SURFACE EMISSION DEVICE, OPTICAL SYSTEM AND ASSOCIATED METHOD

Abstract

SURFACE EMISSION DEVICE, OPTICAL SYSTEM AND ASSOCIATED METHOD One aspect of the invention relates to a surface emission device (1) comprising: a waveguide (11), including a first face (11a) and a second face (11b) and an active region (110); and a diffraction grating (12) extending over the first face (11a) of the waveguide (11), the device (1) being notable in that the diffraction grating (12) is reflective and has an order greater than or equal to two, and in that the second face (11b) is transparent. Figure to be published with the abstract: Figure 1

Description

Title of the invention: SURFACE EMISSION DEVICE, OPTICAL SYSTEM AND ASSOCIATED METHOD TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of the invention is that of surface-emitting electromagnetic radiation sources, such as surface-emitting lasers. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Surface-emitting devices emit electromagnetic radiation, for example a laser beam, in a direction substantially perpendicular to the plane over which they extend. This beam can exhibit good spatial uniformity, particularly in the near field. Surface-emitting devices are therefore advantageously integrated into optical systems.

[0003] The document [“Surface-emitting 10.1 pm quantum cascade distributed feedback lasers” D. Hofstetter et al., Appl. Phys. Lett. 75, 3769-3771 (1999)] discloses a surface-emitting device. The device comprises, from an InP substrate, a stack comprising: • two InGaAs layers, framing a semiconductor multilayer, the whole constituting a waveguide, the semiconductor multilayer being described as an "active region", promoting the emission of an electromagnetic field (in this case by quantum cascade); and • a second-order transmission diffraction grating extending over the free face of the waveguide.

[0004] The second-order transmission diffraction grating favors the emission of a normal component of the electromagnetic field, which is then emitted by the free face of the waveguide, including the diffraction grating. This is why this type of device is called a "surface emission" device. Devices based on a second-order transmission diffraction grating, however, exhibit a problem with the stability of the guided modes, for example, as a function of temperature.

[0005] The document [“Experiment demonstration of high speed 1.3 pm grating assisted surface-emitting DFB lasers” J. Luan et al., Optics Express 25111 Vol. 30, No. 14 (Jul. 2022)] solves the stability problem by means of a transmission diffraction grating comprising three distinct portions of different orders (a first portion of second order flanked by two portions of first order). The first-order portions confine a guided mode of the electromagnetic field by distributed negative feedback, thus forming a laser cavity. The second-order portion performs surface emission of a portion of the guided mode of in a normal manner with respect to the diffraction grating.

[0006] The document [“High power surface emitting terahertz laser with hybrid second- and fourth-order Bragg gratings” Y. Jin et al., Nat. Commun. 9, 1407 (2018)] discloses a device that differs from the previous one in that the transmission diffraction grating is said to be “hybrid” because it has two different orders superimposed on each other. In this case, these are the second and fourth orders.

[0007] Surface emission devices, however, present new challenges. For example, their integration into an existing measurement system is not straightforward. It requires, for instance, the use of mirrors to direct the beam towards the measurement system. There is a need to provide a surface emission device whose integration into a measurement system can be simple. Summary of the invention

[0008] To this end, the invention relates to a surface-emitting device of an electromagnetic field comprising: • a waveguide, extending in a plane and comprising a first face and a second face, opposite the first face, the first and second faces being parallel to the plane, the waveguide comprising an active region extending in the plane and being configured to emit the electromagnetic field; and • a diffraction grating extending over the first face of the waveguide, the device being remarkable in that the diffraction grating is reflective and has, along a first direction parallel to the plane, a diffraction order greater than or equal to two and in that the second face is transparent.

[0009] The active region allows the emission of a field that can propagate through the waveguide. The diffraction grating, interacting with the electromagnetic field (which will also be simply called the "field"), can promote, depending on its diffraction pattern, the appearance of: • a counter-propagating component of the field propagating in the first direction within the waveguide; and / or • a component of the field propagating in an out-of-plane direction.

[0010] The counter-propagating component of the field can be induced by a distributed feedback effect of the diffraction grating on the field. This counter-propagating component allows the establishment of stationary modes of the field in the waveguide.

[0011] By direction out of plane, we mean a direction having an angle greater than 20° with respect to the plane in which the waveguide extends, or even an angle greater than 45° with respect to said plane, or even more preferably an angle greater than 80° in relation to said plan.

[0012] A diffraction order of two primarily favors the out-of-plane component. A diffraction order greater than two favors the counter-propagating component while still maintaining an out-of-plane component. In both cases, emission can be carried out in an out-of-plane direction. However, unlike devices described in the prior art, the diffraction grating reflects the field, which is then emitted only by the second face of the waveguide. Considering the first face as a "top" face and the second face as a "bottom" face, the device according to the invention can therefore be called a "bottom-surface emission device."

