OPTICAL SYSTEM COMPRISING A PHOTOELECTRIC TRANSDUCER COUPLED WITH A WAVEGUIDE AND ASSOCIATED MANUFACTURING METHOD.

Abstract

OPTICAL SYSTEM COMPRISING A PHOTOELECTRIC TRANSDUCER COUPLED WITH A WAVEGUIDE AND ASSOCIATED MANUFACTURING METHOD.One aspect of the invention relates to an optical system (1) comprising a substrate (10) and, formed on said substrate (10): a reflective structure (20) comprising a first internal parabolic reflective face (22), a second internal plane reflective face (23) and a third internal plane reflective face (24), a photoelectric transducer (30) comprising an active region configured to emit light waves or configured to receive light waves and positioned in the reflective structure (20), at the level of a part of the foci (Fi), the material (21) of the reflective structure (20) being chosen to be transparent to light waves, a waveguide (40) arranged so that its longitudinal axis (Xg) is parallel to the optical axes (Xoi) and its proximal end (41) is attached to the reflective structure (20) between the second and third reflective faces (23,24) and at the level of the active region. Figure to be published with the abbreviation: Figure 1.

Description

Title of the invention: OPTICAL SYSTEM COMPRISING A PHOTOELECTRIC TRANSDUCER COUPLED WITH A WAVEGUIDE AND ASSOCIATED MANUFACTURING METHOD. TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of the invention is that of photonic devices for intra- and inter-chip communication.

[0002] The present invention relates to an optical system using a semiconductor photoelectric transducer coupled to a waveguide. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] In the field of inter- or intra-chip communication, the use of light to replace electrical signals makes it possible to overcome physical constraints linked to the impedance and size of metallic interconnections, and thus, to meet the need to transfer large quantities of data at very high speed.

[0004] The photonic devices or platforms developed for this purpose include a light source, generally of micrometric dimensions, electrically excited and capable of being efficiently coupled to a photonic waveguide, which conducts the light and, thus, carries the information.

[0005] In this context, light-emitting diode (LED) architectures, also called LED sources or micro-LED sources, have recently been used as a light source. Compared to laser sources, they have the advantage of being low cost and low power consumption. It is also easier to reduce their dimensions down to the micrometric scale.

[0006] Such LED architectures are generally formed from a hetero structure based on an IILV semiconductor material, i.e. a body composed of an element from column III and an element from column V of Mendeleev's periodic table. For example, LED sources are formed from gallium nitride (or GaN). The hetero structure is further connected to two metal electrodes which, when a voltage is applied between them, allow the IILV semiconductor material to be excited and the emission of light to be generated.

[0007] The use of LED sources, however, poses the problem of efficiently coupling the emitted light into a waveguide.

[0008] A first difficulty is related to the fact that an LED source emits waves that are incoherent with each other. It is therefore not possible to use a so-called surface coupling technique, which consists of exploiting phase-matching phenomena to choose the propagation mode to which one wishes to transfer optical power.

[0009] A second difficulty is linked to the fact that it is often necessary to choose for the waveguide a material having a refractive index lower than that of the LED. Since light tends to propagate in the material having the highest refractive index, it follows that it can only pass very weakly from the LED to the guide, even if the LED and the guide are purely attached to each other, or integrated with each other.

[0010] More specifically, the confinement of light in the LED is due to the refractive indices and the respective dimensions of the LED source and the waveguide.

[0011] Thus, the III-V semiconductor material of the LED has a higher refractive index than the material used for the waveguide: at 532 nm, the III-V GaN semiconductor material has a refractive index of about 2.4, while the silicon nitride SiN generally used for the waveguide has an index close to 2.

[0012] Moreover, the LED generally has much larger dimensions than the dimensions of the guide. For example, the LED source may have a transverse extent of approximately 1 pm x 0.2 pm and the guide an extent of approximately 150 pm x 0.4 pm.

[0013] To overcome this difficulty and improve the efficiency of coupling the radiation emitted by the LED source in the waveguide, one solution is to form the LED source and the guide from the same layer of material, according to a so-called monolithic integration.

[0014] The document “On-Chip photonic System using suspended pn junction InGaN / GaN multiple quantum wells device and multiple waveguides”, by Wang Y. et al., Applied Physics Letters, 2016 and patent US20180267238 describe a planar monolithic integration of an LED source and a waveguide from a layer of n-doped GaN.

[0015] The paper “Waveguide-coupled nanopillar metal-cavity light-emitting diodes on Silicon” by Dolores-Calzadilla V. et al., Nature Communication, 2017, describes a vertical monolithic integration of an LED source and a waveguide from a p-doped InP layer. The LED source is then arranged on a segment of the waveguide.

[0016] However, using monolithic integration does not solve the problem that a large portion of the radiation emitted by the LED source is not transmitted to the waveguide, because the LED also emits radiation in regions that are not adjacent to the end of the waveguide. Without means of collecting and directing this radiation back to the waveguide, it is lost.

[0017] However, it is not easy to efficiently collect this “lost” radiation because it has a high angle range, or, in other words, a large divergence.

[0018] The previously cited US patent 20180267238 describes a cavity formed under a portion of the LED source and waveguide. This cavity creates an air / GaN interface that is favorable for better utilization of the waves emitted in the vertical direction by the LED source. However, the waves emitted laterally in the direction opposite to the waveguide remain unused.

