PHOTOACOUSTIC SYSTEM AND ASSOCIATED PROCESS

Abstract

PHOTOACOUSTIC SYSTEM AND ASSOCIATED METHOD The invention relates to a photoacoustic system (1) comprising: a semiconductor substrate (2) comprising a hollow volume (21a) and a window (22) extending vertically above a portion of the hollow volume (21a); ​​and a surface-emitting device (3) comprising a waveguide (31) and a diffraction grating (32), the waveguide (31) having an upper face (31a) and a lower face (31b), the lower face (31b) being disposed on the window (22), the diffraction grating (32) being disposed on the upper face (31a) or on the lower face (31b). Figure to be published with the abbreviation: Figure 1a

Description

Title of the invention: PHOTOACOUSTIC SYSTEM AND ASSOCIATED METHOD TECHNICAL FIELD OF INVENTION

[0001] The technical field of the invention is that of photoacoustic systems, for example, implemented in the detection of chemical compounds in a gas or in a vascularized skin sample. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Photoacoustic detection systems enable the detection and quantification of target species present in a continuous medium such as a gas or a liquid. These systems rely on the emission of an acoustic wave in the continuous medium in response to the selective excitation of target atoms or molecules in that continuous medium by means of a light beam. The acoustic wave, captured by a microphone, is proportional to the density of target atoms or molecules in the continuous medium.

[0003] The principle of photoacoustics has been known for several decades, but the realization of a photoacoustic detector system that can be miniaturized and precise still raises many technical problems.

[0004] In this context, the document [“Downsizing and Silicon Integration of Photoacoustic Gas Cells” Glière et al., International Journal of Thermophysics (2020) 41:16] discloses a detection system comprising a volume designed to accommodate the continuous medium. The disclosed system is remarkable, however, in that the volume is small. Indeed, the photoacoustic signal is inversely proportional to the volume of the continuous medium in which the acoustic background propagates. A direct way to improve the resolution of a detection system is therefore to reduce the volume involved, ideally to within a few cubic micrometers for a measurement at atmospheric pressure (otherwise the volume is too small for the probability of a photon interacting with a gas molecule to be sufficient). The disclosed system takes advantage of fabrication methods for Si devices, which make it possible to form very small hollow volumes within a silicon substrate.Specifically, it is disclosed that a network of trenches can be etched onto the surface of two separate substrates. The two substrates are then bonded together so that the trench networks are aligned. The two separate substrates thus form a thick substrate, and the two trench networks form a hollow volume within the thick substrate, beneath the surfaces of said thick substrate. The disclosed hollow volume comprises a main, straight cavity and a transparent window located on one slice of the substrate. thick. A light beam aligned with the window (this is called abutment) can then enter the main cavity and interact with the medium present within it. Thus, a laser beam correctly aligned with the first cavity can interact with the continuous medium present in the main cavity and induce a photoacoustic response in the medium.

[0005] However, this geometry poses new problems. Indeed, the response of the detection system is very sensitive to misalignment or angular divergence of the laser beam with the main cavity. In a laboratory environment, it is possible to implement an active beam alignment device and a high-performance collimation system. However, its implementation is complex and does not meet the requirements for miniaturization or integration into a lab-on-a-chip (Lab-on-a-Chip). Integrating a non-collimated laser source (such as a laser diode) near the optical window makes integration into a Lab-on-a-Chip possible. On the other hand, it requires a high-amplitude laser source to compensate for the weak coupling with the cavity and / or its high angular divergence.Furthermore, poor control of beam alignment with the cavity or poor control of the source distance from the optical window leads to a large dispersion of resolution between several detection systems of the same type.

[0006] There is therefore a need to provide a photoacoustic system that is efficient, reproducible and that can be integrated into a lab-on-a-chip. Summary of the invention

[0007] In this context, the invention relates to a photoacoustic system comprising: a semiconductor substrate, the semiconductor substrate comprising: • a face called the "upper face"; and • an internal wall delimiting a hollow volume, the hollow volume extending into the semiconductor substrate at a distance from the upper face of said semiconductor substrate.

[0008] The photoacoustic system is remarkable in that: • the semiconductor substrate includes at least one portion, called a "window", transparent to an electromagnetic field, extending vertically above at least part of the hollow volume and from the upper surface of the semiconductor substrate to the hollow volume; and in that • The photoacoustic system comprises at least one surface-emitting device including an emitting face, said emitting face extending over said at least one window, said at least one device comprising: • a waveguide comprising a first face and a second face, opposite the first face, the waveguide comprising a active region configured to emit the electromagnetic field; and • a diffraction grating exhibiting, along a first direction parallel to the emitting face, a diffraction order greater than or equal to two, • said diffraction grating extending over the first face of the waveguide, the emitting face coinciding with the second face of the waveguide; or said diffraction grating extending over the second face of the waveguide, the emitting face coinciding with the diffraction grating.

[0009] By transparent to the electromagnetic field, it is meant that the window has a transmission coefficient greater than 50%, in other words, at least 50% of the power carried by the electromagnetic field is transmitted through the window.

[0010] The fact that the diffraction grating has a diffraction order greater than or equal to two makes it possible to extract the electromagnetic field generated by the active region. In particular, the diffraction grating allows a component of the field to propagate out of plane. A substantial portion of the power carried by the out-of-plane component (which we will call electromagnetic radiation) is then emitted by the second face of the waveguide.

[0011] Thanks to the window in the semiconductor substrate and the arrangement of the surface-emitting device, the electromagnetic radiation from the surface-emitting device is injected directly into the hollow volume. It can therefore be used to perform a photoacoustic measurement. A surface-emitting device emits electromagnetic radiation that exhibits good near-field homogeneity and low angular dispersion. Consequently, a continuous medium in the hollow volume directly above the surface-emitting device (and more specifically directly above the window) can be efficiently excited by the radiation from the surface-emitting device. Furthermore, the limited thickness of the window also helps to reduce radiation absorption.Finally, the surface-emitting device can be positioned very close to the window (or even on the window) and therefore very close to the hollow volume, so as to effectively illuminate a medium located within the hollow volume. The photoacoustic system according to the invention is therefore efficient.