[0013] By reflective diffraction grating, it is understood that the field is reflected by more than 50% by the diffraction grating, or even more than 70%, or preferably, more than 90%. By transparent face, it is understood that the optical power is transmitted more than 50% by the second face. Thus, a substantial portion of the optical power generated by the device is transmitted by the second face.

[0014] The injection of the electromagnetic field into an optical system (such as a measuring system) can therefore be carried out via the second face (the "lower" face). A surface emission device according to the invention is thus easily integrated into an optical system because it can be deposited directly onto the optical system (the second face being, for example, glued onto an optical window) or fabricated directly onto the optical system (for example, directly onto an optical window).

[0015] Advantageously, the waveguide 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 a second direction, said second direction being parallel to the plane and parallel to the first direction, the device also comprising a first conductive coating and a second conductive coating, the first conductive coating extending over the first flank and the second conductive coating extending over the second flank. By substantially perpendicular or parallel, we mean respectively perpendicular or parallel to within + / - 20°, or even + / - 10°. By conductive layer, we mean, for example, a metallic layer.

[0016] The first and second conductive layers enhance the confinement of the electromagnetic field in the waveguide along the second direction by forming a cavity with high reflectivity in that direction. The diffraction grating can then also serve as a spectral filter for guided modes propagating in the second direction within the waveguide.

[0017] Preferably, the device comprises at least one electrically insulating spacer disposed between the first conductive layer and the first flank and between the The second conductive layer and the second side. Thus, the different layers of the waveguide are not short-circuited by the conductive layers.

[0018] Advantageously, the diffraction grating extends vertically above the active region of the waveguide, the active region having a length, measured along the first direction, and the diffraction grating having a length, also measured along the first direction, the length of the diffraction grating being substantially equal to the length of the active region, the grating having, along the first direction, a single diffraction order. By substantially equal, we mean equal to + / - 20%, or even equal to + / - 10%. By single diffraction order, we mean, for example, that the diffraction grating has a constant pitch (or period) to + / - 10%. This is not, for example, a structure having different portions along the first direction or different pitches superimposed on one another, as disclosed in the document [“High power surface emitting terahertz laser with hybrid second- and fourth-order Bragg gratings” Y. Jin et al., Nat. Commun.9, 1407 (2018)]. .

[0019] Advantageously, the active region has a width, measured along a second direction parallel to the plane and perpendicular to the first direction, and the diffraction grating has a width, measured along the second direction, the width of the diffraction grating being substantially equal to the width of the active region, the grating having, along the second direction, another unique diffraction order greater than or equal to two.

[0020] Advantageously, the diffraction grating exhibits, along the first direction, a diffraction order greater than or equal to three. A diffraction order of two allows for predominantly the out-of-plane component. The spectrum of the field emitted by the second face can therefore be broad or have a multimode signature. A diffraction order greater than two allows for the preference of the counter-propagating component while retaining an out-of-plane component. Only wavelengths corresponding to the stationary modes can be emitted, and the spectrum of the field emitted by the second face is narrow and monochromatic.

[0021] Preferably, the active region emits the field by spontaneous emission and / or stimulated emission. When the field is emitted by both spontaneous and stimulated emission, the device can operate in laser mode. Advantageously, the active region implements field emission by inter-band or intra-band emission. Preferably, the active region is configured to perform a quantum cascade.

[0022] Advantageously, the electromagnetic field comprises a wavelength in [0.8 pm; 20 pm] and preferably in [4 pm; 12 pm].

[0023] Advantageously, the diffraction grating 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. The metallic layer The vertical axis prevents the transmission of the field through the diffraction grating, which therefore becomes purely reflective. It thus allows the diffracted field to be redirected towards the second face (in other words, through the lower face).

[0024] Advantageously, the diffraction grating has a thickness, measured perpendicular to the plane, greater than 2 / (10 neff) and preferably greater than 2 / (4 n^), where 2 is a wavelength of the electromagnetic field emitted by the active region, and neff is an effective index of a guided mode in the waveguide, interacting with the diffraction grating. The thickness of the diffraction grating also allows control of the effective index of the guided mode. It can therefore allow control of the diffraction orders of the guided mode with the diffraction grating.

[0025] Advantageously, the periodic structure comprises alternating first and second portions, the first portions being made of a first semiconductor material having a first refractive index and the second portions being made of a metal or a second material, such as another semiconductor material or a gas, such as air, having a second refractive index different from the first. The freedom in choosing the second material makes it possible to select a material that is easily integrated into the technological processes for manufacturing the array.

[0026] The first portions of the periodic structure are, for example, lines or dots.

