Guided mode extraction structure

Abstract

The invention relates to a structure for extracting a linearly polarized guided mode of wavelength λ, comprising: a primary waveguide for guiding the guided mode; an intermediate liquid crystal waveguide for guiding a coupled mode, extending parallel to the upper surface of a substrate; a flat reflective surface opposite an exit face of the intermediate waveguide, making a non-zero angle with the substrate; and first and second electrodes arranged relative to the core of the intermediate waveguide so as to shift the refractive index of the liquid crystal from a first level to a second level when a variation in the electrical potential difference is applied between the first and second electrodes. The guided mode is coupled by evanescent coupling only when the refractive index of the liquid crystal is equal to the second level. Figure 1A (for the abstract)

Description

Title of the invention: Guided mode extraction structure technical field

[0001] The field of the invention is that of extraction structures for a guided mode propagating in an integrated waveguide. More specifically, the invention relates to an active extraction structure for controlling the extraction of the guided mode. It also relates to a method for manufacturing such an extraction structure and an optical device implementing a plurality of active extraction structures. PRIOR TECHNOLOGY

[0002] Structures for extracting a guided mode are known in the field of integrated photonics. For example, there are surface diffraction gratings. A surface diffraction grating is a periodic structure etched onto the surface of a waveguide in which the guided mode propagates. The period of the surface grating is adapted to selectively diffract the wavelength of the guided mode, allowing at least a portion of it to be extracted. Surface diffraction gratings are used, for example, to extract a guided mode from a waveguide to an optical fiber or vice versa. They are often preferred to edge couplers and are sometimes essential, particularly for testing photonic chips before individualization by slicing. Diffraction gratings are generally passive extraction structures, meaning that their extraction efficiency is fixed once and for all during their fabrication.

[0003] There also exist active extraction structures for a guided mode propagating in an integrated waveguide, that is, controllable between an open state in which at least part of the guided mode's energy is extracted from the waveguide to generate a directional light flux, and a closed state in which the guided mode remains entirely confined within the waveguide. The majority of the energy of the directional light flux propagates within a solid angle strictly less than 2ir steradians around a principal direction. The solid angle can, for example, be less than 0.5ir steradians. When in an open state, the extraction structure is said to be activated.

[0004] Active extraction structures are used, for example, in displays for extended reality applications (augmented reality, virtual reality, or mixed reality). To cite one example, they have recently been used in a new type of microdisplay for augmented reality exploiting a self-focusing effect, such as that described in the paper C. Martinez et al. “See-through holography retinal projection display concept”, Optica, vol. 5, no. 10, p. 1200, Oct. 2018, doi: 10.1364 / OPTICA.5.001200. This type of microscreen makes it possible to do without optical systems to project an image into the eye of a user, and can therefore be integrated into less complex, less bulky and lighter optical systems.

[0005] In general terms, a pixel on such a screen results from the combination of several coherent light waves emanating from a distribution of emission points. The light is brought to the emission points by an array of integrated waveguides, optically connected to a light source. Each emission point has an active extraction structure that allows light to be extracted on command from a corresponding waveguide. A holographic film placed on the active extraction structures allows the phase and direction of the light extracted by the structures to be adjusted. The emission points can, for example, emit light waves having the same phase modulo 2ir and propagating around parallel principal axes. In this case, the observer's eye sees a sharp point corresponding to a virtual pixel located at infinity.A control circuit connected to the light source and an electrode array allows the light source and the extraction structures corresponding to the pixel to be activated simultaneously.

[0006] The document Matthias Colard, et al., "Study of a liquid crystal impregnated diffraction grating for active waveguide addressing", Proc. SPIE 12023, Emerging Liquid Crystal Technologies XVII, 1202302 (3 Tuesday 2022); doi: 10.1117 / 12.2607475 describes two embodiments of an active extraction structure adapted to the realization of micro-screens exploiting a self-focusing effect.

[0007] A first embodiment aims to modify the refractive index contrast of a diffraction grating. It employs a surface diffraction grating formed on a waveguide; the surface grating being immersed in a liquid crystal. The refractive index of the surface grating is, for example, approximately equal to the ordinary refractive index of the liquid crystal. A pair of electrodes allows an electric field to be applied in the liquid crystal, perpendicular to the mean plane of the surface grating. When an electrical potential difference is applied between the two electrodes, the electric field aligns the molecules of the liquid crystal perpendicular to the mean plane of the surface grating. An electrical transverse (ET) mode guided by the waveguide encounters a uniform structure with a refractive index equal to the ordinary refractive index. It therefore remains largely confined within the waveguide.Conversely, when the electrodes are at the same electrical potential, the electric field is zero in the liquid crystal and the liquid crystal molecules align themselves in a direction parallel to the mean plane of the surface lattice. The transverse electric mode (TE) then interacts with a medium whose refractive index is approximately equal to the index. ordinary at the level of a tooth of the surface grating, and a medium with a refractive index equal to the extraordinary refractive index of the liquid crystal between two teeth of the surface grating. It is therefore diffracted and partially extracted from the waveguide. In the case where the electrical transverse (ET) mode does not perceive a refractive index modulation at the level of the diffraction grating, a small part of this mode is nevertheless diffracted by an anchoring layer of the liquid crystal that conforms to the diffraction grating.

[0008] A second embodiment overcomes this drawback. It aims to modify the confinement of a guided mode to make it interact or not with a diffraction grating. In this embodiment, a liquid crystal extends from a lower face to an upper face. A core of a waveguide is in contact with the lower face. A diffraction grating is formed in the liquid crystal and is flush with the upper face of the liquid crystal. The diffraction grating has teeth arranged perpendicular to an optical axis of the waveguide. Interdigitated electrodes made of indium tin oxide (ITO) are arranged in a plane parallel to the upper face, opposite the liquid crystal on a side facing away from the lower face. The electrodes are capable of applying an electric field in the liquid crystal parallel to the optical axis of the waveguide.An anchoring layer allows the liquid crystal molecules to be aligned parallel to the teeth of the diffraction grating in the absence of an electric field in the liquid crystal. The core of the waveguide has a refractive index greater than or equal to an extraordinary refractive index (ne) of the liquid crystal.

[0009] Thus, during operation, when an electrical potential difference is applied across the electrode terminals, the liquid crystal molecules align parallel to the optical axis. A polarized, electrically transverse (TE) mode then interacts with a coating having a refractive index equal to the ordinary refractive index (no) of the liquid crystal. The refractive index contrast between the core and the coating is therefore maximal, and the TE mode remains sufficiently confined so as not to interact with the diffraction grating. No light is then extracted. Conversely, when the electrodes are at the same electrical potential, the electric field is essentially zero in the liquid crystal. The liquid crystal molecules are aligned parallel to the teeth of the diffraction grating. The TE mode then interacts with a coating having a refractive index equal to the extraordinary refractive index (ne) of the liquid crystal.The refractive index contrast between the core and the coating is therefore minimal, and an evanescent part of the TE-type mode extends to the diffraction grating. Part of the TE-type mode is then extracted from the waveguide.

[0010] However, these two embodiments have the disadvantage of diffracting the guided mode into unnecessary or parasitic diffraction orders, which consequently induces undesirable losses and a lack of directivity of the extracted light. by the active extraction structure. For the specific application of microscreens, parasitic diffraction patterns can form spurious images. Furthermore, the maximum power of the extracted light is reached over a long diffraction grating. As a result, the active extraction structure is bulky. Description of the invention

[0011] The invention aims to remedy, at least in part, the drawbacks of the prior art, and more particularly to provide a guided mode extraction structure exhibiting low losses and enabling increased directivity of light extracted by the extraction structure. The invention also aims to provide a brighter and more compact optical device, such as a microscreen.

[0012] For this purpose, the object of the invention is a structure for extracting a guided mode of wavelength X, linearly polarized along a polarization direction, comprising: a support substrate including a substantially flat upper face; a main waveguide suitable for guiding the guided mode; an intermediate waveguide suitable for guiding a mode, called coupled, at wavelength X, including a core in a liquid crystal extending parallel to the upper face, and an output face, the core extending to the output face; a flat surface opposite the output face, reflective at wavelength X, making a non-zero angle with the upper face of the support substrate;a first electrode and a second electrode, arranged with respect to the core of the intermediate waveguide so as to cause a refractive index of the liquid crystal to switch, in a coupling portion of the core of the intermediate waveguide, from a first level to a second level, according to the polarization direction, when a variation of an electrical potential difference is applied between the first and second electrodes. ;

[0013] The first level, the second level and the arrangement of the coupling portion with respect to the main waveguide are such that the guided mode, when present, is coupled at least in part, by evanescent coupling of the main waveguide to the coupling portion only when the refractive index of the coupling portion is equal to the second level.

