Large-area surface-emitting high-power single-mode solid semiconductor photonic crystal lasers

Abstract

Photonic crystal layer, PC layer, (100), for a surface-emitting photonic crystal laser, PCSEL, (200), comprising a semiconductor layer (10) consisting of a first semiconductor material and including a structured region (12) in which a plurality of adjacent square PC unit cells (20) with a common side length A form a periodic refractive index modulation grating structure, wherein each PC unit cell (20) comprises a symmetrical elongated isosceles triangle, SIT, (30) of a second semiconductor material having a lower or higher refractive index than the first semiconductor material, wherein the SIT (30) has a base length B and a base angle θ, the upper corner (31) of the SIT (30) coinciding with a corner (21) of the PC unit cells (20), the base length B being between 0.60A and 0.85A and the base angle θ is between 60° and 80°,and wherein the first base corner (32) of the SIT (30) and the second base corner (33) of the SIT (30) are located outside the PC unit cell (20) or each coincide with a side of the PC unit cell (20).

Description

[0001] The present invention relates to a photonic crystal (PC) layer for a surface-emitting photonic crystal laser (PCSEL) and a large-area, high-power, single-mode solid semiconductor PCSEL with such a PC layer. In particular, the present invention relates to an improved PCSEL that exhibits small one-dimensional (1D) feedback, large two-dimensional (2D) feedback, and large mode discrimination due to a specially optimized PC unit cell design of the PC layer. Technological background

[0002] Single-mode diode laser emitters with a narrow far field (< 1°), a narrow spectral width (< 1 nm), high power conversion efficiency (> 50%), and high continuous-wave (CW) power (several tens of W) are required for many applications. However, with conventional diode laser technologies, it is difficult to provide diode lasers optimized for all these parameters. For example, single GaAs-based edge emitters with lateral single-mode waveguides (e.g., web waveguides) whose wavelength is stabilized by external or internal gratings typically require expensive narrow far-field collimation optics and deliver a CW power of < 5 W (see M. Wilkens et al., “Externally Wavelength-Stabilized Single Mode Lasers with 65% Conversion Efficiency and 50 pm Spectral Width at 1 W Output,” Proc.2021 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO / Europe-EQEC), Munich, Germany, 2021, p. 1-1). Single-mode, vertical cavity surface-emitting multi-junction lasers (VCSELs) typically deliver only low power (< 20 mW) and also require costly collimation optics for operation in the narrow far field (see MM Dummer et al., “Single-mode multi-junction VCSELs with integrated transverse mode filter”, Proc. SPIE 12439, 1243908 (2023)).

[0003] In recent decades, a relatively new diode laser technology has emerged, based on so-called photonic crystal surface-emitting lasers (PCSELs). PCSELs have attracted considerable attention due to their ability to outperform conventional semiconductor lasers by providing high output power while maintaining single-mode emission. PCSELs are a modern type of electrically pumped surface-emitting semiconductor laser that utilizes a two-dimensional photonic crystal (PC) structure. This structure forms an in-plane laser resonator and also diffractes light out of the laser plane to achieve near-diffraction-limited surface emission. The basic architecture of a PCSEL includes a two-dimensional PC structure that acts as an in-plane (lateral) cavity.This structure essentially consists of a thin layer of a semiconductor material (e.g., GaAs, GaN, or InP) comprising a pattern (e.g., square or triangular) of air holes extending over a specific area. Preferably, the light is emitted from the substrate of the device; therefore, the semiconductor material must be transparent (non-absorbing) to the laser radiation. Coupling the PC structure with a thin active layer beneath the PC layer achieves laser amplification through stimulated emission. Typically, the active region is separated from the PC structure only by a thin waveguide layer.

[0004] PCSELs have made significant progress, including the demonstration of large-area coherent oscillation, beam pattern control, beam steering, and the realization of watt-range operation with high beam quality. However, due to losses associated with air-hole-based PC technology, efficiency is typically low. For example, recently introduced large-aperture (3 mm) PCSELs based on double-grate air-hole PC designs offer power conversion efficiencies of around 30% (M. Yoshida et al., “High-brightness scalable continuous-wave single-mode photonic-crystal laser”, Nature 618, 727–732 (2023)).

