GaN-based electrically injected photonic crystal surface emitting laser

Abstract

The application provides a GaN-based electrically injected photonic crystal surface emitting laser, and relates to the technical field of semiconductor optoelectronic devices, and comprises a GaN substrate, an n-type cladding layer, an n-side waveguide layer, a multi-quantum well active region, a p-side transition layer, a first GaN-based layer in which a photonic crystal layer is embedded, a second GaN-based layer, a p-type light confinement layer and a p-type contact layer which are sequentially stacked from bottom to top; the side of the GaN substrate away from the n-type cladding layer is thinned to form an n-side light emitting surface, a ring-shaped electrode is arranged on the n-side light emitting surface, the ring-shaped electrode surrounds a projection area of the photonic crystal layer to form a light emitting window; a multi-stage spiral phase plate is integrated in the light emitting window, and the multi-stage spiral phase plate is used for wavefront modulation of a light beam emitted from the light emitting window. The preparation of the phase modulation structure does not need to etch a main resonance area of the laser, so that the wavefront modulation of the emitted light beam is realized while avoiding damage to the main resonance structure.

Description

Technical Field

[0001] This application relates to the field of semiconductor optoelectronic device technology, and more specifically, to a GaN-based electrically injected photonic crystal surface-emitting laser. Background Technology

[0002] GaN-based blue-green light semiconductor devices are located in the low absorption window of seawater and have significant application value in underwater optical communication, short-range high-speed visible light links, special light field illumination, and on-chip miniature coherent light sources. Photonic crystal surface-emitting lasers can utilize two-dimensional photonic crystals to provide surface emission feedback and mode modulation capabilities, representing an important technological approach for achieving high-brightness, high-directivity on-chip output of blue-green light.

[0003] For GaN-based photonic crystal surface-emitting lasers (PCSELs), the device structure not only needs to meet the requirements of lasing threshold, mode field distribution and current injection continuity, but also needs to take into account the synergistic constraints of multiple factors such as photonic crystal feedback structure, absorption control of the high-loss region on the p side, light-emitting window on the n side and metal electrode arrangement.

[0004] In existing technologies, directly etching complex vortex phase structures on the light-emitting surface or main resonant feedback region of GaN-based PCSELs can easily introduce etching damage, surface roughness, and non-radiative recombination centers, thereby affecting the performance of electro-injection lasing. Especially in the blue-green light band, due to the shorter target wavelength, the step height and sidewall precision requirements of multi-stage phase plates are higher. If GaN is directly etched to form a step structure, the process window is narrow, the damage is large, and the step error can easily lead to a decrease in the purity of the orbital angular momentum mode. Summary of the Invention

[0005] In view of this, this application provides a GaN-based electrically injected photonic crystal surface-emitting laser.

[0006] One aspect of this application provides a GaN-based electrically injected photonic crystal surface-emitting laser, comprising: a GaN substrate, an n-type cladding, an n-side waveguide layer, a multi-quantum-well active region, a p-side transition layer, a first GaN substrate with a buried photonic crystal layer, a second GaN substrate, a p-type light confinement layer, and a p-type contact layer, stacked sequentially from bottom to top; the side of the GaN substrate away from the n-type cladding is thinned to form an n-side light-emitting surface, and an annular electrode is provided on the n-side light-emitting surface, the annular electrode surrounding the projection region of the photonic crystal layer to form a light-emitting window; a multi-level spiral phase plate is integrated within the light-emitting window, the multi-level spiral phase plate being used for wavefront modulation of the light beam emitted from the light-emitting window.

[0007] According to the embodiments of the present application, the p-side transition layer includes a GaN spacer layer and an AlGaN electron blocking layer arranged in a stacked manner, the GaN spacer layer is disposed close to the multi-quantum well active region, and the AlGaN electron blocking layer is disposed close to the first GaN base layer.

[0008] According to the embodiments of the present application, the first GaN base layer includes one of the following: an undoped GaN layer, a p-type GaN layer with a Mg doping concentration lower than that of the p-type contact layer; the distance between the lower surface of the photonic crystal layer and the upper surface of the multi-quantum well active region is 30 nm-100 nm.

[0009] According to the embodiments of the present application, the photonic crystal layer includes a circular hole array arranged in a square lattice; the array period of the photonic crystal layer is configured to be related to the ratio of the target emission wavelength to the effective refractive index, and the effective refractive index is the equivalent refractive index of the light beam in the composite waveguide structure where the photonic crystal layer is located; the thickness of the photonic crystal layer is configured to match the thickness of the first GaN base layer, the thickness of the n-side waveguide layer and the thickness of the p-side transition layer.

[0010] According to the embodiments of the present application, the p-type optical confinement layer is made of a p-type AlGaN material, the refractive index of the p-type optical confinement layer is lower than that of the first GaN base layer, and the p-type optical confinement layer is configured to confine the optical field to the side where the first GaN base layer is located while maintaining continuous p-type current injection.

[0011] According to the embodiments of the present application, a p-side contact electrode formed by sequentially stacking Pd, Pt and Au, and a pad electrode formed of Pd and Au electrically connected to the p-side contact electrode are disposed on the p-type contact layer; the annular electrode is formed by sequentially stacking Ti, Pt and Au, and is configured to inject current into the n-type cladding layer through the GaN substrate.

[0012] According to the embodiments of the present application, the multi-stage spiral phase plate is made of SiNx material, where 1<x<1.5, and at least covers the light-emitting area corresponding to the light-emitting window; the central opening of the annular electrode defines the light-emitting window, and the diameter of the central opening is smaller than the diameter of the projection area of the photonic crystal layer; a gap of 1 is provided between the inner edge of the annular electrode and the outer edge of the multi-stage spiral phase plate ~10 safety interval.