[0014] Some preferred but not limiting aspects of this extraction structure are as follows.

[0015] The first electrode can be said to be buried. The buried electrode, the main waveguide, the intermediate waveguide and the second electrode can extend in distinct planes, parallel to the upper face of the supporting substrate, the main waveguide and the intermediate waveguide being able to be intercalated between the buried electrode and the second electrode.

[0016] The reflective plane surface can be an interface between a first medium and a second medium transparent to wavelength X, the first medium being able to be arranged between the exit face and the reflective plane surface and being able to have a refractive index strictly greater than a refractive index of the second medium, and such that, when the guided mode is present and coupled at least in part, a transmitted light wave from the guided mode can propagate from the exit face to the reflective plane surface along a principal axis which can make an angle α with a normal to the reflective plane surface greater than or equal to a minimum angle of incidence on the reflective plane surface for which the light is totally reflected.

[0017] The intermediate waveguide may have one end opposite the main waveguide, the core being able to extend from the end to the exit face.

[0018] The end can make a non-zero angle with the upper face of the supporting substrate so as to achieve an adiabatic coupling region.

[0019] The flat reflective surface can be a metallized surface.

[0020] The guided mode can be a TM mode, the liquid crystal can include a nematic phase and the second level can be an extraordinary refractive index of the liquid crystal.

[0021] The wavelength X can be included in the visible spectrum.

[0022] The invention also relates to an optical device comprising a first group of several extraction structures according to any one of the preceding characteristics, sharing the supporting substrate, such that the main waveguide of each extraction structure is a portion of a first main waveguide.

[0023] The optical device may further comprise a second group of several extraction structures according to any of the preceding characteristics, which may share the supporting substrate among themselves and with the extraction structures of the first group. The main waveguide of each extraction structure of the second group may be a portion of a second main waveguide distinct from the first main waveguide.

[0024] Each extraction structure of the first group can correspond to a corresponding extraction structure of the second group such that their intermediate waveguides can be two portions of a common intermediate waveguide.

[0025] The reflective flat surface of each extraction structure of the first group and, where applicable, of the second group can be the end of the intermediate waveguide of another extraction structure of the same group.

[0026] All intermediate waveguides of the extraction structures can have equal heights, measured perpendicular to the top face.

[0027] The invention also relates to a method for manufacturing an extraction structure or an optical device according to any one of the preceding characteristics, comprising the following steps: the following steps: provision of a support substrate comprising a main waveguide; provision of an encapsulation substrate; formation of a structured layer on the support substrate or the encapsulation substrate, by a nano-imprinting lithography process, the structured layer comprising protruding parts of identical heights equal to a common height; formation of an adhesive bead on the support substrate or the encapsulation substrate, such that the adhesive bead has a thickness greater than or equal to the common height, delimits a central region, and comprises a through-side opening communicating with the central region;Transfer of the encapsulation substrate onto the support substrate so that the structured layer acts as a spacer, fixing a gap between the encapsulation substrate and the support substrate, and delimiting a continuous volume in the central region intended to be the core of the intermediate waveguide; bonding of the encapsulation substrate to the support substrate by the adhesive bead; introduction of a liquid crystal into the continuous volume through the through-hole.

[0028] The nano-imprint lithography process can employ a reference mold obtained by the following steps: provision of a temporary substrate which may have a top face and trenches which may extend deep into the temporary substrate from the top face; filling of the trenches with a positive photosensitive resin; exposure of the positive photosensitive resin by collimated light which may propagate in the positive photosensitive resin in a direction making an angle 00 between 30° and 60° with the top face. Brief description of the drawings

[0029] Other aspects, objects, advantages and features of the invention will become more apparent upon reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the accompanying drawings in which:

[0030] [Fig.1A] is a schematic cross-sectional view of a first embodiment of an extraction structure according to the invention;

[0031] [Fig.1B] is a schematic cross-sectional view of a detail of the first embodiment;

[0032] [Fig.2] is a schematic cross-sectional view of a second embodiment of an extraction structure according to the invention;

[0033] [Fig.3] is a schematic cross-sectional view of a third embodiment of an extraction structure according to the invention;

[0034] [Fig.4A] is a schematic cross-sectional view of an example of an optical device implementing extraction structures according to the first embodiment;

[0035] [Fig.4B] is a perspective view of a detail of the example optical device;

[0036] Figures 5A to 5D are simulation results leading to the optimization of a extraction structure according to the invention;

[0037] [Fig.6] is a simulation result of the first embodiment;

[0038] Figures 7A to 7H are schematic cross-sectional views of steps in a manufacturing process for a reference mold and a buffer mold that can be used to produce an extraction structure according to the invention;

[0039] Figures 8A to 8D are schematic cross-sectional views of steps in a process of manufacturing of an upper part of an extraction structure according to the invention;

[0040] Figures 9A and 9B are schematic cross-sectional views of steps in a manufacturing process for an extraction structure according to the invention incorporating the upper part.

[0041] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0042] In the figures and in the following description, the same reference numerals represent identical or similar elements. Furthermore, the various elements are not drawn to scale in order to enhance the clarity of the figures. Moreover, the different embodiments and variants are not mutually exclusive and may be combined. Unless otherwise indicated, the terms "approximately," "about," and "in the order of" mean within 10%, and preferably within 5%. Furthermore, the terms "between ... and ..." and equivalents mean that the limits are inclusive, unless otherwise stated.

[0043] The invention relates to a structure for extracting an X-ray guided mode. It comprises a support substrate, a main waveguide, and an intermediate waveguide. The intermediate waveguide has a core comprising a liquid crystal. The liquid crystal has an ordinary refractive index (no) and an extraordinary refractive index (ne). The main waveguide can be separated from the intermediate waveguide by an upper encapsulation layer. The upper encapsulation layer has a refractive index strictly lower than the extraordinary refractive index (ne) and the refractive index of the main waveguide core.

[0044] The extraction structure further comprises a first electrode and a second electrode arranged, with respect to the intermediate waveguide, so as to apply an electric field in a coupling portion of the liquid crystal. In an embodiment illustrative of the invention, the electric field is capable of orienting molecules of the liquid crystal in a direction orthogonal to a direction adopted by the molecules in the absence of an electric field in the liquid crystal. Thus, by changing the amplitude of the electric field, the extraction structure tilts between an open state in which at least part of a mode guided by the main waveguide is coupled by evanescent coupling from the main waveguide to the intermediate waveguide, and a closed state in which the guided mode remains entirely confined within the main waveguide. That is to say, a variation in the electrical potential difference applied between the first and second electrodes changes the refractive index of the intermediate waveguide in such a way as to allow the propagation of a mode of the intermediate waveguide whose propagation constant is equal to the propagation constant of the mode guided by the main waveguide—in this case, the effective indices of the two modes are equal.

[0045] A reflective deflection surface opposite an output face of the intermediate waveguide then deflects the portion of the guided mode towards the medium surrounding the extraction structure. Activating evanescent coupling in combination with the reflective surface allows for efficient light extraction, with minimal losses, along a single angular direction.

[0046] Throughout the description, the expression "effective index of a waveguide-guided mode" is given its common meaning in the technical field of the invention. The effective index is a weighted average of the refractive indices of the materials constituting the waveguide. It is related to the propagation constant [3] of the waveguide-guided mode by the relation "_", where neff is the effective index P~ A and X the wavelength of the guided mode. The propagation constant [3 and / or the effective index neff can for example be determined by simulation.

[0047] Throughout this description, a waveguide is a single-mode or multimode waveguide capable of confining light, as opposed to optical waveguides in which light propagates by total internal reflection. Without further specification, a waveguide can be of any type. For example, it can be a ribbon, edge, or planar waveguide. A waveguide has a core, sometimes called a guiding path, and a coating surrounding the core so as to be in physical contact with it. A contrast or variation in refractive indices between the core and the coating allows the light to be confined. The coating can comprise several distinct parts and be made of one or more different materials. Waveguides are identified by their cores in the figures.Similarly, without further specification, a refractive index of a waveguide is a refractive index of the waveguide core; a distance separating two waveguides is the distance separating the cores of the respective waveguides; the material of a waveguide is the material of the waveguide core; when a waveguide extends in a direction, it is understood that the waveguide core extends in that direction; when a waveguide is in contact with a layer, it is understood that the waveguide core is in contact with the layer.