[0027] Advantageously, the metallic layer comprises portions, each portion of the metallic layer covering one of the first portions of the periodic structure.

[0028] Alternatively, the metallic layer is continuous.

[0029] Advantageously, the waveguide comprises a first semiconductor layer based on the first semiconductor material constituting the first portions of the periodic structure, the first semiconductor layer extending from the active region to the first face. Preferably, the first semiconductor layer is doped.

[0030] Advantageously, the waveguide of the device includes a second semiconductor layer extending from the active region of the waveguide to the second face of the waveguide, the thickness of the second semiconductor layer, measured perpendicular to the plane, is between 2 pm and 100 pm and preferably between 2 pm and 40 pm.

[0031] The invention also relates to an optical system, characterized in that it comprises a surface emission device of an electromagnetic field according to the invention and an optical window transparent to the electromagnetic field, the second face of the waveguide of the surface emission device extending over the optical window.

[0032] Advantageously, the system also includes a thermal regulator configured to regulate the temperature of the device's waveguide when the device is operating in a steady state.

[0033] Preferably, the thermal regulator is configured to regulate the temperature of the waveguide to within 0.1 °C.

[0034] Alternatively, the thermal regulator is configured so that the temporal variation of the system temperature is less than 0.1 °C / s during the operation of the device.

[0035] The invention further relates to a method for manufacturing the surface emission device according to the invention, the method comprising the following steps: • form a waveguide, extending in a plane and comprising a first face and a second face, opposite to the first face, the first and second faces being parallel to the plane, the waveguide comprising an active region extending in the plane and configured to emit an electromagnetic field, the second face being transparent; • form a diffraction grating extending over the first face of the waveguide, the diffraction grating being reflective and exhibiting, along a first direction parallel to the plane, a diffraction order greater than or equal to two.

[0036] Advantageously, the method includes, prior to the formation of the diffraction grating, a step of optimizing the coupling ratio of the diffraction grating with the electromagnetic field emitted by the active region. The opto-geometric parameters of the grating (such as its depth and / or width and / or length) are numerically optimized to obtain high reflectivity and an optimal coupling ratio from the lower surface. This optimization thus makes it possible to couple a maximum of optical power in the optical system.

[0037] 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

[0038] 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.

[0039] [Fig.1] shows, according to a cross-sectional view, an embodiment of a surface emission device according to the invention.

[0040] [Fig.2] shows a first result of a numerical simulation carried out from the device of the [Fig.l].

[0041] [Fig.3] shows a second result of a numerical simulation carried out from the device of the [Fig.l].

[0042] [Fig.4] shows a third result from a numerical simulation carried out using the device of the [Fig.l].

[0043] [Fig.5] schematically shows an optical system. DETAILED DESCRIPTION

[0044] Figure 1 shows, in cross-section, an embodiment of a surface emission device 1 according to the invention. Figure 1 also shows a magnified view of a portion 1' of said device 1. In this embodiment, the device 1 comprises: a waveguide 11 and a diffraction grating 12.

[0045] The waveguide 11 extends in a plane P. This is a plane in which a first direction X and a second direction Y extend. The waveguide 11 comprises a first face 1a and a second face 11b, opposite the first face 11b. The first face 1 can also be called the "upper face" and the second face 11b the "lower face". The first and second faces 1a, 11b extend parallel to the plane P. The first face 1a is, for example, oriented along a third direction Z, perpendicular to the plane P, and the second face 11b is oriented along the same direction but in the opposite direction.

[0046] The waveguide 11 also includes an active region 110. The active region 110 is a portion of the waveguide 11 configured to emit an electromagnetic field (simply referred to as the "field"). The active region 110 is, for example, configured for spontaneous and / or stimulated emission. In the latter case, the active region could also be called the "amplifying medium" because it enables the device 1 to operate in laser mode. The active region 110 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 110 is configured to perform both spontaneous and stimulated emission.

[0047] The active region may have a thickness Wno, measured perpendicular to the plane P, which may be between 1 pm and 5 pm and preferably between 1.5 pm and 2.5 pm.

[0048] The active region 110 is preferably framed by two semiconductor layers 111, 112, extending parallel to the plane P. A first semiconductor layer 111, which may be called the "top cladding," extends over the active region 110 and preferably directly against it. In this embodiment, the face of the top cladding 111 opposite the active region 110 is advantageously the first face 11a. A second semiconductor layer 112, which may be called the "top cladding," lower » or “bottom cladding” in English, also extends over the active region 110 and preferentially against the active region 110. The face of the lower coating 112 which is opposite the active region 110 is then advantageously the second face 11b.

[0049] The upper and lower coatings 111, 112 can allow the field to be guided in the waveguide 11. For this purpose, they advantageously have optical indices (also called refractive indices) strictly lower than the average optical index of the active region 110.