[0019] The document “Waveguide-coupled nanopillar metal-cavity light-emitting diodes on Silicon” by Dolores-Calzadilla V. et al., Nature Communications, 2017, cited above, describes adapting the structure of the LED source by integrating the source into a metallic cavity. In this structure, the cavity consists of a nanopillar made of IILV semiconductor material, which defines the nanometric LED source. This nanopillar is coated with metal to channel a large portion of the spontaneous emission into the fundamental mode of an InP waveguide.

[0020] The document “Inverse design optimization for efficient coupling of electrically injected optical antenna-LED to a single-mode waveguide,” by Andrade et al., Optics Express, 2019, also describes an LED structure integrated into a metal-enclosed cavity. This cavity acts as a resonant cavity, increasing the spontaneous emission of the source. The structure is coupled vertically or laterally to a waveguide. The waveguide walls near the LED structure are then coated with the same metal used for the metal cavity. However, the presence of metal (from the LED structure or the electrodes) in this area is generally undesirable because it introduces more optical losses in the LED or the waveguides.

[0021] In the field of integrated optical interconnections, there is therefore still a need for an LED source capable of being efficiently injected into a waveguide. Summary of the invention

[0022] The invention provides a solution to the problems discussed above by allowing the lateral radiation emitted backwards by an electrically controlled micrometric LED source to be collected and redirected towards the front of the LED source, where the waveguide is located. Thus, this "rear" lateral radiation which normally never reaches the end of the waveguide is channeled into the numerical aperture of the waveguide. Symmetrically, if the electrically controlled micrometric receiver is a photodiode, the invention allows the radiation emitted forwards by the waveguide to be collected and redirected towards the micrometric receiver.

[0023] For this, the LED source is integrated into a reflective structure of cylindro-parabolic shape configured to reflect the light rays coming from the rear face of this LED source and to converge them at a focal point located at the front of the LED source and in front of the end of the waveguide.

[0024] One aspect of the invention relates to an optical system comprising a substrate, a reflective structure, a photoelectric transducer and a waveguide, • the reflective structure being formed by a cylindro-parabolic pillar placed on the substrate and comprising: • a first internal reflection face with a parabolic profile defined by a plurality of parabolic sections parallel to the plane of the substrate, each parabolic section being defined by a parabola having an optical axis and a focus and extending from the vertex of the parabola to the line perpendicular to the optical axis and passing through the focus, • a second internal face and a third internal reflective face, planar, orthogonal to the plane of the substrate and to the plane containing the optical axes of the plurality of parabolic sections and located on either side of said plane containing the optical axes at the level of the foci of the plurality of parabolic sections, • the photoelectric transducer comprising an active region configured to emit light waves or configured to receive light waves, and arranged so that the active region is positioned in the reflective structure, at a part of the foci of the plurality of parabolic sections, the material of the reflective structure being chosen to be transparent to light waves, • the waveguide having a proximal end, a distal end and a longitudinal axis parallel to the optical axes of the plurality of parabolic sections, and arranged on the substrate so that the proximal end is attached to the reflective structure, between the second and third reflection face and at the level of the active region.

[0025] Thanks to the invention, the reflective structure has a first internal reflection face of parabolic profile defined by a plurality of parabolic sections parallel to the plane of the substrate, each parabolic section being defined by a parabola having an optical axis and a focus and extending from the vertex of the parabola to the straight line perpendicular to the optical axis and passing through the focus.

[0026] Thus, this first internal reflection face acts as a two-dimensional parabolic mirror and obeys in each plane parallel to the plane of the substrate the laws of geometric optics specific to the parabolic mirror.

[0027] Thus, in each plane parallel to the plane of the substrate, the first internal reflective face provides at its focus (located on the optical axis) a perfect image of a point located at infinity. This means that the rays originating from an isotropic light source placed at the focus are reflected (on this first internal reflective face) parallel to the optical axis. Symmetrically, the light rays originating from the focal point are reflected parallel to the optical axis. Elements along the optical axis are reflected by the first internal reflection face at a point located at the focus.

[0028] The set of foci of the plurality of parabolic sections defines a line segment forming the locus of foci of the first internal reflection face.

[0029] Since the photoelectric transducer comprises an active region configured to emit light waves or configured to receive light waves, the active region behaves as an isotropic light source or as a light receiver. By "light waves" is meant waves having at least one wavelength in the spectral band between 200 nm and 1200 nm.

[0030] This active region is positioned in the reflective structure, at a portion of the foci of the plurality of parabolic sections. Thus, the active region has a lateral face (called the rear face) oriented toward the first internal reflection face and located near the location of the foci thereof. In each plane parallel to the plane of the substrate, the active region therefore corresponds to a point located at the focus of the parabolic section. According to the above, the light rays originating therefrom are reflected by the first internal reflection face, parallel to the optical axis. Symmetrically, the light rays originating on the first internal reflection face parallel to the optical axis are reflected toward the active region.

[0031] Furthermore, the reflective structure has a second internal face and a third internal reflection face that are planar, orthogonal to the plane of the substrate and to the plane containing the optical axes of the plurality of parabolic sections.