[0013] According to the embodiments of the present application, the multi-stage spiral phase plate has a multi-stage discrete step structure, and the total number of steps of the multi-stage discrete step structure includes one of the following: 8, 16 or 32; the topological charge number of the vortex beam generated by the multi-stage spiral phase plate includes one of the following: 1, 2 or 3.

[0014] According to an embodiment of this application, in a multi-level discrete step structure, the phase delay of multiple steps arranged sequentially along the circumference is distributed in an arithmetic sequence, and the phase difference between adjacent steps is determined by the absolute value of the topological charge number and the total number of steps; the step height of each step is determined by the phase delay corresponding to each step, the target emission wavelength, the refractive index of the SiNx material, and the refractive index of the light-emitting side medium of the multi-level spiral phase plate.

[0015] According to an embodiment of this application, when the topological charge is negative, a vortex beam with chirality opposite to that of a positive topological charge is generated by one of the following methods: arranging the phase sectors of a multi-level spiral phase plate in opposite directions along the circumference; taking the modulus of the phase delay of each step of a multi-level discrete step structure to determine the corresponding non-negative step height.

[0016] According to the embodiments of this application, the first GaN substrate with the embedded photonic crystal layer is located above the active region of the multi-quantum well, close to the p-side, and isolated from the p-type contact layer. This is different from the photonic crystal structure embedded in the ordinary n-side waveguide layer, and also different from the structure in which a surface dielectric photonic crystal capping layer is directly formed on the uppermost surface of the device. The multi-level spiral phase plate is physically integrated in the light-emitting window on the n-side and does not directly contact the embedded photonic crystal layer and the active region of the multi-quantum well. This allows the fabrication of the phase modulation structure to be carried out without etching the main resonant region of the laser, thereby avoiding damage to the main resonant structure while realizing wavefront modulation of the emitted beam. Attached Figure Description

[0017] The above and other objects, features and advantages of this application will become clearer from the following description of embodiments with reference to the accompanying drawings, in which:

[0018] Figure 1 A schematic diagram of a GaN-based electrically injected photonic crystal surface-emitting laser according to an embodiment of this application is shown.

[0019] Figure 2 The schematic diagram illustrates the top view correspondence between the n-side annular electrode, the multi-stage spiral phase plate, and the projection area of ​​the photonic crystal array according to an embodiment of this application.

[0020] Explanation of reference numerals in the attached figures:

[0021] 1. Pad electrode; 2. p-side contact electrode; 3. Silicon dioxide insulating layer; 4. p-type contact layer; 5. p-type light confinement layer; 6. Second GaN substrate; 7. First GaN substrate; 8. GaN spacer layer; 9. AlGaN electron blocking layer; 10. Multi-quantum well active region; 11. n-side waveguide layer; 12. n-type cladding; 13. GaN substrate; 14. n-side ring electrode; 15. Multi-level spiral phase plate; 16. Photonic crystal array projection region. Detailed Implementation

[0022] The embodiments of this application will now be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of this application. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of this application for ease of explanation. However, it will be apparent that one or more embodiments may be implemented without these specific details. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concepts of this application.

[0023] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. The terms “comprising,” “including,” etc., as used herein indicate the presence of features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0024] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0025] If the photonic crystal is placed directly near the surface of a GaN-based electrically injected PCSEL, etching damage and current path discontinuity problems are easily introduced during subsequent electrode formation, surface treatment and regrowth. If the light field is too distributed in the highly Mg-doped p-type contact region, the device threshold will be significantly increased, which is not conducive to stable lasing.

[0026] Underwater communication prioritizes stable, low-order, high-purity orbital angular momentum (OAM) mode output, with preferred topological charge numbers typically around ±1, ±2, or ±3, rather than solely pursuing high-order vortex light. If devices only aim for general vortex light output without designing for low-order, high-purity OAM modes, wavefront accuracy, and electrode compatibility, they will struggle to meet the link stability and mode purity requirements of blue-green underwater communication.

[0027] Furthermore, GaN-based electrically injected PCSELs typically need to balance n-side light emission, n-side ring electrode, current injection, and a central light emission window. If the phase plate is mismatched with the ring electrode or light emission window, it can lead to metal obstruction, mode distortion, alignment difficulties, and decreased vortex mode purity. Especially when the device already employs an n-side light emission structure, how to introduce a fabricable, alignable, and mass-producible phase modulation structure on the n-side light emission surface without disrupting the main resonant feedback structure is a key problem that remains to be solved in current technology.

[0028] Therefore, this application proposes a GaN-based electrically injected PCSEL scheme based on a GaN substrate, facing the n-side light-emitting structure, and integrating a SiNx multi-level spiral phase plate (SPP) on the n-side light-emitting surface after GaN substrate thinning. This scheme aims to achieve compatibility between stable lasing of electrically injected PCSEL and direct output of low-order, high-purity OAM beams suitable for underwater communication without directly damaging the buried photonic crystal and active region.

[0029] Figure 1 A schematic diagram of a GaN-based electrically injected photonic crystal surface-emitting laser according to an embodiment of this application is shown. Figure 1 As shown, the GaN-based electrically injected photonic crystal surface-emitting laser includes: a pad electrode 1, a p-side contact electrode 2, a silicon dioxide insulating layer 3, a p-type contact layer 4, a p-type light confinement layer 5, a second GaN substrate 6, a first GaN substrate 7, a GaN spacer layer 8, an AlGaN electron blocking layer 9, a multi-quantum-well active region 10, an n-side waveguide layer 11, an n-type cladding layer 12, a GaN substrate 13, an n-side ring electrode 14, a multi-level spiral phase plate 15, and a photonic crystal array projection region 16.