[0012] The fact that the surface-emitting device extends over the upper surface of the substrate and not near a slice of the substrate also simplifies its integration into the photoacoustic system. It simply needs to be placed (or fabricated) on the window. Furthermore, the sources of uncertainty regarding alignment are low. Indeed, since the surface-emitting device is placed on the upper surface (or on (an optical base which is itself deposited on the upper surface) the uncertainty regarding its distance or inclination relative to the upper surface is low or even negligible. Furthermore, rotating the device around an axis perpendicular to the window does not necessarily have any effect. The alignment constraint is therefore less stringent than with a prior art system. In addition, the window can be enlarged as much as necessary to compensate for any misalignment of the device relative to it. The measurement or detection results that can be obtained with several photoacoustic systems according to the invention are therefore reproducible.

[0013] The surface emission device can be compact, making it a good candidate for integration into a lab-on-a-chip. Furthermore, the dimensions, particularly the lateral dimensions, of the surface emission device can be reduced as much as necessary to further reduce its size and the size of the photoacoustic system. It should be noted that measurement accuracy increases with a reduction in the excited volume. The photoacoustic system according to the invention is therefore easily integrated, for example, into a lab-on-a-chip.

[0014] Advantageously, the diffraction grating of said at least one device extends over the first face of the waveguide of said at least one device and said diffraction grating is reflective.

[0015] Advantageously, the diffraction grating of said at least one device comprises a periodic structure and a metallic layer, the periodic structure extending over the first face of the waveguide and the metallic layer extending over the periodic structure.

[0016] Alternatively, the diffraction grating of said at least one device extends over the second face of the waveguide of said at least one device and said diffraction grating is non-reflective.

[0017] Advantageously, the periodic structure of the diffraction grating of said at least one device comprises an alternation of first portions and second portions, the first portions being made of a first semiconductor material having a first optical index and the second portions being made of a metal or a second material, such as another semiconductor material or a gas, having a second optical index different from the first optical index.

[0018] Advantageously, the diffraction grating of said at least one device extends vertically from the active region of the waveguide of said at least one device, the active region having a length, measured along the first direction, and the diffraction grating of said at least one device having a length, also measured along the first direction, the length of said diffraction grating being substantially equal to the length of said active region, said grating having, along the first direction, a unique diffraction order.

[0019] Preferably, the diffraction grating of said at least one device has, along the first direction, a diffraction order greater than or equal to three.

[0020] Advantageously, the active region of said at least one device has a width, measured along a second direction, parallel to the emitting face of said at least one device and perpendicular to the first direction, and the diffraction grating of said at least one device has a width, measured along the second direction, the width of said diffraction grating being substantially equal to the width of said active region, said diffraction grating having, along the second direction, another diffraction order, unique and greater than or equal to two.

[0021] Advantageously, the waveguide of said at least one device is delimited by a first flank and a second flank, the second flank being opposite the first flank, the first and second flanks being substantially perpendicular to the second face, said device also comprising a first conductive layer and a second conductive layer, the first conductive layer extending over the first flank and the second conductive layer extending over the second flank.

[0022] Advantageously, said at least one surface emission device is glued to the window and preferably by molecular bonding or by means of a polymerizable glue.

[0023] Advantageously, said at least one surface-emitting device is configured to emit infrared electromagnetic radiation from its emitting face at an angle, measured with respect to the normal to said emitting face, between 0° and 60°.

[0024] Advantageously, the semiconductor substrate comprises a plurality of windows and the photoacoustic system comprises a plurality of surface-emitting devices, each surface-emitting device of the plurality of surface-emitting devices being arranged on a window of the plurality of windows.

[0025] Advantageously, the electromagnetic radiation comprises a wavelength in [0.8 pm; 20 pm] and preferably in [4 pm; 12 pm].

[0026] Advantageously, the photoacoustic system includes a thermal regulator configured to regulate the temperature of said at least one surface-emitting device.

[0027] Preferably, the thermal regulator configured to regulate the temperature of said at least one surface-emitting device when said at least one surface-emitting device is operating in a steady state.

[0028] Preferably, the thermal regulator is configured to regulate the temperature of said at least one surface-emitting device to within 0.1 °C.

[0029] Alternatively, the thermal regulator is configured to regulate the temporal variation of the temperature of said at least one surface-emitting device to within 0.1 °C / s.

[0030] The invention also relates to a method for manufacturing a photoacoustic system comprising, from a semiconductor substrate comprising a so-called "top face": • form an internal wall delimiting a hollow volume, the hollow volume extending in the semiconductor substrate at a distance from the upper face of said semiconductor substrate so that the semiconductor substrate includes at least one portion, called a "window", transparent to an electromagnetic field, extending vertically from at least a part of the hollow volume and from the upper face of the semiconductor substrate to the hollow volume; • manufacture at least one surface-emitting device extending over the window, said at least one device comprising: • a waveguide comprising a first face and a second face, opposite the first face, the second face resting on said at least one window, the waveguide comprising an active region parallel to the second face configured to emit the electromagnetic field; and • a diffraction grating having a diffraction order greater than or equal to two and extending over the first face of the waveguide or over the second face of the waveguide.

[0031] Advantageously, the manufacture of said at least one surface-emitting device comprises: • to form the waveguide of said at least one device on said at least one window of the semiconductor substrate; and • form the diffraction grating of said at least one device on the first face of said waveguide.

[0032] Alternatively, the manufacture of said at least one surface-emitting device comprises: • form the diffraction grating of said at least one device on said at least one window of the semiconductor substrate; and • form the waveguide of said at least one device on said diffraction grating.

[0033] The invention and its various applications will be better understood by reading the following description and examining the accompanying figures. BRIEF DESCRIPTION OF THE FIGURES

[0034] The figures are shown for illustrative purposes only and are not intended to limit the invention. Unless otherwise specified, the same element appearing in different figures has a unique reference numeral.

[0035] Figs.1a, 1b, 3a, 3b, 4, 5 and 6 schematically show the first, second, third, fourth and fifth embodiments of a photoacoustic system according to the invention.

[0036] Fig. 2 schematically shows an embodiment of a surface emission device that can be implemented in the photoacoustic system according to the invention.

[0037] Figures [Fig. 7a] and [Fig. 7b] schematically show two embodiments of a method for manufacturing a photoacoustic system according to the invention. DETAILED DESCRIPTION

[0038] Figures [Fig. 1a] and [Fig. 1b] schematically illustrate an embodiment of a photoacoustic system 1 according to the invention. In this embodiment, the system 1 is adapted to measure the concentration of a target species in a gas. The system 1 is based on measuring a sound wave resulting from the photoacoustic emission of the target species in the gas.