[0005] Although edge-emitting solid semiconductor DFB lasers can be fabricated with negligible losses due to grating technology (see CM Schultz et al., “In situ etched gratings embedded in AlGaAs for efficient high power 970 nm distributed feedback broad-area lasers”, Applied Physics Letters 100, 201115 (2012)), they have not yet been realized as large-area PCSELs with conventional PC designs because they have a low index contrast and etching very small structures is difficult, which usually leads to highly multimodal operation of large-area PCSELs.

[0006] Therefore, there is a need for an improved PCSEL design that overcomes or at least significantly reduces the shortcomings of the state of the art and also enables the provision of diode laser single emitters with narrow far field < 1°, narrow spectrum < 1 nm, high power conversion efficiency > 50% and high CW power (several tens of W).

[0007] US 2019 / 0288483 A1 relates to a single light-emitting semiconductor element with a plurality of light-emitting sections, each of which is capable of producing light with a desired ray projection pattern, and a method for manufacturing the light-emitting semiconductor element. Brief description of the invention

[0008] The present invention solves this objective problem by providing a photonic crystal (PC) layer for a surface-emitting photonic crystal laser (PCSEL) according to claim 1 and a PCSEL comprising such a PC layer according to claim 6. Preferred embodiments of the present invention are specified in the dependent claims.

[0009] According to one aspect of the present invention, a photonic crystal (PC) layer is provided for a surface-emitting photonic crystal laser (PCSEL), wherein the PC layer comprises a semiconductor layer consisting of a first semiconductor material and comprising a structured region in which a plurality of adjacent square PC unit cells with a common side length A form a periodic refractive index modulation grating structure, wherein each PC unit cell comprises a symmetric stretched isosceles triangle (SIT) of a second semiconductor material having a lower or higher refractive index than the first semiconductor material, wherein the SIT has a base length B and a base angle θ.However, of these two alternatives, the one in which the second semiconductor material has a lower refractive index than the first semiconductor material is preferred, wherein the upper corner of the SIT coincides with a corner of the PC unit cells, wherein the base length B is between 0.60A and 0.85A and the base angle θ is between 60° and 80°, and wherein the first base corner of the SIT and the second base corner of the SIT are outside the PC unit cell or each coincide with a side of the PC unit cell.

[0010] The present invention is based on an improvement of a 2D PC structure consisting of a square grid in which each grid element is associated with a hole in the form of an air-filled right-angled isosceles triangle (RIT) (K. Hirose et al., “Watt-class high-power, high-beam-quality photonic-crystal lasers”, Nature Photonics 8, 406–411 (2014)). The RIT design is based on a significantly asymmetric structure in the form of RIT-shaped air holes. Such RIT shapes are advantageous for high-power PCSEL operation because they circumvent a field cancellation effect that occurs in PC structures with a symmetrical shape (such as a circle) and leads to the suppression of diffracted vertical emission from the 2D PC. Therefore, higher extraction efficiency can be achieved with the RIT-shaped air holes.

[0011] However, the presence of air holes (cavities) in semiconductor lasers leads to high stray (optical) and charge carrier (Shockley-Read-Hall) losses and also increases electrical resistance (current cannot flow through the cavities). To enable both higher power conversion efficiency and scaling of the device size for higher power with a narrow far field, the air holes of the original PC design are replaced with a different semiconductor material, resulting in an all-semiconductor PCSEL and thus avoiding losses due to the presence of air holes (cavities). The properties of the PCSEL must then be further optimized with regard to the dimensions of the PC structures (here, the triangular SIT structure), the refractive index contrast, and other design parameters.

[0012] The suitable dimensions for the PC structure design according to the invention were determined using a specially developed simulation tool. A clear process window for symmetrical SIT-shaped structures in the PC unit cell was identified, within which large-area, all-semiconductor PCSELs with high mode discrimination can operate while simultaneously enabling high power, high efficiency, a narrow spectrum, and a narrow far field. Since these properties correlate in the PC structures and a large number of different parameters must be considered during optimization, an analytical approach is generally not feasible. Therefore, numerical methods had to be used to simulate the properties of RIT- or SIT-based PC structures.

[0013] The upper corner of the SIT coincides with a corner of the PC unit cells. The upper corner of each triangle thus has a clearly defined position relative to its respective PC unit cell.