[0048] A layer is defined as an area consisting of one or more sublayers of a material whose thickness along a Z-axis is less than, for example, ten times or even twenty times, its longitudinal dimensions of width and length in a plane (X, Y) perpendicular to the Z-axis. A layer may be conformal, in which case it follows the topology of the surface on which it rests. When it consists of several sublayers, the sublayers may be made of different materials. The sublayer(s) extend in planes substantially parallel to the (X, Y) plane.

[0049] Particular embodiments will be described relating to an X-ray guided mode extraction structure comprising a main waveguide and an intermediate waveguide made of a material having a refractive index that can vary between a high and a low value under the effect of a controllable physical stimulus. In these particular examples, the physical stimulus is an electric field and the material is a liquid crystal. However, these embodiments can be adapted to other optoelectronic devices, for example, an optical switch capable of sending two optical signals guided by the main waveguide to two separate output channels and of simultaneously switching each optical signal from one output channel to the other. In this example, the optical signals can be of different wavelengths and / or different polarizations.

[0050] A first embodiment of an extraction structure lO.i according to the invention will be described with reference to Figures IA and IB. The extraction structure lO.i comprises a support substrate 100, a main waveguide 115, an intermediate waveguide 130.i, a reflective surface 133.i, a first electrode 105 and a second electrode 205.i.

[0051] For the sake of clarity, this first embodiment, as well as the following ones, are described for a specific operation in which the guided mode is polarized of the transverse magnetic (TM) type, and the extraction structure is activated when a non-zero potential difference is applied between the first electrode 105 and the second electrode 2O5.i, without the invention being limited to this type of operation. The extraction structure lO.i according to the invention can also be adapted to extract a guided polarized mode of the transverse electrical (TE) type when a zero or non-zero potential difference is applied between the first electrode 105 and the second electrode 2O5.i. The extraction structure lO.i according to the invention can also be adapted to extract a polarized mode of the transverse magnetic (TM) type when the first and second electrodes are at the same potential.For some of these variants within the reach of a person skilled in the art, it is necessary to modify the arrangement of the first and second electrodes (105, 205.i) with respect to the intermediate waveguide 130.i and / or the orientation of an extraordinary axis of the liquid crystal in the absence of an electric field in the liquid crystal.

[0052] The substrate 100 can, for example, be made from a plate, possibly after a cutting and / or thinning step. The plate can, for example, be made of glass, silicon, or quartz. The substrate 100 can be sufficiently thin to be curved by applying a force. It has a top face and a bottom face opposite the top face. The top and bottom faces are substantially flat and substantially parallel to each other. The main waveguide 115 rests on the substrate 100 on a side opposite the bottom face and extends parallel to the top face of the substrate 100. In this example, the main waveguide 115 is straight. It can also be curved.

[0053] A three-dimensional orthogonal (X, Y, Z) direct coordinate system is defined here and for the remainder of this description, where the X and Y axes form a plane parallel to the upper face of the support substrate 100, the X axis being parallel to the main waveguide 115, and where the Z axis is oriented substantially orthogonally to the upper face of the support substrate 100, from the lower face to the upper face. In the remainder of this description, the terms "vertical" and "vertically" are understood to refer to a direction substantially parallel to the Z axis, and the terms "horizontal" and "horizontally" to refer to an orientation substantially parallel to the (X, Y) plane. Furthermore, the terms "lower" and "upper" are understood to refer to an increasing positioning as one moves away from the support substrate 100, along the +Z direction.

[0054] In the orthogonal three-dimensional direct coordinate system (X, Y, Z), a unit vector normal to a face, surface, or plane has the following spherical coordinates: x = sin₂o₁, y = sin₂sin₂, and z = cos₀. In the description, the orientation of a face, surface, or plane is defined by the spherical coordinates (O, μ) of a unit vector normal to the face, surface, or plane, such that O ∈ [0°; 180°]. The face, surface, or plane is said to have the angular orientation (O, μ). For a plane diopter separating a first medium from a second medium with a refractive index strictly less than one refractive index of the first medium, the reference unit vector, normal to the diopter, with spherical coordinates (O, μ), is oriented from the second medium to the first medium.

[0055] The first electrode 105 rests here on the upper surface of the support substrate 100, optionally separated from it by one or more layers, for example an electrically insulating layer. The first electrode 105 is made of an electrically conductive material, for example a metal or a metal oxide, such as indium tin oxide (ITO) or aluminum-doped zinc oxide (AZO).

[0056] The main waveguide 115 is advantageously a single-mode waveguide. For example, it has dimensions along the Y and Z axes on the order of the wavelength X, or less than or equal to X, for example between 0.5X and 1.5X. The waveguide The main electrode 115 is separated from the first electrode 105 by a lower encapsulation layer 110. The lower encapsulation layer 110 is in physical contact with both the first electrode 105 and the main waveguide 115. In this configuration, the first electrode 105 is said to be buried. Here, the main waveguide 115 is a ribbon waveguide, but it could be of another type, for example, a ribbed waveguide. It is, for example, made of silicon (Si) or, as in this example, silicon nitride (SiN). The lower encapsulation layer 110 is made of one or more dielectric materials transparent to wavelength X. The dielectric material(s) have refractive indices strictly lower than one refractive index of the main waveguide 115. Here, the lower encapsulation layer 110 is made of silicon oxide. It has a thickness between 100 nm and 2 |am, for example equal to 1 pm.

[0057] The intermediate waveguide 130.i comprises a core, separated from the main waveguide 115 by an upper encapsulation layer 120. The core extends parallel to the upper face of the support substrate 100 from an end 131.i to an exit face 132.i. It comprises a liquid crystal. A proximal liquid crystal portion of the core is at least opposite the main waveguide 115; that is, there exists a straight line parallel to the Z-axis passing through the core of the main waveguide 115 and through the proximal portion, for any section of the proximal portion parallel to the (Y, Z) plane. The main waveguide 115 is preferably entirely opposite the proximal portion. Here, the end 131.i and the exit face 132.i are interfaces between the liquid crystal and the media surrounding the intermediate waveguide 130.i. In this particular example, the proximal portion extends from the end 131.i to the exit face 132.i, so that the end 131.i and the output face 132.i are opposite the main waveguide 115. .

[0058] The liquid crystal, for example, has a nematic phase. It has an ordinary refractive index (no) and an extraordinary refractive index (ne). The ordinary refractive index (no) is the refractive index affecting a light wave propagating in the liquid crystal, linearly polarized in a direction perpendicular to the average orientation of the electric dipoles of the liquid crystal molecules. The extraordinary refractive index (ne) is the refractive index affecting a light wave propagating in the liquid crystal, linearly polarized in a direction parallel to the average orientation of the electric dipoles of the liquid crystal molecules. By "orientation of the molecules of a liquid crystal," we mean the average orientation of the electric dipoles of the liquid crystal molecules exhibiting an electric dipole.In the absence of an electric field in the liquid crystal, one or more anchoring layers, not shown in figures IA, IB, 2, 3, 4A, 4B and 6, orient the liquid crystal molecules along a predominant, or preferred, direction.

[0059] The liquid crystal is, for example, a 5CB liquid crystal (4'-Pentyl-4-biphenylcarbonitrile or 4-Cyano-4'-pentylbiphenyl or 4-Pentyl-4'-cyanobiphenyl). For example, the ordinary refractive index (n0) is equal to 1.545 and the extraordinary refractive index (n0) is equal to 1.735 for a wavelength X equal to 532 nm.

[0060] The upper encapsulation layer 120 is in contact with the main waveguide 115 and the core of the intermediate waveguide 130i. It is made of one or more dielectric materials transparent at wavelength X. The dielectric material(s) have refractive indices strictly less than one refractive index of the main waveguide 115 and the extraordinary refractive index (ne) of the liquid crystal. Preferably, the refractive index(es) of the dielectric material(s) is / are greater than or equal to the ordinary refractive index (no) of the liquid crystal and less than or equal to 1.1 times the ordinary refractive index (no) of the liquid crystal. Here, the upper encapsulation layer 120 is made of silicon dioxide. For example, it has a thickness between 10 nm and 100 nm, measured directly above the main waveguide 115.

[0061] The second electrode 205.i is arranged in an encapsulation substrate 200 and extends parallel to the upper face of the support substrate 100. The main waveguide 115 and the intermediate waveguide 130i are interposed between the buried electrode 105 and the second electrode 205i. Due to its position relative to the support substrate 100, in this particular example, the second electrode 205i is called the upper electrode 205i. At least a portion of the upper electrode 205i is opposite the proximal portion of the core of the intermediate waveguide 130i and opposite the buried electrode 105. It is located at a distance des>i, preferably non-zero, from the intermediate waveguide 130i. The distance des>i is, for example, between 100 nm and 2 pm, for example equal to 1 pm. The upper electrode 2O5.i is made of an electrically conductive material, for example a metal or a metal oxide, such as indium tin oxide (ITO) or aluminum-doped zinc oxide (AZO).

