SYSTEM COMPRISING A LIGHT SOURCE ON A SUBSTRATE WITH A HIGH OPTICAL INDEX AND ASSOCIATED METHOD

A system includes a light source configured to have at least one stationary mode of an electromagnetic field parallel to a plane; a substrate, parallel to the plane and having a high optical index; and an anti-resonant reflector for the at least one stationary mode, the reflector extending over the substrate, the source extending over the reflector.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to French Patent Application No. 2213549, filed Dec. 16, 2022, the entire content of which is incorporated herein by reference in its entirety.

FIELD

The technical field of the invention is the integration of a light source onto a substrate having a high optical index.

BACKGROUND

Light sources that can be integrated onto a substrate have many applications, such as the photoacoustic detection of chemical compounds. In order to benefit from the advantages provided by several distinct technologies, it is sought to integrate light sources manufactured using a particular technology onto a substrate not derived from that technology. This is known as “hybrid” technology. One example is the integration of so-called “III-V” light sources onto a silicon semiconductor substrate.

III-V light sources are made from a semiconductor alloy also called “III-V”, that is, comprised of a semiconductor material belonging to group III (column 13 of the Periodic Table of Elements) and a semiconductor material belonging to group V (column 15 of the Periodic Table of Elements). Common alloys comprise, for example, InP, InAs, GaAs, GaN or InSb. III-V light sources are good candidates for emitting a beam in an extended spectral range, such as mid-infrared. However, these sources have major integration restrictions, especially when they have to be integrated into the silicon substrate.

A laser type light source, whether of the III-V type or not, comprises a cavity formed by a stack of layers comprising a so-called “active” or “amplifying” region and two so-called “cladding” semiconductor layers. The active region is configured to emit an electromagnetic field by spontaneous and/or stimulated emission. The cladding layers are adjacent to the active region and disposed on either side of the same to confine certain modes of the electromagnetic field, called “guided modes”. In order to effectively confine the guided mode(s), cladding layers have optical indices that are strictly less than the average optical index of the active region. For example, when the active region is comprised of a multilayer of InGaAs/AlInAs or InAlAs/AlGaInAs, the cladding layers are made of InP.

The light source, directly disposed on a Si substrate, can suffer high optical losses, which are a handicap for its use. Indeed, silicon has a higher optical index than III-V materials, such as InP, and therefore higher than the cladding layers. Thus, without any special provision and when the penetration of the guided mode into the cladding layers is significant, said guided mode can couple to the Si substrate and reduce the effectiveness of the confinement provided by the cladding layers.

To overcome this problem, it is provided that the cladding layer separating the active region from the Si substrate, called the lower cladding layer, is relatively thick. This thus compensates for the depth of penetration of the guided mode into the cladding layers and reduces the coupling of the guided mode to the Si substrate. For a guided mode having a wavelength of 4.5 μm, the thickness of the lower cladding layer has to be at least 3.5 μm. This represents a significant cost in terms of raw material and also an additional restriction to the integration of the source. For a guided mode having a wavelength greater than 10 μm (far infrared), the thickness of the lower cladding layer would have to be greater than 20 μm, which can make source integration even more complex.

One solution provided by document [Coutard, J. G., Brun, M., Fournier, M. et al. Volume Fabrication of Quantum Cascade Lasers on 200 mm-CMOS pilot line”. Sci Rep 10, 6185 (2020)], avoiding thickening of the lower cladding layer, consists in inserting a layer of low optical index between the lower cladding layer and the Si substrate. Said low index layer acts as a barrier to the propagation of the guided mode in the Si substrate and makes it possible to reduce the thickness of the lower cladding layer. This low index layer is for example made of oxide or nitride. While decoupling between the III-V source and the Si substrate is effective for a guided mode having a wavelength of less than 4 μm, the situation is different when the guided mode has a longer wavelength. Indeed, oxides or nitrides induce high losses when the wavelength of the guided mode is greater than 4 μm, which greatly limits spectral range of the source.

There is therefore a need to provide a means for integrating a light source capable of operating over an extended spectral range, such as a III-V source, onto a substrate having a high optical index, such as a Si substrate.

SUMMARY

An aspect of the invention solves the above problems by means of an anti-resonant reflector disposed between the light source and the substrate. Since the reflector is based on interference conditions, it can be made from materials that do not absorb the radiation emitted by the source. The light source can therefore operate in a wide spectral range.