[0050] The upper coating 111 has a thickness, measured perpendicular to plane P, of between 1 pm and 2 pm. The lower coating 112 has a thickness, measured perpendicular to plane P, of between 2 pm and 100 pm and preferably between 2 pm and 40 pm.

[0051] The lower coating 112 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 112 is, for example, formed from InP. The lower coating 112 can also be doped, for example, with n-type doping, that is, with impurities acting as electron donors. The lower coating 112 is, for example, doped with sulfur.

[0052] The upper coating 111 can be, in the same way as the lower coating 112, a type IILV semiconductor heterostructure, such as InP. Similarly, the upper coating 111 can be doped, for example with type n.

[0053] In the embodiment of [Fig. 1], the waveguide 11 is therefore a stack of layers 110, 111, 112 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 Lu, a width, and a thickness Wn. The length of the waveguide 11 is, for example, measured along the first direction X. The width of the waveguide 11 is, for example, measured along the second direction Y, perpendicular to the first direction X. The thickness Wn of the waveguide 11 is, for example, measured along the third direction Z, perpendicular to the two aforementioned directions X and Y.

[0054] The length Lu of the waveguide 11 can be between 1000 pm and 5000 pm. In one embodiment, the length Lu and the width of the waveguide 11 can be equal, for example, to within 10%. Thus, viewed from above, the first face 1la can have a square shape. Alternatively, the length Lu of the waveguide 11 can be greater than the width of the waveguide 11 and, for example, greater than twice the width of the waveguide 11, 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 (not shown in the figure) can be between 10 pm and 50 pm.

[0055] The active region 110 has a length Ln0, measured along the first direction X, substantially equal to the length Lu of the waveguide 11. Similarly, the active region 110 has a width, measured along the second direction Y, substantially equal to the width of the waveguide IL. By substantially equal, we mean equal to within 20%, or even 10%. Flanks perpendicular to the plane P, for example, delimit the waveguide 11 and the active region 110.

[0056] The diffraction grating 12 extends over the first face 11 of the waveguide 11. In the embodiment of [Fig.1], the diffraction grating 12 extends over the upper coating 111. The diffraction grating is advantageously arranged vertically above the active region 110 of the waveguide 11 and preferably centered with respect to the latter.

[0057] The diffraction grating 12 can have a length Ln and a width. The length of the diffraction grating 12 is, for example, measured along the first direction X. The width of the diffraction grating 12 is, for example, measured along the second direction Y, perpendicular to the first direction X. The length Li2 and the width of the diffraction grating 12 are advantageously chosen so that the diffraction grating 12 completely covers the active region 110. In other words, the lengths and widths of the diffraction grating 12 are, respectively, substantially equal to the lengths and widths of the active region 110. In this way, the diffraction grating 12 can be coupled homogeneously to the field emitted by the active region 110. It makes it possible, for example, to provide homogeneously distributed negative feedback over the entire length Ln0 of the active region 110.

[0058] Device 1 is remarkable in that the second face 11b of the waveguide is transparent to the field which can be emitted by the active region 110. By transparent, we mean that the face 11b has a spectral transmission window and that this spectral transmission window corresponds to at least a part of the spectrum of the field which can be emitted by the active region 110.

[0059] Device 1 is also notable in that the diffraction grating 12 is reflective and has a diffraction order, along the first direction X, greater than or equal to two. By reflective, it is understood that at least 50% of the field is reflected by the diffraction grating 12. The diffraction order greater than or equal to two along the direction X implies that the field propagating along the first direction X couples to the diffraction grating 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 12 reflects the component of the field propagating along +Z, this component of the field is therefore oriented towards the second face 11b. Device 1 can thus perform emission from the second surface 11b.

[0060] Advantageously, the diffraction grating 12 comprises a periodic structure 121. The periodic structure 121 is, for example, a layer extending over the first face 11 of the waveguide 11. Coupled with the field emitted by the active region 110, it induces the diffraction effects. The coupling ratio between the field and the diffraction grating 12 is advantageously between 10 cm⁻¹ and 100 cm⁻¹.

[0061] The periodic structure 121 includes, for example, first portions 1211 and second portions 1212. These first and second portions 1211, 1212 are arranged periodically and form an alternation along the first direction X and along the second direction Y where applicable.

[0062] For example, the first and second portions 1211, 1212 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. 1] illustrates this example.

[0063] According to another example, the first portions 1211 are plots arranged in a rectangular grid. The second portions 1212 are tori, surrounding each plot 1211 and filling the space between the plots 1211. According to a variant of this example, the plots 1211 may have a large length along the first X direction and a small length along the second Y direction, or vice versa.