[0062] The encapsulation substrate 200 is, in this example, transparent to wavelength X. It is, for example, predominantly made of silicon (Si), germanium (Ge), or, as in this example, of quartz or glass. It may optionally include layers transparent to wavelength X, such as one or more layers of silicon dioxide or silicon nitride (SiN). The encapsulation substrate 200 rests on the intermediate waveguide 130i on a side opposite the buried electrode 105, for example, in physical contact with it or, as shown here, separated from it by a portion of a material in physical contact with the intermediate waveguide 130i and the encapsulation substrate 200. If the material has a refractive index greater than or equal to the extraordinary refractive index (ne) of the liquid crystal, the portion of the material preferably has a thickness of less than 20 nm,

[0063]

[0064]

[0065]

[0066]

[0067] even less than 10 nm. The refractive index of the encapsulation substrate 200 is strictly less than the extraordinary refractive index (ne) over a whole lower region of the encapsulation substrate 200 extending from the upper electrode 205.i to a lower face of the encapsulation substrate 200. The reflective surface 133.i is a substantially flat surface. It is positioned opposite the output face 132.i. It is in physical contact with the upper encapsulation layer 120. As shown in [Fig.1B], the reflective surface 133.i has an angular orientation (03>i, 4>3>i) such that, in operation, a luminous flux from the guided mode transmitted through the output face 132.i and reflected by the reflective surface 133.i propagates in the half-space opposite the support substrate 100 and delimited by an upper face of the encapsulation substrate 200, or in the half-space opposite the encapsulation substrate 200 and delimited by the lower face of the support substrate 100. The end 13 Li has the angular orientation of angles (0, ¢14). Advantageously, 0i4 falls within an angle range that allows adiabatic coupling of a portion of the guided mode from the main waveguide 115 to the intermediate waveguide 130i when the extraction structure 10i is activated. In this example, this is achieved when 0 is between 30° and 50°, for example, 35°. The core of the intermediate waveguide 130i therefore comprises a straight portion extending from the end 131i to the exit face 132i of constant height, measured parallel to the Z-axis. The output face 132.ia for angular orientation the angles (02>i, ¢23)- In this example, = = 0 ; ^2 / = 90 0 ; is strictly between 0° and 90°. The exit face 132.i is therefore consequently orthogonal to the (X, Y) plane. The reflecting surface 133.ia has a height HP4 greater than or equal to a height Hgi of the intermediate waveguide 130.i, the heights being measured parallel to the Z-axis. Advantageously, as in this example, Hpi is equal to Hgi. The reflecting surface 133.i is located at a distance eP4 from the output face 132.i, measured parallel to the (X, Y) plane. eP4 is the smallest distance separating the reflecting surface 133.i from the output face 132.i in a horizontal plane. The distance eP4 is, for example, less than 20 nm, preferably less than 10 nm, advantageously the smallest possible, it being understood that it can be zero. The reflecting surface 133.ia has a length LP4 measured parallel to the X-axis, such that , n . 4 tan^^- Advantageously, as in this example, the reflective surface 133.i can be a plane interface between a first medium and a second medium transparent at wavelength X. The first medium is a high-index region 260.i extending parallel to the upper face of the support substrate 100 from the exit face 132.i to the reflective surface 133.i. The high-index region 26O.i has a refractive index strictly greater than the refractive index of the second medium. In this example, the high-index region 26O.i is part of a structured layer 250 resting on the upper encapsulation layer 120 and encapsulating the core of the intermediate waveguide 130.i. The structured layer 250 is in contact with the entire end 131.i, the entire exit face 132.i, and possibly with the upper encapsulation layer 120, as shown here. In this example, when the encapsulation substrate 200 is separated from the intermediate waveguide 130.i by the portion of material, the structured layer 250 includes the portion of material. The refractive index of the high-index region is 26O.i is for example between 1.5 and 2, preferably between 1.8 and 2, for example equal to 1.9.

[0068] In this example, although not strictly necessary, the reflective surface 133.i is a diopter between the high-index region 26O.i and the core of the intermediate waveguide 130.i+l of an additional extraction structure 10.i+l according to the first embodiment. The reflective surface 133.i is coplanar with the end 131.i+1 of the intermediate waveguide 130.i+1 of the additional extraction structure 10.i+l. Similarly, the end 131.i is coplanar with the reflective surface 133.i-1 of another additional extraction structure 10.i-1 according to the first embodiment. Thus, the additional extraction structures 10.i1, 10.i+l respectively comprise additional upper electrodes 205.i-1, 205.i+1. The high-index region 26O.i-1 of the supplementary extraction structure lO.i-1 extends from the exit face 132.L 1 of the intermediate waveguide 13O.i-1 of the supplementary extraction structure lO.i-1 up to the end 13l.i of the intermediate waveguide 130.i of the extraction structure lO.i. .

[0069] Alternatively, the reflective surface 133.i can be a metallized surface comprising a metal, for example aluminium (Al) or silver (Ag). In this case, the refractive index of the high-index region 26O.i can be arbitrary, and preferably substantially equal to ne.

[0070] In operation, a polarized mode of the transverse magnetic (TM) type and of wavelength X is guided along the +X axis by the main waveguide 115 towards the end 131.i of the intermediate waveguide 130.i. A non-zero potential difference is applied between the buried electrode 105 and the upper electrode 205.i so as to create an electric field sufficient to orient liquid crystal molecules parallel to the electric field. Since the upper electrode 205.i and the buried electrode 105 are opposite each other and facing the proximal portion, the electric field is substantially parallel to the Z-axis in a substantial part of the proximal portion, defining a coupling portion of the waveguide core. intermediate 13O.i. The molecules of the liquid crystal are therefore mostly oriented parallel to the Z axis in the coupling portion, which is the polarization direction of the guided mode.

[0071] The thickness of the upper encapsulation layer 120 is sufficiently thin to allow an evanescent portion of the guided mode to interact with the coupling portion. Due to the orientation of the molecules parallel to the polarization direction of the guided mode, the refractive index of the coupling portion allows a mode excited by the evanescent portion to propagate in the intermediate waveguide 130i; that is, the propagation constants of the excited mode and the guided mode are substantially equal in the coupling portion. Thus, a portion of the guided mode is optically coupled to the intermediate waveguide 130i by evanescent coupling and exits through the output face 132i to generate a transmitted light wave.

[0072] The transmitted light wave propagates in free space in the high-index region 26O.i along a principal axis until it reaches the reflective surface 133.i. If the reflective surface 133.i is a diopter, the principal axis makes an angle α with a normal to the plane reflective surface 133.i greater than or equal to a minimum angle of incidence on the plane reflective surface 133.i at which the light is totally reflected. This constraint is not necessary if the reflective surface 133.i is metallized.

[0073] The ratio between the energy of the light wave transmitted in the intermediate waveguide and the energy of the guided mode in the main waveguide defines a coupling efficiency. The coupling efficiency is a function of several factors, such as the refractive indices of the waveguides, their spacing, etc. It depends in particular on the length of the coupling portion Lc. The length Lci of the coupling portion is also the length measured along the X-axis of the proximal portion within which the propagation constants of the excited mode and the guided mode are equal. The length Lci depends on the arrangement of the buried electrode 105 and the upper electrode 205.i relative to the liquid crystal. In this example, the buried electrode 105 is opposite the liquid crystal along the entire length of the core of the intermediate waveguide 13i, the upper electrode 205.i is opposite the liquid crystal only at the level of the entire right portion of the core of the intermediate waveguide 130.i. The length Lci is therefore here equal to the length of the right portion measured along the X axis.

[0074] The coupling efficiency initially increases as the length Lc,i increases from zero to reach a maximum coupling efficiency for a length Le, called the optimal coupling length. It can then decrease as the length LCji increases further, to increase and so on. The optimal coupling length Lc increases with the thickness of the top encapsulation layer 120. It can be determined by simulation. For some applications, such as testing a photonic chip, a low coupling efficiency, for example less than or equal to 5%, may be desired. For other applications, such as microdisplays, a higher, or even maximum, coupling efficiency may be desired. The length Lci can then be adjusted accordingly.