[0039] The system 1 includes, in particular, a surface-emitting device 3, two sound sensors 4 (i.e., microphones), and a Helmholtz resonator 21 into which the gas containing the target species can be introduced. The surface-emitting device 3 is coupled to the Helmholtz resonator so as to inject electromagnetic radiation capable of exciting the target species in the gas. The sound sensors 4 are coupled to the resonator 21 in order to measure the amplitude of the sound waves propagating in the resonator 21 and resulting from the photoacoustic emission by the target species.

[0040] System 1 illustrated in [Fig.la] and [Fig.lb] is partially truncated, according to two different configurations, to show the elements of system 1.

[0041] The system 1 as illustrated comprises a semiconductor substrate 2 including the resonator 21. The substrate 2 extends in a plane {X; Y] formed by a first direction X and a second direction Y, orthogonal to the first direction X. The substrate 2 comprises two large faces 2a, 2b, opposite each other. That is to say, a face 2a, called the "upper face", oriented along a third direction Z, and a face 2b, called the "lower face", oriented along the third direction -Z and opposite the upper face 2a. The two large faces 2a, 2b are delimited by a side 2c (also called the "edge"), forming the perimeter of the substrate 2.

[0042] The substrate 2 may have a thickness measured along the Z direction, within [0.5 mm; 3 mm]. The substrate 2 may have a length L2 and a width L2, respectively measured along the first and second directions X and Y, within [5 mm; 20 mm].

[0043] The resonator 21 comprises two hollow volumes 21a, 21b, also referred to as "main cavities." The main cavities 21a, 21b extend parallel to each other along X. They extend within the volume of the substrate 2, beneath the upper surface 2a of the substrate 2. In other words, they extend at a distance from the upper surface 2a. Each of the main cavities 21a, 21b is delimited by an internal wall within the substrate 2. The resonator 12 also comprises channels 211 connecting the main cavities 21a, 21b. The illustrated channels 211 each have an opening 212, formed in the upper surface 2a of the substrate 2. These openings 212 can be used to introduce the gas and the target species into the resonator 21.

[0044] The semiconductor substrate 2 can be formed from two thin semiconductor substrates 2', 2" joined together. A line 20 illustrates, for example, the interface between the two thin substrates 2', 2" joined together. To form the thick substrate 2, each of the thin substrates 2', 2" is etched to form trenches that will compose the resonator 21. Joining the two substrates 2', 2" against each other, so that their respective trenches are facing each other, forms the substrate 2 and the resonator 21.

[0045] Substrate 2 is preferentially made of Si.

[0046] The substrate 2 comprises a portion 22 called the "window". By window, it is understood that it is adapted to transmit electromagnetic radiation into the resonator 21 and in particular into one of the main cavities 21a, 21b. The window 22 extends vertically from at least a portion of one of the main cavities 21a, 21b. By extending vertically, we mean that the window is aligned with a portion of a cavity 21a, 21b along the Z direction (perpendicular to the upper surface 2a). The window 22 extends into the substrate 2 and connects the upper surface 2a of the substrate 2 to one of the cavities 21a, 21b. The window 22 is therefore not an opening made in the substrate 2 so as to expose a cavity 21a, 21b. The window 22 can be made of the same material as the substrate 2, for example Si. The transmission coefficient of the window 22 can vary depending on the thickness W22 of the window 22, measured along the Z direction.A Si window 22 can be considered transparent to infrared radiation in the range [0.8 pm; 20 pm] when its thickness W22 is less than or equal to 100 pm. A smaller window thickness W22, for example less than or equal to 50 pm, may be preferred to further improve infrared radiation transmission. The window can also be... made from another material, particularly when that material offers better transmission of electromagnetic radiation for certain wavelengths. For example, it could be a portion made of silicon nitride or silicon carbide.

[0047] In the embodiment of [Fig. 1a] and [Fig. 1b], the window 22 is associated with a cavity 21a of the resonator 21. In one development, the substrate 2 may comprise a plurality of windows associated with a cavity. For example, the windows extend from the upper face 2a of the substrate 2 to the same cavity 21a, but to different portions of said cavity 21a. Thus, the system 1 may comprise several surface-emitting devices 3, each arranged on one of the windows. Thus, the plurality of devices is coupled to the same cavity. This can be useful for exposing the same cavity to radiation with different wavelengths (for example, to detect different species).

[0048] The surface emission device 3 allows electromagnetic radiation to be injected into the resonator 21 and in particular into the main cavity 21a, 21b to which the window 22 is associated.

[0049] A surface-emitting device is defined as an electro-optical device configured to emit electromagnetic radiation from one of its faces, which is generally called the "emitting face". It differs from an edge-emitting device in which the emission of electromagnetic radiation is carried out at an edge or a side of said device.

[0050] The advantage of a surface-emitting device 3 is twofold. First, it is possible to emit radiation in an out-of-plane direction. That is, in the case of [Fig. 1a] and [Fig. 1b], out of the {X; Y} plane. For example, in the Z direction. The device 3 also allows radiation to be emitted from a surface (the so-called "emitting face") that has a larger area than an edge. The emitting face generally has a substantial area and makes it possible to obtain spatially homogeneous electromagnetic radiation, especially in the near field. It is therefore advantageous to use this type of device to inject electromagnetic radiation into the resonator 21 and efficiently excite the targeted species in the gas.

[0051] The device 3 is configured to emit through a face 3b, which will be called the "emitting face". The electromagnetic radiation emanating from the emitting face 3b is substantially perpendicular to the emitting face 3b. By substantially perpendicular, it is understood that the radiation has an angle greater than or equal to 30° with the emitting face 3b (i.e., between 30° and 90°).

[0052] In order to inject electromagnetic radiation into the resonator 21 and in particular into the cavity 21a, the surface-emitting device 3 is arranged on the window 22. The emitting face 3b of the surface-emitting device 3 is positioned directly in contact with window 22a. In this way, the electromagnetic radiation emitted by device 3 is directly transmitted to cavity 21a. The emitted radiation can then excite the targeted species present in resonator 21.

[0053] Figure 2 shows, in cross-section, an embodiment of a surface emission device 3 that can be implemented in the photoacoustic system 1. The device 3 is shown in cross-section in the {X; Z} plane. Figure 2 also shows a magnified view of a portion 3' of said device 3, in the same plane.

