant deformation of the support due to the stresses imposed by the dielectric layer. The compensation layer 3' can help maintain the flatness of the photonic substrate in such a situation of a relatively thick dielectric layer 3.The compensation layer 3' can be of a dielectric nature, for example of the same nature as that of the dielectric layer 3 or of a different dielectric nature, but this is not necessarily the case and in general this layer can be formed of any material bringing stresses to the support 5 aimed at compensating those brought by the dielectric layer 3. .

[0029] In the embodiment of [Fig. 1], the dielectric layer 3 consists of a single layer of silicon oxide with the properties just mentioned. This embodiment does not include a compensation layer 3'.

[0030] In the embodiment of [Fig. 3], the dielectric layer consists of a first dielectric layer 3a made of silicon oxide, having the same properties as the single dielectric layer 3 of the embodiment of [Fig. 1], and a second dielectric layer 3b. This second dielectric layer 3b forms a barrier to the diffusion of certain electrically conductive species present in the electro-optical layer 2, for example, lithium when this layer 2 is made of lithium niobate. This prevents these species from migrating to the trapping layer 4 and / or to the support 5, which would affect the electrical properties (particularly radio frequency) of this layer 4 and this support 5, and therefore of the photonic substrate 1 as a whole. This second dielectric layer 3b is particularly useful when the trapping layer 4 is made of polycrystalline silicon, which is susceptible to accommodating electrically conductive species. 。

[0031] The second dielectric layer 3b can thus comprise nitrogen and be formed, for example, of silicon nitride or silicon oxynitride. Preferably, it has a relatively small thickness, smaller than that of the first dielectric layer 3a, in order to limit its optical impact.

[0032] The photonic substrate 1 also includes a support 5 on which rests the trapping layer 4, which will be described in a later section of this description. The support 5 provides mechanical stability to the relatively thin layers forming the photonic substrate 1, and for this purpose has a thickness of several hundred microns.

[0033] Preferably, to contribute to the radio frequency performance of the photonic substrate 1, the support 5 has a high resistivity, greater than 750 ohms centimeters, and more preferably, greater than 2000 ohms centimeters. This limits the density of charges, holes or electrons, that are likely to move within it.

[0034] For reasons of availability and cost, the support 5 is preferably made of monocrystalline silicon. For example, and by way of illustration only, it may be a CZ silicon substrate with a low interstitial oxygen content of between 6 and 10 ppm, or an FZ silicon substrate which, in particular, has a naturally very low interstitial oxygen content. It may also be a CZ silicon substrate with a high interstitial oxygen content (referred to as "High Oi") exceeding 26 ppm.

[0035] According to the description of the embodiments illustrated in Figures 1, 2, and 3, and in an important respect, the photonic substrate 1, which is the subject of this description, comprises an electrical charge trapping layer 4, disposed between the dielectric layer 3 and the support 5, on the side of the first face of the support 5. This trapping layer 4 limits the movement of charges under the influence of the electromagnetic fields produced by the electrodes of the modulation device when the latter operates at very high frequencies. By limiting the electrical coupling between the modulator electrodes and the support 5, electrical losses and the linearity of the modulation device are thus reduced. This is particularly the case when the electrical signals have a frequency below 10 GHz or above 50 GHz.

[0036] The trapping layer 4 can be of a very diverse nature. Generally, it is a non-single-crystal layer exhibiting structural defects such as dislocations, grain boundaries, amorphous zones, interstices, inclusions, pores, etc. These structural defects form traps for charges that may circulate in the material, for example, at incomplete or dangling chemical bonds. This prevents conduction in this layer 4, which consequently exhibits high resistivity.

[0037] Generally, the trapping layer 4 can have a thickness between 10 nm and 30 micrometers, preferably between 50 nm and 5 micrometers.