More particularly, an aspect of the invention relates to a system comprising:a substrate extending in parallel to a plane, said substrate having an optical index n2;a light source configured to have at least one stationary mode of an electromagnetic field, said mode being parallel to the plane, the source being also configured to have, for said at least one stationary mode, an effective optical index neffsuch that neff<n2.

The system is remarkable in that it also comprises an anti-resonant reflector for said at least one stationary mode, said reflector extending over the substrate and the source extending over the reflector, the reflector comprising at least one semiconductor bilayer extending in parallel to the plane, said at least one bilayer comprising: a first semiconductor sublayer and a second semiconductor sublayer, extending against the first sublayer, the first sublayer being disposed between the source and the second sublayer, the first and second sublayers respectively having optical indices n301and n302such that n301>n302and n301>neff, the first sublayer also having a thickness d301configured to form an anti-resonant cavity for said at least stationary mode.

By stationary mode, it is meant a mode of an electromagnetic field that is confined within a resonant cavity.

By mode parallel to the plane, it may be meant that the electromagnetic field is transverse to the plane. By electromagnetic mode transverse to the plane, it can be meant that the guided mode exclusively has an electrical polarisation of the electromagnetic field normal to the plane or a magnetic polarisation of the magnetic field normal to the plane.

By optical index, it is meant also the refractive index.

By effective optical index, it is meant the optical index observed by the stationary mode in the source.

By “reflector extending over the substrate”, it is meant that the reflector and the substrate face each other and that the substrate can either extend directly against the substrate or be separated from the substrate by an intermediate layer.

By anti-resonant reflector, it is meant that the reflector is based on the principle of destructive interference of said at least one stationary mode, blocking the transmission of the guided mode through the substrate.

By “thickness d301configured to form an anti-resonant cavity for said at least one mode”, it is meant that the thickness of the first sublayer of each bilayer is chosen so as not to promote resonance in the cavity formed by each first sublayer. A thickness d301allowing resonance is for example λ/4n301. The thickness d301of the first sublayer of said is therefore in

where λ is a wavelength of said at least one stationary mode and p ∈.

Decoupling between the source and the substrate is carried out by utilising an anti-resonant reflector. This is an unexpected characteristic for the person skilled in the art. Indeed, a person skilled in the art wanting to use a reflector would prefer a resonant reflector, such as a Bragg mirror. For example, the person skilled in the art know vertical cavity light sources utilising Bragg mirrors to confine a vertical guided mode.

Unexpectedly, the reflector utilises a bilayer whose sublayers have optical indices greater than the effective optical index observed by the stationary mode in the source. This is also an unexpected characteristic for the person skilled in the art who would think that this tends to promote transmission of the mode to the substrate rather than preventing it.

Thus, the stationary mode is not transmitted to the substrate, even if the latter has an optical index greater than the effective index of the source. In addition, the reflector does not require the use of oxide or nitride to operate. The stationary mode is therefore not absorbed by the reflector at wavelengths greater than or equal to 4 μm.

The reflector provides a means for integrating a light source capable of operating over an extended spectral range, such as a III-V source, onto a substrate having a high optical index, such as a Si substrate.

The anti-resonant reflector also differs from a resonant reflector, such as a Bragg mirror, in that only one of the thicknesses of the sublayers making up said at least one bilayer is constrained. This makes it easier to manufacture said anti-resonant reflector.

Beneficially, the second sublayer of said at least one bilayer can be the same as the substrate. In other words, it cannot be distinguished from the substrate. For example, it extends directly against the substrate and is made from the same material as the substrate.

In an embodiment, the thickness d301of the first sublayer of said at least one bilayer is such that

where λ is a wavelength of said at least one stationary mode, p ∈and δ=λ×10%, for example δ=λ×20%, or even δ=λ×50%.

According to one development, the thickness d301of the first sublayer of said at least one bilayer is

with N ∈, λ a wavelength of said at least one stationary mode and deffa thickness of the stationary mode.

According to one alternative, the thickness d301of the first sublayer of said at least one bilaver is

with δ=λ×50%, for example δ=λ×20%, or even δ=λ×10%.

Beneficially, the anti-resonant reflector comprises a plurality of bilayers, the second sublayer of a first bilayer of the plurality of bilayers extending against the first sublayer of a second bilayer of the plurality of bilayers.