[0030] This application proposes a GaN-based electrically injected photonic crystal surface-emitting laser with an n-side integrated SiNx multi-level spiral phase plate 15, which can achieve direct output of low-order, high-purity OAM vortex light without changing the PCSEL master resonant feedback structure.

[0031] The GaN-based electrically injected photonic crystal surface-emitting laser provided according to the embodiments of this application includes:

[0032] The GaN substrate 13, n-type cladding 12, n-side waveguide layer 11, multi-quantum well active region 10, p-side transition layer, first GaN base layer 7 with buried photonic crystal layer 6, second GaN base layer 6, p-type optical confinement layer 5 and p-type contact layer 4 are stacked sequentially from bottom to top.

[0033] The GaN substrate 13 can be a thinned GaN substrate, with the side away from the n-type cladding 12 being thinned to form an n-side light-emitting surface. A ring electrode is provided on the n-side light-emitting surface, and the ring electrode surrounds the projection area of ​​the photonic crystal layer to form a light-emitting window.

[0034] The light-emitting window integrates a multi-stage spiral phase plate 15, which is used to perform wavefront modulation on the light beam emitted from the light-emitting window.

[0035] In this embodiment, the core structure of the GaN-based electrically injected photonic crystal surface-emitting laser is sequentially stacked along the epitaxial growth direction (i.e., from bottom to top). The bottom layer is a GaN substrate 13, which serves as the epitaxial growth substrate for the entire device. A low-loss GaN substrate can be used to facilitate subsequent n-side current injection and light emission. On the GaN substrate 13, an n-type cladding layer 12, an n-side waveguide layer 11, a multi-quantum-well active region 10, a p-side transition layer, a first GaN base layer 7 with a buried photonic crystal layer, a second GaN base layer 6, a p-type optical confinement layer 5, and a p-type contact layer 4 are sequentially grown.

[0036] A photonic crystal layer is embedded within the first GaN substrate 7. This photonic crystal layer serves as a two-dimensional distributed feedback structure, providing in-plane optical feedback and modulating the lasing mode. Encased in GaN material, it does not directly contact the active region or the external environment, thus avoiding etching damage. The second GaN substrate 6 is located between the first GaN substrate 7 and the p-type light confinement layer 5, acting as a buffer and modulating the optical field. The p-type light confinement layer 5 typically uses a material with a lower refractive index than the adjacent GaN substrate to confine the optical field to the active region and the vicinity of the photonic crystal layer, reducing p-side free carrier absorption loss. Above the p-type contact layer 4 is a silicon dioxide insulating layer 3 and a p-side contact electrode 2, used to form a good ohmic contact.

[0037] After completing the epitaxial structure fabrication and device front-side processes (including photonic crystal patterning and p-type electrode fabrication), the device is thinned.

[0038] Specifically, the side of the GaN substrate 13 away from the n-type cladding 12 is ground, polished, and cleaned until the required thickness is achieved (e.g., thinned to approximately 100 μm or less), thereby forming a flat n-side light-emitting surface. This n-side light-emitting surface is the final light-emitting interface of the laser.

[0039] A ring electrode is disposed on the n-side light-emitting surface. This ring electrode typically employs a multilayer metal structure (e.g., Ti / Pt / Au), which forms an electrical contact with the underlying n-type cladding 12 through a thinned GaN substrate 13, for injecting n-type charge carriers into the device. The central region of the ring electrode has a hollow structure, which faces the projection region of the photonic crystal layer, thus forming a central light-emitting window without metal obstruction. This ensures that the light beam can be efficiently emitted from the central region of the device, avoiding obstruction of the light path by the metal electrode.

[0040] Within the light-emitting window, a multi-stage spiral phase plate 15 is integrated. This multi-stage spiral phase plate 15 can be made of a low-loss dielectric material and is formed in the central region of the n-side light-emitting surface using a low-temperature back-end process, completely or partially covering the light-emitting window. The multi-stage spiral phase plate 15 itself does not participate in the feedback process of the laser's main resonant cavity, but rather acts as a wavefront modulation element. When the surface-emitted beam generated by the photonic crystal layer feedback exits through the n-side light-emitting window, it passes through the multi-stage spiral phase plate 15, and its wavefront is modulated by the spatially variable phase delay introduced by the multi-stage spiral phase plate 15, thereby converting the ordinary surface-emitted beam into a vortex beam carrying orbital angular momentum.

[0041] This design, which places the phase modulation structure on the light-emitting surface, effectively avoids the process damage and performance degradation caused by directly etching complex phase morphologies on the gain region or photonic crystal structure.

[0042] In this embodiment, the first GaN substrate 7 with the embedded photonic crystal layer is located above the multi-quantum well active region 10, close to the p-side, and isolated from the p-type contact layer 4. This differs from the photonic crystal structure embedded in the ordinary n-side waveguide layer, and also from the structure in which a surface dielectric photonic crystal capping layer is directly formed on the uppermost surface of the device. The multi-level spiral phase plate 15 is physically integrated in the light-emitting window on the n-side and does not directly contact the embedded photonic crystal layer and the multi-quantum well active region 10. This allows the fabrication of the phase modulation structure to be carried out without etching the main resonant region of the laser, thereby avoiding damage to the main resonant structure while achieving wavefront modulation of the emitted beam.