[0064] The first portions 1211 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 111. The second portions 1212 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.

[0065] The diffraction grating 12 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 1211 are arranged is constant over the entire length Ln of the grating 12. The first portions 1211 then advantageously all have the same width A^n, measured along the first direction X. The second portions 1212, separating the first portions 1211, can also have the same width A1212, also measured along the first direction X. The arrangement period A of the first portions 1211, which corresponds to the period of the diffraction grating 12 along the first direction X, is therefore equal to Ann+Ann-. A unique diffraction order means that the period A is constant over the entire length Li2 of the diffraction grating.

[0066] The active region 110 is preferably configured to emit in the range Infrared, that is, 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 a micrometer. A diffraction grating 12 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).

[0067] The diffraction grating 12 also includes a metallic layer 122 that extends over the periodic structure 121 and preferably over the entire periodic structure 121. It is, for example, made of Ti or Au. The reflective effect of the diffraction grating 12 can also be provided by the metallic layer 122 extending over the periodic structure 121. The metallic layer can prevent the transmission of the field through the diffraction grating 12. In particular, the metallic layer 12 extends over each first portion 1211 of the periodic structure 121. In this way, it prevents the transmission of the field through the first portions 1211.

[0068] The embodiment of [Fig. 1] illustrates a continuous metallic layer 122 extending along the first direction X. Alternatively, the metallic layer 122 can be discontinuous and comprise a plurality of portions. Each portion thus advantageously extends over each first portion 1211 of the periodic structure 1211. If, for example, the first portions 1211 of the periodic structure 121 are aligned along the second direction Y, then the metallic layer 122 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 1211.

[0069] The metallic layer 122 can also extend beyond the periodic structure 121 because it can also be used as a contact electrode of the device 1. It can allow an electric field to be applied uniformly to the active region 110 in order to inject carriers into the active region 110. The carriers can de-excite by emitting photons into the active region 110.

[0070] The metallic layer 122 also makes it possible to improve the confinement of the field in the waveguide 11 thanks to the plasmonic interaction of the field with the metal.

[0071] According to one embodiment, the diffraction grating 12 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 1211 of the periodic structure 121 are plots arranged in a rectangular mesh. This embodiment is advantageous when the width and length of the active region (and therefore of the diffraction grating) are approximately equal, for example within 20%. On the other hand, if the length of the active region 110 is much greater than its width, for example 20 times greater than its width (ridge-type waveguide), it is preferable for the diffraction grating 12 to exhibit order only along the first X direction and no order along the second Y direction. In other words, the first portions 1211 of the periodic structure 121 can be lines aligned along the second Y direction and distributed along the first X direction.

[0072] If necessary, the diffraction grating 12 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.

[0073] Lateral field confinement can also be achieved by conductive layers extending over the sides of the waveguide 11. The waveguide 11 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 1 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 direction of propagation and allow the field to be confined along the second Y direction.

[0074] To prevent a short circuit within the waveguide (for example, a short circuit between the upper coating 111 and the lower coating 112), the device 1 advantageously comprises at least one electrically insulating spacer. This is, for example, a dielectric layer. Each spacer is, for example, arranged on the lateral sides, between the conductive layers and the lateral sides.

[0075] The device 1 may also include third and fourth conductive layers to contribute to field confinement along the first direction. These layers extend, for example, along a third and fourth flank, respectively, delimiting the waveguide 11 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. Each spacer is 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.

[0076] Figures [Fig. 2] and [Fig. 3] present a simulation result obtained from a Device 1 as described with reference to [Fig. 1]. The reference symbols for the different layers are shown in [Fig. 2] and correspond to the layers described previously. The upper and lower coatings 111, 112 and the active region 110 have the same refractive index n = 3.1. The diffraction grating 12 considered is of odd order m, and more specifically in this example, equal to three. The first portions 1211 of the periodic structure 121 have a refractive index n = 3.1, while the second portions 1212 of the periodic structure 122 are metallic, for example, Au. The field considered is monochromatic and its wavelength is 4.5 pm. The period A of the periodic structure 121 (with A = mA / 2neff where m is the diffraction order of the grating, neff the effective refractive index, and 2 is the wavelength considered) is A = 3.3 pm. The diffraction grating has a thickness W[2 = 400 nm.The duty cycle D of the network (calculated as D = A1211 / A) is equal to 0.5.

[0077] To facilitate the interpretation of the results, the simulation considers only a wave train emitted by the active region 110 at X = 0 pm and propagating along increasing X with a positive wave vector kx. In reality, the emitted field propagates in both directions. The simulation is stopped when the wave train reaches 1450 pm (Fig. 2 is only a truncated portion of this 1450 pm). Three regions of space, which will be called "sensors," are considered to integrate the intensity of the electromagnetic field as a function of time. These are virtual sensors 31, 32, 33, which have no interaction with the propagation of time in device 1.