Beneficially, the source comprises a cavity in which said at least one stationary mode of the electromagnetic field can be established, the cavity comprising a first layer called the “active region” and a second layer called the “lower cladding layer”, the active region extending in parallel to the plane and over the lower cladding layer, the lower cladding layer being disposed between the active region and the reflector, the active region being also configured to emit the electromagnetic field.

Beneficially, the lower cladding layer is made of III-V material, such as InP, the first sublayer of said at least one bilayer being made of Ge and the second sublayer of said at least one bilayer being made of Si.

Beneficially, the system comprises a bonding layer separating the source and the reflector, the bonding layer having an optical index n5such that n5≤n301and for example n5<n301.

Beneficially, the bonding layer comprises a face, parallel to the plane comprising a diffraction grating, the lower cladding layer extending against said face of the bonding layer, against the diffraction grating.

An aspect of the invention also relates to a method for manufacturing a system comprising the following steps of:forming, from a substrate extending in parallel to a plane and having an optical index n2less than an effective index neff, an anti-resonant reflector for at least one stationary mode parallel to the plane of an electromagnetic field, said reflector extending over the substrate, the reflector comprising at least one semiconductor bilayer extending in parallel to the plane, said at least one bilayer comprising: a first semiconductor sublayer and a second semiconductor sublayer extending against the first sublayer, the second sublayer being disposed between the first sublayer and the substrate, the first and second sublayers respectively having optical indices n301and n302such that n301>n302and n301>neff, the first sublayer also having a thickness d301configured to form an anti-resonant cavity for said at least one stationary mode; andforming, on the reflector, a light source configured to have said at least one stationary mode parallel to the plane of the electromagnetic field, the source being also configured to have, for said at least one stationary mode, the effective optical index neffsuch that neff<n2.

The invention and its various applications will be better understood upon reading the following description and examining the accompanying figures.

DETAILED DESCRIPTION

FIG.1shows a first embodiment of a system6according to an embodiment of the invention. It also shows an enlarged view of a portion of the system6, as well as an optical index curve n(y) as a function of a depth y. It furthermore shows an orthonormal reference frame {X; Y; Z}.

The system6comprises a light source1, a substrate2and a reflector3.

The substrate2extends in parallel to a plane P. With respect to the orthonormal reference frame {X; Y; Z}, the plane P corresponds to the plane {X; Z}.FIG.1represents only a portion of the substrate2. Since the latter may be a bulk substrate, it may have very large dimensions compared to the other elements of the system6. For example, the substrate2is made of silicon.

By light source1, or simply source1, it is meant a source of electromagnetic radiation. For example, it is a laser source or an electroluminescent source. The source1is beneficially configured to emit a light beam in a wavelength range between 0.8 μm and 20 μm. This range comprises, for example, the near infrared and part of the mid-infrared. Emission can be carried out from a side surface of the source1or from a surface parallel to the plane P. In the first case, the source1is called a “side emitting” source, whereas in the latter case, the source1is called a “surface emitting” source.

In the embodiment inFIG.1, the source1has a ridge shape, stretching in the direction Z. In other words, it has a large aspect ratio with a length, measured along Z, of more than 200 μm and a width, measured along X, of less than 50 μm. The source1may have a height, measured along Y, of less than 20 μm, or even less than 10 μm.

The source1is configured to emit an electromagnetic field and to confine this electromagnetic field in one or more stationary modes. These may be stationary modes, such as those observable in a laser cavity. The source is distinctive in that each stationary mode is parallel to the plane P. By mode parallel to the plane, it is meant that the field has, exclusively:a polarisation of the electric field perpendicular to the plane P; ora polarisation of the magnetic field perpendicular to the plane P.

It is meant that the field does not have the two polarisations (of the electric field and the magnetic field) which are both substantially parallel to the plane P (that is, parallel to within 10°, or even 5°). In the mode parallel to the plane P, it is meant that the mode is for example transverse to the plane P. In other words, the electromagnetic field comprises for example:a polarisation of the electric field normal to the plane (that is, parallel to the direction Y) and a polarisation of the magnetic field parallel to the plane P; ora polarisation of the electric field EXparallel to the plane P and a polarisation of the magnetic field HYnormal to the plane P (as illustrated inFIG.1).