[0014] Preferably, the first refractive index n1 of the first semiconductor material and the second refractive index n2 of the second semiconductor material differ by between 2% and 20%, more preferably by between 2% and 25%, and even more preferably by between 2% and 35% or more. In particular, the material with the lower index can correspond to the SIT structures, and the material with the higher index can correspond to the surrounding semiconductor.

[0015] Preferably, the first semiconductor material is GaAs and the second semiconductor material is a compound of In x Ga 1-x As 1-y P y , for example In x Ga 1-xP (InGaP), where x is chosen to be a near-grating match to GaAs. In the visible (VIS) and near-infrared (NIR) spectrum, for example, the refractive index of GaAs can be in a range between 3.3 and 3.6, while a typical range for the refractive index of In x Ga 1-x P can lie in a range between 3.5 and 3.2. However, the present invention is not limited to these specific materials, and other semiconductor materials can also be combined to achieve the advantageous effects of the present invention. In particular, GaAs and AlAs, GaAs and AlGaAs, InP and AllnAs, InP and InGaAsP, or more generally, any two semiconductor compounds from the Al-In-Ga-As-P-Sb-N compound semiconductor material systems can be combined as the first semiconductor material and the second semiconductor material.

[0016] Preferably, the structured region has a diameter D > 0.5 mm, more preferably D > 1 mm, and / or the thickness d of the PC layer is between 50 nm and 1,000 nm. The diameter can be defined directly in the case of a circular structured region, or as the lateral extent (e.g., the diameter of a circumscribed circle) of a structured region with a different shape (e.g., rectangular, square, or elliptical). These are typical dimensions for PC layers in PCSEL, and they enable the simple fabrication of the PC structures with conventional methods in high precision and with aperture-wide structural uniformity.

[0017] In a conventional RIT design, the ratio of the base length B to the side length A of the PC unit cell is typically between 0.72A and 2A, where the base angle θ is 45°. For PCSELs based on RIT designs, the maximum PCSEL size, and therefore the maximum achievable performance in a narrow far field, is severely limited by the occurrence of higher-order modes.

[0018] By using a stretched isosceles triangle in the novel SIT design instead of a conventional right-angled isosceles triangle (RIT) design, the PC structure can be optimized using fewer parameters, while maintaining narrow far-field operation even with significantly larger device sizes. In particular, the defined process window was found by simulating various structures exhibiting small one-dimensional (1D) feedback, large two-dimensional (2D) feedback, and large mode discrimination due to a specially optimized photonic crystal unit cell design of the PC structure. One-dimensional feedback could easily lead to the oscillation of separated modes, resulting in wider far fields, while large 2D coupling improves the spectral filtering of the PC structure.

[0019] The inventors found that, with low 1D coupling and high 2D coupling, the base length B is preferably between 0.60A and 0.85A, more preferably between 0.65A and 0.80A, and even more preferably between 0.70A and 0.75A. The base angle θ can preferably be between 60° and 80°, more preferably between 65° and 75°, and even more preferably between 67° and 73°.

[0020] According to a further aspect of the present invention, a surface-emitting photonic crystal laser (PCSEL) is provided comprising: a PC layer according to the invention; an active layer (i.e., a layer in which opposite charge carriers are combined to generate photons); and a filter layer (or reflector layer) (e.g., a vertical filter / reflector layer) with a wavelength-selective element (e.g., a distributed Bragg reflector, DBR), wherein the PC layer, the active layer, and the filter layer form a stacked layer structure (e.g., a three-dimensional, 3D, layer structure) configured such that electromagnetic radiation emitted by the active layer is coupled into and guided within the stacked layer structure.

[0021] Photons generated in the active layer are coupled into the PC structure, which acts as a 2D resonator for the photons. The triangles provide effective in-plane feedback but simultaneously function as scattering structures, allowing coupling in and out in both directions perpendicular to the plane of the PC layer. Thus, the PC structures act as a filter element, enabling optical feedback with the active layer on one side (e.g., from the bottom) and allowing the filtered electromagnetic radiation to be coupled out on both sides of the active layer.