[0032] Thus, the second and third internal reflection faces act as plane mirrors arranged in front of the two-dimensional parabolic mirror in a plane perpendicular to the plane of the substrate. This means that, in each plane parallel to the plane of the substrate, the light rays having been reflected by the first internal reflection face parallel to the optical axis are reflected in the same direction by one or other of the second or third internal reflection faces. According to the above, these light rays coming from one or other of the second or third internal reflection faces are then directed towards the active region by a new reflection on the first internal reflection face.

[0033] The fact that the second inner face and the third inner reflecting face are located on either side of the plane containing the optical axes at the foci of the plurality of parabolic sections allows the waveguide to be attached to the reflective structure at the level of the active region. Thus, the face of the active region opposite the rear face, called the front face, is attached to, or at least located near, the proximal end of the waveguide.

[0034] It should be added that using a material transparent to light rays emitted or received by the active region for the reflective structure makes it possible to achieve the internal reflection faces from a solid pillar. The advantage is that such a pillar can be made using standard microelectronic manufacturing techniques (e.g. layer structuring). In other words, the entire optical system is compatible with microelectronic manufacturing processes.

[0035] The combination of these different characteristics therefore makes it possible to obtain an integrated optical interconnection (or optical system) in which the photoelectric transducer is efficiently coupled to the waveguide, thereby increasing the fraction of the optical power transmitted to the waveguide (when the photoelectric transducer is a light source) or to the photoelectric transducer (when the photoelectric transducer is a light receiver).

[0036] Advantageously, the photoelectric transducer comprises an upper electrode and a lower electrode, the upper electrode being positioned on an upper face of the reflective structure directly above the predetermined area for the active region, and having lateral dimensions corresponding to the predetermined dimensions for the active region.

[0037] Thus positioned, the upper electrode is located at a distance from the active region and the proximal end of the waveguide. The metal from which it is formed causes little or no disturbance to the light rays, which reduces optical losses and promotes better coupling efficiency. It should also be noted that the position of the upper electrode preserves the transparency of the reflective structure. Furthermore, the size and position of the active region are defined by the size and position of the upper electrode. Thus, no etching operation on the sides of the transducer is necessary, allowing for a simple manufacturing process.

[0038] Advantageously, a first medium is arranged outside the reflective structure in a space adjacent to the first internal reflection face and a second medium is arranged outside the reflective structure and the waveguide in the space adjacent to the second internal reflection face and in the space adjacent to the third internal reflection face, the first and second medium having a refractive index strictly lower than the refractive index of the transparent material chosen for the reflective structure.

[0039] Thus, the internal reflective faces are obtained by an interface between two media. The advantage is that these internal reflective faces can be produced from a solid pillar, simply and in a manner compatible with standard microelectronic manufacturing processes.

[0040] According to a first variant embodiment, the first medium is formed from a dielectric material and the second medium comprises air.

[0041] The dielectric material makes it possible to define the first internal reflection face in at the same time as it allows the sides of the reflective structure to be electrically insulated.

[0042] Air makes it possible to define the second and third internal reflection faces without requiring a metallic material likely to disturb the propagation of light rays near the active region and / or the waveguide. Indeed, unlike the first internal reflection face, the second and third internal reflection faces are located near the waveguide and the active region, and it is preferable to avoid any addition of metal in this area.

[0043] According to a second variant, the first and second mediums are formed from a dielectric material.

[0044] The dielectric material makes it possible to define the internal reflection faces at the same time as it makes it possible to electrically insulate the sides of the reflective structure.

[0045] Advantageously and in a manner compatible with the two previous embodiments, a wall of metallic material is disposed in the first medium and near the first internal reflective face.

[0046] Thus, the metallic material wall allows the first internal reflective face to operate in specular reflection or total internal reflection. The fraction of reflected radiation is increased compared to a transparent material / dielectric medium interface. Since this face is far from the active region and the waveguide, the metal wall does not affect the confinement of light rays at the active region or the proximal end of the waveguide.

[0047] In addition to the characteristics just mentioned in the preceding paragraph, the optical system according to one aspect of the invention may have one or more complementary characteristics from among the following, considered individually or according to all technically possible combinations.

[0048] The photoelectric transducer is formed of a PN junction and quantum wells arranged in the PN junction.

[0049] The PN junction material is a III-V material such as a material based on GaN.

[0050] The transparent material chosen for the reflective structure is the material of the PN junction.

[0051] Thus, the reflective structure and the photoelectric transducer form a single solid piece exhibiting a homogeneous refractive index. The advantage is that the figure of merit of the reflective structure is improved, leading to better coupling efficiency.

[0052] The waveguide is formed from the PN junction material and the quantum wells.

[0053] Thus, the waveguide is attached to the reflective structure without the need to etch the quantum wells of the photoelectric transducer. The advantage is that electrical problems are avoided.

[0054] A layer of metallic material is disposed in the second medium, near the second internal reflection face and near the third internal reflection face.

[0055] Thus, the second and third internal reflection faces behave like total reflection surfaces.

[0056] The reflective structure has a lateral dimension of height in the plane of the substrate strictly greater than a size of the waveguide.

[0057] Preferably, the height is 18 times greater than the size of the guide.