Unless otherwise stated, only one stationary mode of the field in source1will be considered, in order to simplify the description of the invention. However, the teachings apply to each stationary mode when the source1has a plurality of stationary modes.

Different layers110,111,112making up the source1may have different optical indices. An example is plotted in the optical index curve n(y). The different indices plotted are considered along the dotted line A. However, each stationary mode in the source1may have an effective optical index neffwhich may have a different value from the optical indices of the layers of the source1, especially due to the geometry of the different layers. This is why the effective optical index neffof the stationary mode is considered. The effective optical index neffcan be determined digitally. An estimate of the effective optical index neffcan be determined from the average optical index in the source1, calculated along the line A. It is also desirable to take account of the elements that enable confinement of the electric field in the source1, such as the geometry (thickness and/or width) of the other so-called “cladding” semiconductor layers or the presence of reflective layers (such as metal electrodes or a diffraction grating).

The substrate2has an optical index n2. When neff>n2, the stationary mode of the source1is not likely to couple with the substrate2. By contrast, when neff<n2, the stationary mode is likely to couple with the substrate2, increasing the optical losses and reducing the effectiveness of the source1.

To avoid this drawback, an aspect of the invention provides for the reflector3to be disposed between the substrate2and the source1. The reflector3is distinctive in that it is configured to be anti-resonant in at least one stationary mode of the source1and, in an embodiment, all the stationary modes of the source1. In this way, the transmission of the stationary modes to the substrate2is reduced and the effectiveness of the source1is maintained, even if the substrate2has a high optical index enabling it to couple with the stationary modes of the source.1.

The reflector3comprises, for example, at least one optical cavity configured so as to be anti-resonant in the stationary mode. The optical cavity, extending in parallel to the plane P and between the source1and the substrate2, prevents transmission of the stationary mode to the substrate2. The source1is entirely superimposed, in the direction normal to the plane P, on the reflector3. Thus, the reflector3extends under the source1, beyond the source1, so as to effectively block the transfer of the stationary mode to the substrate2.

InFIG.1, the reflector3comprises three semiconductor bilayers31,32,33, each extending in parallel to the plane P. Each bilayer31,32,33comprises a first semiconductor sublayer301and a second semiconductor sublayer302.FIG.1shows an enlarged view of the reflector and a portion of the substrate2. For each bilayer31,32,33, the first sublayer301extends against the second sublayer302. Each bilayer31,32,33is oriented such that the first sublayer301is disposed between the second sublayer302and the source1. A single bilayer31may be sufficient to form the non-resonant optical cavity. However, it is beneficial to have a plurality of bilayers31,32,33(and therefore a plurality of anti-resonant cavities) to improve decoupling between the source1and the substrate2. In the case of several bilayers31,32,33, said bilayers are, in an embodiment, in contact in twos, that is, stacked directly one on top of the other, so as to form a vertical stack of bilayers31,32,33.

In the embodiment inFIG.1, the second sublayer302of one33of the bilayers is the same as the substrate2. Indeed, to simplify manufacture, it may be beneficial for one of the second sublayers302to be a portion of the substrate2.

In order to form an optical cavity, each first sublayer301has an optical index n301strictly greater than the optical index n302of the second sublayer302or of the second sublayers with which it is in contact.

For each bilayer31,32,33, the optical index n301of the first sublayer301is strictly greater than the optical index n302of the second sublayer302. Otherwise, for each bilayer31,32,33, the optical indices verify n301>n302.

The optical index curve n(y) curve inFIG.1shows an example of alternation for the optical indices n301, n302of the first and second sublayers301,302. This is a simple embodiment of the reflector3in which the first sublayers301all have the same optical index n301. Similarly, the second sublayers302also have the same optical index n302.

In this example, the first sublayers301can be made of germanium and the second sublayers302can be made of silicon. This embodiment is all the more beneficial when the substrate2is also made of silicon. Thus, one of the second sublayers302can be the same as the substrate2.

The first bilayer31disposed in the vicinity of the source1interacts directly with the stationary mode in the source1. The anti-resonance condition requires its first sublayer301to have an optical index n301strictly greater than the effective optical index neffassociated with the stationary mode. Otherwise, for each bilayer31,32,33, n301>neff.

Thus, the first sublayer301, having an index n301>neffand n301>n302form a plurality of optical cavities.