[0078] Figure 2 shows, in greyscale, the amplitude E of the electromagnetic field in the different layers at the end of the simulation. The field E is in arbitrary units. Figure 3 shows the intensity as a function of time measured at the three sensors 31, 32, 33.

[0079] In [Fig. 2], the propagation of a wave train along a wave vector opposite to the initial wave train +kx is observed. This is the wave resulting from the distributed feedback of the diffraction grating 12 on the first wave train (along +kx). A wave propagating with a wave vector -kz perpendicular to the initial wave vector kx and in the direction of decreasing Z is also observed. Less visibly, a wave propagating with a wave vector +kz perpendicular to the initial wave vector kx and in the direction of increasing Z is also observed. However, this latter wave is less obvious to observe since it is reflected by the diffraction grating 12 and thus redirected in -kz. In the absence of the metallic layer 122 on the periodic structure 121, the wave train propagating along +kz would be more obvious.It would propagate in the same way as the wave train propagating along -kz (with an adaptation of the period as well as the optical indices of the media it would pass through, such as air for example, extending in place of said metallic layer).

[0080] The presence of the metallic layer 122 reflects the radiation propagating with a +kz vector, which means that the device 1 shown emits only from the lower surface 11b. In the absence of the metallic layer 122, the device 1 would also emit a field along +kz. The device modeled in [Fig. 2] is therefore particularly suitable for emitting a field, such as a laser beam, from the lower surface 11b of the lower coating 11.

[0081] Figure 3 shows that the intensity L of the E field as a function of time is very high at the first sensor 31, as soon as the wave train propagating along +kx reaches this sensor 31. The intensity I32 of the E field, measured by the second sensor 32, is, on the other hand, lower, because it is the result of the distributed negative feedback of the diffraction grating 12. The intensity could, however, be sufficient to maintain a standing wave in the device 1 and activate the stimulated emission in the active region 110.

[0082] The intensity I33 measured by the third sensor 33 increases over time and as the initial wave train (propagating along +kx) moves. The transient regime illustrated in [Fig. 3] ends after a few picoseconds.

[0083] The operating and structural conditions of the simulation were chosen to highlight the propagation of the field towards the lower face 11b. An adjustment of the parameters of the diffraction grating 11 could increase the negative feedback of the grating 12 on the field propagating in the guide 11. It could also decrease the coupling ratio of the field, while ensuring that most of the optical power carried by the field would be directed towards the lower face 11b (and then towards a point of interest).

[0084] Figure 4 shows complementary results from a simulation carried out using the same numerical model as that used to obtain the results of Figures 2 and 3. This numerical model differs, however, from the previous model in that it varies the coupling ratio of the diffraction grating 12 with the field E propagating in the device 1. The parameter studied is more precisely the duty cycle D of the diffraction grating 12. Indeed, the coupling ratio depends on several parameters such as the duty cycle of the grating 12, the thickness Wi2i of the periodic structure 121, or the refractive index of the materials constituting the periodic structure 121. In Figure 4, the duty cycles considered are D = 0.5 (identical to that of Figures 2 and 3) and D = 0.35.We observe that a duty cycle D < 0.5 tends to increase the intensities I32 and I33 measured by the second and third sensors 32, 33 (which correspond to the intensities of the wave trains propagating along -kx and -kz). The intensity I31 measured by the first sensor 31 is, on the other hand, reduced, because the power it carries dissipates towards the other propagating waves (in this case along -kx and -kY).

[0085] A simulation carried out using the same numerical model shows that the intensity of the field propagating normally to the plane of the grating 12 also depends on the thickness of the periodic structure 121. The greater its thickness Wm, the better the coupling between the grating 12 and the field E. The diffraction grating 12 has a thickness preferably greater than 2 / 10 (where 2 is a wavelength of the field E and neff is the effective index of the guided mode considered) and even more preferably, greater than 2 / 4rzeff.

[0086] A manufacturing process for the device 1 may include, firstly, a waveguide formation step 11. The waveguide 11 may be formed from an optical system, for example, on an optical window. The fabrication of the waveguide 11 includes, for example, the formation of the lower coating 112, followed by the formation of the active region 110, followed by the formation of the upper coating 111. These layers are, for example, produced by molecular beam epitaxy or chemical vapor deposition using metal-organic precursors (MOCVD). An etching step may be carried out to define the waveguide 11.

[0087] The lower coating layer 112, for example made of InP, can also be produced in a preliminary step and then bonded to a substrate, for example made of Si, or an optical window, for example made of Si, by molecular bonding. The other layers 110, 111 are then formed from the lower coating 111. Molecular bonding can provide a good surface finish on the second face 11b of the waveguide 11 and thus avoids the need for an anti-reflective coating.