[0058] Thus, even highly divergent light rays (or those making a large angle with respect to the optical axis of the first internal reflecting face) are collected and efficiently redirected towards the active region or waveguide. The reflective structure also remains compact.

[0059] The waveguide comprises a proximal end enlarged relative to the dimensions of the waveguide.

[0060] This waveguide configuration allows for more efficient coupling of the light emitted by the active region and redirected towards the proximal end of the waveguide by the reflective structure.

[0061] Another aspect of the invention relates to a method of producing an optical system according to the first aspect of the invention comprising the steps of: • Providing a substrate comprising a substrate layer, a buffer layer arranged on the substrate layer and a PN junction arranged on the buffer layer, • Three-dimensional structuring of the PN junction to define a top face of the waveguide and part of the parabolic-cylindrical pillar forming the reflective structure, the structuring including lithography substeps • Vertical etching of the PN junction on either side of the parabolic-cylindrical pillar formed to complete the definition of the waveguide and the parabolic-cylindrical pillar forming the reflective structure, and to define a bonding layer, • Passivation by conformal deposition of the first dielectric medium on the outside of the parabolic-cylindrical pillar, at the level of the first internal reflective face, to obtain the reflective structure, • Forming an upper electrode on the upper face of the PN junction at the foci of the plurality of parabolic sections, and a lower electrode on the bonding layer to complete the photoelectric transducer and the optical system.

[0062] The invention and its various applications will be better understood upon reading the description which follows and the examination of the figures which accompany it. BRIEF DESCRIPTION OF THE FIGURES

[0063] The figures are presented for information purposes only and in no way limit the invention. • [Fig.l] schematically represents, in top view, a first embodiment of the optical system, • [Fig.2] shows an enlarged top view of the reflective structure and photoelectric transducer of the optical system of [Fig.l], • [Fig.3] is a sectional view of the optical system of [Fig.l], • [Fig.4] schematically represents, in top view, a second embodiment of the optical system, • Figures 5A to 5E represent in sectional view the steps or sub-steps of manufacturing the optical system of [Fig.l].

[0064] Unless otherwise specified, the same element appearing in different figures has a single reference. DETAILED DESCRIPTION

[0065] The first aspect of the invention relates to an optical system based on a photoelectric transducer formed by a PN junction and laterally integrated with a waveguide. The optical system according to the invention is remarkable in particular in that it makes it possible to improve the lateral coupling of this photoelectric transducer with the waveguide.

[0066] A first embodiment of the optical system will be described with reference to [Fig.l], [Fig.2] and [Fig.3].

[0067] A second embodiment will then be described with reference to [Fig.4].

[0068] In a manner common to the first and second embodiments, the system optical 1 comprises a photoelectric transducer 30, a waveguide 40 and a reflective structure 20 carried by a substrate 10.

[0069] These elements 10, 20, 30 and 40 are described individually below.

[0070] With reference to [Fig. 3], the substrate 10 comprises a substrate layer 11, for example a sapphire substrate layer, a buffer layer 12, for example made of aluminum nitride, arranged on the substrate layer 11. This buffer layer 12 carries the photoelectric transducer 30, the waveguide 40 and the reflective structure 20.

[0071] With further reference to [Fig. 3], the photoelectric transducer 30 comprises a lower region 31, an active region 32 and an upper region 33 formed in a PN junction 13, as well as two electrodes 34, 35, one, referred to as the upper electrode 34, being connected to the P layer 13c of the PN junction 13 and the other, referred to as the lower electrode 35, being connected to the N layer 13a of this PN junction 13 via a bonding layer 16.

[0072] The photoelectric transducer 30 is a receiving optical system when the active region 32 is capable of transforming an optical signal (illustrated by the rays r6 and r8 in [Fig.l]) received from the waveguide 40 into an electrical signal, or an emitting optical system when the active region 32 is capable of generating an optical signal (illustrated in particular by the rays r5 and r7 in [Fig.l]) in the direction of the waveguide 40 under the effect of an electrical signal. The electrical signals are conducted / applied by the electrodes 34, 35.

[0073] In the remainder of the description, a photoelectric transducer 30 of the emitter type will be considered, therefore having an active emitter region 32. The general principle of the invention, however, naturally applies to a receiving photoelectric transducer comprising an active receiving region 32.

[0074] It should be noted that the terms “optical signal” or “light waves” designate electromagnetic waves comprising at least one wavelength in the spectral band ranging from 200 nm to 1200 nm.

[0075] The material of the photoelectric transducer corresponds to the material 13 forming the PN junction.

[0076] This material 13 is a semiconductor material such as a III-V semiconductor material or an IV-IV semiconductor, or even an ILVI semiconductor, having a high refractive index in the spectral band 200 nm - 1200 nm, for example greater than 2.

[0077] Preferably, the material 13 of the photoelectric transducer is an IILV semiconductor material.

[0078] As used herein, the term “IILV semiconductor material” means and includes any semiconductor material that is at least primarily composed of one or more elements from Group IIIA of the periodic table (B, Al, Ga, In, and Ti) and one or more elements from Group VA of the periodic table (N, P, As, Sb, and Bi). For example, IILV semiconductor materials include, but are not limited to, GaN, GaP, GaAs, InN, InP, etc.

[0079] For example, the photoelectric transducer material is based on gallium nitride GaN. The refractive index of such a material is 2.4 at a wavelength of 532 nm.