[0043] In some embodiments, the p-side transition layer includes a GaN spacer layer 8 and an AlGaN electron blocking layer 9 stacked together. The GaN spacer layer 8 is disposed near the multi-quantum well active region 10, and the AlGaN electron blocking layer 9 is disposed near the first GaN substrate 7.

[0044] In this embodiment, the p-side transition layer may be composed of a GaN spacer layer 8 and an AlGaN electron blocking layer 9. The GaN spacer layer 8 and the AlGaN electron blocking layer 9 are disposed between the multi-quantum-well active region 10 and the first GaN substrate 7 to which the photonic crystal layer is buried, forming a transition structure between the multi-quantum-well active region 10 and the photonic crystal layer that balances electron confinement and longitudinal optical field modulation.

[0045] The GaN spacer layer 8 primarily serves as a buffer and modulates the optical field. On one hand, it provides a smooth growth interface for the multi-quantum-well active region 10, which helps improve the crystal quality of the subsequent AlGaN electron blocking layer 9. On the other hand, by appropriately setting its thickness (e.g., 30nm-60nm), the vertical distribution range of the optical field generated by the multi-quantum-well active region 10 can be adjusted, allowing the optical field to effectively penetrate into the buried photonic crystal layer above, while avoiding excessive exposure of the optical field to the highly doped p-type contact region, which would lead to increased absorption loss.

[0046] AlGaN electron blocking layer 9 is typically made of p-type AlGaN material, which blocks electrons from leaking from the multi-quantum-well active region 10 to the p-side, thereby improving the radiative recombination efficiency of charge carriers in the active region and reducing the threshold current of the device.

[0047] Meanwhile, the refractive index of AlGaN is typically lower than that of GaN. The introduction of this layer also helps to confine the light field within the low-refractive-index waveguide core region containing the active region and the photonic crystal layer, enhancing the photonic crystal layer's ability to control the light field. By co-designing the thickness and doping concentration of the GaN spacer layer 8 and the AlGaN electron blocking layer 9, a transition structure that can effectively confine electrons and optimize the longitudinal light field distribution can be established between the multi-quantum-well active region 10 and the first GaN substrate 7 with the buried photonic crystal layer.

[0048] In some embodiments, the first GaN substrate 7 includes one of the following: an undoped GaN layer, a p-type GaN layer with a Mg doping concentration lower than that of the p-type contact layer 4; and the spacing between the lower surface of the photonic crystal layer and the upper surface of the multi-quantum well active region 10 is 30 nm to 100 nm.

[0049] In this embodiment, the first GaN substrate 7 with the embedded photonic crystal layer is a low-loss GaN substrate. This layer can be an undoped GaN, a low-Mg-doped p-type GaN, or a low-absorption GaN modulating layer. Its Mg doping concentration is lower than that of the p-type contact layer 4, so as to support the photonic crystal layer and reduce the overlap between the light field and the high-Mg-doped p-type contact region.

[0050] By setting the first GaN substrate to an undoped or low Mg-doped state, energy loss during optical field transmission can be effectively reduced and the quality factor of the photonic crystal layer can be improved without affecting the vertical conduction of the current.

[0051] The distance between the lower surface of the photonic crystal layer and the upper surface of the multi-quantum-well active region 10 is 30nm-100nm, and the etching depth or equivalent thickness of the photonic crystal layer is 130nm-150nm. These two, together with the thickness of the n-side waveguide layer 11, the thickness of the low-loss GaN substrate (the first GaN substrate 7 and the second GaN substrate 6 with the photonic crystal layer embedded), and the refractive index parameters of the secondary epitaxial p-AlGaN low-refractive-index upper cladding (p-type optical confinement layer 5), jointly determine the longitudinal mode field distribution.

[0052] A spacing of 30 nm to 100 nm is maintained between the lower surface of the photonic crystal layer and the upper surface of the multi-quantum-well active region 10. This spacing is optimized through precise optical field simulation. If the spacing is too small (less than 30 nm), the etching process of the photonic crystal layer can easily damage the active region and introduce non-radiative recombination centers. If the spacing is too large (greater than 100 nm), the exponential decay of the optical field in the longitudinal direction will significantly reduce the mode overlap integral between the photonic crystal layer and the active region, weakening the feedback efficiency provided by the two-dimensional photonic crystal. By controlling the spacing within the range of 30 nm to 100 nm, it is possible to ensure that the photonic crystal layer is in a region with high optical field intensity, giving it sufficient coupling coefficient to achieve stable two-dimensional distributed feedback, while maintaining a safe process isolation distance from the active region, thus balancing the optical performance and process reliability of the device.

[0053] In some embodiments, the photonic crystal layer includes an array of circular holes arranged in a square lattice; the array period of the photonic crystal layer is configured to be related to the ratio of the target emission wavelength to the effective refractive index, the effective refractive index being the equivalent refractive index of the beam in the composite waveguide structure in which the photonic crystal layer is located; the thickness of the photonic crystal layer is configured to match the thickness of the first GaN substrate 7, the thickness of the n-side waveguide layer 11, and the thickness of the p-side transition layer.

[0054] In photonic crystal structures, the filling factor (FF) refers to the proportion of the area occupied by the dielectric material (or air holes / etched holes) within a single unit cell of a two-dimensional photonic crystal array. The square lattice array of circular holes used in this embodiment sets the filling factor in the range of 0.10–0.20.

[0055] This range ensures that the photonic crystal has a suitable refractive index contrast, thereby opening a sufficiently wide photonic bandgap at the target emission wavelength (450-500 nm), providing the necessary conditions for two-dimensional distributed feedback. If the fill factor is too low (e.g., less than 0.10), the refractive index modulation is too weak, making it difficult to form an effective optical field confinement; if the fill factor is too high (e.g., greater than 0.20), although the refractive index contrast increases, it may lead to increased mode leakage and increase the risk of adjacent circular holes connecting during fabrication.