[0054] The device 3 comprises: a waveguide 31 and a diffraction grating 32.

[0055] The waveguide 31 extends in a plane P which is, for example, parallel to the upper surface 2a of system 1 (i.e., the plane {X; Y}). The waveguide 31 comprises a first face 31a and a second face 31b, opposite the first face 31b. The first and second faces 31a, 31b extend parallel to the plane P. The first face 31a is, for example, oriented along the third direction Z, perpendicular to the plane P, and the second face 31b is oriented along the same direction but in the opposite direction.

[0056] The waveguide 31 also includes an active region 310. The active region 310 is a portion of the waveguide 31 configured to emit an electromagnetic field (also called "electromagnetic radiation" or simply "field" or "radiation"). The active region 310 is, for example, configured for spontaneous and / or stimulated emission. In the latter case, the active region could also be called an "amplifying medium" because it allows the device 3 to operate in laser mode. The active region 310 is, for example, a stack of subshells, such as [InGaAs / AlInAs]xN, where N is the number of InGaAS / AlInAs subshell pairs, for example, one hundred. In this way, the active region 310 is configured to achieve both spontaneous and stimulated emission.

[0057] The active region may have a thickness W31Q, measured perpendicular to plane P, which may be between 1 pm and 5 pm and preferably between 1.5 pm and 2.5 pm.

[0058] The active region 310 is preferably framed by two semiconductor layers 311, 312, extending parallel to the plane P. A first semiconductor layer 311, which can be called the "top cladding," extends over the active region 310 and preferably directly against it. In this embodiment, the face of the top cladding 311 opposite the active region 310 is advantageously the first face 31a. A second semiconductor layer 312, which can be called the "bottom cladding," also extends over the active region 310 and preferably against the active region 310. The face of the lower coating 312 which is opposite the active region 310 is then advantageously the second face 31b.

[0059] The upper and lower coatings 311, 312 can allow the field to be guided in the waveguide 31. For this purpose, they advantageously have optical indices (also called refractive indices) strictly lower than the average optical index of the active region 310.

[0060] The upper coating 311 has a thickness, measured perpendicular to plane P, of between 1 µm and 2 µm. The lower coating 312 has a thickness, measured perpendicular to plane P, of between 2 µm and 100 µm and preferably between 2 µm and 40 µm.

[0061] The lower coating 312 is, for example, a type IILV semiconductor heterostructure, that is, materials classified in groups III B and III B of the periodic table of elements (which, according to another convention, corresponds to columns 13 and 15 of the periodic table of elements). This coating 312 is, for example, formed from InP. The lower coating 312 can also be doped, for example, with n-type doping, that is, with impurities acting as electron donors. The lower coating 312 is, for example, doped from S.

[0062] The upper coating 311 can be, in the same way as the lower coating 312, a type IILV semiconductor heterostructure, such as InP. Similarly, the upper coating 311 can be doped, for example with type n.

[0063] In the embodiment of [Fig. 2], the waveguide 31 is therefore a stack of layers 310, 311, 312 extending parallel to the plane P. It can have the shape of a rectangular parallelepiped delimited by its sides. For example, it has a length L31, a width, and a thickness Wgp. The length of the waveguide 31 is, for example, measured along the first direction X. The width of the waveguide 31 is, for example, measured along the second direction Y, perpendicular to the first direction X. The thickness W3j of the waveguide 31 is, for example, measured along the third direction Z, perpendicular to the two aforementioned directions X and Y.

[0064] The length L31 of the waveguide 31 can be between 1000 pm and 5000 pm. In one embodiment, the length L31 and the width of the waveguide 31 can be equal, for example, to within 10%. Thus, viewed from above, the first face 31a can have a square shape. Alternatively, the length L31 of the waveguide 31 can be greater than the width of the waveguide 31 and, for example, greater than twice the width of the waveguide 31, or even greater than one hundred times the width of the waveguide. In this case, it is called a ridge-type waveguide. For example, the width of the waveguide 31 (not shown in the figure) can be between 10 pm and 50 pm.

[0065] The active region 310 has a length L3i0, measured along the first direction X, substantially equal to the length L31 of the waveguide 31. Similarly, the active region 310 has a width, measured along the second direction Y, substantially equal to the width of the waveguide 31. By substantially equal, we mean equal to within 20%, or even 10%. Flanks perpendicular to the plane P, for example, delimit the waveguide 31 and the active region 310.

[0066] In the embodiment of [Fig. 2], the diffraction grating 32 extends over the first face 31a of the waveguide 31 and in particular over the upper coating 311. The emitting face 3b of the device 3 then coincides with the second face 31b of the waveguide. By coincide, it is understood that the faces are coplanar.

[0067] The diffraction grating is advantageously arranged directly above the active region 310 of the waveguide 31 and preferably centered with respect to the latter.

[0068] The diffraction grating 32 can have a length L32 and a width. The length L32 of the diffraction grating 32 is, for example, measured along the first direction X. The width of the diffraction grating 32 is, for example, measured along the second direction Y, perpendicular to the first direction X. The length L32 and the width of the diffraction grating 32 are advantageously chosen so that the diffraction grating 32 completely covers the active region 310. In other words, the lengths and widths of the diffraction grating 32 are, respectively, substantially equal to the lengths and widths of the active region 310. In this way, the diffraction grating 32 can be coupled homogeneously to the field emitted by the active region 310. It makes it possible, for example, to provide homogeneously distributed negative feedback over the entire length L^q of the active region 310.

[0069] Device 3 is remarkable in that the second face 31b of the waveguide is transparent to the field that can be emitted by the active region 310. By transparent, we mean that the face 31b has a spectral transmission window and that this spectral transmission window corresponds to at least a part of the spectrum of the field that can be emitted by the active region 310. The second face 31b of the waveguide 31 also constitutes the emitting face 3b of device 3.

[0070] Device 3 is also remarkable in that the diffraction grating 32 is reflective and has a diffraction order, along the first X direction, greater than or equal to two. By reflective, it is understood that at least 50% of the field is reflected by the diffraction grating 32. The diffraction order greater than or equal to two along the X direction implies that the field generated by the active region propagating along the first X direction couples to the diffraction grating 32 and induces a component of the field that propagates out of the plane P, that is, along +Z and / or -Z. Since the diffraction grating 32 reflects the component of the field Propagating along +Z, this component of the field is therefore oriented towards the second face 31b. The device 3 can thus emit from the second face 31b. The second face 31b of the waveguide 31 therefore coincides with the emitting face 3b of the device 3.