The anti-resonance condition can be obtained by controlling the thickness d301of each first sublayer301, measured perpendicularly to the plane P, in other words, the thickness of each anti-resonant cavity in a direction perpendicular to the stationary mode in the source. The thickness d301may be a function of the wavelength λ of the stationary mode of the source1. λ is for example between 0.8 μm and 20 μm. The wavelength λ corresponds for example to a maximum amplitude of the electromagnetic field in the stationary mode. It can be set by the source1according to the established stationary mode.

For example, the thickness d301of each first sublayer301may be such that

where p ∈and ∪ is the union operator.

The thickness d301of the first sublayer301can take a value other than λ/4n301, this value promotes resonance of the mode in the first sublayers. Indeed, a thickness, for example equal to d301=(2p+1)λ/4n301would promote the transfer of the stationary mode through the reflector3and would therefore reduce the effectiveness of the source1.

In order to optimise the anti-resonance of the reflector3, each first sublayer301beneficially has a thickness d301such that

where δ=10% λ. Thus, the thicknesses d301of the first sublayers301have values far from (2p+1)λ/4n301in order to limit the resonance of the mode in the first sublayers301and thus decouple the stationary mode from the substrate2. In an embodiment, δ=20% λ or even δ=50% λ.

The thickness d301can be adjusted within the above interval by performing an optimisation, for example digital optimisation, to minimise the transmission of the stationary mode to the substrate2.

In one embodiment, the thickness d301can be in the range

For example, the thickness d301can be equal to

This thickness is the exact opposite to the thickness that can be implemented in a resonant reflector such as a Bragg reflector.

In an embodiment, the thickness d301is:

where N ∈, deffand neffare respectively an effective thickness of the stationary mode in the source1and the effective optical index observed by the stationary mode in source1. The effective thickness deffof the mode corresponds to a thickness that the stationary mode has in the source1. It corresponds, for example, to a thickness d11of a cavity11of the source1in which the stationary mode is established. The effective index neffcan be determined from the optical indices of the elements making up the cavity11in which the stationary mode is established. The effective index neffmay be equal to the average of the optical indices of the components of the cavity11.

In the embodiment inFIG.1, the source1is a laser source. It comprises a laser cavity11configured to emit an electromagnetic field and to confine this electromagnetic field in a stationary mode. The cavity11extends in parallel to the plane P. For example, it has a width, measured in the direction X, parallel to the plane (and in the plane ofFIG.1) of between2and 50 μm. It may furthermore have a height, also referred to as the thickness d11measured perpendicularly to the plane P, of between 2 μm and 10 μm. It may also have a length, measured in the direction Z, of between 20 μm and 1000 μm, or even between 100 μm and 5000 μm (this is a “ridge” type laser).

In the embodiment inFIG.1, the source1comprises a cavity11in which the stationary mode(s) can be established. The cavity11comprises a plurality of layers extending in parallel to the plane P. For example, the cavity11comprises a first layer110, which is referred to as an “active region” or “amplifying region”. The active region110is configured to emit the electromagnetic field. The cavity11also comprises a second semiconductor layer111, called the “lower cladding layer”, extending between the active region110and the reflector3. The active region110rests on the internal cladding layer111. The cavity11also comprises a third semiconductor layer112, called the “upper cladding layer”, resting on the active region110. The active region110separates the cladding layers111,112. The three layers thus form a vertical stack. Said stack is delimited by one or more flanks11a,11bsuch that the three aforementioned layers110,111,112are in vertical alignment with each other.

The active region110can be configured so that the emission of the electromagnetic field is at least spontaneous and, in an embodiment, spontaneous and stimulated. The latter case enables the source1to operate in laser mode. The emission can be based on interband cascade emission. It may be based on inter-subband emission, also known as quantum cascade emission. To enable quantum cascade emission, the active region110comprises, for example, a stack of sublayers of III-V material, forming a succession of quantum wells and potential barriers. For example, the stack of sublayers extends in parallel to the plane P.

The cladding layers111,112make it possible to confine the electromagnetic field in the cavity11, especially along the direction Y, normal to the plane. In this way, the electromagnetic field remains localised at the active region110and makes it possible, for example, to promote stimulated emission from the active region110. For this, the cladding layers111,112are for example configured to have optical indices n111, n112strictly less than an average optical index n110of the active region110. By average optical index n110of the active region110, it is meant an optical index taking account of the indices of the layers or sublayers making up the active region110. In this way, the electromagnetic field emitted by the active region110is confined to the vicinity of the active region110.