[0075] Optionally, upstream of the coupling portion relative to the propagation direction of the guided mode, another evanescent portion of the guided mode interacts with an adiabatic coupling portion of the core of the intermediate waveguide 130.i so as to excite a fundamental mode of the intermediate waveguide 130.i. The adiabatic coupling portion includes the end 131.i. Since the end 131.i is inclined with respect to the (X, Y) plane, the surface area of ​​a cross-section of the core gradually increases in the direction of the coupling portion, such that the effective index of the fundamental mode in the intermediate waveguide 130.i reaches that of the guided mode. It can therefore propagate in the intermediate waveguide 130.i and promote the transfer of energy from the evanescent coupling to this fundamental mode at the coupling portion.Thus, the light wave guided in the main guide does not undergo a discontinuity of perceived index at the level of its evanescent part, which reduces the losses of the extraction structure lO.i. The part of the energy of the guided mode transferred to the fundamental mode at the level of the adiabatic coupling portion contributes to increasing the coupling efficiency of the main waveguide 115 to the intermediate waveguide 130.i. .

[0076] Conversely, when a zero potential difference is applied between the buried electrode 105 and the upper electrode 205.i, the electric field is essentially zero inside the liquid crystal. Consequently, the liquid crystal molecules are mostly oriented parallel to a direction favored by one or more anchoring layers, parallel to the (X, Y) plane, here parallel to the Y axis. Due to this molecular orientation, the guided mode interacts with a medium having a refractive index equal to the ordinary refractive index (n0) in the coupling portion, and no mode of the intermediate waveguide 130.i is excited or guided. Thus, the guided mode remains confined within the main waveguide 115.

[0077] The dimensioning of the elements of the extraction structure lO.i can be optimized using electromagnetic wave propagation simulation tools implementing algorithms such as FDTD (Finite Difference Time Domain), FDE (Finite Difference Eigenmode), or EME (Eigenmode Expansion). The behavior of the liquid crystal, and therefore its refractive index, when a potential difference is applied between the buried electrode 105 and the upper electrode 205.Î, can be deduced from simulation results obtained by a finite element method, such as that offered by the commercial software COMSOL®.

[0078] Now, a second embodiment will be described in relation to [Fig.2], in which the guided mode can propagate in two opposite directions of the main waveguide 115. Only the differences with the first embodiment are explicitly described.

[0079] In this embodiment, 02>i is strictly less than 90° and strictly greater than 0°. This facilitates the manufacture of a reference mold 540 and a buffer mold 550 using the manufacturing process shown in Figures 7A to 7H, and facilitates the manufacture of an upper part of the extraction structure lO.i according to the manufacturing process shown in Figures 8A to 8D. The angle 02>i is, for example, substantially equal to the angle 03>i.

[0080] A third embodiment will be described in relation to [Fig.3]. Only the differences with the first embodiment are explicitly described.

[0081] In this embodiment, the buried electrode 105 and the support substrate 100 are transparent to wavelength X. 03i is strictly greater than 90°. Thus, the reflective surface 133i directs the transmitted light wave towards the support substrate 100, after reflection, to extract it through the lower face of the support substrate 100.

[0082] The first, second and third embodiments have been described in connection with a guided polarization mode TM, extracted when a non-zero potential difference is applied between the first electrode 105 and the second electrode 205.i.

[0083] To extract a guided TM mode only when a zero potential difference is applied between the first electrode 105 and the second electrode 205.i, the first electrode 105 and the second electrode 205.i are, for example, both arranged in a plane parallel to the (X, Y) plane in the encapsulation substrate 200, one opposite an upstream region of the intermediate waveguide 130.i and the other opposite a downstream region of the intermediate waveguide 130.i, closer to the output face 132.i than the upstream region. The direction favored by the anchoring layer(s) can be parallel to the Z-axis. This configuration is commonly described under the English term "in-plane switching (IPS)".

[0084] To extract a guided TE mode only when a zero potential difference is applied between the first electrode 105 and the second electrode 205.i, the first electrode 105 and the upper electrode 205.i can for example be arranged in the same way as in Figures 1 to 3, and the direction favored by the anchoring layer(s) can be parallel to the Y axis.

[0085] To extract a TE guided mode only when a non-zero potential difference is applied between the first electrode 105 and the second electrode 205.i, the first electrode 105 and the second electrode 205.i are, for example, both arranged in a plane parallel to the (X, Y) plane in the encapsulation substrate 200, on either side of a plane orthogonal to the (X, Y) plane comprising an optical axis of the main waveguide 115. The direction favored by the anchoring layer(s) can be parallel to the X axis.

[0086] An optical device implementing lO.i extraction structures according to the first embodiment will now be described, with reference to Figures 4A and 4B. Alternatively, this optical device may implement lO.i extraction structures according to the second embodiment and / or the third embodiment, possibly in combination with one or more lO.i extraction structures according to the first embodiment. The diopter orientations shown in these figures represent only one specific case; other orientations may be considered based on the teachings of the first, second, and third embodiments.

[0087] Figure 4A is a cross-sectional view of the perspective view of Figure 4B, passing through an optical axis of a first principal waveguide 315. An example of light propagation is schematically represented in Figure 4A by gray arrows. The widths of the arrows schematically illustrate the relative energies of the light flux in the different branches of the optical device for a particular operation of the optical device.

[0088] The optical device comprises a first main waveguide 315 and at least a second main waveguide 316. The second main waveguide 316 extends here parallel to the main waveguide 315 in a plane coplanar with the main waveguide 315. In this example, although not essential, the first and second main waveguides 315, 316 are straight.

[0089] The optical device comprises a first group of several extraction structures lO.i according to the first embodiment, sharing the support substrate 100, such that the main waveguide 115 of each extraction structure is a portion of the first main waveguide 315. An extraction structure lO.i is intercalated between two additional extraction structures 10.il, 10.i+l according to the same embodiment as that of the extraction structure lO.i, here of the first embodiment. The end 131.i+1 of the additional extraction structure 10.i+l is the reflective surface 133.i of the extraction structure lO.i. Similarly, the end 131.i of the extraction structure lO.i is the reflective surface 133.i-1 of the additional extraction structure 10.il.

[0090] Alternatively, the reflective surface 133.i-1 of an extraction structure 10.i-1 can be an interface, possibly metallized, between the high-index region 26O.i-1 and any second medium, for example an adhesive. The adhesive can be a UV adhesive. The second medium can extend over the upper encapsulation layer 120 and have a thickness measured parallel to the Z-axis substantially equal to the height Hg l । of the intermediate waveguide 13O.i-1 of the extraction structure 10.il.

[0091] In this example, the additional extraction structure lO.i-1 is intended to be activated and deactivated simultaneously with another extraction structure, here the additional extraction structure 10.i+1. The length Ll,., 1 of the coupling portion of the additional extraction structure lO.i-1 is strictly less than the length Lc i+1 of the coupling portion of the additional extraction structure 10.i+1. Preferably, the difference between the lengths Ll,., 1 and Lc i+1 is such that the light waves transmitted by the additional extraction structures lO.i-1, 10.i+1 have equal intensities. The difference in lengths to achieve this can be established by photometric measurements of test structures or by simulation. In this preferred case, the additional extraction structures lO.i-1, 10.i+l can, for example, be emission point extraction structures from a microscreen belonging to a set of emission points corresponding to a pixel of an image.

[0092] In the case where several extraction structures lO.i of the first group are intended to be activated simultaneously, the length Lc 4 of the coupling portion of the extraction structure lO.i located furthest downstream with respect to the progression of the guided mode is preferably equal to the optimal coupling length Lc, in particular for a micro-screen type application.

[0093] Here, the optical device further comprises a second group of several extraction structures lO.i according to the first embodiment, sharing the support substrate 100, such that the main waveguide 115 of each extraction structure is a portion of the second main waveguide 316. Just as with the first group of extraction structures, an extraction structure lO.i of the second group is intercalated between two additional extraction structures lO.i-1, 10.i+1 of the second group. More precisely, the end 131.i+1 of the additional extraction structure 10.i+1 is, in this example, the reflective surface 133.i of the extraction structure lO.i. Similarly, the end 13l.i of the extraction structure lO.i is the reflective surface 133.L 1 of the additional extraction structure lO.i-1. The optical device can comprise any number of groups, each comprising a plurality of extraction structures lO.i.