[0071] Advantageously, the diffraction grating 32 comprises a periodic structure 321. The periodic structure 321 is, for example, a layer extending over the first face 31a of the waveguide 31. Coupled with the field emitted by the active region 310, it induces the diffraction effects. The coupling ratio between the field and the diffraction grating 32 is advantageously between 10 cm1 and 100 cm'.

[0072] The periodic structure 321 includes, for example, first portions 3211 and second portions 3212. These first and second portions 3211, 3212 are arranged periodically and form an alternation along the first direction X and along the second direction Y where applicable.

[0073] For example, the first and second portions 3211, 3212 can be lines, oriented along the second direction Y. These lines are arranged side by side along the first direction X so as to form an alternation along the first direction X. In other words, they form two combs nested one inside the other. [Fig. 2] illustrates this example.

[0074] According to another example, the first portions 3211 are plots arranged in a rectangular grid. For example, the second portions 3212 are tori surrounding each plot 3211 and filling the space between the plots 3211. According to a variant of this example, the plots 3211 may have an elliptical shape, having a large length along the first direction X and a small length along the second direction Y, or vice versa.

[0075] The first portions 3211 are made of a first semiconductor material having a first refractive index. The first semiconductor material is, for example, a type III-V semiconductor material, such as InP. It may also be the same material as the top coating 311. The second portions 3212 are made of a second material or a metal. The second material may be another semiconductor material or a gas, such as air. In this case, it has a second refractive index different from the first refractive index.

[0076] The diffraction grating 32 advantageously exhibits, along the first direction X, a unique diffraction order. That is to say, the period A (or "step") with which the first portions 3211 are arranged is constant over the entire length L32 of the grating 12. The first portions 3211 then advantageously all have the same width A32ip measured along the first direction X. The second portions 3212, separating the first portions 3211, can also exhibit a same width A32i2' also measured along the first direction X. The period A of arrangement of the first portions 3211, which corresponds to the period of the diffraction grating 32 along the first direction X is therefore equal to 11 + A^212- A unique diffraction order means that the period A is constant, within + / - 10%, over the entire length L32 of the diffraction grating.

[0077] The active region 310 is preferably configured to emit in the infrared range, i.e., in the range [0.8 pm; 20 pm] or preferably [4 pm; 12 pm]. Such wavelengths imply a period A of the grating 12 on the order of at least one micrometer. A diffraction grating 32 applicable to the infrared spectrum is also simpler to manufacture than a grating with a much shorter period (for example, in the blue or ultraviolet spectrum). Moreover, this type of range is advantageously suited to photoacoustic measurement or detection. In addition, a large family of semiconductor materials is transparent to infrared radiation.

[0078] The diffraction grating 32 also includes a metallic layer 322 extending over the periodic structure 321 and preferably over the entire periodic structure 321. It is, for example, made of Ti or Au. The reflective effect of the diffraction grating 32 can also be provided by the metallic layer 322 extending over the periodic structure 321. The metallic layer can prevent the transmission of the field through the diffraction grating 32. The metallic layer 12 extends, in particular, over each first portion 3211 of the periodic structure 321. In this way, it prevents the transmission of the field through the first portions 3211.

[0079] The embodiment of [Fig. 2] illustrates a continuous metallic layer 322 extending along the first direction X. Alternatively, the metallic layer 322 can be discontinuous and comprise a plurality of portions. Each portion thus advantageously extends over each first portion 3211 of the periodic structure 3211. If, for example, the first portions 3211 of the periodic structure 321 are aligned along the second direction Y, then the metallic layer 322 will comprise a plurality of portions also extending along the second direction Y, with a portion of the metallic layer extending over an upper surface of a first portion 3211.

[0080] The metallic layer 322 can also extend beyond the periodic structure 321 because it can also be used as a contact electrode of the device 3. It can allow an electric field to be applied uniformly to the active region 310 in order to inject carriers into the active region 310. The carriers can de-excite by emitting photons into the active region 310.

[0081] The metallic layer 322 also makes it possible to improve the confinement of the field in the waveguide 31 thanks to the plasmonic interaction of the field with the metal.

[0082] According to one embodiment, the diffraction grating 32 can be configured so that, along the second direction Y, perpendicular to the first direction X, it also has an order greater than or equal to two. Thus, the field propagating along the second direction Y also couples to the diffraction grating and induces a component of the field that also propagates out of the plane P, i.e., along +Z and / or -Z. This embodiment can be obtained when the first portions 3211 of the periodic structure 321 are plots arranged in a rectangular grid. This embodiment is advantageous when the width and length of the active region (and therefore of the diffraction grating) are substantially equal, for example, within 20%.On the other hand, if the length of the active region 310 is much greater than its width, for example 20 times greater than its width (ridge-type guide), it is preferable that the diffraction grating 32 exhibits order only along the first X direction and no order along the second Y direction. In other words, the first portions 3211 of the periodic structure 321 can be lines aligned along the second Y direction and distributed along the first X direction.

[0083] If necessary, the diffraction grating 32 may also exhibit a unique diffraction order along the second direction Y. The arrangement periods of the first and second portions along the first and second directions X and Y are then constant in both directions. However, they may be different, so that the order along the first direction X is different from the order along the second direction Y.

[0084] Lateral field confinement can also be achieved by conductive coatings extending over the sides of the waveguide 31. The waveguide 31 is, for example, delimited by a first side and a second side, opposite each other. The first and second sides are, for example, substantially perpendicular to the second Y direction. These are then referred to as "lateral sides" or "lateral facets". The device 3 comprises first and second conductive layers extending respectively over the first and second sides. The conductive layers are, for example, metallic layers of Ti or Au, extending perpendicularly to the plane P. They form a cavity along the second Y propagation direction and allow the field to be confined along the second Y direction.

[0085] To prevent a short circuit within the waveguide (for example, a short circuit between the upper coating 311 and the lower coating 312), the device 3 advantageously comprises electrically insulating spacers. These are, for example, dielectric layers. Each spacer is, for example, arranged on the lateral sides, between the conductive layers and the lateral sides.