[0038] According to one possible approach, and for reasons of simplicity of implementation, the trapping layer 4 is formed of a polycrystalline silicon layer. This layer can be formed by deposition on the first face of the support 5. In this case, and in order to preserve the polycrystalline quality of this layer during the thermal treatments that the photonic substrate 1 may undergo, an amorphous layer, of silicon dioxide for example, can advantageously be provided on the support 5 before the deposition of the trapping layer 4. In this case also, and as illustrated in [Fig. 3], the deposition of the trapping layer on the first face of the support can lead to the formation of a complementary polycrystalline layer 4' on the second face of the support 5.This complementary polycrystalline layer 4' has the same characteristics (or at least similar characteristics) as the trapping layer 4, particularly in terms of thickness, size and grain density.... .

[0039] Alternatively, the trapping layer 4 can be formed by implanting a relatively heavy substance, such as argon, into a surface layer of the support 5, on the side of its first face, in order to create the structural defects constituting the electrical traps. This trapping layer 4 can also be formed by porosifying a surface layer of the support 5, on the side of its first face, or by any other method suitable for creating structural defects in a surface layer of the support, these structural defects being capable of trapping electrical charges.

[0040] According to a particularly advantageous approach, the trapping layer 4 consists of a silicon-rich oxide deposited on the first face of the support 5. "Silicon-rich" refers to an oxide having an atomic concentration of silicon between 50% and 99.9%. Oxygen and, optionally, nitrogen are present in this layer 4 in substoichiometric atomic concentrations. Preferably, this oxide has an atomic concentration of silicon between 70% and 90% and an atomic concentration of oxygen between 10% and 30%. It may contain nitrogen in an atomic concentration between 8% and 45%.

[0041] A silicon-rich layer is usually formed from an amorphous matrix. This amorphous matrix may include crystalline silicon inclusions or crystalline silicon grains, the density and size of these inclusions and / or grains in the amorphous matrix depending on the relative proportions of oxygen, nitrogen, and silicon in the layer and the heat treatments it has undergone. In extreme cases, a silicon-rich layer may be entirely amorphous or entirely polycrystalline, the amorphous matrix in this latter case is reduced to amorphous inclusions present between the grains of the polycrystalline structure. Such a layer has the advantage of exhibiting a particularly high defect density, forming traps for electrical charges. It is therefore likely to exhibit very high resistivity, up to approximately 1OA12 ohm.cm. Further details on the formation of the trapping layer 4 in the form of a silicon-rich layer can be found in publications WO2024115410, WO2024115411, and WO2024115414.

[0042] The manufacture of the different modes of implementation of the photonic substrate 1 which have just been presented does not pose any particular problem, it is perfectly accessible to a person skilled in the art.

[0043] The support 5 is first prepared to form, in successive steps, the trapping layer 4 (and optionally the complementary polycrystalline layer 4') and the dielectric layer 3. A compensation layer 3' may also be formed, and this formation can be simultaneous with the formation of the dielectric layer 3 if these two layers are of the same type. The resulting stack is referred to as the "base substrate" in the remainder of this description.

[0044] Regardless of the nature of the trapping layer 4, when it is formed by deposition on the support 5, this deposition can be carried out using a PECVD or LPCVD technique, or even by epitaxy. This deposition can occur on the first face of the support 5 or simultaneously on the two opposite faces of this support 5.

[0045] Thus, when the trapping layer 4 is a polycrystalline silicon layer, it can be formed by deposition at a moderate temperature, for example strictly between 600°C and 950°C. This can be a polycrystalline silicon layer deposited using an LPCVD technique. Alternatively, it can be formed at a relatively higher temperature, for example between 950°C and 1100°C, in an epitaxial reactor.

[0046] When the trapping layer 4 is a silicon-rich oxide, the deposition step includes introducing, into the deposition chamber, a first silicon precursor gas (for example, silane, with the formula SiH4) and at least one second oxygen precursor gas (nitrous oxide or dioxygen), and optionally nitrogen (for example, nitrous oxide, with the formula N2O). These precursor gases may be supplemented by a carrier gas, for example, nitrogen, argon, or helium. Separate precursor gases can, of course, be used for oxygen and nitrogen. As is well known, the precursor gases react in the deposition chamber, under controlled pressure and temperature conditions, to progressively form the trapping layer 3a. When this deposition is carried out using the PECVD technique, it is performed at a moderate temperature. typically between 150°C and 600°C, and at a subatmospheric pressure of, for example, a few Torr (i.e., a few hundred Pascals). When carried out using the LPCVD technique, it is performed at a temperature typically between 550°C and 750°C, also at a subatmospheric pressure of, for example, a few Torr (i.e., a few hundred Pascals).