[0088] The manufacturing process for the device 1 may include, following the formation of the waveguide 11, the formation of the diffraction grating 12 on the first face 1la of the waveguide 11. The periodic structure 121 may be fabricated on the first face 1la of the waveguide 11, for example, on the upper coating 111. Advantageously, the periodic structure 121 may be etched directly into the upper coating 111, for example, by forming a series of parallel trenches distributed along the first X direction. The metallic layer 122, deposited subsequently, fills these metal trenches, for example. The upper coating 111 is then slightly thicker to account for the final thickness W121 of the periodic structure 121.

[0089] Alternatively, another semiconductor material can be deposited in these trenches. After a planarization step, the metallic layer 122 is deposited on the periodic structure 121.

[0090] According to another alternative, the metallic layer 122 can be deposited directly onto the upper coating 111 and the trenches are then made in the re upper layer 111, through the metallic layer 122. Thus, the first portions 1211 of the periodic structure 121 are formed by portions of the upper layer 111 arranged beneath the portions of the metallic layer 122 and framed by two trenches. The portions of the metallic layer arranged on the first portions 1211 of the periodic structure 121 block the transmission of the field through the grating 12 and redirect the laser beam to the lower face 11b (or a point of interest or an optical system if applicable).

[0091] The parameters of the diffraction grating 12, such as the thickness Wi2i of the periodic structure 121, the materials constituting the first and second portions 1211, 1212 of the periodic structure 121, or the duty cycle D of the grating, can influence the coupling between the diffraction grating 12 and the field emitted by the active region 110. The manufacturing process then advantageously includes a step of optimizing the coupling ratio of the diffraction grating 12 with the field emitted by the active region 110. This optimization step preferably takes place before the formation of the diffraction grating 12. The expected coupling ratio is, for example, between 10 cm⁻¹ and 100 cm⁻¹.

[0092] Figure 5 schematically represents an optical system 2. The optical system 2 comprises, for example, a fiber or a resonant cavity. The injection of a light beam E into the fiber or cavity can be used to measure a parameter of interest.

[0093] The system 2 comprises a surface-emitting device 1 as described above. It also comprises an optical window 20 transparent to the light beam E that can be emitted by the active region 110 of the device 1. In the embodiment of [Fig. 5], the optical window 20 is embedded in a support 21, which surrounds, for example, the optical window 20 and the device 1.

[0094] The device 1 is located on the optical window 20. The second face 11b of the waveguide 11 therefore extends over the optical window 20 and preferably directly against the optical window 20. Indeed, the device 1, carrying out the emission by its second face 11b, does not require an anti-reflective layer to efficiently transmit the radiation to the optical window 20. The second face 11b of the device 1 can also be glued against the window 20 and preferably by molecular bonding.

[0095] The face of the optical window 20 intended to receive the second face 11b of the device is preferably flat. For example, when the device 1 is placed on the window, it extends parallel to the plane P of the device 1.

[0096] The optical window 20 is, for example, made of Si. It has a thickness W2o, measured perpendicular to the plane of the device 1, of between 10 pm and 50 pm. In order to limit optical losses, the sum of the thickness W2o of the optical window 20 and the thickness Wn2 of the lower coating 112, measured perpendicular to the plan P, is less than 100 pm.

[0097] The waveguide 11 is delimited by first and second flanks 1 le, 1 Id in the second direction Y. On each flank 1 le, 1 Id extends a conductive coating 131, 132 allowing to improve the lateral confinement of the field in the waveguide 11. Each flank 1 le, 1 Id is isolated from the conductive coating by means of an electrically insulating spacer 14, disposed between each conductive coating and the flank on which it extends.

[0098] In an advantageous embodiment, the system 2 also includes a thermal controller 22 configured to regulate the temperature of the waveguide 11 when the device 1 is operating, and in particular during steady-state operation. Surface-emitting devices are sensitive to temperature variations. It is therefore advantageous to maintain a constant temperature during the illumination of the sample 3, and preferably a variation of less than 0.1 °C. Since the illumination may last for a few seconds, the thermal controller 22 can be configured so that the temperature variation of the system 2 is less than 0.1 °C / s during the device's operation.

[0099] The embodiment of [Fig. 5] shows that the thermal regulator 22 comprises active cells 221, such as Peltier cells, and a heat sink 222, such as a finned heat sink. The cells 221 are, for example, in contact with the support 21, provided that the heat transfer offered by the support is sufficient. If, for example, it is made of silicon, the heat transfer is approximately 150 W / m / K, which may be sufficient. The heat sink 222 is in contact with the cells 221 so as to dissipate the heat drawn from the support 21.