[0094] In this example, the intermediate waveguide 130.i of each extraction structure lO.i of the first group and the second group is a waveguide planar. More precisely, the intermediate waveguide 130.i of each extraction structure lO.i of the first group, together with the intermediate waveguide 130.i of a corresponding extraction structure lO.i of the second group, constitutes two parts of a common, planar intermediate waveguide 130.i. The common intermediate waveguide 130.i is opposite the first main waveguide 315 and the second main waveguide 316. Here, the end 131.i of each extraction structure lO.i of the first group is coplanar with the end 131.i of the corresponding extraction structure lO.i of the second group. Similarly, the exit face 132.i of each extraction structure lO.i of the first group is coplanar with the exit face 132.i of the corresponding extraction structure lO.i of the second group. Note that it is possible to activate independently of each other, two lO.i extraction structures sharing an intermediate waveguide 130.The respective coupling portions can be delimited by the geometry of the upper electrodes 205.i. That is, an electrical potential difference applied between an upper electrode 205.i and the buried electrode 105 can orient the liquid crystal molecules into a corresponding coupling portion, without affecting the orientation of the molecules in the other coupling portions of the common intermediate waveguide 130.i. Any number of extraction structures lO.i belonging to distinct groups of extraction structures lO.i can have their respective intermediate waveguides 130.i included within the common intermediate waveguide 130.i. In this example, 5 extraction structures lO.i belonging to distinct groups share a common planar intermediate waveguide 130.i.

[0095] The buried electrodes 105 of the extraction structures of the first group and / or the second group can each be a portion of a common buried electrode extending continuously in a plane parallel to the (X, Y) plane. The common electrode can be electrically connected to a fixed electrical potential, for example to ground.

[0096] In connection with figures 5A to 5D, simulation results will be described. These results are useful for sizing an extraction structure lO.i according to the invention.

[0097] In [Fig. 5A], a two-dimensional map is shown representing the ratio of the energy of the light wave transmitted in the intermediate waveguide to the initial energy of the main guided mode (white iso-value lines), as a function of the height of the main waveguide 115 measured along the Z-axis (abscissa axis, in pm) and the height Hgi of the intermediate waveguide 130i (ordinate axis, in pm). For these simulation results, the upper encapsulation layer 120 is made of silicon oxide and has a thickness of 100 nm. The wavelength X is 532 nm. The length Lci of the coupling portion is in each The point is equal to the optimal coupling length Lc. The liquid crystal is a 5CB. The main waveguide 115 is made of silicon nitride (SiN). The guided mode is a magnetic transverse polarized (TM) mode. These results show that it is possible to adjust the maximum coupling efficiency over a wide range of values. The maximum coupling efficiency can be greater than 0.9.

[0098] Figure 5B shows a two-dimensional map representing the ratio of the energy of the light wave transmitted after reflection by the reflecting surface 133.i to the energy of the light wave transmitted in the intermediate waveguide (white iso-value lines), as a function of the height Hp>i (abscissa axis, in pm) and the length Lp>i (ordinate axis, in pm) of the reflecting surface 133.i. For these simulation results, 4>3ji is equal to 0° and 03>i is variable and equal to , as specified above. Two dashed lines indicate the angles 03; tan r equal to 20° and 50°, respectively. The high-index region 26O.i comprises titanium oxide (TiO2) and has a refractive index of 1.9. The liquid crystal is a 5CB. The height Hgi of the intermediate waveguide 13O.i is equal to the height Hp>i of the reflecting surface 13O.i. The reflecting surface 13O.i is a diopter separating the high-index region 26O.i from a second medium with a refractive index equal to the ordinary refractive index (n0) of the liquid crystal. The guided mode is a magnetic transverse polarized (TM) mode. The wavelength X is 532 nm. These results show that it is possible to determine a value of θ3i that maximizes the reflectivity of the reflecting surface 13O.i.

[0099] Figure 5C is the result of combining Figures 5A and 5B for a length Lp>i of the reflecting surface 133.i fixed at 2 pm. It shows a two-dimensional map giving the ratio of the background light energy transmitted after reflection by the reflecting surface 133.i to the energy of the guided mode (black iso-value lines), as a function of the height of the main waveguide 115 (abscissa axis, in pm) and the height Hpi (ordinate axis, in pm) of the reflecting surface 133.i. The height Hpi is also equal to the height of the intermediate waveguide 130.i. The other parameters for obtaining these simulation results take the same values ​​as those mentioned in connection with Figure 5A and Figure 5B. A maximum of 90% (white star) is reached for a main waveguide height of 115 equal to 130 nm, an intermediate waveguide height of 130.i equals 1.32 pm, a thickness of the upper encapsulation layer 120 equals 100 nm and an inclination of the reflective surface 133.i.

[0100] TM-type polarization of the guided mode is advantageous when the transmitted light wave is extracted through the upper face of the encapsulating substrate 200 or through the lower face of the support substrate 100 into a surrounding medium with a low refractive index, such as a gas or air. In this case, the reflection of the light wave at the interface between the surrounding medium and, respectively, the encapsulating substrate 200 or the support substrate 100 is reduced, for example, 5 times lower, or even 6 times lower, than the reflection obtained with a TE-type guided mode.

[0101] In [Fig. 5D], a two-dimensional map is shown representing the ratio of the guided mode energy downstream of the extraction structure lO.i to the guided mode energy upstream of the extraction structure lO.i (black iso-value lines), as a function of the height of the main waveguide 115 (x-axis, in pm) and the height Hpi (y-axis, in pm) of the reflecting surface 133.i. The simulation results are obtained when the buried electrode 105 and the upper electrode 2O5.i are at the same electrical potential. The operating point identified in [Fig. 5C] is shown in [Fig. 5D], for which 96% of the guided mode energy remains confined within the main waveguide 115 as it passes through the extraction structure lO.i.

[0102] In [Fig. 6], a simulation map of the electric field (oriented dashed lines) is shown within a region of the first embodiment including the end 131.i of the intermediate waveguide 130.i. For these simulation results, the direction favored by the anchoring layer(s) is parallel to the X-axis. The distance from the >i is equal to 1 pm. The thickness of the lower encapsulation layer 110 is equal to 1 pm. An electrical potential difference of 5 V is applied between the upper electrode 205.i and the buried electrode 105. The upper electrode 205.i is not opposite the end 131.i. Therefore, a region with a right-angled triangular cross-section in a plane parallel to the (X, Z) plane is delimited by the end 131.i and a lower face of the intermediate waveguide 130.i.i has an intermediate refractive index, varying within a range of values ​​between the ordinary refractive index (n0) and the extraordinary refractive index (n0). The refractive index gradually increases with distance from the 13l.i end, which promotes the formation of an adiabatic coupling region. This also helps to limit a diffraction effect of the guided mode at the interface between the high-index region 26O.i and the liquid crystal, at the 13l.i end.

[0103] An example of a method for making an extraction structure lO.i as illustrated in [Fig. 1A] or [Fig. 2] is now described. This method includes the fabrication of a reference mold 540 and a buffer mold 550 (Figures 7A to 7H), the fabrication of an upper part of the extraction structure lO.i (Figures 8A to 8D) and the actual fabrication of the extraction structure lO.i incorporating the upper part (figures 9A to 9C).

[0104] In [Fig. 7A], a photosensitive resin is deposited on a temporary substrate 500. The photosensitive resin is locally exposed and developed to obtain a mask 510 in contact with the temporary substrate 500. Alternatively, the mask 510 can be a structured mineral layer, for example, silicon nitride (SiN) or silicon oxide, obtained by photolithography and etching steps. The mask 510 has openings 515 passing through the mask 510 to expose regions of an upper face of the temporary substrate 500. The temporary substrate 500 can be a silicon substrate, for example, a silicon wafer, for example, with a diameter of 150 mm, 200 mm, or 300 mm. The openings 515 may, for example, have rectangular shapes in a plane parallel to the upper face of the temporary substrate 500.In this particular example, the 515 openings are rectangular and extend lengthwise in a common direction.

[0105] In [Fig. 7B], the temporary substrate 500 is partially etched through the openings 515 to obtain trenches 520. The etching is anisotropic, for example, reactive ion etching. Each trench 520 has a bottom and walls substantially orthogonal to the bottom. They have substantially the same depth, for example, greater than or equal to 10 pm, for example, between 20 pm and 25 pm.

[0106] In [Fig. 7C], an absorbent layer 525 is deposited conformally, for example by PVD or CVD, on the upper surface of the temporary substrate 500, as well as on the bottoms and walls of the trenches 520. The absorbent layer 525 is optional. When present, the absorbent layer 525 may comprise alternating chromium (Cr) and silicon dioxide underlayers.