[0047] In a subsequent step, the dielectric layer 3 can be formed on the trapping layer 3 to finalize the preparation of the base substrate. When a compensation layer 3', for example a dielectric layer, is planned for the second face of the support 5, it can be produced first, for example by PECVD. We can then subsequently form the dielectric layer 3 on the side of the first face of the support 5, on the trapping layer 4, for example by the same PECVD technique, in a thickness advantageously chosen between 2 microns and 10 microns.

[0048] Alternatively, the second dielectric layer 3b, for example a SiON layer, can be deposited onto the trapping layer 4 located on the first face of the support 5 to form a barrier, for example in LPCVD. Then, the first dielectric layer 3a, typically silicon oxide in PECVD, is deposited. This deposition can be carried out simultaneously on both faces of the support as already mentioned, and in this case, it is not necessary to have previously deposited the compensation layer 3' on the second face of the support 5.

[0049] Whether or not the second dielectric layer 2b is present, as already stated, the base substrate including the dielectric layer 3 can be annealed to improve its optical quality. Typically, this annealing can take place at a temperature higher than the deposition temperature of the dielectric layer 2b, under a dry or humid oxidizing atmosphere, to improve its stoichiometry. Annealing at low pressure is preferable. This pressure is typically less than 500 Torr (approximately 66 MPa), and preferably less than 100 Torr (approximately 13 MPa), to promote the desorption of chemical reaction byproducts trapped during deposition in the dielectric layer(s) 3, 3a, 3b.

[0050] Following these initial preparation steps of the base substrate, the photonic substrate 1 can be constituted by transferring an electro-optical layer 2. This transfer can be obtained by any suitable technique, for example by assembling a so-called "donor" substrate composed of, or comprising, the optical-quality single-crystal material intended to form the electro-optical layer with the base substrate.

[0051] The donor substrate, after being assembled with the base substrate, can be thinned or fractured to form the electro-optical layer 2. The thinning may include grinding and / or polishing steps aimed at progressively reducing the thickness of the donor substrate to provide the electro-optical layer 2 in the desired thickness and roughness, and thus complete the photonic substrate 1.

[0052] As an alternative to this thinning, a process conforming to the well-known Smart Cut™ technology can be deployed. This process involves introducing so-called "light" particles into the thickness of the donor substrate to create a weakening plane. This introduction is generally achieved by implanting the light species in ionic form. These species can be helium and / or hydrogen. After assembling the implanted donor substrate onto the base substrate, the donor substrate is fractured at the weakening plane by applying mechanical and / or thermal stresses.

[0053] A finishing sequence can then be applied, combining thinning steps (polishing, etching, etc.), annealing, and surface treatment to form the electro-optical layer 2 in the chosen thickness and roughness level. This approach also allows for precise control of thickness uniformity, which is mainly defined by the depth of introduction of the light species into the donor substrate, a depth that can be precisely chosen when this step is performed by implantation.

[0054] This completes the fabrication of the photonic substrate 1.

[0055] As stated in the introduction to this application, the photonic substrate can be used to form photonic components, and in particular thin-film optical modulation devices.

[0056] Thus, portions of the electro-optical layer 2 of the photonic substrate 1 can be selectively removed to define, within this layer 2, waveguides that allow at least one optical mode to propagate in the modulator. Deposition steps, particularly of metals, can also be carried out to form the modulation electrodes that allow a variable electrical quantity (in particular an electric field) to be imparted to these waveguides. This variable electrical quantity is controlled to modify the optical properties of the material forming the waveguides and in order to modulate the optical mode(s) propagating therein.

[0057] Of course the invention is not limited to the modes of implementation described and alternative embodiments can be made without departing from the scope of the invention as defined by the claims.