The optical indices n111, n112and n110of the cladding layers111,112and the active region110are plotted on the index curve n(y).

In order to activate spontaneous emission from the active region110, the source1comprises a system for circulating an electric current in the active region110. This system comprises, for example, the cladding layers111,112when the latter are doped. They thus enable the electric current to be conducted through the active region110.

The source1may also comprise conductive electrodes12allowing the circulation of electric current in the cladding layers111,112and in particular in the active region110. A first conductive electrode121, for example made of Au or Ti, extends in contact with the upper cladding layer112(if the latter is doped) so as to make electrical contact. Similarly, two second conductive electrodes122, for example made of Au or Ti, extend against a portion of the lower cladding layer111so as to make electrical contact. The two second electrodes122also extend in parallel to the flanks11a,11bof the cavity11. To prevent the cladding layers111,112and the active region110from being short-circuited by the second electrodes122, the source1also comprises two insulating spacers, for example made of SiN, disposed between the cavity11and each second electrode122. The spacers extend for example in contact with each flank11a,11bof the cavity11and separate the cavity11from the second electrodes122.

When the source1is a III-V type source. The cladding layers111,112are for example made of a III-V alloy such as InP. The active region110comprises, for example, a multilayer of InGaAs/AlInAs or InAlAs/AlGaInAs. To integrate the III-V source onto the Si substrate2, each first sublayer301of the reflector3may be made of Ge and each second sublayer302of the reflector3may be made of Si.

Indeed, it is easy to make alternating Si and Ge sublayers from a Si bulk substrate, for example by growth. In addition, this growth can be made using standard methods implemented in the so-called “CMOS” (Complementary Metal-Oxide-Semiconductor) technology.

In order to facilitate the transfer of the source1to the reflector3, the system6may comprise a bonding layer5, disposed between the source1and the reflector3. The bonding layer5moreover extends, in an embodiment, in contact with the reflector3, the source1resting directly on the bonding layer5. By “bonding layer”, it is meant a layer whose function is to enable transfer, also known as bonding, to the reflector3. Indeed, it can be difficult to transfer a layer of III-V material (such as the lower cladding layer111) directly onto a Ge layer. The bonding layer5is therefore made of a material which, on the one hand, makes it easier to transfer the source1onto it and which, on the other hand, does not interfere negatively with the reflector3or with the stationary mode in the source1.

In the embodiment illustrated, the bonding layer5extends over a first sublayer301of the reflector3. To avoid interference, the bonding layer5has, in an embodiment, an optical index n5such that n5<n301. Thus the second sublayer301of the reflector3, which is in contact with the bonding layer5, can act as an optical cavity. In an embodiment, n5≤n302. It may be beneficial that n5≤neff, however, it is expected that the bonding layer5is not made of a material, such as an oxide or a nitride, which can absorb the stationary mode when it belongs to a wavelength range greater than 4 μm. Thus, a bonding layer5having an index n5holding neff≤n5≤n302provides a good compromise. For example, the bonding layer5can be made from the same material as the second sublayers302of the reflector3, such as Si.

The source1can be a distributed feedback (DFB) laser source. In a DFB source, the stationary mode is established in a cavity, in response to the action of a diffraction grating extending over one face of said cavity. In the case of the source1, the diffraction grating extends over one of the faces of the cavity11and along the direction Z. Thus, the stationary mode is established in the cavity11, along the direction Z.

For example, the diffraction grating extends over the upper cladding layer112. For example, the diffraction grating extends over a face of one of the cladding layers111,112, said face being opposite to the active region110. According to a first example, the diffraction grating extends over a face of the upper cladding layer112, between said layer112and the upper electrode121extending over the cavity11. In one development, the diffraction grating may be etched into the face of said upper cladding layer112. The diffraction grating is for example formed by trenches, oriented in the direction X, and spaced apart at a constant pitch in the direction Z.

According to one alternative, the diffraction grating extends over the lower cladding layer111. For example, it extends over one face of the lower cladding layer111, between said layer111and the bonding layer5. The diffraction grating may be etched into the face of said lower cladding layer111. Alternatively, the diffraction grating may be formed in, for example etched into, the bonding layer5.