[0107] In [Fig. 7D], a positive photosensitive resin 530 is deposited on the absorbent layer 525, or directly onto the temporary substrate 500 when the absorbent layer 525 is absent. The positive photosensitive resin 530 completely fills the trenches 520; advantageously, it covers the upper surface of the temporary substrate 500. The positive photosensitive resin 530 is then exposed by collimated light propagating within the positive photosensitive resin 530 in a direction making an angle θ strictly between 0° and 90° with the upper surface of the temporary substrate 500, in order to expose only a portion of the positive photosensitive resin 530 inside the trenches 520. The angle θ is preferably between 30 degrees and 60 degrees, or 30 degrees and 50 degrees. In this example, the direction of the collimated light is orthogonal to the common direction.

[0108] In [Fig. 7E], the positive photosensitive resin 530 is developed. At the end of this step, residual, unexposed portions 535 of the positive photosensitive resin 530 remain inside the trenches 520. Each residual portion 535 covers the bottom, preferably entirely, of a respective trench 520. Each residual portion 535 has an inclined face 536 making a non-zero angle θi with the upper face of the temporary substrate 500, substantially equal to θ. The inclined face 536 is flush with the upper face of the temporary substrate 500 and covers the bottom of the corresponding trench 520. The presence of an absorbing layer 525 during the exposure of the positive photosensitive resin 530 prevents the formation of interference fringes induced by reflection on the walls of the trench 520. The inclined faces 536 are then smoother.

[0109] In [Fig. 7F], a non-conforming layer is formed on the temporary substrate 500 to create a reference mold 540. The layer can be a metallic layer, for example, nickel (Ni). It can be deposited by CVD or PVD. It can also be obtained by an electroplating growth process, optionally preceded by the conforming deposition of an electrically conductive seed. In the case where the absorbing layer 525 is present and comprises alternating chromium (Cr) and silicon dioxide sublayers, the layer can advantageously be grown by electroplating from a chromium (Cr) sublayer terminating the alternating sublayers of the absorbing layer 525.

[0110] In [Fig. 7G], the reference mold 540 is removed. The reference mold 540 has protruding parts, raised in relief from a principal face 543 of the reference mold 540. Each protruding part comprises an inclined face 541 and a straight face 542. The inclined faces 541 each correspond to an inclined face 536 of a residual part 535. The straight faces 542 each correspond to a wall of a trench 520. In this example, the protruding parts are right prisms with a right triangular cross-section. Thus, each inclined face 541 defines an edge with a straight face 542. Here, all the edges of the protruding parts are parallel to each other and extend in the same plane parallel to the principal face 543.

[0111] In [Fig. 7H], a flexible layer, for example made of an elastomer such as polydimethylsiloxane (PDMS), is formed on the reference mold 540 so as to be in contact with the protruding parts and the main face 543 of the reference mold 540. The flexible layer constitutes a buffer mold 550 comprising cavities, each with a shape corresponding to a protruding part of the reference mold 540. Thus, each cavity comprises an inclined face 551 and a straight face 552 corresponding respectively to an inclined face 541 and a straight face 542 of a protruding part of the reference mold 540. The step in [Fig. 7H] can be repeated several times to produce several buffer molds 550. In this example, the The cavities form right prisms with a right triangular cross-section. Each inclined face 551 defines an edge with a straight face 552. All the edges of the cavities are parallel to each other and extend in the same plane parallel to a principal face 553 of the buffer mold 550 corresponding to the principal face 543 of the reference mold 540. Each inclined face 551 makes an angle θi with the principal face 553.

[0112] In [Fig. 8A], an electrically conductive layer is deposited on an upper face of a support 600. The support 600 is, in this example, made of a material transparent to wavelength X. For example, it is silicon (Si) or germanium (Ge) if wavelength X is in the infrared. For example, it can be quartz or glass if wavelength X is in the visible spectrum.

[0113] The electrically conductive layer can be made of a metal, or of a metal oxide, such as for example indium tin oxide (ITO). It is locally etched over its entire thickness to create upper electrodes 2O5.i.

[0114] In [Fig. 8B], an encapsulation layer 610 is formed on the support 600 so as to be in contact with the upper electrodes 205.i and the upper face of the support 600. The encapsulation layer 610 has, on a side opposite the upper electrodes 205.i, a flat face substantially parallel to the upper face of the support 600. The encapsulation layer 610 is made of a material transparent to the X wavelength. For example, it is made of the same material as the support 600. Here, it is silicon oxide. The support 600 and the encapsulation layer 610 together define the encapsulation substrate 200.

[0115] In [Fig. 8C], a structured layer 250 is produced by a nano-imprint lithography (NIL) process. For example, it is possible to deposit an imperfectly crosslinked xerogel layer comprising titanium dioxide (TiO2), optionally with added organic stabilizing agents and / or fluidizers and / or polycondensation inhibitors. The xerogel layer is then molded by the buffer mold 550, optionally after slight heating, to obtain the structured layer 250. During this step, the buffer mold 550 is brought into contact with the xerogel, and pressure is applied to the buffer mold 550, perpendicular to the upper face of the support 600, until the buffer mold 550 is possibly brought into contact with the encapsulation layer 610.When the buffer mold 550 is not in contact with the encapsulation layer 610, the pressure is uniform so as to keep the main face 553 of the buffer mold 550 substantially parallel with a top face of the encapsulation layer 610 opposite the support 600. Therefore, in all cases, the main face 553 of the buffer mold 550 is substantially parallel with the top face of the encapsulation layer 610 during the shaping of the xerogel.

[0116] In [Fig. 8D], an upper portion of an extraction structure lO.i is obtained. The buffer mold 550 is removed, and the structured layer 250 is heated to a high temperature to solidify and stabilize it. A concentration of titanium dioxide (TiO2) in the xerogel allows the refractive index of the structured layer 250 to be adjusted, for example, to a value of 1.9.

[0117] After the heating substep, the structured layer 250 comprises protruding parts, each corresponding to a cavity in the buffer mold 550. The protruding parts of the structured layer 250 therefore have edges corresponding to the edges of the cavities. Since the main face 553 of the buffer mold 550 is parallel to the upper face of the encapsulating layer 610, the edges of the protruding parts of the structured layer 250 are coplanar and parallel to the upper face of the layer 610. Consequently, the protruding parts of the structured layer 250 have identical heights, equal to a common height.

[0118] In this example, the buffer mold 550 has not been brought into contact with the encapsulation layer 610, so that the protruding parts of the structured layer 250 are raised relative to a principal face 253 of the structured layer 250. Each protruding part includes an inclined face 251 intended to be an end 131.i and / or a reflective surface 133.i of an extraction structure 10.i. It also includes a straight face 252 intended to be an exit face 132.i of an extraction structure 10.i. The inclined face 251 makes an angle 03>i with the principal face 253 which may be different from Op. The angle 03>i increases as 0B, and therefore 0O, increase.

[0119] A polyimide layer is then formed in contact with the main face 253, or in contact with the encapsulation layer 610 when the buffer mold 550 has been brought into contact with it during the nano-imprint lithography step. The polyimide layer is brushed in a direction intended to be a preferred direction for the orientation of the liquid crystal molecules when no electric field is present in the liquid crystal. The polyimide layer is thus intended to be a top anchoring layer 625 of the liquid crystal.

[0120] In [Fig. 9A], a lower part of an extraction structure lO.i is fabricated. An electrically conductive layer is deposited on an upper surface of a support substrate 100. The support substrate 100 can, for example, be made of silicon, for example, a silicon wafer. The electrically conductive layer can be made of a metal, or of a metal oxide, such as indium tin oxide (ITO). It is locally etched through its entire thickness to create a buried electrode 105.

[0121] A lower encapsulation layer 110 is then deposited on the support substrate 100, in contact with the support substrate 100 and the buried electrode 105. The lower encapsulation layer 110 is, for example, made of silicon oxide. It can be polished, for example by chemical polishing.

[0122] A dielectric or semiconducting layer is then formed on the support substrate 100, by a layer transfer technique or by deposition, possibly followed by a planarization step, for example by chemical polishing. The layer is then locally etched over its entire height to obtain the core of a main waveguide 115.

[0123] An upper encapsulation layer 120 is then deposited on the lower encapsulation layer 110, in contact with the lower encapsulation layer 110 and the main waveguide 115. The upper encapsulation layer 120 is, for example, made of silicon oxide. It can be polished, for example by chemical polishing.

[0124] An additional polyimide layer is then formed in contact with the upper encapsulation layer 120, over at least a portion of the upper encapsulation layer 120. The additional polyimide layer is brushed in a direction intended to favor the orientation of the liquid crystal molecules when no electric field is present in the liquid crystal. The additional polyimide layer is thus intended to be a lower anchoring layer 630 of the liquid crystal.