[0022] The described PCSEL structure corresponds to a conventional PCSEL design but utilizes the advantages of an improved PC layer according to the invention. Furthermore, the filter layer can act as a reflector, further reducing laser losses. In a preferred embodiment, the PC layer can be located between the active layer and the filter layer. In this case, any electromagnetic radiation coupled out from the PC layer in the direction away from the laser's active layer can still be reflected back as useful emission. Thus, established filter layers (e.g.,vertical reflector layers), as known from vertical cavity surface-emitting lasers (VCSELs), are combined with new PC layers according to the present invention to provide a large-area high-power single-mode solid semiconductor PCSEL with a narrow far field < 1°, a narrow spectrum < 1 nm, a high power conversion efficiency > 50% and at the same time a high continuous wave power (several 10 W).

[0023] Preferably, the side length A of the PC unit cells corresponds to one period of a second-order grating for reflecting an emission wavelength of the PCSEL. The filter function of the PC structure must therefore be adapted to the emission wavelength of the PCSEL to accommodate the filter function of the PC structure described above. It has been shown that if the side length A of the PC unit cell corresponds to one period of a second-order grating for reflecting an emission wavelength of the PCSEL, the resulting structure sizes can be easily fabricated using standard methods, while the emission wavelength of the laser can be easily varied by simply changing the side length A of the PC unit cell and the parameters that depend on the side length A.

[0024] Preferably, the wavelength-selective element comprises a distributed Bragg reflector (DBR) for reflecting an emission wavelength of the PCSEL. The use of DBRs for VCSELs is known and can be directly transferred to the PCSELs according to the invention.

[0025] Further preferred embodiments of the invention result from the features mentioned in the dependent claims.

[0026] The various embodiments and aspects of the invention mentioned in this application can be advantageously combined with one another, unless otherwise specified in individual cases. Brief description of the drawings

[0027] The invention is described in more detail below with reference to the drawings. The examples given serve to describe the invention and are not intended to limit the present invention to the described embodiments. The drawings show: Fig. 1 a schematic representation of an embodiment of a photonic crystal (PC) layer; Fig. 2 a schematic representation of PC unit cells based on a right-angled isosceles triangle (RIT) design according to the prior art; Fig. 3 a schematic representation of a PC unit cell with a RIT according to the state of the art; Fig. 4 a schematic representation of a PC unit cell with an embodiment of a SIT according to the invention; Fig. 5 a schematic representation of a PC unit cell with a further embodiment of a SIT according to the invention; Fig. 6 a schematic representation of a PC unit cell with yet another embodiment of a SIT according to the invention; Fig. 7 a schematic representation of an exemplary embodiment of a surface-emitting photonic crystal laser (PCSEL) according to the invention; Fig. 8 an exemplary vertical cross-section of a refractive index profile of a PCSEL according to the invention; Fig. 9 exemplary representations of optical 2D (|κ 2D+ | and |κ 2D- |)- and 1D (|κ 1D |)-Coupling strengths in the plane for an infinite PC layer according to the present invention; Fig. 10 exemplary representations of mode gain separation and mode radiation loss in an infinite PC layer according to the present invention and Fig. 11 Diagrams of mode separation, external differential efficiency and threshold current for an exemplary PCSEL of finite size according to the present invention. Detailed description of the invention

[0028] Fig. Figure 1 shows a schematic representation of an embodiment of a photonic crystal (PC) layer 100. The PC layer 100 comprises a semiconductor layer 10, which consists of a first semiconductor material (with a first refractive index n1) and includes a structured region 12 in which a plurality of adjacent square PC unit cells 20 with a common side length A form a periodic refractive index modulation grating structure. The structured region 12 can be a subregion of the semiconductor layer 10, as shown, for example, in the central region of the semiconductor layer 10. It can have various shapes, such as rectangular, square, circular, elliptical, or the like, and comprise the periodic refractive index modulation grating structure, the periodicity being determined by the arrangement of the square PC unit cells 20.The PC unit cells 20 can thus be defined solely by the periodicity of the structures within the structured region itself and do not need to be defined by providing physical "cells" with a structural boundary or similar. In some embodiments, the structured region 12 can extend over the entire surface of the semiconductor layer 10. The thickness d of the PC layer 100 can range from 50 nm to 1,000 nm. The side length A can range from 100 nm to 1,000 nm, and the structured region 12 can have a diameter D > 0.5 mm.

