WO2024115447A1

WO2024115447A1 - Surface-emitting device, optical system and associated method

Info

Publication number: WO2024115447A1
Application number: PCT/EP2023/083281
Authority: WO
Inventors: Badhise Ben Bakir; Olivier Lartigue; Christophe Constancias
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2022-11-29
Filing date: 2023-11-28
Publication date: 2024-06-06
Also published as: FR3142618A1

Abstract

According to one aspect, the invention relates to a surface-emitting device (1) comprising: a waveguide (11), comprising a first face (11a), 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 characterised 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.

Description

DESCRIPTION

TITLE: SURFACE EMITTING DEVICE, OPTICAL SYSTEM AND ASSOCIATED METHOD

TECHNICAL FIELD OF THE INVENTION

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

TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0002] Surface emission devices emit electromagnetic radiation, for example a laser beam, with a direction substantially perpendicular to the plane on which they extend. This beam can have 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 & 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 assembly constituting a waveguide, the semiconductor multilayer being described as an “active region”, promoting emission an electromagnetic field (in this case by quantum cascade); and a second-order transmission diffraction grating extending on the free face of the waveguide.

The diffraction grating in second-order transmission promotes the emission of a normal component of the electromagnetic field which is then emitted by the free face of the waveguide, comprising the diffraction grating. This is the reason why this type of device is called “surface emitting”. Devices based on a transmission diffraction grating of order two, on the other hand, show a problem of 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 using a diffraction grating in transmission comprising three distinct portions and presenting different orders (a first portion presenting an order two framed by two portions presenting an order one). The order one portions make it possible to confine a guided mode of the electromagnetic field by distributed feedback and thus form a laser cavity. The second order portion carries out surface emission of part of the guide mode normal to the plane of the diffraction grating.

[0006] The document [“High power surface emitting terahertz laser with hybrid second- and fourth-order Bragg gratings” Y. Jin & al., Nat. Common. 9, 1407 (2018)] discloses a device distinguishing itself from the previous one in that the transmission diffraction grating is called “hybrid” because it presents two different orders but superimposed on one another. These are orders two and four.

[0007] Surface emission devices, however, pose new problems. For example, their integration into an existing measurement system is not easy. It requires, for example, the use of mirrors to direct the beam towards the measuring system. There is a need to provide a surface emitting device whose integration into a measurement system can be simple.

SUMMARY OF THE INVENTION

[0008] For this, the invention relates to a device for surface emission 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, in 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 makes it possible to emit a field which can propagate in the waveguide. The diffraction grating, interacting with the electromagnetic field (which we will also simply call "field"), can promote, depending on its diffraction order, the appearance of: a counter-propagating component of the field propagating in the first direction in the waveguide; and/or a component of the field propagating in an out-of-plane direction.

The counter-propagating component of the field can be induced by a distributed counter-reaction 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 the plane, we mean a direction having an angle greater than 20° relative to the plane in which the waveguide extends, or even an angle greater than 45° relative to said plane, or even still preferred manner an angle greater than 80° relative to said plane.

[0012] A diffraction order equal to two mainly favors the out-of-plane component. A diffraction order greater than two makes it possible to favor the counter-propagating component while retaining an out-of-plane component. In both cases, the emission can be carried out in an out-of-plane direction. However, unlike the devices described in the prior art, the diffraction grating reflects the field which is then emitted only by the second face of the waveguide. By considering the first face as an “upper” face and the second face as a “lower” face, the device according to the invention can therefore be called a “lower surface emission device”.

[0013] By reflective diffraction grating, we mean 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, we mean that more than 50% of the optical power is transmitted by the second face. Thus, a substantial part of the optical power generated by the device is transmitted through 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 can therefore be easily integrated into an optical system because it can be deposited directly on the optical system (the second face being for example glued to an optical window) or manufactured directly on the optical system (for example example directly on 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 on the first sidewall and the second conductive coating extending on the second sidewall. 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.