[0080] The lower 31, active 32 and upper 33 regions are respectively formed in the N layer 13a, the quantum wells 13b and the P layer 13c of the PN junction 13. They are connected successively from bottom to top.

[0081] For example, the N 13a layer is an nGaN layer, the P 13c layer is a pGaN layer and the quantum wells 13b are formed from an alternation of InGaN and deGaN layers.

[0082] The active region 32 is defined by the upper electrode 34 and the configuration of the PN junction.

[0083] Thus, the position and lateral dimensions of the upper electrode 34 determine the position and dimensions of the active region 32 and of the upper and lower regions 31,32 on the substrate 10. In other words, the upper electrode 34 defines the lateral dimensions and the position of the photoelectric transducer 30.

[0084] These lateral dimensions Lel, Le2 of the upper electrode 34 are shown in [Fig.2]. Preferably, they are of the order of a few hundred nanometers, for example 200 nm or 400 nm. For the sake of simplicity, the first lateral dimension Lel can be taken as being equal to the second lateral dimension Le2. As will be described a little later in the description, the lateral dimensions Lel and Le2 of the upper electrode 34 are linked to the dimensions of the reflective structure 20 and are therefore determined as a function of the dimensions of this reflective structure 20.

[0085] The configuration of the PN junction determines the optical and photoelectric properties of the active region 32.

[0086] The configuration of the PN junction is thus chosen so that the active region behaves, under the effect of an electrical excitation provided by the electrodes 34, 35, like a point and isotropic light source, this in each plane parallel to the plane of the substrate 10. The wavelength of the radiation is for example located in the green, around 532 nm.

[0087] The radiation emitted by the active region 32 in a plane i parallel to the plane of the substrate 10 is illustrated in [Fig. 1] using a geometric optical description. The radiation is emitted with a large angular range θ, as illustrated by the rays ri0, r20, r5, r7 shown in [Fig. 1]. Part of the rays is emitted (for example, the rays ri0, r20) by a lateral face, referred to as the rear face 321 (see [Fig. 3]), of the active region 32. Another part is emitted by a lateral face opposite the rear face 321 (see [Fig. 3]), referred to as the front face 322 of the active region 32.

[0088] With reference to [Fig.l], the reflective structure 20 surrounds the photoelectric transducer 30 at the level of the stack formed by the lower 31, active 32 and upper 33 regions.

[0089] Specifically, the reflective structure 20 forms, with the lower 31, active 32, and upper 33 regions of the photoelectric transducer 30, a solid pillar 20 of cylindro-parabolic shape. In other words, the pillar 20 is a truncated right cylinder, with its axis and generatrices perpendicular to the plane of the substrate 10, and having as its directrix a parabola with axis Xoi parallel to the plane of the substrate 10 and having as its focus a point E located on the axis Xoi. This right cylinder is truncated by a plane parallel to the generatrices and including the focus F. The pillar 20 is thus delimited by a cylindrical surface comprising a parabolic lateral surface 25 and a flat lateral surface 26 and by two flat faces 27, 28 (see [Fig.3]) parallel to the plane of the substrate 10.

[0090] The material 21 chosen for the reflective structure 20 is a material transparent to light waves emitted by the active region 32. It has a refractive index close to or equal to the refractive index of the photoelectric transducer material in the wavelength range 400 nm - 800 nm. By "close to" is meant that the difference in index is less than 0.2.

[0091] Preferably, the transparent material 21 chosen for the reflective structure 20 is identical to the material 13 of the photoelectric transducer 30. When the latter is based on GaN, the refractive index of the reflective structure is 2.4.

[0092] The reflective structure 20 comprises, near the parabolic lateral surface 25, a first internal reflection face 22 and, near the flat lateral surface 26, a second internal reflection face 23 and a third internal reflection face 24.

[0093] With reference to [Fig. 1], the first internal reflection face 22 has a parabolic profile defined by a plurality of parabolic sections parallel to the plane of the substrate 10. Such a section is illustrated in [Fig.l]. Each parabolic section is defined by the parabola having the optical axis Xoi and the focus F;, and extends from the vertex Si of the parabola to the line y perpendicular to the optical axis Xoi and passing through the focus F;.

[0094] The second internal face 23 and the third internal reflecting face 24 are planar faces, orthogonal to the plane of the substrate 10 and to the plane containing the optical axes Xoi of the plurality of parabolic sections. These faces 23, 24 are further situated on either side of said plane containing the optical axes Xoi at the level of the foci F of the plurality of parabolic sections.

[0095] The internal reflective faces 22, 23, 24 are capable of reflecting light waves emitted by the active region inside the reflective structure 20 into the interior of the reflective structure 20. In other words, the internal faces 22, 23 and 24 act as semi-reflective surfaces.

[0096] To this end, a first medium 51 is disposed outside the reflective structure 20 in a space adjacent to the first internal reflection face 22. In other words, the second medium 51 encloses the parabolic lateral surface 25 of the reflective structure 20 along its entire height. Furthermore, a second medium 52 is disposed outside the reflective structure on the side of the flat lateral surface 26, in the space adjacent to the second internal reflection face 23 and in the space adjacent to the third internal reflection face 24.