[0029] Fig. Figure 2 shows a schematic representation of PC unit cells based on the prior art design of a right-angled isosceles triangle, RIT 30. The representation may, in particular, refer to a top view of a partial area of ​​the Fig. Refer to the structured region 12 shown in Figure 1. Each PC unit cell 20 comprises a RIT 30 formed by an air hole within the semiconductor material of semiconductor layer 10, which has a lower refractive index than the semiconductor material of semiconductor layer 10 (see K. Hirose et al., “Watt-class high-power, high-beam-quality photonic-crystal lasers”, Nature Photonics 8, 406–411 (2014)). Thus, the second refractive index n2 of the triangle is approximately 1, while the first refractive index n1 of the semiconductor material is typically between 3 and 4.

[0030] Fig. Figure 3 shows a schematic representation of a PC unit cell 20 with a RIT 30 according to the state of the art. The depicted PC unit cell 20 and the RIT 30 can be used in Fig. The two unit cells 20 shown correspond to the square PC unit cells 20. The square PC unit cells 20 have a common side length A and include a first corner 21, a second corner 22, a third corner 23, and a fourth corner 24. The RIT is defined within this square structure and includes a top corner 31, a first base corner 32, and a second base corner 33. The top corner 31 of the RIT 30 coincides with the first corner 21 of the PC unit cell 20. Furthermore, the first base corner 32 of the RIT 30 coincides with the second corner 22 of the PC unit cell 20, and the second base corner 33 of the RIT 30 coincides with the third corner 23 of the PC unit cell 20. The ratio of the base length B to the side length A of the PC unit cell 20 is typically between 0.72A and 2A, where the base angle is θ45°.

[0031] Fig. Figure 4 shows a schematic representation of a PC unit cell 20, which comprises an embodiment of a SIT 30 (symmetric stretched isosceles triangle, SIT) according to the invention. The basic structure of the PC unit cell 20 is the same as that of the RIT 30 from Figure 4. Fig. 3, however, instead of a right-angled isosceles triangle, there is a symmetrical stretched isosceles triangle in the PC unit cell 20. The SIT 30 is defined within the square structure of the PC unit cell 20 and comprises an upper corner 31, a first base corner 32, and a second base corner 33, with the upper corner 31 of the SIT 30 coinciding with the first corner 21 of the PC unit cell 20. In contrast to the one in Fig. In contrast to the conventional RIT design shown in Figure 2, in the novel SIT design the first base corner 32 and the second base corner 33 are located on the sides of the PC unit cell 20 opposite the coinciding corner (first corner 21) and do not coincide with the other corners of the PC unit cell 20. The base length B can thus be defined as a parameter that depends on the common side length A of the PC unit cells 20, with the base length B preferably being in a range between 0.65A and 0.804. The base angle θ can be in a range between 65° and 75°.

[0032] Another difference of the novel SIT design is that the air holes of the RIT structures are replaced by a second semiconductor material with a different refractive index than the first semiconductor material. Preferably, the first refractive index n1 of the first semiconductor material and the second refractive index n2 of the second semiconductor material differ by between 2% and 35%. In particular, the material with the lower index can correspond to the second refractive index n2 of the second semiconductor material, and the material with the higher index can correspond to the first refractive index n1 of the first semiconductor material. Specifically, the first semiconductor material can be GaAs and the second semiconductor material can be InGaP.

[0033] Fig. Figure 5 shows a schematic representation of a PC unit cell 20, which comprises a further embodiment of a SIT 30 according to the invention. The SIT 30 shown corresponds essentially to the SIT 30 of the in Fig. 4. The previous embodiment shown differs, however, in that the first base corner 32 of the SIT 30 and the second base corner 33 of the SIT 30 do not coincide with a side of the PC unit cell 20, but are located within the PC unit cell 20. In particular, in this embodiment, the SIT 30 has only one corner (the upper corner 31) that coincides with a corner (the first corner 21) of the PC unit cell 20. In this way, the effective refractive index difference between the triangles and the background material in the PC unit cells of the structure can be further reduced.