[0056] The array period of the photonic crystal layer is not arbitrarily set, but is determined based on the target emission wavelength and the equivalent refractive index of the beam in the composite waveguide structure where the photonic crystal layer is located. Its design basis satisfies the following relationship:

[0057]

[0058] in, Indicates the array period. This indicates the target emission wavelength (i.e., the operating wavelength of a GaN-based electrically injected photonic crystal surface-emitting laser). Indicates the equivalent refractive index. It is a preset fluctuation range value, which can be 15nm.

[0059] The equivalent refractive index reflects the refractive index of the light field propagating in the composite waveguide composed of the n-side waveguide layer 11, the multi-quantum-well active region 10, the p-side transition layer, and the first GaN substrate.

[0060] The thickness (or etching depth) of the photonic crystal layer is not independent but needs to be coordinated and matched with multiple longitudinal structural parameters. Specifically, it requires comprehensive optimization by combining the thickness of the first GaN substrate 7 where the photonic crystal layer is embedded, the thickness of the n-side waveguide layer 11, and the thickness of the p-side transition layer. The purpose of this matching design is to control the longitudinal optical field distribution so that the optical field generated by the multi-quantum-well active region 10 can penetrate into the photonic crystal layer with an appropriate intensity, ensuring that the mode coupling efficiency between the two reaches its optimal level. If the thickness of the photonic crystal layer is mismatched with the thicknesses of the aforementioned layers, the optical field may not be able to effectively enter the photonic crystal region, or it may cause the optical field to extend excessively into the high-loss region, thereby degrading the threshold characteristics and mode purity of the laser. Through this multi-dimensional thickness matching, a balance is achieved between the photonic crystal feedback intensity and the active region gain.

[0061] In some embodiments, the p-type light confinement layer 5 is made of p-type AlGaN material, the refractive index of the p-type light confinement layer 5 is lower than the refractive index of the first GaN substrate 7, and the p-type light confinement layer 5 is configured to confine the light field to the side where the first GaN substrate 7 is located while maintaining continuous injection of p-type current.

[0062] In this embodiment, the p-type light confinement layer 5 can be a secondary epitaxial p-AlGaN low-refractive-index cladding. The refractive index of the secondary epitaxial p-AlGaN low-refractive-index cladding at the target emission wavelength is lower than that of the adjacent GaN substrate. The difference in refractive index between the two is expressed by the following formula:

[0063]

[0064] This represents the difference in refractive index, preferably 0.03–0.10. Indicates the refractive index of the first GaN substrate. This represents the refractive index of the low-refractive-index cladding layer of the secondary epitaxial p-AlGaN (i.e., the refractive index of the p-type light confinement layer 5).

[0065] By combining a low-refractive-index p-AlGaN cladding layer with a low-loss GaN substrate through secondary epitaxy, the fundamental mode can be pulled back towards the active region and the photonic crystal layer without disrupting continuous current injection.

[0066] In terms of refractive index characteristics, the refractive index of the p-type AlGaN material is lower than that of the first GaN substrate 7 with the embedded photonic crystal layer. This refractive index difference forms a high-low refractive index interface of the dielectric waveguide on the cross section perpendicular to the epitaxial growth direction, forming an anti-guiding effect. This confines the light field to the first GaN substrate with a higher refractive index and the active region below it, effectively suppressing the light field from spreading to the highly doped p-type contact layer 4 and reducing the absorption loss of free carriers.

[0067] To further quantify the characteristics of the light field distribution, the definition of the longitudinal mode field confinement factor is introduced. The longitudinal mode field confinement factor is used to characterize the proportion of light field energy of a specific optical mode that is confined to a specific structural region.

[0068]

[0069] in, Represents a specific structural region. Indicates the pattern order. Represents structural region The integration domain, Represents the integral field of the whole structure. Indicates refractive index, Indicates the order of the pattern The electric field strength.

[0070] Based on this definition, the optimization and screening of the device's longitudinal structure must meet the following mode field confinement conditions: the confinement factor of the fundamental mode in the multi-quantum-well active region is 10. To ensure sufficient overlap between the optical field and the gain medium; the confinement factor of the fundamental mode in the photonic crystal layer. To ensure the effectiveness of the two-dimensional distributed feedback; the limiting factor of the fundamental mode in the high-loss region on the p-side. This is to suppress free carrier absorption; at the same time, the confinement factor of the fundamental mode in the active region is greater than that of the first-order mode in the active region. ).

[0071] By constraining the parameters mentioned above, the p-type optical confinement layer 5, together with the first GaN substrate and the n-side waveguide layer 11, localizes the fundamental mode optical field to the maximum extent in the active region and near the photonic crystal layer, thereby reducing the device threshold current and improving lasing stability.

[0072] In some embodiments, the p-type contact layer 4 is provided with a p-side contact electrode 2 formed by sequentially stacking Pd, Pt and Au, and a pad electrode 1 formed of Pd and Au and electrically connected to the p-side contact electrode 2; the annular electrode is formed by sequentially stacking Ti, Pt and Au, and is configured to inject current into the n-type cladding layer 12 via the GaN substrate.

[0073] In terms of the electrode structure, a p-side contact electrode 2 formed by sequentially stacking Pd, Pt and Au is formed above the p-type contact layer 4, for forming a stable electrical contact with the p-type contact layer 4. The pad electrode 1 formed of Pd and Au is electrically connected to the p-side contact electrode 2, for external probe testing or wire bonding. An n-side annular electrode of Ti / Pt / Au is disposed on the periphery of the n-side light exit surface after the GaN substrate is thinned, for forming an n-side injection channel, and reserving an unobstructed area for the central light exit window at the same time.