The first and second conductive layers make it possible to reinforce the confinement of the electromagnetic field in the waveguide in the second direction by forming a cavity in the second direction with high reflectivity. The diffraction grating can then also serve as a spectral filter for the guided modes propagating in the second direction in the waveguide.

Preferably, the device comprises at least one electrically insulating spacer disposed between the first conductive layer and the first side and between 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 directly above the active region of the waveguide, the active region having a length, measured in the first direction, and the diffraction grating having a length, also measured according to the first direction, the length of the diffraction grating being substantially equal to the length of the active region, the grating presenting, according to 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 step (or period) of +/- 10%. It is not, for example, a structure having different portions in the first direction or different pitches superimposed on each other, as disclosed by the document [“High power surface emitting terahertz laser with hybrid second- and fourth-order Bragg gratings » Y. Jin & al., Nat. Common. 9, 1407 (2018)]. [0019] Advantageously, the active region has a width, measured in a second direction parallel to the plane and perpendicular to the first direction, and the diffraction grating has a width, measured in the second direction, the width of the diffraction grating being substantially equal to the width of the active region, the grating presenting, in the second direction, another unique diffraction order greater than or equal to two.

Advantageously, the diffraction grating has, in the first direction, a diffraction order greater than or equal to three. A diffraction order equal to two mainly favors 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 makes it possible to favor the counter-propagating component while retaining an out-of-plane component. Only wavelengths relating to stationary modes can be emitted and the spectrum of the field emitted by the second face is narrow and monochromatic.

Preferably, the active region emits the field by spontaneous emission and/or by stimulated emission. When the field is emitted by spontaneous and stimulated emissions, the device can operate in laser mode. Advantageously, the active region implements the emission of the field by inter-band emission or intra-band emission. Preferably, the active region is configured to carry out a quantum cascade.

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

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 prevents the transmission of the field through the diffraction grating which therefore becomes only reflective. It therefore makes it possible to reorient the diffracted field towards the second face (in other words, from the lower face).

Advantageously, the diffraction grating has a thickness, measured perpendicular to the plane, greater than Â/(10 n _eff ) and preferably greater than 2/(4 n _eff ), where 2 is a wavelength of the field electromagnetic emitted by the active region, and n _eff is an effective index of a guided mode in the guide wave, interacting with the diffraction grating. The thickness of the diffraction grating also makes it possible to control the effective index of the guided mode. It can therefore make it possible to control the diffraction orders of the guided mode with the diffraction grating.

[0025] Advantageously, the periodic structure comprises an alternation of first portions and second portions, the first portions being made up of a first semiconductor material having a first optical index and the second portions being made up of a metal or a second material, such as another semiconductor material or a gas, such as air, having a second optical index different from the first optical index. Freedom in the choice of the second material makes it possible to select a material that can easily be integrated into the network's technological manufacturing processes.

The first portions of the periodic structure are, for example, lines or plots.

Advantageously, the metal layer comprises portions, each portion of the metal layer covering one of the first portions of the periodic structure.

Alternatively, the metal layer is continuous.

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.

Advantageously, the waveguide of the device comprises 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 device for surface emission of an electromagnetic field according to the invention and an optical window transparent to the electromagnetic field, the second face of the guide wave of the surface emitting device extending over the optical window. [0032] Advantageously, the system also includes a thermal regulator configured to regulate the temperature of the waveguide of the device when the device operates in a steady state.

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

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

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

Advantageously, the method comprises, before the formation of the diffraction grating, a step of optimizing a coupling rate 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 its width and/or its length) are digitally optimized to obtain high reflectivity and an optimal coupling rate from the underside. This optimization thus makes it possible to couple maximum optical power into the optical system.

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

[0038] The figures are presented for information purposes only and in no way limit the invention. Unless otherwise specified, the same element appearing in different figures presents a unique reference.

