Self-latching photonic crystal surface-emitting laser and method for manufacturing the same

Abstract

The application relates to the technical field of semiconductor lasers, in particular to a self-locked mode photonic crystal surface-emitting laser and a preparation method thereof. The self-locked mode photonic crystal surface-emitting laser comprises a pump light incident antireflection micro-nano structure layer, a window layer, an active region, gain light, a photonic crystal layer, a first light component, a second light component, a saturable absorber layer and a DBR layer. The gain light is generated by the active region after the pump light is incident on the window layer. The gain light is transmitted downward to the photonic crystal layer to form the first light component and the second light component which are transmitted upward and downward respectively. The second light component reaches the DBR layer after passing through the saturable absorber layer, is reflected by the DBR layer, is transmitted upward and passes through the photonic crystal layer, and the first light component and the second light component are combined and transmitted upward. The photonic crystal layer is used for filtering high-order transverse modes, passing through a base transverse mode and controlling the intensity ratio of the first light component and the second light component. The saturable absorber layer is used for passively locking or passively Q-switching the base transverse mode. The application is at least beneficial to realizing stable ultrashort pulse output with high efficiency, high beam quality and self-starting.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor laser technology, and particularly relates to a self-mode-locked photonic crystal surface-emitting laser and its fabrication method. Background Technology

[0002] Vertical external cavity surface-emitting lasers (VECSELs) have the unique advantages of high photoelectric conversion efficiency of traditional semiconductor lasers and high output power and high beam quality of solid-state lasers. By introducing an external resonant cavity, VECSELs can not only effectively control transverse modes and achieve fundamental transverse mode output close to the diffraction limit, but also facilitate the insertion of various optical components into the cavity to achieve wavelength tuning, efficient frequency doubling, and ultrashort pulse output.

[0003] In terms of driving method, optically pumped external cavity surface-emitting lasers have significant structural and performance advantages over electrically pumped devices. Optically pumped external cavity surface-emitting lasers do not require the fabrication of complex electrode structures and highly doped semiconductor layers near the active region, thereby greatly reducing absorption losses and thermal effects caused by free carriers. At the same time, optical pumping can provide more uniform carrier injection, avoiding the current crowding effect commonly found in electric pumping, enabling the device to maintain high power and high beam quality single-mode output even under large-aperture pumping.

[0004] To obtain ultrashort pulse output, passive mode-locking components are usually introduced into the external resonant cavity. Traditional passive mode-locked VECSELs require an independent external semiconductor saturable absorber mirror (SESAM) as the cavity mirror. However, this multi-mirror cavity structure not only significantly increases the collimation difficulty and overall size of the system, but also reduces the device's ability to resist environmental vibration and temperature drift. To simplify the system, the solution of integrating the saturable absorber and the gain medium on the same semiconductor chip has become a research hotspot. This monolithic integration solution enables the device to achieve self-mode-locked output in a simple straight cavity, greatly improving the system's compactness and stability.

[0005] However, existing integrated optically pumped surface-emitting lasers still face technical challenges in achieving high-performance, high-stability ultrashort pulse output, such as surface Fresnel reflection and pump efficiency loss, transverse mode control and polarization instability at high power, and difficulties in functional coordination and field strength design of quantum wells and saturable absorbers. In particular, the reflection loss at the pump light incident surface can be as high as 30% or more, which seriously reduces pump efficiency. The lack of an effective mode selection mechanism leads to the oscillation of higher-order modes during high-power pumping, which degrades beam quality. In addition, the thermal load generated by absorbing residual pump light after the saturable absorber is integrated with the gain region often leads to mode-locking failure or device degradation. Summary of the Invention

[0006] In view of this, the present invention aims to provide a self-mode-locked photonic crystal surface-emitting laser and its fabrication method, which utilizes the mode narrowing and filtering capabilities of photonic crystals and combines them with the gain characteristics of VECSELs to achieve high-efficiency and high-stability self-mode-locked pulse output.