[0086] The device 3 may also include third and fourth conductive layers that contribute to field confinement along the first direction. These layers extend, for example, along a third and fourth flank, respectively, delimiting the waveguide 31 along the first direction X. The third and fourth flanks may be called the "front facet" and "back facet." The third and fourth flanks are, for example, perpendicular to the first direction X. The spacers are then advantageously arranged on the third and fourth flanks, between the third and fourth conductive layers and the third and fourth flanks, so as to avoid any short circuit.

[0087] In an alternative embodiment of the device 3, the diffraction grating 32 does not extend over the first face 31a of the waveguide 31. It extends over the second face 31b of the waveguide 31. This is, in this case, the face of the waveguide 31 that is oriented towards the window 22a of the photoacoustic system 1. In other words, the diffraction grating 32 coincides with the emitting face 3b of the device 3. The diffraction grating 32 is, for example, located between the device 3 and the window 22a. In order to allow the propagation of the field towards the window 22a, the diffraction grating 32 is non-reflective. That is to say, it transmits a portion of the power carried by the electromagnetic field.

[0088] The diffraction grating 32 may include a periodic structure 321 as described above. In contrast, this periodic structure extends over the second face 31b of the waveguide 31, for example, over the lower coating 312. The device 3 may also include a metallic layer. However, the metallic layer does not extend over the periodic structure 321, as this would block field transmission. The metallic layer may, however, extend over the first face 31a of the waveguide 31. In this way, it reflects the field propagating towards the first face 31a and increases the power passing through the periodic structure 321 on the second face 31b.

[0089] Figures 3a and 3b show an embodiment of system 1. This embodiment differs from the one illustrated in Figures 1a and 1b in that the window 22 has a thickness W22' measured along the Z direction of less than 50 pm, in this case 10 pm. To achieve this, a trench 220 is cut into the substrate 2, starting from the upper surface 2a of the substrate 2, so as to reduce the thickness W22 of the window 22. Thus, the transmission of radiation from the device 3 is increased.

[0090] Applicable to embodiments of [Fig. 1a], [Fig. 1b], [Fig. 3a], and [Fig. 3b], the device 3 can be glued to the window to improve coupling and avoid the use of an anti-reflective coating. The gluing is, for example, carried out by at the molecular level (more commonly known as "molecular bonding" or "intimate bonding"). It can also be achieved using a polymerizable adhesive, for example, an epoxy-based adhesive. The adhesive is preferably polymerizable under UV radiation, so as to remain passive to infrared radiation.

[0091] In the embodiment of [Fig. 3a] and [Fig. 3b], the system 1 also includes a thermal regulator 6 configured to regulate the temperature of the device 3. The thermal regulator 6 is, for example, a Peltier cell. A cold source of the thermal regulator 6 is the surface-emitting device 3, and a hot source of the thermal regulator 6 is a thermal bath (such as a finned heat sink in contact with the ambient environment). In this embodiment, the thermal regulator 6 is located under the substrate 2 and in direct contact with its lower face 2b. More specifically, the regulator 6 is positioned directly above the device 3 to be regulated. The substrate 2 then serves as a thermal bridge between the device 3 and the thermal regulator 6. In this particular example, the portions of the substrate 2 adjacent to the cavity 21a act as the thermal bridge between the device 3 and the regulator 6.Indeed, substrate 2, which can be made of Si, has a thermal conductivity between 60 W / m / K and 150 W / m / K, which allows for the efficient transfer of heat from device 3 to thermal regulator 6.

[0092] The surface-emitting device 3 is advantageously configured so that the electromagnetic radiation (also called a "beam" or "brush") is emitted in a direction substantially normal to the emitting face 3b. In other words, the radiation has, for example, an angle, measured with respect to the normal to said emitting face 3b, that is between 0° and 60°. This angle can also be between 0° and 20°. Thus, a portion of the resonator 21 substantially directly above the emitting face 3b can be illuminated by the beam. The emission angle can be controlled by slightly modifying the pitch of the diffraction grating 32.

[0093] Figure 4 shows an embodiment of system 1 that differs from the embodiments of Figures 3a and 3b in that the device 3 includes an additional semiconductor layer 33 called the "optical base." The optical base 33 extends between the window 22 and the emitting face 3b of the device 3, that is, between the window 22 and the second face 31b of the waveguide 31 of said device 3. In this embodiment, the base 33 rests at the bottom of the trench 220. Thus, the additional thickness created by the base 33 is compensated for. The optical base 33 can be used to match the refractive index of the waveguide 31 to that of the window 22.

[0094] In order to improve regulation, the thermal regulator 6 can be placed on the upper face 2a of the substrate 2, for example near the device 3, or even in direct contact with the device 3.

[0095] The thermal controller 6 is advantageously configured to regulate the temperature of the surface-emitting device 3. For example, the thermal controller 6 is configured to regulate the temperature of said device 3 to within 0.1 °C when the latter is operating in steady state. The modulated operation of the device 3 can be included in the steady state. Indeed, in the context of a photoacoustic measurement, the stimulation of a molecule or a medium is carried out with an acoustic frequency (for example, between 20 Hz and 20 kHz). The operation of the device 3 is therefore modulated (for example, switched on / off periodically) according to this acoustic frequency.

[0096] Alternatively, whether the device is in steady state or transient regime, the thermal regulator 6 can be configured to regulate the temperature of said device 3 so that the time variation of the temperature of said device is less than 0.1 °C / s.

[0097] Figure 5 shows, in a cross-section along a plane {X; Z], an embodiment of system 1 in which the beam E (black arrows) is emitted at an angle to the normal to the emitting surface 3b. The beam E is thus injected into the main cavity 21a of the resonator 21, which is not directly above the device 3. The device 3 is, for example, advantageously configured to emit the radiation E at an angle α, measured with respect to the normal to said emitting face 3b, of between 30° and 60°. Thus, reflections of the radiation E against the inner wall delimiting the cavity 21a can allow the radiation E to propagate throughout the entire cavity 21a.

[0098] To reduce the absorption of the beam E in the substrate 2, the resonator 21 may have a metallic surface 23 positioned along the optical path of the radiation E to promote reflections of the beam E within the resonator 21. For example, the metallic surface 23 is positioned on a portion of the inner wall delimiting the main cavity 21a and oriented towards the emitting face 3b of the device 3 (the orientation convention of the inner wall being defined as oriented from the substrate 2 towards the cavity 21a). Advantageously, the metallic surface 23 is positioned over the entire inner wall delimiting the cavity 21a, except for the portion in contact with the window 22. In this way, the electromagnetic radiation can penetrate the resonator 21 unimpeded and illuminate a large portion of the volume of the cavity 21a through multiple reflections.