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

[0040] [Fig. 2] shows a first result of a digital simulation carried out using the device of [Fig. 1 ],

[0041] [Fig. 3] shows a second result of a digital simulation carried out using the device in [Fig. 1 ],

[0042] [Fig. 4] shows a third result of a digital simulation carried out using the device in [Fig. 1 ],

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

DETAILED DESCRIPTION

[0044] [Fig. 1] presents, in a sectional view, an embodiment of a surface emitting device 1 according to the invention. [Fig. 1] also presents a magnification of a portion T of said device 1. In this embodiment, the device 1 comprises: a waveguide 11 and a diffraction grating 12.

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 11 a and a second face 11 b, opposite the first face 11 b. We can also call the first face 11a “upper face” and the second face 11b “lower face”. The first and second faces 11 a, 11 b extend parallel to the plane P. The first face 11 a is for example oriented in a third direction Z, perpendicular to the plane P, and the second face 11 b is oriented in the same direction but in an opposite direction.

The waveguide 11 also comprises an active region 110. The active region 110 is a portion of the waveguide 11 configured to emit an electromagnetic field (simply called "field"). The active region 110 is for example configured so that the emission is spontaneous and/or stimulated. In the latter case, the active region could also be called “amplifying medium” because it can allow the device 1 to operate in laser mode. The active region 110 is for example a stack of sublayers, such as [lnGaAs/AllnAs]xN, where N is the number of pairs of InGaAS/AllnAs sublayers, for example equal to one hundred. In this way, the active region 110 is configured to carry out spontaneous and stimulated emission.

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.

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 can be called "top cladding" in English, is extends over the active region 110 and preferably directly against the active region 110. In this embodiment, the face of the upper coating 111 which is opposite the active region 110 is then advantageously the first face 11a. A second semiconductor layer 112, which can be called "bottom cladding" or "bottom cladding" in English, also extends over the active region 110 and preferably 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 11 b.

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

The upper coating 111 has a thickness, measured perpendicular to the plane P, which can be between 1 pm and 2 pm. The lower coating 112 has a thickness, measured perpendicular to the plane P, which can be between 2 pm and 100 pm and preferably between 2 pm and 40 pm.

The lower coating 112 is for example a semiconductor heterostructure of type III-V, that is to say materials classified in groups III B and VB of the periodic table of elements (which, according to another convention, corresponds in columns 13 and 15 of the periodic table of elements). Said coating 112 is for example formed from InP. The lower coating 112 can also be doped, for example n-type, that is to say with impurities acting as electronic donors. The lower coating 112 is for example doped with sulfur.

The upper coating 111 can be, in the same way as the lower coating 112, a semiconductor heterostructure of type III-V, such as InP. In the same way, the upper coating 111 can be doped, for example n-type.

[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 from the sides. For example, it has a length Lu, a width and a thickness Wn. The length of the waveguide 11 is for example measured in the first direction X. The width of the waveguide 11 is for example measured in the second direction Y, perpendicular to the first direction The wave 11 is for example measured in the third direction Z, perpendicular to the two aforementioned directions X, Y.

The length Lu of the waveguide 11 can be between 1000 pm and 5000 pm. According to one embodiment, the length Lu and the width of the waveguide 11 can be equal, for example to within 10%. Thus, seen from above, the first face 11a 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 a hundred times the width of the waveguide 11. wave. In this case, we speak of a ridge type waveguide. For example, the width of the waveguide (not shown in the figure) can be between 10 pm and 50 pm.

The active region 110 has a length Lno, measured in the first direction X, substantially equal to the length Lu of the waveguide 11. In the same way, the active region 110 has a width, measured in the second direction Y, substantially equal to the width of the waveguide 11. By substantially equal, we mean equal to within 20%, or even within 10%. Flanks, perpendicular to the plane P, delimit for example the waveguide 11 and the active region 110.

The diffraction grating 12 extends over the first face 11a 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 directly above the active region 110 of the waveguide 11 and preferably centered relative to the latter.

The diffraction grating 12 can have a length L12 and a width. The length of the diffraction grating 12 is for example measured along the first direction 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 a homogeneous distributed counter-reaction over the entire length L110 of the active region 110.