[0074] In some embodiments, the multi-stage spiral phase plate 15 is made of SiNx material, where 1<x<1.5, and at least covers the light exit area corresponding to the light exit window; the central opening of the annular electrode defines the light exit window, and the diameter of the central opening is smaller than the diameter of the projection area of the photonic crystal layer; a distance of 1 is provided between the inner edge of the annular electrode and the outer edge of the multi-stage spiral phase plate 15 ~10 safety interval.

[0075] reference Figure 2 , Figure 2 schematically shows a schematic top-view corresponding relationship among the n-side annular electrode, the multi-stage spiral phase plate 15 and the photonic crystal array projection area 16 according to the embodiment of the present application, including: the n-side annular electrode 14, the multi-stage spiral phase plate 15, and the photonic crystal array projection area 16, wherein the dashed box represents the photonic crystal array projection area 16, and 0, h, 2h to 7h marked in different sectors are used to represent the discrete step height distribution of the eight-segment spiral phase plate.

[0076] In the embodiment of the present application, the center of the multi-stage spiral phase plate 15, the center of the photonic crystal array, and the center of the annular electrode are coaxial or substantially coaxial, and the maximum center deviation thereof is not more than 20 , preferably not more than 10 .

[0077] The multi-stage spiral phase plate 15 is fabricated using a SiNx dielectric material, where x in the chemical formula has a value range of 1<x<1.5. SiNx within this composition range has a low extinction coefficient in the 450-500 nm blue-green wavelength band, which can effectively reduce optical transmission loss. Meanwhile, the SiNx material can be deposited on the surface of the GaN substrate at low temperature via a plasma-enhanced chemical vapor deposition process, and this low-temperature process will not damage the already fabricated front-side structure of the device.

[0078] The coverage area of the multi-stage spiral phase plate 15 on the n-side light exit surface at least covers the light exit window defined by the central opening of the annular electrode, ensuring that all or most of the light beams emitted from the light exit window undergo phase modulation.

[0079] The central opening of the Ti / Pt / Au n-side annular electrode is arranged corresponding to the light exit window formed by the projection area of the photonic crystal layer, and the multi-stage spiral phase plate 15 is located within or covers the light exit area corresponding to the central opening. With this structure, while ensuring n-side current injection, the influence of metal shading on the central exit light beam and OAM mode purity can be reduced.

[0080] The relationship between the central opening of the n-side annular electrode and the light exit window formed by the projection area of the photonic crystal layer can be expressed by the following formula:

[0081]

[0082] wherein, represents the diameter of the central opening of the annular electrode, represents the diameter of the light exit window formed by the projection area of the photonic crystal layer.

[0083] The central opening of the annular electrode defines the boundary of the light exit window on the top-view plane, and the diameter of this central opening is designed to be smaller than the diameter of the projection area of the photonic crystal layer. This dimensional relationship ensures that the diffracted light beam generated by the photonic crystal layer can fully fall into the light exit window without metal shading when exiting, avoiding physical shading and absorption loss of the light field by the annular electrode metal layer. In addition, between the inner edge of the annular electrode and the outer edge of the multi-stage spiral phase plate 15, 1 ~10 radial safety intervals are provided. This interval is used to accommodate photolithography alignment deviation and etching process deviation during the manufacturing process, preventing the annular electrode metal layer from being exposed at the edge and etched, or causing metal short circuit due to poor step coverage, and also avoiding additional scattering loss or mode distortion caused by the metal edge being too close to the center of the light field, so as to ensure the quality of the output beam and the device yield.

[0084] In some embodiments, the multi-level spiral phase plate 15 is a multi-level discrete step structure, and the total number of steps in the multi-level discrete step structure includes one of the following: 8, 16, or 32; the topological charge of the vortex beam generated by the multi-level spiral phase plate 15 includes one of the following: 1. 2 or 3.

[0085] The multi-stage spiral phase plate 15 employs a multi-stage discrete step structure, dividing the circumferential direction into multiple sector-shaped regions. Within each sector unit, a specific physical step height is etched to approximate a continuous spiral phase distribution. The total number of steps N in the multi-stage discrete step structure is selected as 8, 16, or 32. These number of steps are chosen to balance phase fitting accuracy and fabrication complexity: an 8-stage structure has a larger fabrication tolerance, suitable for low-cost preparation; 16-stage and 32-stage structures have smaller discretization errors for the ideal spiral phase, enabling higher purity of the orbital angular momentum mode.

[0086] Selection of topological charge of the vortex beam generated by the multi-stage spiral phase plate 15 1. 2 or Any value from 3. The topological charge number is an integer; its absolute value determines the order of the phase singularity of the vortex beam, and its sign determines the rotational chirality of the beam wavefront (i.e., the direction in which the phase increases along the azimuth). A low-order topological charge number is chosen ( 1. 2 or 3) Rather than higher-order values, this is to meet the requirements of link stability and mode purity in application scenarios such as underwater communication, because lower-order orbital angular momentum modes have stronger anti-interference capabilities and lower mode crosstalk during transmission.

[0087] In a multi-stage discrete step structure, the first Phase delay of each sector From the topological load number Total number of steps The calculation formula is jointly determined as follows:

[0088]

[0089] in, The sector number has a value range of 1. ; For modulo operations, the phase value is normalized within the interval [0, 2π) to ensure that the calculated physical step height is positive. This formula defines the phase delay of each sector relative to the reference point. By periodically arranging sectors with different phase delays along the azimuth angle, spiral phase modulation of the incident plane wavefront is achieved.