[0097] The first and second media 51,52 have a refractive index strictly lower than the refractive index of the transparent material 21 chosen for the reflective structure (or of the material 13 of the photoelectric transducer 30).

[0098] Preferably, the first medium 51 is formed of a dielectric material and the second medium 52 includes air.

[0099] The dielectric material is for example silicon oxide.

[0100] Thus the refractive index of the first and second media 51,52 is close to 1, while the refractive index of the reflective structure 20 and the photoelectric transducer 30 is 2.4. The first dielectric medium 51 then forms a dielectric wall 51 surrounding the parabolic lateral surface of the reflective structure 20.

[0101] Another possibility is that the first and second mediums 51,52 are both formed of a dielectric material (not shown in Figures 1, 2 or 3).

[0102] To complete the description of the reflective structure 20, it should be specified that it has lateral dimensions (see [Fig.2]) of length Lsrl along the optical axis Xoi, and of height Lsr2 along the line y, as well as a height Lsr3 (along an axis perpendicular to the plane of the substrate 10, see [Fig.3]).

[0103] For example, the lateral length dimension Lsrl is 1 micron and the lateral height dimension Lsr2 is 4 microns. The lateral height dimension Lsr2 is related to the lateral length dimension Lsrl by the equation of the parabola.

[0104] With reference to [Fig.1], the waveguide 40 has a proximal end 41, a distal end 42 (not shown in [Fig.1]), and a longitudinal axis Xg.

[0105] The waveguide 40 is formed of a semiconductor material 43 having a refractive index, called the refractive index of the waveguide, of the same order of magnitude as the refractive index of the material 13 of the photoelectric transducer 30. The waveguide 40 is further surrounded by one or more media 52, 12 having a refractive index strictly lower than the refractive index of the waveguide 40 (and of the material 13 of the photoelectric transducer).

[0106] The waveguide 40 is for example formed from silicon nitride SiN.

[0107] Preferably, the waveguide 40 is formed of the same material as the material 13 of the photoelectric transducer 30 and includes the quantum wells 13b.

[0108] The arrangement of the photoelectric transducer 30, the reflective structure 20 and the waveguide 40 in the optical system 1 relative to each other is described below.

[0109] First, the photoelectric transducer 30 is arranged so that the active region 32 is positioned in the reflective structure 20, at a portion of the foci F of the plurality of parabolic sections. The lateral dimensions Le1 and Le2 of the upper electrode 34 (and therefore of the active region 32) are at least ten times smaller than the lateral length and height dimensions Ls1 and Ls2 of the reflective structure 20.

[0110] In [Fig.1], the active region 32 occupies an area corresponding to the upper electrode 34. This is positioned laterally to or near the focus F; of the first internal reflection face 22.

[0111] Thus, the rear lateral face 321 of the active region is oriented towards the parabola formed by the first internal reflection face 22.

[0112] Secondly, the waveguide 40 is arranged so that its longitudinal axis Xg is parallel to the optical axes Xoi of the plurality of parabolic sections and that its proximal end 41 is abutted to the reflective structure 21 between the second and third internal reflection faces 23, 24 (laterally, see [Fig. 1]) and at the level of the active region 32 (vertically, see [Fig. 3]). The term "abutted" refers to the fact that the proximal end 41 of the waveguide 40 is in contact with the reflective structure and the active region 32.

[0113] When the waveguide 40 is formed of the same material 13 as the photoelectric transducer 30 and includes the quantum wells 13b, these are aligned with the longitudinal axis Xg of the waveguide (see [Fig. 3]). In this case, the waveguide 30 is joined to the reflective structure 20 and to the active region 32 by monolithic integration.

[0114] Thirdly, the lateral dimensions Lsrl and Lsr2 (see [Fig. 2]) of the reflective structure 20 are strictly greater than the size Lg2 of the waveguide. For example, the lateral height dimension Lsr2 of the reflective structure 20 is 4 microns for a waveguide size Lg2 of 400 nm. In another example, the lateral height dimension Lsr2 of the reflective structure 20 is 4 microns for a waveguide size Lg2 of 220 nm. The lateral height dimension Lsr2 is then 18 times greater than the size Lg2 of the waveguide 40.

[0115] Thanks to the elements just described and their particular arrangement, the optical system 1 makes it possible, in each plane parallel to the substrate and over the entire height Lsr3 of the reflective structure 20, to channel the light rays coming from the rear face 321 of the active region 32 and direct them towards the proximal end 41 of the waveguide 40. It should be noted that in the absence of the reflective structure 20, these rays do not reach the proximal end 41 of the waveguide 40. Thus, the brightness (or optical power) at the output of the assembly formed by the photoelectric transducer and the reflective structure is increased.

[0116] The path followed by the light rays coming from the rear face 321 of the active region 32 is illustrated using the rays ri0 and r20 shown in [Fig.1].

[0117] As the active region is located at the foci of the first internal reflection face 22, the rays ri0 and r20 are reflected on the first internal reflection face 22 parallel to the optical axis Xoi. This first reflection gives rise to the rays ru and r2[ shown in [Fig.1].

[0118] Since the second and third reflecting faces 23,24 are planar and orthogonal to the plane of the substrate and to the plane containing the optical axes of the plurality of parabolic sections, the rays reflected once ri2 and r2i are reflected a second time and redirected in the same direction, which is therefore parallel to the axis Xoi optics. This second reflection gives rise to rays rn and r23.