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

[0059] The device 1 is also remarkable in that the diffraction grating 12 is reflective and in that it has a diffraction order, in the first direction X, greater than or equal to two. By reflective, we mean that at least 50% of the field is reflected by the diffraction grating 12. The diffraction order greater than or equal to two in the direction to the diffraction grating and induces a component of the field which propagates outside the plane P, that is to say along +Z and/or -Z. Since the diffraction grating 12 reflects the component of the field propagating along +Z, said component of the field is therefore oriented towards the second face 11 b. The device 1 can therefore carry out emission via the second surface 11 b.

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

[0061] The periodic structure 121 comprises for example first portions

1211 and second portions 1212. These first and second portions 1211,

1212 are arranged periodically and form an alternation in the first direction X and in the second direction Y where appropriate.

[0062] For example, the first and second portions 1211, 1212 can be lines, oriented in the second direction Y. Said lines are arranged next to each other in the first direction 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 pads arranged in a rectangular mesh. The second portions 1212 are tori, surrounding each pad 1211 and filling the space between the pads 1211. According to a variant of this example, the pads 1211 can have a long length in the first direction Y or vice versa.

The first portions 1211 are made up of a first semiconductor material having a first optical 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 upper covering 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 optical index different from the first optical index.

The diffraction grating 12 advantageously presents, in the first direction X, a single diffraction order. That is to say that the period A (or “step”) with which the first portions 121 1 are arranged is constant over the entire length L12 of the network 12. The first portions 1211 then advantageously all have the same width A1211, measured in the first direction direction X. The period A of arrangement of the first portions 1211, which corresponds to the period of the diffraction grating 12 in the first direction A single diffraction order means that the period A is constant over the entire length L12 of the diffraction grating.

The active region 110 is preferably configured to emit in the infrared range, that is to say in a range [0.8 pm; 20 pm] or preferably [4 pm; 12 p.m.]. Such wavelengths imply a period A of the network 12 of the order of a micrometer at least. A diffraction grating 12 applicable to the infrared spectrum is also simpler to produce than a grating having a much lower period (for example in the blue or ultraviolet spectrum).

The diffraction grating 12 also comprises a metal layer 122 which 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 metal layer 122 extending over the periodic structure 121. The metal layer can prevent the transmission of the field through the diffraction grating 12. The metal layer 12 extends in particular 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 metal layer 122 which extends in the first direction X. Alternatively, the metal layer 122 can be discontinuous and comprise a plurality of portions. Each portion therefore advantageously extends over each first portion 1211 of the periodic structure 1211. If for example the first portions 1211 of the structure of the periodic structure 121 are aligned in the second direction Y, then the metal layer 122 will comprise a plurality of portions also extending in the second direction Y, a portion of the metal layer extending over an upper surface of a first portion 1211.

[0069] The metal 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 make it possible to apply an electric field uniformly. on the active region 110 in order to inject carriers into the active region 110. The carriers can de-excite by emitting photons in the active region 110.

[0070] The metal 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 a variant, the diffraction grating 12 can be configured so that it has, along the second direction Y, perpendicular to the first direction X, an order greater than or equal to two as well. Thus the field propagating along the second direction Y also couples to the diffraction grating and induces a component of the field which also propagates outside the plane P, that is to say along +Z and/or -Z. This variant can be obtained when the first portions 1211 of the periodic structure 121 are pads arranged in a rectangular mesh. This variant is advantageous when the width and length of the active region (and therefore of the diffraction grating) are substantially equal, for example to 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 guide), it is preferable that the diffraction grating 12 only presents an order according to the first direction

[0072] If necessary, the diffraction grating 12 can also present, along the second direction Y, a single diffraction order. The periods of arrangement of the first and second portions according to the first and second directions X and Y are then constant in the two directions. They can, however, be different, so that the order according to the first direction X is different from the order according to the second direction Y.

[0073] The lateral confinement of the field can also be provided by conductive layers extending on the sides of the waveguide 11. The waveguide 11 is, for example, delimited by a first side and a second side, opposed to each other. The first and second flanks are for example substantially perpendicular to the second direction Y. We then speak of “lateral flanks” or “lateral facets”. The device 1 comprises first and second layers conductive which extend respectively on the first and second sides. The conductive layers are for example metallic layers of Ti or Au, extending perpendicular to the plane P. They form a cavity in the second direction Y of propagation and make it possible to confine the field in the second direction Y.