FIGS.2to13show the results of digital simulations carried out by considering several embodiments of the reflector3.FIGS.6to9are moreover based on a reflector of prior art.

All the simulations were carried out with a same source1. It comprises a cavity11as described with reference toFIG.1. The cavity has a width, measured along X, of 10 μm and a height, measured along Y, of3μm. The cavity11has a length, measured along Z, of 250 μm.

The lower cladding layer111is made of InP and has a thickness of 0.6 μm (unless otherwise indicated, thicknesses indicated are measured perpendicularly to the plane P). The active region110has a thickness of 1.72 μm. The upper cladding layer112is made of InP and has a thickness of 1.3 μm. An InGaAs layer separates the active region110from the lower cladding layer111. It has a thickness of 0.3 μm. An InP layer separates this InGaAs layer from the active region110and has a thickness of 0.2 μm. An InGaAs layer also separates the active region from the upper cladding layer112. It has a thickness of 0.02 μm. A 0.1 μm InGaAs layer separates the upper cladding layer from the first conductive electrode121. This electrode121is made of Au and has a thickness of 0.5 μm. The second conductive electrodes122extending facing the flanks of the cavity11are also made of Au and have thicknesses, measured in parallel to the plane P, of 0.9 μm. A SiN spacer separates each second electrode122from the cavity11. The spacer has a thickness, also measured in parallel to the plane P, of 0.9 μm.

The substrate2is made of Si.

The stationary mode established in the cavity11is monochromatic and has a wavelength of 4.5 μm. The optical index of Si is 3.47. The optical index of Ge is 4. The optical index of InP is 3.11. The average optical index of the active region110is 3.35.

FIGS.2to5set forth the first results of digital simulation carried out from a first embodiment of the reflector3. The reflector3comprises a stack of three bilayers31,32,33. The stack extends over the substrate2. The first sublayers301are made of Ge and the second sublayers302are made of Si. The first sublayers301each have a thickness d301=0,496 μm. The second sublayers302each have a thickness, measured perpendicularly to the plane, of 0.607 μm.

The bilayer extending directly against the substrate2comprises a second sublayer302which is indistinguishable from the substrate2.

The system also comprises a bonding layer5, made of Si, with a thickness of 0.255 μm. The bonding layer5extends directly against the reflector3(and in this case on a Ge sublayer) and the source1(and more particularly the lower cladding layer111) extends against the bonding layer5.

The dimensions of the reflector3, and in particular the first and second sublayers301,302, have been determined by means of an optimisation algorithm. The algorithm is moreover based on genetic optimisation. The minimised optimisation criterion is proportional to the optical losses of the stationary mode in the substrate2. It is in this case the imaginary part of the effective optical index of the stationary mode.

FIGS.2and3show, in grey scale, an amplitude of the electric field in the direction X (parallel to the plane P) and the direction Y (normal to the plane P) respectively. In other words,FIG.2shows the amplitude of the electric field for a mode TE00andFIG.3shows the amplitude of the electric field for a mode TM00.

FIGS.2and3show that the modes considered are confined in the cavity11. Moreover, they extend little, if at all, into the reflector3, and negligibly into the substrate2.

The imaginary part Im(neff) of the effective optical index of the stationary mode in the cavity11is proportional to the losses experienced by the stationary mode. It is therefore proportional to its level of coupling with the substrate2.

FIGS.4and5show the propagation of a wave TM00in the cavity11and in particular along the direction Z along which the cavity11extends. The mode considered is not stationary but propagative. However, it allows the effect of the reflector3on the propagation of the field in the system6to be visualised. The wave is generated at Z=0 μm and propagates over 250 μm.

The amplitude of the field EYis shown in grey scale inFIG.5. The corresponding power is plotted inFIG.4.FIG.5shows that the amplitude of the field decreases as the wave propagates along Z. The power loss, illustrated inFIG.4, is less than 40%.

By way of comparison,FIGS.6to9set forth results of digital simulation carried out from a system6comprising an oxide layer instead of the reflector3, as taught in prior art.

FIGS.6and7also show, in grey scale, an amplitude of the electric field of the modes TE00and TM00.

UnlikeFIGS.2and3, confinement is less effective with the oxide layer. The stationary modes extend significantly beyond the cavity11and the oxide layer, into the substrate2. The result is particularly significant for the mode TM00inFIG.7.