[0119] Finally, these rays from two successive reflections are reflected a third time on the first internal reflection face 22. As they arrive at the first internal reflection face 22 parallel to the optical axis Xoi, the reflected rays rM and r24 are directed towards the focus F; in front of which is located the proximal end 41 of the waveguide 40.

[0120] The fact that the lateral height dimension Lsr2 of the reflective structure 20 is much greater, for example 18 times greater, than the size Lg2 of the waveguide 40 allows to channel rays emitted with a large angle 0 with respect to the optical axis Xoi.

[0121] The second embodiment illustrated in [Fig.4] differs from the first embodiment in two points.

[0122] First, the first internal reflective face 22 is obtained by means of a wall of metallic material 60 disposed in the first medium 51 and close to the first internal reflective face 22. The first internal reflective face 22 thus operates in total reflection.

[0123] Next, the waveguide 40 has an enlarged proximal end 41 and a funnel shape 44 at this end 4L. In this configuration, the lateral dimensions Lel, Le2 of the active region 32 are adapted to the size of the enlarged end 41, this via the size of the upper electrode 34. This configuration is favorable to better coupling of the incident optical power at the end 41 of the waveguide.

[0124] It should be noted that a layer of metallic material (not shown in [Fig.3]) can also be disposed in the second medium 52, near the second internal reflection face 23 and near the third internal reflection face 24 so that the second and third faces 24,25 also work in total reflection.

[0125] A second aspect of the invention relates to a method 500 for manufacturing the optical system 1.

[0126] Figures 5A to 5E illustrate in cross-sectional view steps or sub-steps of manufacturing the optical system of [Fig.1], according to a preferred embodiment.

[0127] The process 500 begins at step S501 illustrated in [Fig. 5A] with the provision of a substrate 10 comprising the substrate layer 11, the buffer layer 12 and the PN junction 13, the latter comprising a lower N layer 13a, quantum wells 13b and an upper P layer connected successively from bottom to top. The PN junction 13 is obtained, for example, by epitaxial growth and doping.

[0128] This step S501 is extended with step S502 shown in [Fig. 5B]. Step S502 consists of structuring the PN 13 junction in three dimensions by lithography to create recesses and thus define an upper face 45 of the waveguide 40 and delimit a part of the lateral surfaces of the reflective structure 20.

[0129] Following this initial three-dimensional structuring S502, a cylindrical-parabolic pillar 20a, conforming to the pillar forming the reflective structure 20, is created in the PN junction. This pillar 20a thus comprises the parabolic lateral face 25 and the planar lateral face 26 described previously.

[0130] Step S503 follows step S502. It is illustrated in [Fig.5C]. Step S503 is a vertical etching step, also called mesa etching, carried out on either side of the pillar 20a formed in step S502 to define the waveguide 40 at the level of the flat lateral face 26 of the pillar 20a, to complete the definition of the cylindro-parabolic pillar 20b forming the reflective structure 20, and to define the bonding layer 16.

[0131] The vertical etching step S503 is followed by a passivation step S504, illustrated in [Fig. 5D], aimed at conformally depositing a dielectric layer 51 on the parabolic lateral surface 25 of the pillar 20b. The flat lateral surface 26 as well as the lateral and upper faces of the waveguide are in contact with air 52.

[0132] The dielectric layer 51 electrically isolates the parabolic lateral surface 26 from the reflective structure 20 while simultaneously creating an interface between the material 13 of the PN junction and the dielectric, the latter having very contrasting refractive indices (2.4 for example for a PN junction in GaN, and 1 for the dielectric).

[0133] At the end of step S504, the substrate 10 carries the waveguide 40 and the reflective structure 20. The reflective structure also includes the first internal reflection face 22 and the second and third internal reflection faces 23, 24. Finally, the waveguide is attached laterally to the reflective structure 20 at its planar lateral surface 26, between the second and third internal faces 24, 25 (not shown in [Fig. 5D]) and at the quantum wells 13b, which are distributed along its longitudinal axis Xg. A bonding layer 16 connected to the lower region 13a has also been defined on the side of the parabolic lateral surface 25 of the reflective structure 20.

[0134] Step S505, which follows step S504 of passivation, is illustrated in [Fig. 5E]. This step S505 consists of the formation of the upper electrodes 34 and lower electrodes 35.

[0135] For this purpose, a first metallic motif 34 is formed on the upper face of the cylindrical-parabolic pillar 20b, near the flat lateral surface 26, which corresponds to the level of the foci of the first internal reflection face 22.

[0136] A second metallic wall-shaped pattern 35 is also formed on the bonding layer 16. The metallic wall 35 may or may not take the shape of the parabolic lateral surface 25.

[0137] A combination of photolithography, resin deposition and removal can be used at this step S505.

[0138] At the end of this step S505, the photoelectric transducer is obtained and the system Optics 1 is finalized.

[0139] The method 500 which has just been described relates to a monolithic integration of the waveguide, the reflective structure and the photoelectric transducer.