[0074] In order to avoid a short circuit within the waveguide (for example a short circuit of the upper coating 111 with 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 side walls, between the conductive layers and the side walls.

[0075] The device 1 can also include third and fourth conductive layers making it possible to contribute to the confinement of the field in the first direction. They extend for example on, respectively, a third and fourth flanks, delimiting the waveguide 11 in the first direction X. The third and fourth flanks can be called "front facet" and "rear facet". The third and fourth sides are for example perpendicular to the first direction X. Each spacer is then advantageously arranged on the third and fourth sides, between the third and fourth conductive layers and the third and fourth sides so as to avoid any short circuit.

[0076] The [Fig. 2] and [Fig. 3] present a simulation result obtained from a device 1 as described with reference to [Fig. 1], The reference signs of the different layers are indicated 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 optical index n = 3.1. The diffraction grating 12 considered is of order m odd and more particularly in this example equal to three. The first portions 1211 of the periodic structure 121 have an optical index n = 3.1 while the second portions 1212 of the periodic structure 122 are metallic, for example made of Au. The field considered is monochromatic and its wavelength is equal to 4.5 pm. The period A of the periodic structure 121 (with A = mA/2n _eff where m is the diffraction order of the grating, n _eff the effective optical index and Â is the wavelength considered) is A = 3.3 p.m. The diffraction grating has a thickness W12 = 400 nm. The duty cycle D of the network (calculated as D = A1211/A) is equal to 0.5. [0077] In order to facilitate the interpretation of the results, the simulation only considers a wave train emitted by the active region 110 at X = 0 pm and propagating along the 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 part of these 1450 pm). Three regions of space, which we will call “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 the device 1.

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

[0079] We observe, in [Fig. 2], the propagation of a wave train according to an opposite wave vector -kx to the initial train +kx. This is the wave resulting from the distributed feedback of the diffraction grating 12 on the first wave train (according to +kx). We also observe a wave propagating with a wave vector -kz perpendicular to the initial wave vector kx and in the direction of decreasing Z. Less visibly, we also observe a wave propagating with a wave vector +kz perpendicular to the initial wave vector kx and in the direction of increasing Z. However, the latter is less obvious to observe since it is reflected by the diffraction grating 12 and therefore 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 to see. It would propagate in the same way as the wave train propagating along -kz (by adapting the period as well as the optical indices of the media it would pass through, such as air for example, extending to the place of said metallic layer).

The presence of the metal layer 122 reflects the radiation propagating with a vector +kz, which means that the device 1 presented only emits via the lower surface 11 b. In the absence of the metal layer 122, the device 1 would also emit a field along +kz. The modeled device of [Fig. 2] is therefore particularly suitable for emitting a field, such as a laser beam, through the lower surface 11 b of the lower coating 11. [0081] [Fig. 3] shows that the intensity I31 of the field E as a function of time is very high at the level of the first sensor 31, since the wave train propagating along +kx reaches this sensor 31. The intensity I32 of the field E , measured by the second sensor 32, is on the other hand less, because it is the result of the distributed counter-reaction 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 active region 110.

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

[0083] The operational and structural conditions of the simulation were chosen to highlight the propagation of the field towards the lower face 11 b. An adjustment of the parameters of the diffraction grating 11 could make it possible to increase the counter-reaction of the grating 12 on the field propagating in the guide 11. It could also make it possible to reduce the coupling rate of the field, while ensuring that the major part of the optical power conveyed by the field would be directed towards the lower face 11 b (and then towards a point of interest).