[0007] To achieve the above objectives, the technical solution created by this invention is implemented as follows: This invention provides a self-mode-locked photonic crystal surface-emitting laser, comprising: a DBR layer, a saturable absorber layer, a photonic crystal layer, an active region, a window layer, and an anti-reflection micro / nanostructure layer stacked sequentially along the thickness direction; a pump light is incident on the anti-reflection micro / nanostructure layer, and then passes through the window layer and incident on the active region to generate gain light. The anti-reflection micro / nanostructure layer is used to reduce Fresnel reflection of the pump light at its incident surface to improve the absorption efficiency of the pump light; the gain light is transmitted downward into the photonic crystal layer, forming an upwardly transmitted first light component and a downwardly transmitted second light component. The second light component passes through the saturable absorber layer and reaches the DBR layer, is reflected by the DBR layer, and then propagates upward and through the photonic crystal layer. The first light component and the second light component... The two optical components merge and propagate upwards sequentially through the active region, window layer, and antireflection micro / nano structure layer. The photonic crystal layer is located between the active region and the saturable absorber layer. The photonic crystal layer is used to filter out higher-order transverse modes, allow the fundamental transverse mode to pass through, and control the intensity ratio of the first and second optical components. The saturable absorber layer is located between the photonic crystal layer and the DBR layer. The saturable absorber layer is used to passively mode-lock or passively Q-switch the fundamental transverse mode after it has been filtered by the photonic crystal layer. The photonic crystal layer is configured to give the fundamental transverse mode after it has been filtered a higher local light intensity at the saturable absorber layer than the higher-order transverse mode, so that the additional absorption loss of the saturable absorber layer on the fundamental transverse mode is less than the additional absorption loss on the higher-order transverse mode.

[0008] Furthermore, the antireflective micro / nano structure layer comprises multiple micro / nano units arranged periodically or quasi-periodically. The Fresnel reflectivity of the antireflective micro / nano structure layer to pump light is less than 1%, and the equivalent refractive index of the antireflective micro / nano structure layer gradually increases from top to bottom. The antireflective micro / nano structure layer includes, but is not limited to, one-dimensional gratings, two-dimensional columnar arrays, two-dimensional aperture arrays, or metasurface structures.

[0009] Furthermore, the antireflection micro / nano structure layer is a two-dimensional subwavelength grating with a duty cycle ranging from 0.3 to 0.7, and a grating depth d. g Satisfy: 0.1λ p ≤d g ≤0.3λ p The grating period P of a two-dimensional subwavelength grating satisfies: P < λ p / n and P is at 0.15λ p up to 0.28λ p Within the range, where λp Where λ is the center wavelength of the pump light, and n is the refractive index of the window layer for the pump light; or, the antireflection micro / nano structure layer is a one-dimensional subwavelength grating or an asymmetric metasurface structure, and the antireflection micro / nano structure layer has different reflectivities for pump light polarized in different directions.

[0010] Furthermore, the photonic crystal layer includes a plurality of periodically arranged holes and / or a plurality of periodically arranged columnar structures. The lattice constant, duty cycle and etching depth of the photonic crystal layer are configured such that the photonic bandgap of the photonic crystal layer covers the frequency range corresponding to the higher-order transverse modes, and the frequency corresponding to the fundamental transverse mode is located within the passband of the photonic crystal layer.

[0011] Furthermore, the bandgap energy of the window layer is greater than the photon energy of the pump light, and the thickness of the window layer is in the range of 50nm~2000nm.

[0012] Furthermore, the saturable absorber layer includes a quantum well structure, a quantum dot structure, graphene, carbon nanotubes, or a topological insulator. The absorption wavelength of the saturable absorber layer is greater than the lasing laser wavelength, and the position of the saturable absorber layer relative to the reflected standing wave field of the DBR layer is configured to adjust the modulation depth and saturation flux of the saturable absorber layer. The reflectivity of the DBR layer to the pump light is less than 50%, and the reflectivity of the DBR layer to the gain light is greater than 99.5%.

[0013] Furthermore, the gain chip also includes: a first buffer layer located between the photonic crystal layer and the saturable absorber layer; and a second buffer layer located between the saturable absorber layer and the DBR layer.