[0090] In some embodiments, in a multi-level discrete step structure, the phase delay of multiple steps arranged sequentially along the circumference is distributed in an arithmetic sequence, and the phase difference between adjacent steps is determined by the absolute value of the topological load number and the total number of steps.

[0091] The height of each step is determined by the phase delay corresponding to each step, the target emission wavelength, the refractive index of the SiNx material, and the refractive index of the light-emitting medium of the multi-stage spiral phase plate 15.

[0092] In a multi-level discrete step structure, the phase delay of multiple steps arranged sequentially along the circumference exhibits an arithmetic progression. Specifically, the multi-level spiral phase plate 15 is divided into equal angular sections along the circumference. Each sector corresponds to There are discrete steps. The phase difference between two adjacent steps is a constant, the value of which is determined by the absolute value of the topological load number and the total number of steps.

[0093] The physical step height of each step The phase delay corresponding to this step Target emission wavelength The refractive index of SiNx materials and the refractive index of the light-emitting side medium of the multi-stage spiral phase plate 15 Determined jointly. The specific calculation formula is as follows:

[0094]

[0095] in, For the first Phase delay of each sector ( ), The target emission wavelength for the device design (e.g., a specific wavelength in the 450-500 nm band), For deposited SiNx dielectric materials in The refractive index at that point, The refractive index of the light-emitting side medium of the multi-stage spiral phase plate 15.

[0096] This formula is based on the principle of optical path difference. By precisely controlling the thickness difference of SiNx layers in different sectors, the beam generates a preset phase delay after passing through media of different thicknesses, thereby converting the surface-emitted beam into a vortex beam carrying a specific orbital angular momentum.

[0097] In some embodiments, when the topological charge is negative, a vortex beam with the opposite chirality to that of a positive topological charge is generated by one of the following methods: arranging the phase sectors of the multi-level spiral phase plate 15 in opposite directions along the circumference; or taking the modulus of the phase delay of each step of the multi-level discrete step structure to determine the corresponding non-negative step height.

[0098] When the topological charge of the vortex beam to be generated by the multi-stage spiral phase plate 15 is negative, one of the following two process-compatible methods is used to achieve the vortex beam output with the opposite chirality to the positive topological charge.

[0099] The first method involves arranging the phase sectors in a reverse circumferential direction. During the layout design phase, the step height of each sector is kept consistent with the design for the corresponding positive topology charge, but the circumferential arrangement of the sectors is reversed. For example, if the phase delay corresponding to the positive topology charge increases counterclockwise, then in the design for the negative topology charge, sectors with the same phase delay are arranged clockwise. This change in sector arrangement direction directly causes the spiral rotation direction of the beam wavefront to be reversed, thus physically generating vortex beams with opposite chirality, without requiring adjustments to process parameters such as etching depth.

[0100] The second method involves determining the non-negative step height after taking the modulus of the phase delay. According to the aforementioned phase delay calculation formula, when the topological charge number is negative, the calculated phase value may also be negative. In this case, the phase delay calculated for all sectors is uniformly modulated to normalize it to the non-negative range of zero to twice pi, resulting in new non-negative phase values. Subsequently, based on these non-negative phase values, the physical etching depth of each sector is determined using the step height calculation formula. Since the step height must be non-negative in physical processing, this modulus taking method ensures process feasibility. Furthermore, by assigning a specific non-negative phase delay distribution to each sector, it is also possible to achieve the technical effect of generating a vortex beam with chirality opposite to that of a positive topological charge number.

[0101] According to an embodiment of this application, a joint design method for the above-mentioned GaN-based electrically injected photonic crystal surface-emitting laser is provided. This method incorporates the optical mode characteristics, electrical injection characteristics, and layout of the back-end phase modulation structure of the device into a unified optimization process.

[0102] Initial design parameters are input, including the target emission wavelength, target topological charge number, discrete order of the multi-stage spiral phase plate 15, candidate thickness range of each epitaxial layer, structural constraints of the n-side annular electrode opening, and fabrication tolerances. Based on the input parameters, a refractive index profile model along the epitaxial growth direction is established, and the longitudinal optical field distribution of the fundamental mode and first-order mode is obtained by solving the wave equation. Based on this, the confinement factors of the fundamental mode and first-order mode in the multi-quantum-well active region 10, the buried photonic crystal layer, and the p-side high-loss region are calculated respectively.

[0103] Candidate epitaxial structures are screened based on the calculated confinement factors: ensuring that the confinement factor of the fundamental mode in the active region is higher than that of the first-order mode in the active region, so as to ensure the optimal selection of the fundamental mode and suppress competition of higher-order modes; ensuring that the confinement factor of the fundamental mode in the photonic crystal layer reaches a preset lower limit, so as to maintain sufficient two-dimensional distribution feedback intensity; and ensuring that the confinement factor of the fundamental mode in the high-loss region on the p side is lower than a preset upper limit, so as to reduce free carrier absorption loss.

[0104] After determining the epitaxial structure that meets the optical performance requirements, the design of the subsequent phase modulation structure is initiated. Based on the input target topological charge and discrete order, the phase delay of each sector of the multi-stage spiral phase plate 15 is calculated and further converted into the corresponding physical step height for each sector. Next, a joint check at the layout level is performed, focusing on verifying the coaxiality constraints and safety spacing between the central opening of the n-side annular electrode, the central light-emitting window of the device, and the multi-stage spiral phase plate 15.