[0140] Naturally, this process can also be adapted in the case where the waveguide 40 and the reflective structure are made of materials different from the material of the photoelectric transducer. In this case, the structuring step S502 comprises a combination of structuring and epitaxial substeps allowing the definition of 1) the photoelectric transducer, 2) the parabolic cylindro-pillar 20b and 3) the upper face 45 of the waveguide 40.

Claims

Claims

1. Optical system (1) comprising a substrate (1), a reflective structure (20), a photoelectric transducer (30) and a waveguide (40), - the reflective structure (20) being formed by a cylindro-parabolic pillar (20b) arranged on the substrate (10), the cylindro-parabolic pillar being delimited by a cylindrical surface comprising a parabolic lateral surface (25) and a flat lateral surface (26) and by two flat faces (27, 28) parallel to a plane of the substrate (10), and comprising: • a first internal reflection face (22) of parabolic profile defined by a plurality of parabolic sections parallel to the plane of the substrate (10), each parabolic section being defined by a parabola having an optical axis (Xoi) and a focus (FO and extending from the vertex (Si) of the parabola to the straight line (y) perpendicular to the optical axis (Xoi) and passing through the focus (FO, • a second internal face (23) and a third internal face (24) of planar reflection, orthogonal to the plane of the substrate (10) and to the plane comprising the optical axes of the plurality of parabolic sections and situated on either side of said plane comprising the optical axes at the level of the foci (FO) of the plurality of parabolic sections, - the photoelectric transducer (30) comprising an active region (32) configured to emit light waves or configured to receive light waves, and arranged such that the active region (32) is positioned in the reflective structure (20), at a portion of the foci (Fi) of the plurality of parabolic sections, the material (21) of the reflective structure (20) being chosen to be transparent to light waves, - the waveguide (40) having a proximal end (41), a distal end (42) and a longitudinal axis (Xg) parallel to the optical axes (Xoi) of the plurality of parabolic sections, and arranged on the substrate (10) so that the proximal end (41) is attached to the reflective structure (20), between the second and third reflection faces (23,24) and at the level of the active region (32).

2. Optical system (1) according to claim 1 characterized in that a first medium (51) is arranged outside the reflective structure (20) in a space adjacent to the first internal reflection face (22) and a second medium (52) is arranged outside the reflective structure (20) and the waveguide (40) in the space adjacent to the second internal reflection face (23) and in the space adjacent to the third internal reflection face (24), the first and second medium (51, 52) having a refractive index strictly lower than the refractive index of the transparent material (21) chosen for the reflective structure (20).

3. Optical system (1) according to claim 2 characterized in that the first medium (51) is formed of a dielectric material and the second medium (52) comprises air.

4. Optical system (1) according to claim 2 characterized in that the first and second medium (51, 52) are formed from a dielectric material.

5. Optical system according to one of claims 3 to 4 characterized in that a wall (60) of metallic material is arranged in the first medium (51) and close to the first internal reflection face (22).

6. Optical system (1) according to one of claims 1 to 5 characterized in that the photoelectric transducer (30) is formed of a PN junction (13) and quantum wells (13b) arranged in the PN junction (13) and the transparent material (21) chosen for the reflective structure (20) is the material (13) of the PN junction.

7. Optical system (1) according to claim 6 characterized in that the waveguide (40) is formed from the material (13) of the PN junction and the quantum wells (13b).

8. Optical system according to one of claims 6 to 7 characterized in that the material (13) of the PN junction is a III-V material such as a GaN-based material.

9. Optical system (1) according to one of claims 1 to 8 characterized in that the photoelectric transducer (30) comprises an upper electrode (34) and a lower electrode (35), the upper electrode (34) being positioned on an upper face (27) of the reflective structure

10.

11.

12. (20) directly above the predetermined area (32) for the active region, and having lateral dimensions (Lel, Le2) corresponding to the predetermined dimensions for the active region (32). Optical system (1) according to one of claims 1 to 9, characterized in that the reflective structure (20) has a lateral dimension (Lsr2) of height in the plane of the substrate (10) strictly greater than a size (Lg2) of the waveguide. Optical system (1) according to one of claims 1 to 10, characterized in that the waveguide (40) comprises a proximal end (41) widened relative to the dimensions (Lg2) of the waveguide. Method (500) for producing an optical system (1) according to one of claims 1 to 11 comprising the steps of: Providing (S501) a substrate (10) comprising a substrate layer (11), a buffer layer (12) arranged on the substrate layer (11) and a PN junction (13) arranged on the buffer layer (12), Three-dimensional structuring (S502) of the PN junction (13) so as to define an upper face (45) of the waveguide (40) as well as a part of the cylindro-parabolic pillar (20a) forming the reflective structure (20), the structuring (S502) comprising lithography sub-steps, Vertical etching (S503) of the PN junction (13) on either side of the cylindro-parabolic pillar (20a) formed to complete the definition of the waveguide (40) and the cylindro-parabolic pillar (20b) forming the reflective structure (20), and defining a bonding layer (16), Passivation (S504) by conformal deposition of the first dielectric medium (51) outside the cylindrical-parabolic pillar (20b), at the level of the first internal reflection face (22), to obtain the reflective structure (20), Forming (S505) an upper electrode (34) on the upper face (27) of the PN junction at the foci (FO) of the plurality of parabolic sections, and a lower electrode (35) on the bonding layer (16) to finalize the photoelectric transducer (30) and the optical system (1).

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