[0014] Furthermore, the self-mode-locked photonic crystal surface-emitting laser also includes: an external output mirror, which is located on the side of the anti-reflection micro / nano structure layer away from the window layer, and the external output mirror is spaced apart from the gain chip; wherein, the outgoing light output from the gain chip, including a first light component and a second light component, is incident on the external output mirror, and the external output mirror reflects part of the light back into the gain chip to maintain oscillation, while the other part of the light passes through the external output mirror as laser output. The DBR layer and the external output mirror together form a resonant cavity and provide the main feedback of the gain light in the cavity.

[0015] Another aspect of this invention provides a method for fabricating a self-mode-locked photonic crystal surface-emitting laser (SMCS). The method comprises: sequentially epitaxially growing a DBR layer and a saturable absorber layer on a substrate; epitaxially growing a photonic crystal initial layer on the saturable absorber layer; etching the photonic crystal initial layer to form a plurality of periodically arranged holes to obtain the photonic crystal layer; sequentially epitaxially growing an active region and a window layer on the photonic crystal layer through secondary epitaxy, retaining air or filling the holes with a low-refractive-index medium; and forming an anti-reflection micro / nano structure layer on the side of the window layer away from the active region, the anti-reflection micro / nano structure layer being used to reduce Fresnel reflection of the pump light at its incident surface.

[0016] Compared with the prior art, the present invention can achieve the following beneficial effects: First, by integrating an anti-reflection micro / nano structure layer designed for the pump light wavelength onto the surface where the pump light is incident, the Fresnel reflectivity of the pump light incident surface is reduced from approximately 31% to below 1%, significantly improving the coupling efficiency of the pump light and the absorption efficiency of the pump light in the active region. Simultaneously, the period of the micro / nano unit cells in the anti-reflection micro / nano structure layer satisfies P < λ. p / n ensures that the pump light has only zero-order transmission at the incident surface without generating higher-order diffraction, thus preventing the pump light from deviating from the gain region of the active region. Furthermore, due to the suppression of incident surface reflection, the standing wave contrast of the pump light along the thickness direction is reduced and the absorption distribution is more uniform, which helps to alleviate local overheating and improve the gain uniformity under high-power pumping. Secondly, by placing the photonic crystal layer between the active region and the saturable absorber layer, the intracavity optical field propagates in the order of "photonic crystal mode filtering - saturable absorber nonlinear modulation - DBR reflection feedback - re-mode filtering". The photonic crystal layer pre-filters out higher-order transverse modes and makes the basic transverse mode have a higher local light intensity at the saturable absorber layer. This allows the saturable absorber layer to obtain a larger mode-dependent loss difference by nonlinearly modulating the basic transverse mode. This avoids the problem that when the saturable absorber layer directly receives the unfiltered mixed-mode optical field, multiple modes consume its saturation capability at the same time, thereby reducing its ability to suppress non-target modes. As a result, the stability of the single transverse mode output is improved while reducing the mode-locking threshold and intracavity additional loss, and the pulse sequence has high repetition frequency stability and low time jitter. Third, while providing mode screening, the photonic crystal layer's two-dimensional periodic hole structure creates additional heat diffusion channels in the lateral direction. The combination of the high thermal conductivity semiconductor framework and the periodic air holes effectively reduces the longitudinal thermal resistance, allowing the heat generated in the active region to dissipate rapidly along the lateral direction of the photonic crystal layer. This alleviates the thermal lensing effect under high-power pumping, thereby achieving high-efficiency pulse output while maintaining an excellent beam quality factor, making it suitable for high-power continuous pulse operation. Fourth, the antireflection micro / nano structure layer, photonic crystal layer, saturable absorber layer and DBR layer are all integrated in the same semiconductor chip. With the help of an external output mirror located on the outside of the chip, self-starting passive mode-locking or passive Q-switching output can be achieved in a simple straight cavity. There is no need to insert an independent semiconductor saturable absorber mirror in the external resonant cavity, which helps to reduce the system size, reduce the collimation difficulty and improve the resistance to environmental disturbances. In summary, the self-mode-locked photonic crystal surface-emitting laser provided by this invention solves problems such as high pump light reflection loss, insufficient mode control, significant thermal effects, and poor mode-locking stability by vertically integrating the anti-reflection micro / nano structure on the surface and the internal photonic crystal layer and saturable absorber layer. It achieves high-efficiency, high-beam-quality, self-starting, stable ultrashort pulse output. Attached Figure Description