Claims

Demands

1. Photonic substrate (1) comprising: a. a support (5) having a first face and a second face, opposite to the first; b. an electric charge trapping layer (4) disposed on the first face of the support (5); c. a dielectric layer (3;3a,3b) on and in contact with the electric charge trapping layer; d. An electro-optical layer (2) formed of a single-crystalline material of optical quality, the electro-optical layer (2) being disposed on and in contact with the dielectric layer (3;3a,3b).

2. Photonic substrate (1) according to claim 1 in which the electro-optical layer (2) has an absorption coefficient less than 10A-3 in the spectral band between 0.7 micrometer and 1.6 micrometer, preferably less than 10A-2.

3. Photonic substrate (1) according to any one of the preceding claims wherein the electro-optical layer (2) is made of lithium niobate.

4. Photonic substrate (1) according to the preceding claim in which the electro-optical layer (2) of lithium niobate has a crystalline X section.

5. Photonic substrate (1) according to any one of the preceding claims wherein the electro-optical layer (2) has a thickness between 300nm and 600nm.

6. Photonic substrate (1) according to any one of the preceding claims wherein the electro-optical layer (2) has a free face whose roughness is less than 0.5 nm in root mean square value measured over a field of 10 micrometers by 10 micrometers.

7. Photonic substrate (1) according to any one of the preceding claims wherein the electro-optical layer (2) has a unit thickness variability of less than 50 nm over its entire extent.

8. Photonic substrate (1) according to any one of the preceding claims wherein the dielectric layer (3) comprises or is made up of a first dielectric layer (3a) of silicon oxide having an absorption coefficient of less than 10A-3 in the spectral band between 0.7 micron and 1.6 micron.

9. Photonic substrate (1) according to the preceding claim in which the dielectric layer (3) comprises a second dielectric layer (3b), disposed between the first dielectric layer (3a) and the electric charge trapping layer (4), the second dielectric layer (3b) comprising nitrogen.

10. Photonic substrate (1) according to any one of the preceding claims wherein the dielectric layer (3;3a,3b) has a thickness of between 2 microns and 10 microns, preferably between 2 microns and 5 microns.

11. Photonic substrate (1) according to any one of the preceding claims wherein the electrical charge trapping layer (4) is made of polycrystalline silicon.

12. Photonic substrate (1) according to the preceding claim comprising a complementary polycrystalline layer (4') disposed on and in contact with the second face of the support (5).

13. Photonic substrate (1) according to any one of claims 1 to 10 wherein the electrical charge trapping layer (4) is made up of a silicon-rich oxide having an atomic concentration of silicon between 50% and 99.9%, preferably in an atomic concentration of silicon between 70% and 90% and of oxygen in an atomic concentration between 10% and 30%.

14. Photonic substrate (1) according to the preceding claim in which the electrical charge trapping layer (4) also comprises nitrogen in an atomic concentration of between 8% and 45%.

15. Photonic substrate (1) according to any one of the preceding claims wherein the electrical charge trapping layer (4) has a thickness of between 10 nm and 30 micrometers, preferably between 50 nm and 5 micrometers.

16. Photonic substrate (1) according to any one of the preceding claims wherein the support (5) is made of monocrystalline silicon.

17. Photonic substrate (1) according to any one of the preceding claims wherein the support (5) has an electrical resistivity greater than 750 Ohm.cm, preferably greater than 2000 Ohm.cm.

18. Photonic substrate (1) according to any one of the preceding claims comprising a compensation layer (3') disposed on the side of the second face of the support (5).

19. Use of a photonic substrate according to any one of the preceding claims to form a thin-film optical modulation device.

20. Thin-film optical modulation device comprising: a. a support (5) having a first face and a second face, opposite to the first; b. an electric charge trapping layer (4) disposed on the first face of the support (5); c. a dielectric layer (3;3a,3b) on and in contact with the electric charge trapping layer (4); d. at least one waveguide defined in an electro-optical layer formed of a single-crystal material of optical quality, the at least one waveguide being disposed on and in contact with the dielectric layer (3;3a,3b) and being associated with modulation electrodes.

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