[0105] In some embodiments, the effect of thickness deviation of the secondary epitaxial layer on the uniformity of current spread also needs to be checked. If a candidate epitaxial structure meets the screening criteria for optical confinement factor, but cannot meet the coaxial constraint or safety spacing requirements in the layout, the structure will be eliminated and will not be output as the final recommended solution.

[0106] The final design data, after undergoing the aforementioned multiphysics coupling optimization and layout compliance checks, is output as a GDS layout file or mask layout file for tape-out. It should be noted that although the multi-stage spiral phase plate 15 in this embodiment preferably uses SiNx material, it can be replaced with other low-absorption transparent media without departing from the technical concept of this application.

[0107] The embodiments of this application have been described above. However, these embodiments are merely illustrative and not intended to limit the scope of this application. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. Without departing from the scope of this application, those skilled in the art can make various substitutions and modifications, all of which should fall within the scope of this application.

Claims

1. A GaN-based electrically injected photonic crystal surface-emitting laser, characterized in that, comprising: a GaN substrate, an n-type cladding layer, an n-side waveguide layer, a multi-quantum well active region, a p-side transition layer, a first GaN base layer embedded with a photonic crystal layer, a second GaN base layer, a p-type light confinement layer and a p-type contact layer stacked in sequence from bottom to top; the side of the GaN substrate away from the n-type cladding layer is thinned to form an n-side light exit surface, an annular electrode is arranged on the n-side light exit surface, and the annular electrode surrounds the projection area of the photonic crystal layer to form a light exit window; A multi-stage spiral phase plate is integrated in the light exit window, and the multi-stage spiral phase plate is configured to perform wavefront modulation on the light beam emitted from the light exit window.

2. The GaN-based electrically injected photonic crystal surface-emitting laser according to claim 1, characterized in that, The p-side transition layer includes a GaN spacer layer and an AlGaN electron blocking layer arranged in a stacked manner, the GaN spacer layer is disposed close to the multi-quantum well active region, and the AlGaN electron blocking layer is disposed close to the first GaN base layer.

3. The GaN-based electrically injected photonic crystal surface-emitting laser according to claim 1, characterized in that, The first GaN base layer includes one of the following: an undoped GaN layer, a p-type GaN layer with a Mg doping concentration lower than that of the p-type contact layer; The distance between the lower surface of the photonic crystal layer and the upper surface of the multi-quantum well active region is 30 nm to 100 nm.

4. The GaN-based electrically injected photonic crystal surface-emitting laser according to claim 1, characterized in that, The photonic crystal layer includes a circular hole array arranged in a square lattice; The array period of the photonic crystal layer is configured to be related to the ratio of the target emission wavelength to the effective refractive index, and the effective refractive index is the equivalent refractive index of the light beam in the composite waveguide structure where the photonic crystal layer is located; The thickness of the photonic crystal layer is configured to match the thickness of the first GaN base layer, the thickness of the n-side waveguide layer and the thickness of the p-side transition layer.

5. The GaN-based electrically injected photonic crystal surface-emitting laser according to claim 1, characterized in that, The p-type light confinement layer is made of p-type AlGaN material, the refractive index of the p-type light confinement layer is lower than the refractive index of the first GaN base layer, and the p-type light confinement layer is configured to confine the optical field to the side where the first GaN base layer is located while maintaining continuous p-type current injection.

6. The GaN-based electrically injected photonic crystal surface-emitting laser according to claim 1, characterized in that, The p-type contact layer is provided with a p-side contact electrode formed by sequentially stacking Pd, Pt and Au, and a pad electrode formed of Pd and Au electrically connected to the p-side contact electrode; The annular electrode is formed by sequentially stacking Ti, Pt and Au, and is configured to inject current into the n-type cladding layer through the GaN substrate.

7. The GaN-based electrically injected photonic crystal surface-emitting laser according to claim 1, characterized in that, The multi-stage spiral phase plate is made of SiNx material, where 1<x<1.5, and at least covers the light exit area corresponding to the light exit window; The central opening of the annular electrode defines the light-emitting window, and the diameter of the central opening is smaller than the diameter of the projected region of the photonic crystal layer; a distance of 1 is provided between the inner edge of the annular electrode and the outer edge of the multi-level spiral phase plate. ~10 Safe distance.

8. The GaN-based electrically injected photonic crystal surface-emitting laser according to claim 7, characterized in that, The multi-stage spiral phase plate is a multi-stage discrete step structure, and the total number of steps of the multi-stage discrete step structure includes one of the following: 8, 16 or 32; The topological charge of the vortex beam generated by the multi-stage spiral phase plate includes one of the following:

1. 2 or 3.

9. The GaN-based electrically injected photonic crystal surface-emitting laser according to claim 8, characterized in that, In the multi-stage discrete step structure, the phase retardation of a plurality of steps arranged sequentially along the circumferential direction is distributed in an arithmetic sequence, and the phase difference between adjacent steps is jointly determined by the absolute value of the topological charge number and the total number of steps; The step height of each step is jointly determined by the phase retardation corresponding to each step, the target emission wavelength, the refractive index of the SiNx material and the refractive index of the light exit side medium of the multi-stage spiral phase plate.

10. The GaN-based electrically injected photonic crystal surface-emitting laser according to claim 8, characterized in that, When the topological charge number is a negative value, a vortex beam with chirality opposite to that of a positive topological charge number is generated by one of the following methods: The phase sectors of the multi-stage spiral phase plate are arranged in opposite directions along the circumference; The phase delay of each step in the multi-level discrete step structure is moduloed to determine the corresponding non-negative step height.

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