[0017] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 The schematic diagram of the self-mode-locked photonic crystal surface-emitting laser described in the embodiment of the present invention shows the incident pump light, the beam splitting of the gain light at the photonic crystal layer, the selection and transmission process of the first and second optical components, and the periodic hole structure of the photonic crystal layer. Figure 2 A schematic diagram showing the correspondence between the photonic crystal layer and the antireflection micro / nano structure layer in the self-mode-locked photonic crystal surface-emitting laser described in the embodiment of the present invention; Figure 3 This is a schematic diagram of light field propagation and cascaded mode modulation in a self-mode-locked photonic crystal surface-emitting laser as described in an embodiment of the present invention.

[0018] The reference numerals in the figures include: DBR layer 6; saturable absorber layer 5; photonic crystal layer 4; active region 3; window layer 2; antireflection micro / nano structure layer 1; pump light 8; external output mirror 7; first optical component 10; second optical component 11; output light 9; fundamental transverse mode 12; higher-order transverse mode 13. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0020] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0021] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0022] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0023] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0024] refer to Figure 1 and Figure 2This invention provides a self-mode-locked photonic crystal surface-emitting laser, comprising: a DBR layer 6, a saturable absorber layer 5, a photonic crystal layer 4, an active region 3, a window layer 2, and an anti-reflection micro / nano structure layer 1 stacked sequentially along the thickness direction; a pump light 8 is incident on the anti-reflection micro / nano structure layer 1, and then passes through the window layer 2 before being incident on the active region 3 to generate gain light. The anti-reflection micro / nano structure layer 1 is used to reduce Fresnel reflection of the pump light 8 at its incident surface, thereby improving the absorption efficiency of the active region 3 for the pump light 8; the gain light is transmitted downward into the photonic crystal layer 4, forming an upwardly transmitted first light component 10 and a downwardly transmitted second light component 11. The second light component 11 passes through the saturable absorber layer 5 and the window layer 2, respectively. After the absorber layer 5, the light reaches the DBR layer 6, is reflected by the DBR layer 6, and is transmitted upward and passes through the photonic crystal layer 4. The first light component 10 and the second light component 11 merge and pass through the active region 3, the window layer 2 and the anti-reflection micro / nano structure layer 1 in sequence to form the output light 9 transmitted upward. The photonic crystal layer 4 is located between the active region 3 and the saturable absorber layer 5. The photonic crystal layer 4 is used to filter out higher-order transverse modes, allow the fundamental transverse mode to pass through, and control the intensity ratio of the first light component and the second light component. The saturable absorber layer 5 is located between the photonic crystal layer 4 and the DBR layer 6. The saturable absorber layer 5 is used to passively mode-lock or passively Q-switch the fundamental transverse mode after it has been filtered by the photonic crystal layer 4.

[0025] The antireflection micro / nano structure layer 1 is located on the incident side of the pump light 8. The antireflection micro / nano structure layer 1 is composed of multiple micro / nano units arranged periodically or quasi-periodically at the subwavelength scale. The antireflection micro / nano structure layer 1 includes, but is not limited to, one-dimensional gratings, two-dimensional columnar arrays, two-dimensional aperture arrays, or metasurface structures. The cross-sectional shapes of the micro / nano units include, but are not limited to, stripes, circles, ellipses, polygons, and cones. The antireflection micro / nano structure layer 1 is positioned relative to the center wavelength λ of the pump light 8. p By designing the period, duty cycle, height, and cross-sectional geometry of the micro / nano units, a longitudinally gradient equivalent refractive index is formed at the wavelength of the pump light 8, gradually increasing from the equivalent refractive index of air to the equivalent refractive index of the window layer 2. This reduces the Fresnel reflectivity of the pump light 8 at the incident surface to below 1% (when the anti-reflection micro / nano structure layer 1 is not set, the Fresnel reflectivity of this interface is about 31%), significantly improving the coupling efficiency of the pump light 8 and the absorption efficiency of the pump light 8 by the active region 3.