[0099] Figure 6 schematically represents an embodiment of system 1 for performing a photoacoustic measurement of a sample 3. The sample 5 is, for example, a vascularized skin sample. System 1 is configured to illuminate a portion of the sample and induce excitation of a target species in the sample, such as glucose in the blood vessels flowing through it. Sample 5. The photoacoustic response of glucose can induce a vibration of the surface of sample 5 which, coupled to a cavity 21a, can be captured by means of sound sensors 4 (not shown).

[0100] Unlike the embodiments described above, in this embodiment the substrate 2 comprises a hollow volume 21a which is open. That is to say, open on the lower face 2b of the substrate 2. This hollow volume 21a, once pressed against the sample 5, forms a closed cavity in which a sound wave can propagate.

[0101] The device 3 rests on the window 22. The thickness W22 of the window 22 is reduced, for example less than or equal to 50 pm, thanks to the trench 220 made in the substrate 2, from the upper surface 2a.

[0102] In this embodiment, the system 1 also includes a thermal regulator 6, configured to regulate the temperature of the device 3 during operation and preferably in steady state. The embodiment in [Fig. 6] shows that the thermal regulator 6 comprises active cells 61, such as Peltier cells, and a heat sink 62, such as a finned heat sink. The cells 61 are, for example, in contact with the substrate 2, provided that the heat transfer offered by the latter is sufficient. If, for example, it is made of silicon, it offers a heat transfer coefficient of approximately 150 W / m / K, which may be sufficient when a few millimeters separate the cells 61 and the device 3. The heat sink 62 is in contact with the cells 61 so as to dissipate the heat pumped from the substrate 2.Thermal insulators 63 can be added between substrate 2 and sample 5 to prevent sample 5 from heating device 3 or vice versa.

[0103] In one embodiment, the cells 61 can be arranged against the substrate 2, in place of the thermal insulators 63. In this case, the sample 5 can act as a heat sink. The thermal regulator 61 then comprises only the cells 61.

[0104] Figures [Fig.7a] and [Fig.7b] schematically show two modes of implementation of a process 7 for manufacturing a system 1 according to the invention.

[0105] Transversally to the two embodiments illustrated, the method 7 includes a step 71 of supplying a substrate 2 comprising an upper face 2a and at least one window 22 associated with a hollow volume 21a (which may be part of a resonator 21 or may be open). The hollow volume 21a (or the resonator 21) may be formed as described above, for example by engraving a network of trenches on the surface of two thin substrates 2', 2" which, once bonded together, form the substrate 2 and allow the hollow volume 21a (or the resonator 21) to be formed. The wall is formed so that the hollow volume 21a extends into the substrate 2 under the upper face of the substrate 2 and that at least one window 22 as defined previously, extends vertically over at least part of the hollow volume 21a.

[0106] The process 7 also includes a manufacturing step 72 of the surface-emitting device 3 on the window 22. The manufacturing is carried out so that the emitting face 3b of the device 3 rests on the window 22.

[0107] In the embodiment of [Fig. 7a], the fabrication 72 of the device 3 includes a substep 7211 of forming the waveguide 31 on the window 22. The waveguide 31 is, for example, formed on a different substrate and then bonded to the window 22. Alternatively, the layers composing the waveguide 31 can be deposited one after the other from the window 22. For example, the formation 7211 of the waveguide 31 may initially include a molecular bonding step of a first semiconductor layer, for example of InP, onto the upper face 2a of the substrate 2, for example of Si. A stack of semiconductor sublayers can then be formed on said first semiconductor layer of InP to enable the formation of the active region 310. A second semiconductor layer, for example of InP, is formed on the stack of semiconductor sublayers.Finally, etching the first and second InP semiconductor layers and the stacking of semiconductor sublayers allows the formation of the waveguide 31.

[0108] In this embodiment, the fabrication 72 also includes a step 7212 of forming the diffraction grating 32 on the waveguide 31. The diffraction grating 32 can be formed 7212 on the waveguide 31 when the latter is formed on a substrate different from substrate 2. Thus, the assembly forming the device 3 is cut and deposited (and bonded) onto the window 22. Alternatively, when the layers composing the waveguide 31 are deposited on the window 22, the grating 32 can be formed by depositing a metallic layer on the second InP semiconductor layer. A first etching of the metallic layer forms the diffraction grating 32. A second, deeper etching defines the perimeter of the diffraction grating 32 and the waveguide 31, thus forming the device 3.

[0109] In the embodiment of [Fig. 7b], the fabrication 72 of the device 3 first comprises a step 7221 of forming the diffraction grating 32. The diffraction grating 32 is, for example, formed on a different substrate and then transferred to the window 22. Advantageously, the diffraction grating 32 is formed directly on the window 22. For example, a metallic layer is deposited on the window 22 and etched to form the grating 32. Alternatively, a grid of parallel trenches can be etched into the window 22 and filled with a material metallic or semiconducting (but, in the latter case, different from the semiconductor material of window 22).

[0110] In this embodiment, the fabrication 72 of the device 3 then includes the formation 7222 of the waveguide 31 on the diffraction grating 32. The waveguide 31 is, for example, formed on a different substrate and then transferred to the window 22 and the grating 32. Alternatively, the layers composing the waveguide 31 can be deposited directly onto the grating 32. For example, the formation 7222 of the waveguide 31 includes a molecular bonding step of a first semiconductor layer, for example of InP, onto the upper face 2a of the substrate 2 and thus onto the diffraction grating 32. Then, a stack of semiconductor sublayers is formed on said first semiconductor layer of InP to allow the formation of the active region 310. A second semiconductor layer, for example of InP, is formed on the stack of semiconductor sublayers.Finally, etching the first and second InP semiconductor layers and the stack of semiconductor sublayers allows the waveguide 31 to be delimited, directly above the lattice 32.

[0111] When the embodiments of [Fig. 7a] and [Fig. 7b] involve transferring the waveguide 3 onto the window 22 or onto the grating 32, the waveguide 3 can be bonded. This can be done either by molecular bonding or by means of a polymerizable adhesive. In the latter case, a portion of adhesive is deposited on the window 22 (or the diffraction grating 32), and the device 3 is then placed and pressed against this portion of adhesive.

Claims

1.