[0034] Fig. Figure 6 shows a schematic representation of a PC unit cell 20, which further comprises another embodiment of a SIT 30 according to the invention. The SIT 30 shown essentially corresponds to the SIT 30 of the in Fig. 4 and Fig. 5 embodiments shown. However, it differs in that the first base corner 32 of the SIT 30 and the second base corner 33 of the SIT 30 do not coincide with a side of the PC unit cell 20, but are located outside the PC unit cell 20. In particular, in this embodiment, the SIT 30 has only one corner (the upper corner 31) that coincides with a corner (the first corner 21) of the PC unit cell 20, and each PC unit cell 20 includes a first base corner 32 and a second base corner 33 of adjacent PC unit cells 20. Each PC unit cell 20 thus includes a SIT 30; that is, even if the SIT 30 is not structurally unitary, the inclusion can at least relate to a region of the PC unit cells 20 that, in plan view, is covered by a piecewise complete SIT 30.

[0035] Fig. Figure 7 shows a schematic representation of an exemplary embodiment of a surface-emitting photonic crystal laser (PCSEL) 200 according to the present invention. The PCSEL 200 shown comprises a PC layer 100 according to the invention; an active layer 110 and a filter layer 120 with a wavelength-selective element, wherein the PC layer 100, the active layer 110 and the filter layer 120 form a stacked layer structure, which is configured such that electromagnetic radiation emitted by the active layer 110 is coupled into and guided within the stacked layer structure.In particular, the stacked layer structure shown here as an example comprises a P-contact 130; a P-contact layer 132 above the P-contact 130; the filter layer 120 above the P-contact layer 132; a p-cladding layer 134 above the filter layer 120; the active layer 110 above the p-cladding layer 134; an n-cladding layer 144 above the active layer 110; An n-substrate (or an n-contact layer) 142 is located above the n-cladding layer 144, and an n-contact 140 is located above the n-substrate (or an n-contact layer) 142. A driver current is injected between the P-contact 130 and the n-contact 140, wherein the P-contact 130 is configured as a single strip located below the structured region of the PC layer 100, while the n-contact 140 can be configured as a ring structure defining an opening (optical aperture) above the P-contact 130.The PCSEL 200 can be designed to have an optical aperture corresponding to a cavity length L > 0.5 mm.

[0036] The PCSEL structure shown is essentially a conventional PCSEL structure, but takes advantage of the improved design of the PC layer 100 according to the invention. Furthermore, the filter layer 120 acts as a reflector, which makes it possible to further reduce losses within the diode laser. In a preferred embodiment, the PC layer 100 can be located between the PC-active layer 110 and the filter layer 120. In this case, electromagnetic radiation coupled out from the PC layer 100 in a direction away from the active layer 110 can still be reflected back towards the active layer 110 by the filter layer 120.

[0037] Fig. Figure 8 shows an exemplary vertical cross-section of a refractive index profile of a PCSEL 200 according to the invention. The refractive index profile and the position of the PC layer 100 in an exemplary all-semiconductor PCSEL 200 according to the invention (e.g., in the one shown in Figure 8) are shown. Fig. The layers of the PCSEL 200 shown in Figure 7 are recognizable. The P-contact 130 is located on the left side of the diagram (at 0 µm), while the n-contact 140 is located on the right side (at 7 µm). The total height of the layer structure of the PCSEL 200 is therefore 7 µm in this example, but may differ in other embodiments. It is clearly visible in the figure that the PC layer 100 is arranged between the PC-active layer 110 and the filter layer 120, with the layers separated by sheath layers (134, 144) that have a common thickness t. clad They can exhibit being separate from each other.