Claims

Demands

1. Device (1) for surface emission of an electromagnetic field comprising: - a waveguide (11), extending in a plane (P), having a length, measured along a first direction parallel to the plane, and a width, measured along a second direction parallel to the plane and perpendicular to the first direction, the length of the waveguide being greater than the width of the waveguide, the waveguide comprising a first face (1la) and a second face (11b), opposite to the first face, the first and second faces being parallel to the plane, the second face being transparent, the waveguide comprising an active region (110) extending in the plane and configured to emit the electromagnetic field;and - a reflecting diffraction grating (12) extending over the first face of the waveguide, the device being characterized in that the diffraction grating has, along a first direction (X) parallel to the plane, a diffraction order greater than or equal to three.

2. Device (1) according to the preceding claim, characterized in that the waveguide (11) is delimited by a first flank (11e) and a second flank (11d), the second flank being opposite the first flank, the first and second flanks being substantially perpendicular to a second direction (Y), said second direction being parallel to the plane (P) and perpendicular to the first direction (X), the device also comprising a first conductive coating (131) and a second conductive coating (132), the first conductive coating extending over the first flank and the second conductive coating extending over the second flank.

3. Device (1) according to any one of the preceding claims, characterized in that the diffraction grating (12) extends vertically above the active region (110) of the waveguide (11), the active region having a length (Ln0), measured along the first direction (X), and the diffraction grating having a length (Li2), also measured along the first direction, the length of the diffraction grating being sen- possibly equal to the length of the active region, the grating exhibiting, along the first direction, a unique diffraction order.

4. Device (1) according to the preceding claim, characterized in that the active region (110) has a width, measured along a second direction (Y) parallel to the plane (P) and perpendicular to the first direction (X), and the diffraction grating (12) has a width, measured along the second direction, the width of the diffraction grating being substantially equal to the width of the active region, the grating having, along the second direction, another unique diffraction order greater than or equal to two.

5. Device (1) according to any one of the preceding claims, characterized in that the diffraction grating (12) comprises a periodic structure (121) and a metallic layer (122), the periodic structure extending over the first face (1la) of the waveguide (11) and the metallic layer extending over the periodic structure.

6. Device (1) according to claim 5, characterized in that the periodic structure (121) comprises an alternation of first portions (1211) and second portions (1212), 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.

7. Device (1) according to claim 6, characterized in that the metallic layer (122) comprises portions, each portion of the metallic layer covering one of the first portions (1211) of the periodic structure (121).

8. Device (1) according to claim 6, characterized in that the metallic layer (122) is continuous.

9. Device (1) according to any one of the three preceding claims, characterized in that the waveguide (11) comprises a first semiconductor layer (111) based on the first semiconductor material constituting the first portions (1211) of the periodic structure (121), the first semiconductor layer extending from the active region to the first face.

10. Device according to any one of the preceding claims, characterized in that the electromagnetic field comprises a wavelength within [0.8 pm; 20 pm].

11. Device according to any one of the preceding claims, characterized in that the waveguide (11) of the device (1) comprises a second semiconductor layer (112) extending from the active region (110) of the waveguide to the second face (11b) of the waveguide and in that the thickness (Wn2) of the second semiconductor layer, measured perpendicular to the plane (P), is between 2 pm and 100 pm.

12. Optical system (2), characterized in that it comprises a surface-emitting device (1) of an electromagnetic field according to any one of the preceding claims and an optical window (20) transparent to the electromagnetic field, the second face (11b) of the waveguide (11) of the surface-emitting device extending over the optical window.

13. System (2) according to the preceding claim, characterized in that it also comprises a thermal regulator (22) configured to regulate the temperature of the waveguide (11) of the device (1) when the device (1) is operating in a steady state.

14. A method for manufacturing a surface-emitting device (1) according to any one of claims 1 to 12, the method comprising the following steps: - forming a waveguide (11), extending in a plane (P), having a length, measured along a first direction parallel to the plane, and a width, measured along a second direction parallel to the plane and perpendicular to the first direction, such that the length of the waveguide is greater than the width of the waveguide, the waveguide comprising a first face (11a) and a second face (11b), opposite the first face, the first and second faces being parallel to the plane, the second face being transparent, the waveguide comprising an active region (110) extending in the plane and configured to emit an electromagnetic field, the second face being transparent - forming a diffraction grating (12) extending over the first face of the waveguide,the diffraction grating being reflective and exhibiting, along a first direction (X) parallel to the plane, a diffraction order greater than or equal to three.

15. Method according to the preceding claim, comprising, before the formation of the diffraction grating (12), a step of optimizing a coupling ratio of the diffraction grating with an electromagnetic field emitted by the active region (110).

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