[0125] In [Fig. 9B], a bead of adhesive (not shown) is formed on the lower anchoring layer 630 or in contact with the upper encapsulation layer 120. The adhesive bead is closed, i.e., it delimits a central region. It has at least one through-opening lateral opening communicating with the central region. The central region may, for example, have a substantially rectangular shape.

[0126] The upper portion of [Fig. 8D] is transferred onto the upper encapsulation layer 120, so that the protruding parts of the structured layer 250 are in contact with the lower anchoring layer 630 in the central region. Preferably, all the protruding parts are fully aligned with the central region. Sufficient pressure can be applied to the upper portion to drive the protruding parts into the lower anchoring layer 630, possibly until the protruding parts are in contact with the upper encapsulation layer 120. The adhesive bead secures the upper portion to the lower portion. The structured layer 250 acts as a spacer, maintaining a gap between the support substrate 100 and the encapsulation substrate 200.A liquid crystal 640 is then introduced into the central region through the through-side opening so as to fill the entire volume delimited by the adhesive bead and the lower and upper anchoring layers 630, 625. The through-side opening is then sealed.

[0127] At the end of this step, a continuous volume, delimited by the structured layer 250, the upper encapsulation layer 120 and, optionally, the encapsulation layer 610 when the buffer mold 550 has not been brought into contact with the The encapsulation layer 610 defines a core of an intermediate waveguide 130.i. The core extends between two protruding parts of the structured layer 250. The inclined face 251 of a protruding part constitutes a reflective surface 133.i of the intermediate waveguide 130.i. The straight face 252 of the same protruding part constitutes an exit face 132.i of the intermediate waveguide 130.i. If the structured layer 250 has at least three protruding parts, several cores belonging to respective intermediate waveguides 130.i are defined in this way simultaneously. They all have the same height measured orthogonally to the principal plane of the supporting substrate 100, since the edges of the structured layer 250 are coplanar and parallel to the upper face of the layer 610.

[0128] Alternatively, the nano-imprint lithography step of [Fig. 8C] can be carried out on the lower part of [Fig. 9A]. The adhesive bead is formed on the lower part after the nano-imprint lithography step. The step of [Fig. 8C] is omitted, and the upper part of [Fig. 8B] is transferred to the lower part and fixed by the adhesive bead. This embodiment is advantageous for carrying out the third embodiment of [Fig. 3], in particular for more precisely aligning the protruding parts of the structured layer 250 with the main waveguide 115.

[0129] Specific embodiments have just been described. Various variants and modifications will be apparent to those skilled in the art. For example, the simulation results were obtained with a wavelength in the visible range, suitable for the realization of a micro-screen, but similar results can be obtained with a useful wavelength in the field of optical telecommunications, for example approximately equal to 1550 nm.

Claims

Demands

1. Extraction structure (10.i) of a guided mode of wavelength X, linearly polarized along a polarization direction, comprising: • a support substrate (100) having a substantially flat upper face, • a main waveguide (115) for guiding the guided mode, • an intermediate waveguide (130.i) for guiding a coupled mode at wavelength X, comprising a liquid crystal core extending parallel to the upper face, and an exit face (132.i), the core extending to the exit face (132.i), • a flat surface (133.i) opposite the exit face (132.i), reflective at wavelength X, making a non-zero angle with the upper face of the support substrate (100), • a first electrode (105) and a second electrode (105.i), arranged with respect to the waveguide core intermediate wave (130.i) so as to switch, in a coupling portion of the core of the intermediate waveguide (130.i), a refractive index of the liquid crystal, according to the polarization direction, from a first level to a second level, when a variation of an electrical potential difference is applied between the first and second electrodes (105, 205.i), the first level, the second level and the arrangement of the coupling portion with respect to the main waveguide (115) being such that the guided mode, when present, is coupled at least in part, by evanescent coupling of the main waveguide (115) to the coupling portion only when the refractive index of the coupling portion is equal to the second level.

2. Extraction structure (10.i) according to claim 1, wherein the first electrode (105) is said to be buried, and wherein: • the buried electrode (105), the main waveguide (115), the intermediate waveguide (130.i) and the second electrode (205.i) extend in distinct planes, parallel to the upper face of the support substrate (100), • the main waveguide (115) and the intermediate waveguide (130.i) being intercalated between the buried electrode (105) and the second electrode (205.i).

3. Extraction structure (10.i) according to any one of claims 1 or 2, wherein the plane reflective surface (133.i) is an interface between a first medium (26O.i) and a second medium transparent to wavelength X, the first medium (26O.i) being arranged between the exit face (132.i) and the plane reflective surface (133.i) and having a refractive index strictly greater than a refractive index of the second medium, and such that, when the guided mode is present and coupled at least in part, a transmitted light wave from the guided mode propagates from the exit face (132.i) to the plane reflective surface (133.i) along a principal axis making an angle α with a normal to the plane reflective surface (133.i) greater than or equal to a minimum angle of incidence on the plane reflective surface (133.i) for which the light is totally reflected.

4. Extraction structure (10.i) according to any one of the preceding claims, wherein the intermediate waveguide (130.i) has an end (131.i) opposite the main waveguide (115), the core extending from the end (131.i) to the exit face (132.i).

5. Extraction structure (lO.i) according to claim 4, wherein the end (131.i) makes a non-zero angle with the upper face of the support substrate (100) so as to achieve an adiabatic coupling region.

6. Extraction structure (lO.i) according to any one of claims 1 or 2, wherein the reflective plane surface (lO3.i) is a metallized surface.

7. Extraction structure (lO.i) according to any one of the preceding claims, wherein, the guided mode is a TM mode, the liquid crystal comprises a nematic phase and the second level is an extraordinary refractive index of the liquid crystal.

8. Extraction structure (lO.i) according to any one of the preceding claims, wherein the wavelength X is included in the visible spectrum.

9. Optical device comprising a first group of several extraction structures (lO.i) according to any one of claims 4 to 8, sharing the support substrate (100), such that the main waveguide (115) of each extraction structure is a portion of a first main waveguide (315).

10. Optical device according to claim 9, further comprising a second group of several extraction structures (lO.i) according to any one of claims 4 to 8, sharing the support substrate (100) among themselves and with the extraction structures (lO.i) of the first group, such that the main waveguide (115) of each extraction structure of the second group is a portion of a second main waveguide (316) distinct from the first main waveguide (115).

11. Optical device according to claim 10, wherein each extraction structure (lO.i) of the first group corresponds to a corresponding extraction structure of the second group such that their intermediate waveguides (lO.i) are two portions of a common intermediate waveguide.

12. Optical device according to any one of claims 9 to 11, wherein the plane reflective surface (133.i) of each extraction structure (10.i) of the first group and, where applicable, of the second group is the end (131.i+1) of the intermediate waveguide (130.i+1) of another extraction structure (10.i+1) of the same group.

13. Optical device according to claim 11, wherein the extraction structures (lO.i) are extraction structures according to claim 5.

14. Optical device according to any one of claims 9 to 13, wherein all intermediate waveguides (130.i) of extraction structures (10.i) have equal heights, measured perpendicular to the top face.

15. A method for manufacturing an extraction structure (lO.i) according to any one of claims 1 to 8 or an optical device according to any one of claims 9 to 14, comprising the following steps:

16. • provision of a support substrate (100) comprising a main waveguide (115), • supply of an encapsulation substrate (200), • formation of a structured layer (250) on the support substrate (100) or the encapsulation substrate (200), by a nano-imprinting lithography process, the structured layer (250) comprising protruding parts of identical heights equal to a common height, • formation of an adhesive bead on the support substrate (100) or the encapsulation substrate (200), such that the adhesive bead has a thickness greater than or equal to the common height, delimits a central region, and has a through-side opening communicating with the central region, • transfer of the encapsulation substrate (200) onto the support substrate (100) so that the structured layer (250) acts as a spacer fixing a gap between the encapsulation substrate (200) and the support substrate (100), and delimits a continuous volume in the central region intended to be the core of the intermediate waveguide (130.i), • bonding of the encapsulation substrate (200) to the support substrate (100) by the adhesive bead, • introduction of a liquid crystal into the continuous volume through the through-side opening. A manufacturing method according to claim 14, wherein the nano-imprinting lithography process uses a reference mold (540) obtained by the following steps: • provision of a temporary substrate (500) comprising an upper face and trenches (520) extending deep into the temporary substrate (500) from the upper face, • filling the trenches (520) with a positive photosensitive resin (530), • exposure of the positive photosensitive resin (530) by collimated light propagating through the resin photosensitive positive (530) in a direction making an angle 00 between 30° and 60° with the upper face.

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