[0084] [Fig. 4] shows complementary results of a simulation carried out using the same numerical model as that used to obtain the results of [Fig. 2] and [Fig. 3], This digital model, however, differs from the previous model in that it varies the coupling rate of the diffraction grating 12 with the field E which propagates in the device 1. The parameter studied is more precisely the duty cycle D of the diffraction grating 12. Indeed, the coupling rate depends on several parameters such as the duty cycle of the grating 12, the thickness W121 of the periodic structure 121 or even the index optics of the materials constituting the periodic structure 121. In [Fig. 4], the duty cycles considered are D = 0.5 (identical to that of [Fig. 2] and [Fig. 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 131 measured by the first sensor 31 is, however, reduced, because the power it carries is dissipated towards the other propagation waves (in this case according to -kx and -ky). [0085] A simulation carried out using the same digital model shows that the intensity of the field propagating normally to the plane of the network 12 also depends on the thickness W121 of the periodic structure 121. The higher its thickness W121, the better the coupling between the grating 12 and the field E. The diffraction grating 12 has a thickness preferably greater than Â/10 n _eff (where Â is a wavelength of the field E and n _eff is the effective index of the guided mode considered ) and even more preferably, greater than A/4n _eff .

[0086] A method of manufacturing the device 1 may comprise, firstly, a step of forming the waveguide 11. The waveguide 11 can be formed from an optical system, for example on an optical window. The manufacture of the waveguide 11 comprises 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 deposition chemical vapor phase using metal-organic precursors (called “MOCVD” in English for “Metal Organic Chemical Vapor Deposition”). An etching step can be implemented in order to delimit the waveguide

I I .

The lower coating layer 112, for example in InP, can also be produced in a preliminary step and then glued to a substrate, for example in Si, or an optical window, for example in Si, by molecular bonding. The other layers 110, 111 are then produced from the lower coating 111. Molecular bonding can provide a good surface condition of the second face 11b of the waveguide 11 and thus makes it possible to avoid the implementation of anti-reflective treatment.

[0088] The method of manufacturing the device 1 may comprise, following the formation of the waveguide 11, the formation of the diffraction grating 12, on the first face 11a of the waveguide 11. The periodic structure 121 may be manufactured on the first face 11 a of the waveguide 11, for example on the upper coating

III. Advantageously, the periodic structure 121 can be etched directly in the upper coating 111, for example by forming a series of trenches parallel to each other and distributed along the first direction X. The metal layer 122, deposited in a second step, for example, fills these trenches with metal. The upper coating 111 is then a little thicker in order to take into account the final thickness W121 of the periodic structure 121. Alternatively, another semiconductor material can be deposited in these trenches. After a planarization step, the metal layer 122 is deposited on the periodic structure 121.

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

[0091] The parameters of the diffraction grating 12, such as the thickness W121 of the periodic structure 121, the materials constituting the first and second portions 1211, 1212 of the periodic structure 121 or even the cyclic ratio D of the grating, can have an influence on the coupling between the diffraction grating 12 and the field emitted by the active region 110. The manufacturing process then advantageously comprises a step of optimizing a rate of coupling of the diffraction grating 12 with the field emitted by the active region 110. This optimization step preferably occurs before the formation of the diffraction grating 12. The expected coupling rate is for example between 10 cm' ¹ and 100 cm' ¹ .

[0092] [Fig. 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 the cavity can make it possible to measure a parameter of interest.

[0093] System 2 comprises a surface emitting device 1 as described above. It also includes an optical window 20 transparent to the light beam E which 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 11 b of the waveguide 11 therefore extends over the optical window 20 and preferably directly against the optical window 20. In fact, the device 1, carrying out the emission through its second face 11 b, does not require an anti-reflective layer to effectively transmit the radiation to the optical window 20. The second face 11 b of the device 1 can also be glued against the window 20 and preferably by molecular bonding.

The face of the optical window 20 intended to receive the second face 11 b of the device is preferably planar. It extends for example, when the device

1 is placed on the window, parallel to the plane P of the device 1.

[0096] The optical window 20 is for example made of Si. It has a thickness W20, measured perpendicular to the plane of the device 1, between 10 pm and 50 pm. In order to limit optical losses, the sum of the thickness W20 of the optical window 20 and the thickness W112 of the lower coating 112, measured perpendicular to the plane P, is less than 100 pm.