2. Demands Photoacoustic system (1) comprising: a semiconductor substrate (2), the semiconductor substrate (2) comprising: a face (2a) referred to as the "upper face"; and an internal wall delimiting a hollow volume (21a), the hollow volume (21a) extending within the semiconductor substrate (2) at a distance from the upper face (2a) of said semiconductor substrate (2), the photoacoustic system (1) being characterized in that: - the semiconductor substrate (2) comprises at least one portion (22a), referred to as the "window", transparent to an electromagnetic field, extending vertically above at least a portion of the hollow volume (21a) and from the upper face (2a) of the semiconductor substrate (2) to the hollow volume (21a); ​​and in that - the photoacoustic system (1) comprises at least one surface-emitting device (3) comprising an emitting face (3b), said emitting face (3b) extending directly into contact with said at least one window (22a), said at least one device comprising: - a waveguide (31) comprising a first face (31a) and a second face (31b), opposite the first face, the waveguide comprising an active region (310) configured to emit the electromagnetic field; and - a diffraction grating (32) having, along a first direction (X) parallel to the emitting face (3b) of said device (3), a diffraction order greater than or equal to two, - said diffraction grating (32) extending over the first face (31a) of the waveguide (31), the emitting face (3b) coinciding with the second face (31b) of the waveguide (31); or said diffraction grating (32) extending over the second face (31b) of the waveguide (31), the emitting face (3b) coinciding with the diffraction grating (32). System (1) according to claim 1, characterized in that the diffraction grating (32) of said at least one device (3) extends on the first face (31a) of the waveguide (31) of said at least one device (3) and in that said diffraction grating (32) is reflective.

3. System (1) according to the preceding claim, characterized in that the diffraction grating (32) of said at least one device (3) comprises a periodic structure (321) and a metallic layer (322), the periodic structure (321) extending over the first face (31a) of the waveguide (31) and the metallic layer (322) extending over the periodic structure (321).

4. System (1) according to claim 1, characterized in that the diffraction grating (32) of said at least one device (3) extends over the second face (31b) of the waveguide (31) of said at least one device (3) and in that said diffraction grating (32) is non-reflective.

5. System (1) according to any one of the preceding claims, characterized in that the diffraction grating (32) of said at least one device (3) extends vertically above the active region (310) of the waveguide (31) of said at least one device (3), the active region (310) having a length, measured along the first direction (X), and the diffraction grating of said at least one device (3) having a length, also measured along the first direction, the length of said diffraction grating being substantially equal to the length of said active region, said grating having, along the first direction, a unique diffraction order.

6. System (1) according to the preceding claim, characterized in that the diffraction grating of said at least one device (3) has, along the first direction, a diffraction order greater than or equal to three.

7. System (1) according to any one of the preceding claims, characterized in that the active region (310) of said at least one device (3) has a width, measured along a second direction (Y), parallel to the emitting face (3b) of said at least one device (3) and perpendicular to the first direction (X), and the diffraction grating (32) of said at least one device (3) has a width, measured along the second direction (Y), the width of said diffraction grating (32) being substantially equal to the width of said active region (310), said diffraction grating (32) having, along the second direction (Y), another diffraction order, unique and greater than or equal to two.

8. System (1) according to any one of the preceding claims, characterized in that the waveguide of said at least one device (3) is delimited by a first flank and a second flank, the second flank being opposite the first flank, the first and second flanks being substantially perpendicular to the second face (31b), said device also comprising a first conductive layer and a second conductive layer, the first conductive layer extending over the first flank and the second conductive layer extending over the second flank.

9. Photoacoustic system (1) according to any one of the preceding claims, characterized in that said at least one surface emission device (3) is glued to the window (22) and preferably by molecular bonding or by means of a polymerizable glue.

10. Photoacoustic system (1) according to any one of the preceding claims, characterized in that said at least one surface-emitting device (3) is configured to emit infrared electromagnetic radiation from its emitting face (3b) with an angle (a), measured with respect to the normal to said emitting face (3b), of between 0° and 60°.

11. Photoacoustic system (1) according to any one of the preceding claims, characterized in that the semiconductor substrate (2) comprises a plurality of windows and the photoacoustic system (1) comprises a plurality of surface-emitting devices, each surface-emitting device of the plurality of surface-emitting devices being disposed on a window of the plurality of windows.

12. Photoacoustic system (1) according to any one of the preceding claims, characterized in that the electromagnetic radiation comprises a wavelength within [0.8 pm; 20 pm].

13. Photoacoustic system (1) according to any one of the preceding claims, characterized in that the photoacoustic system (1) comprises a thermal regulator (4) configured to regulate the temperature of said at least one surface-emitting device (3).

14. Method (7) of manufacturing a photoacoustic system (1) comprising, from a semiconductor substrate (2) comprising a face (2a) referred to as the "top face": - form (71) an internal wall delimiting a hollow volume (21a), the hollow volume (21a) extending in the semiconductor substrate (2) at a distance from the upper face (2a) of said semiconductor substrate (2) so that the semiconductor substrate (2) includes at least one portion (22a), called a "window", transparent to an electromagnetic field, extending vertically from at least a part of the hollow volume (21a) and from the upper face (2a) of the semiconductor substrate (2) to the hollow volume (21a); - manufacture (72) at least one surface-emitting device (3) extending directly into contact with said window (22a), said at least one device comprising: - a waveguide (31) comprising a first face (31a) and a second face (31b), opposite the first face, the second face (31b) resting on said at least one window (22a), the waveguide comprising an active region (310) parallel to the second face (31b) configured to emit the electromagnetic field; and - a diffraction grating (32) having a diffraction order greater than or equal to two and extending over the first face (31a) of the waveguide (31) or over the second face (31b) of the waveguide (31).

15. A manufacturing method according to claim 14, wherein the manufacture (72) of said at least one surface-emitting device (3) comprises: - form (7211) the waveguide (31) of said at least one device (3) on said at least one window (22) of the semiconductor substrate (2); and - form (7212) the diffraction grating (32) of said at least one device (3) on the first face (31a) of said waveguide (31).

16. A manufacturing method according to claim 14, wherein the manufacture (72) of said at least one surface emission device (3) comprises: - forming (7221) the diffraction grating (32) of said at least one device (3) on said at least one window (22) of the semiconductor substrate (2); and - forming (7222) the waveguide (31) of said at least one device (3) on said diffraction grating (32).

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