[0038] Fig. Figure 9 shows exemplary representations of the optical 2D (|κ 2D+| and |κ 2D- |)- and 1D (|κ 1D |)-Coupling strengths in the plane for an infinite PC layer according to the present invention, wherein κ is 1D , k 2D+ und k 2D- These are complex numbers, as defined in T. Inoue et al. (“General recipe to realize photonic-crystal surface-emitting lasers with 100-W-to-1-kW single-mode operation”, Nature Communications 13.1, 3262 (2022)). In particular, they show Fig. 9A the optical 2D (|κ 2D+ |)-Coupling strengths in the plane, Fig. 9B the optical 2D (|κ 2D- |)-Coupling strengths in the plane and Fig. 9C the optical 1D (|κ 1D|)-Coupling strengths in the plane. These are the results of numerical simulations derived with a specially developed simulation tool for simulating PCSELs with infinite cavity according to the present invention, based on M. Radziunas et al. (“Optical Mode Calculation in Large-Area Photonic Crystal Surface-Emitting Lasers”, IEEE Photonics J. 16, 0601209 (2024)). For the intended application as a resonator element in a PCSEL, the 1D feedback must be small and the 2D feedback large. Feedback in this case refers to the respective coupling between modes of the PC structure in 1D or 2D. High 2D feedback means that an excitation in the PC structure is highly likely to couple to 2D modes, while high 1D feedback means that an excitation in the PC structure is highly likely to couple to 1D modes of the PC structure.Since single-mode laser emission requires the excitation of 2D modes, the respective coupling strengths must be optimized accordingly. Therefore, an optimal operating window must be found in which the 2D feedback is strong and the 1D feedback is weak. The figures show that the conditions are largely met for a design window with a base length B between 0.60A and 0.804A and a base angle θ between 60° and 80°. Considering further aspects relating to mode-gain separation and mode radiation losses within the PC structure, a preferred operating window is determined for a base length B between 0.65A and 0.80A and a base angle θ between 65° and 75°. The preferred operating window is also indicated by a square in the figures.

[0039] Fig. Figure 10 shows exemplary representations of mode-gain separation and mode-radiation loss in an infinite PC layer according to the present invention. In particular, Figure 10 shows Fig. 10A the total optical mode loss due to surface coupling between the lowest-loss infinite cavity ground mode and the next lowest-loss mode (mode gain separation), and Fig. Figure 10B shows the total radiation loss of the fundamental mode (radiation loss of mode A). For single-mode operation, a large mode separation (> 2 cm) is required. -1 ) in the total optical mode loss between the fundamental mode and the next lowest-loss mode. Furthermore, for operation with a low threshold and high efficiency, the total radiation loss of the fundamental mode should not be too high or too low (> 5 cm). -1 , < 20 cm -1Further calculations using the aforementioned simulation tool have confirmed that these requirements are met within the determined preferred operating window.

[0040] Fig. Figure 11 shows diagrams of the mode separation, external differential efficiency, and threshold current for an exemplary PCSEL of finite size according to the present invention. The diagrams illustrate the dependence of these values ​​on the cavity length L for a PCSEL according to the present invention with an inventive SIT structure and a conventional RIT structure in the PC layer. In particular, they show Fig. 11A mode separation, Fig. 11B the external differential efficiency and Fig. 11C the threshold current.

[0041] Sufficient fashion discrimination (> 2 cm) -1The threshold current I between the fundamental mode and the first higher-order modes must be achieved for the defined PCSEL structure with a finite area within the design window. Furthermore, the threshold current I th be low (< 10 A for the test up to an exemplary operating current of I = 100 A, cf. M. Yoshida et al., “High-brightness scalable continuous-wave single-mode photonic-crystal laser”, Nature 618, 727-732 (2023)) for a threshold contribution to the efficiency of [(II th ) / I] ≥ 90%), and the external differential efficiency should be high (η D> 80%, corresponding to a slope efficiency > 0.9 W / A at λ = 1070 nm). The representations result from calculations performed for an exemplary PCSEL structure with a total outer diameter (i.e., a cavity length L) of up to 3 mm and for a design based on a conventional RIT-based unit cell (referred to as "RIT") and an inventive SIT-based unit cell (referred to as "Tri_70"), which lie within the specified preferred operating window with a base length B of 0.75A and a base angle θ of 70°.

[0042] Out of Fig. 11A shows that the mode separation up to the maximum calculated cavity length L = 3 mm for a PCSEL according to the present invention with the SIT structure according to the invention > 2 cm -1 is, whereas for a cavity length L of more than 500 µm, it is less than 2 cm for a conventional RIT-based design. -1This limits the current for RIT-based designs to small device sizes and is therefore unsuitable for operating power levels of several tens of watts (e.g., due to the difficulty of cooling them). Fig. 11B shows that the external differential efficiency η D for the PCSEL according to the present invention, which has the SIT structure according to the invention, increases sharply with the cavity length L and is at > 80% for a cavity length L > 1 mm (L > 400 µm for a conventional RIT-based design). Fig. 11C it is evident that a PCSEL according to the present invention, comprising the SIT structure according to the invention, has a threshold current I up to a cavity length L = 2.5 mm. tn < 10 A is maintained, whereas in the conventional RIT design for cavity lengths L > 1.4 mm a threshold current I th> 10 A occurs. The laser power is calculated based on measured characteristic parameters for a vertical structure with λ = 1070 nm, as in Fig. 7 shown (design based on optical loss = 1.41 cm) -1 , Modal profit factor Γg0 = 22.7 cm -1 and transparency flux density J tr = 56 A / cm -2 ).