[0097] The waveguide 11 is delimited by first and second flanks 11 c, 11 d in the second direction Y. On each flank 11c, 11d extends a conductive coating 131, 132 making it possible to improve the lateral confinement of the field in the waveguide 11. Each side 11 c, 11 d is isolated from the conductive coating thanks to an electrically insulating spacer 14, placed between each conductive coating and the side on which it extends.

[0098] In an advantageous embodiment, the system 2 also comprises a thermal regulator 22 configured to regulate the temperature of the waveguide 11 when the device 1 is in operation and in particular in a stationary regime. Indeed, surface emitting devices are sensitive to temperature variations. It is therefore advantageous to maintain the fixed temperature during the illumination of sample 3 and preferably at a variation of less than 0.1°C. The illumination can be a few seconds, the thermal regulator 22 can be configured so that the temporal variation of the temperature of the system

2 is less than 0.1 °C/s during operation of the device.

[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 sink. The cells 221 are for example in contact with the support 21, since the thermal transfer offered by the support is sufficient. If it is made of Si for example, the heat transfer is around 150 W/m/K, which may be sufficient. The heatsink 222 is in contact with the cells 221 so as to dissipate the heat pumped from the support 21.

Claims

[Claim 1] Device (1) for surface emission of an electromagnetic field comprising:

- a waveguide (11), extending in a plane (P), having a length, measured in a first direction parallel to the plane, and a width, measured in 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 (11 a) and a second face (11 b), 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 the electromagnetic field; And

- a diffraction grating (12), reflecting, extending on the first face of the waveguide, the device being characterized in that the diffraction grating presents, in a first direction (X) parallel to the plane, an order diffraction greater than or equal to three.

[Claim 2] Device (1) according to the preceding claim, characterized in that the waveguide (11) is delimited by a first flank (11 c) and a second flank (11 d), the second flank being opposite the first sidewall, the first and second sidewalls 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 sidewall and the second conductive coating extending over the second sidewall.

[Claims] Device (1) according to one of the preceding claims, characterized in that the diffraction grating (12) extends directly above the active region (110) of the waveguide (11), the active region having a length (Lno), measured in the first direction (X), and the diffraction grating having a length (L12), also measured in the first direction, the length of the grating diffraction being substantially equal to the length of the active region, the grating presenting, in the first direction, a unique diffraction order.

[Claim 4] Device (1) according to the preceding claim, characterized in that the active region (110) has a width, measured in 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 in the second direction, the width of the diffraction grating being substantially equal to the width of the active region, the grating presenting, in the second direction, another diffraction order unique and greater than or equal to two.

[Claims] Device (1) according to 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 (11 a) of the waveguide (11) and the metal layer extending over the periodic structure.

[Claim s] 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 up of a first material semiconductor 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.

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

[Claims] Device (1) according to claim 6, characterized in that the metal layer (122) is continuous.

[Claim 9] Device (1) according to 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.

[Claim 10] Device according to one of the preceding claims, characterized in that the electromagnetic field comprises a wavelength comprised in [0.8 pm; 8 p.m.],

[Claim 11] Device according to 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 guide wave to the second face (11 b) of the waveguide and in that the thickness (W112) of the second semiconductor layer, measured perpendicular to the plane (P), is between 2 pm and 100 p.m.

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

[Claim 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 ) operates in a steady state.

[Claim 14] Method for manufacturing a surface emitting device (1) according to one of claims 1 to 12, the method comprising the following steps:

- form a waveguide (11), extending in a plane (P), having a length, measured in a first direction parallel to the plane, and a width, measured in 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 ; - form a diffraction grating (12) extending over the first face of the waveguide, the diffraction grating being reflective and having, in a first direction (X) parallel to the plane, a diffraction order greater than or equal to three. [Claim 15] Method according to the preceding claim, comprising, before the formation of the diffraction grating (12), a step of optimizing a coupling rate of the diffraction grating with an electromagnetic field emitted by the active region (110) .