ForFIGS.6and7, Im(neff) is equal to 4×10−3and 4×10−3respectively. The imaginary part is therefore significantly higher. The reflector3therefore reduces optical losses in the stationary mode.

FIGS.8and9show the propagation of a wave TM00in the cavity11decoupled from the substrate2by means of the oxide layer.

The amplitude of the field EYis shown in grey scale inFIG.9. The corresponding power is plotted inFIG.8.FIG.9shows that the amplitude of the field decreases rapidly as the wave propagates along Z. The power loss, illustrated inFIG.8, is drastic, since close to 100% for Z=250 μm.

FIGS.10and11set forth results of digital simulation carried out from a second embodiment of the reflector3. Unlike the embodiment inFIGS.2to5, the thickness d301of each first sublayer301follows:

Herein, the thickness of each first sublayer301considered is 2.12 μm. The thickness of each second sublayer302is 2.56 μm.

FIGS.10and11also show, in grey scale, an amplitude of the electric field of modes TE00and TM00. Im(neff) is equal to 0.1×10−3and 0.3×10−3respectively. The imaginary part is therefore reduced compared to that obtained inFIGS.6and7. Thus, since the reflector3is anti-resonant, it significantly reduces optical losses in the stationary mode.

By way of comparison,FIG.12sets forth a result of digital simulation obtained from a system6in which the reflector3is replaced with a Bragg mirror. Unlike the embodiment inFIGS.2to5, the thicknesses of the Si and Ge layers are configured so that the mirror is resonant, rather than anti-resonant.

FIG.12shows, in grey scale, an amplitude of the electric field in the mode TM00. The imaginary part of the effective optical index Im(neff) is equal to 4×10−3, which is equivalent to that obtained when the anti-resonant reflector3is replaced with an oxide layer. The Bragg mirror is therefore of no interest compared with an oxide layer as implemented according to teachings of prior art.

The implementation of an anti-resonant reflector3therefore provides good means for effectively decoupling a stationary mode established in a source1disposed on a substrate with a high optical index.

FIG.13shows schematically an example of the implementation of a method7for manufacturing the system6. The method7comprises two major steps: forming71the reflector3and forming72the source1on the reflector3.

Forming71the reflector3is beneficially carried out from the substrate2. The reflector3is, in an embodiment, moreover formed full plate, that is, over the entire surface of the substrate2. Moreover, when the formation of the reflector3is complete, the substrate1comprises, on one of its faces, at least one bilayer31,32,33, or even a bonding layer5. The set of substrate2and reflector3moreover forms what can be called a functionalised substrate, in the sense that the substrate2is prepared to receive a source1such as a III-V source.

Forming71the reflector3comprises, for example, successively depositing sublayers and layers according to the above mentioned teachings. The manufacture of these layers involves methods known to the person skilled in the art.

Characteristics of the source1and of the stationary mode are, in an embodiment, known before performing forming71the reflector3. In this way, the characteristics of the reflector3, which are for example the optical indices of the bilayers making up the reflector3or the thicknesses of the sublayers making up each bilayer, can be determined before they are manufactured. Alternatively, when the characteristics of the source1and/or the stationary mode are not known in advance, the reflector3can be formed by considering a target effective optical index and a target wavelength. In this way, the functionalised substrate can be manufactured and matching between the reflector3and the source1is thereby based on the choice of this source1.

Forming72the source1may involve a bonding step, such as molecular bonding. For example, the source1can be made separately from the reflector3, for example from a III-V substrate. It can then be transferred onto the functionalised substrate resulting from the step71of forming the reflector3. Alternatively, the source1can be made directly from the free surface of the reflector3(or the bonding layer5).

In order to form a laser source1, the bonding layer5of the functionalised substrate may have a diffraction grating. For example, the diffraction grating is etched into the bonding layer5after the latter has been deposited on the reflector3. This etching step is beneficially part of the step of forming the reflector3.

It will be appreciated that the various embodiments and aspects of the inventions described previously are combinable according to any technically permissible combinations.

The articles “a” and “an” may be employed in connection with various elements, components, processes or structures described herein. This is merely for convenience and to give a general sense of the elements, processes or structures. Such a description includes “one or at least one” of the elements or components. Moreover, as used herein, the singular articles also include a description of a plurality of elements or components, unless it is apparent from a specific context that the plural is excluded.