[0043] Further efficiency scaling can be expected, however, when multiple active layers are included in a PCSEL based on the SIT structure according to the present invention. The multiple active layers can be interconnected by tunnel connections. Such a surface-emitting multi-junction PC laser can operate in a single vertical mode and be stabilized by a single PC structure, so that the resistance overhead of the filter layer (e.g., including a DBR grating) is distributed across multiple laser structures. Reference symbol list 10 Semiconductor layer 12 structured area 20 PC unit cells 21 first corner (PC unit cell) 22 second corner (PC unit cell) 23 third corner (PC unit cell) 24 fourth corner (PC unit cell) 30 right-angled or stretched isosceles triangle (RIT / SIT) 31 upper corner (RIT / SIT) 32 First Base Corner (RIT / SIT) 33 second base corner (RIT / SIT) 100 Photonic Crystal (PC) Layer 110 active shift 120 filter layers 130 P-Contact 132 P-contact layer 134 P-mantle layer 140 N-Contact 142 N-substrate or N-contact layer 144 N-cladding layer 200 surface-emitting photonic crystal lasers (PCSELs) A Side length (PC unit cell) d thickness (semiconductor layer) B Base length (RIT / SIT) D diameter (e.g. diameter of the circumference of the structured area) L cavity length θ Base angle (RIT / SIT)

Claims

[1] Photonic crystal layer, PC layer, (100), for a surface-emitting photonic crystal laser, PCSEL, (200), comprising a semiconductor layer (10) consisting of a first semiconductor material and including a structured region (12) in which a plurality of adjacent square PC unit cells (20) with a common side length A form a periodic refractive index modulation grating structure, wherein each PC unit cell (20) comprises a symmetrical elongated isosceles triangle, SIT, (30) of a second semiconductor material having a lower or higher refractive index than the first semiconductor material, wherein the SIT (30) has a base length B and a base angle θ, wherein the upper corner (31) of the SIT (30) coincides with a corner (21) of the PC unit cells (20), wherein the base length B is between 0.60A and 0.85A and the The base angle θ is between 60° and 80°.and wherein the first base corner (32) of the SIT (30) and the second base corner (33) of the SIT (30) are located outside the PC unit cell (20) or each coincide with a side of the PC unit cell (20). [2] PC layer (100) according to claim 1, wherein the first refractive index n1 of the first semiconductor material and the second refractive index n2 of the second semiconductor material differ by between 2% and 35%. [3] PC layer (100) according to claim 1 or 2, wherein the first semiconductor material is GaAs and the second semiconductor material is InGaP; or GaAs and AlAs, GaAs and AlGaAs, InP and AllnAs, InP and InGaAsP, or more generally, any two semiconductor compounds from the Al-In-Ga-As-P-Sb-N compound semiconductor material systems are combined as the first semiconductor material and second semiconductor material. [4] PC layer (100) according to one of the preceding claims, wherein the structured region (12) has a diameter D > 0.5 mm and / or a thickness d of the PC layer is between 50 nm and 1,000 nm. [5] PC layer (100) according to one of the preceding claims, wherein the side length A is between 100 nm and 1,000 nm. [6] Surface-emitting photonic crystal laser, PCSEL, (200) comprising: a PC layer (100) according to one of the preceding claims; an active layer (110) and a filter layer (120) with a wavelength-selective element, wherein the PC layer (100), the active layer (110) and the filter layer (120) form a stacked layer structure which is designed such that electromagnetic radiation emitted by the active layer (110) is coupled into and guided within the stacked layer structure. [7] PCSEL (200) according to claim 6, wherein the side length A of the PC unit cells (20) corresponds to one period of a second order grating for reflecting one emission wavelength of the PCSEL (200). [8] PCSEL (200) according to claim 6 or 7, wherein the wavelength-selective element comprises a distributed Bragg reflector, DBR, for reflecting an emission wavelength of the PCSEL (200).

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