[0026] Furthermore, since the antireflection micro / nano structure layer 1 suppresses Fresnel reflection of pump light 8 at the incident surface, the longitudinal standing wave contrast formed by the superposition of reflected light and incident light inside the chip is significantly reduced, and the absorption distribution of pump light 8 along the thickness direction is more uniform. This avoids local overheating and longitudinal gain non-uniformity caused by pump standing waves, which is beneficial to improving mode stability under high-power pumping.

[0027] It should be noted that the antireflection micro / nano structure layer 1 is primarily designed for the wavelength of the pump light 8 and is used to improve the coupling and absorption efficiency of the pump light 8. It is not used to provide the main optical feedback to the intracavity gain light; the feedback of the intracavity gain light is mainly provided by the resonant cavity composed of the DBR layer 6 and the external output mirror 7. This is different from the method of setting an antireflection layer between the photonic crystal layer and the active layer and increasing the light field intensity within the photonic crystal layer through the antireflection layer, in terms of placement, target, and technical purpose.

[0028] Furthermore, the period P of the micro / nano unit satisfies: P < λ p / n, where λp is the center wavelength of the pump light 8, and n is the refractive index of the window layer 2 for the pump light 8. When the above conditions are met, the antireflection micro / nano structure layer 1 exhibits only zero-order transmission at the wavelength of the pump light 8 without generating higher-order diffraction. The pump light 8 will not deviate from the gain region of the active region 3 due to higher-order diffraction. Macroscopically, the antireflection micro / nano structure layer 1 exhibits an antireflection structure with an equivalent refractive index that gradually changes along the thickness direction. Preferably, the period P is 0.15λ p up to 0.28λ p Within the range.

[0029] In some embodiments, the antireflection micro / nano structure layer 1 is a one-dimensional subwavelength grating or an asymmetric metasurface structure. By utilizing the morphological birefringence effect, pump light 8 polarized in different directions has different equivalent refractive indices, thereby providing different reflectivities for pump light 8 polarized in different directions. Antireflection conditions can be matched according to the polarization direction of the pump light 8.

[0030] In some embodiments, the size, duty cycle, or rotation angle of the micro / nano unit gradually changes laterally, so that the reflectivity of the antireflection micro / nano structure layer 1 to the pump light 8 has a preset distribution in the lateral direction, thereby shaping the lateral intensity distribution of the pump light 8 so that the intensity distribution of the pump light 8 matches the gain region in the active region 3.

[0031] Furthermore, the Fresnel reflectivity of the antireflective micro / nano structure layer 1 to the pump light 8 is less than 1%.

[0032] Furthermore, the antireflective micro / nano structure layer 1 is a one-dimensional subwavelength grating or a two-dimensional subwavelength grating. The equivalent refractive index of the antireflective micro / nano structure layer 1 gradually changes in the longitudinal direction (thickness direction) to form a broadband antireflection effect at the pump light wavelength 8. The duty cycle of the antireflective micro / nano structure layer 1 is in the range of 0.3 to 0.7, and the grating depth d of the antireflective micro / nano structure layer 1 is... g Satisfy: 0.1λ p ≤d g ≤0.3λ p , where λ p The center wavelength of pump light 8.

[0033] The window layer 2 is located below the anti-reflection micro / nano structure layer 1. It is used to improve the transmission conditions of the pump light 8 and plays a transition and protection role in the structure. It reduces the adverse effects of the upper optical field on the lower active region 3 and prevents carriers from escaping to the chip surface and undergoing nonradiative recombination.

[0034] In some embodiments, the window layer 2 is a wide bandgap semiconductor material, the thickness of which satisfies the condition of pushing the first antinode of the cavity gain light field away from the top surface of the device to reduce the surface state nonradiative recombination loss. The material bandgap of the window layer 2 is larger than the material bandgap of the active region 3, and it is transparent to both the pump light 8 and the gain light. The window layer 2 isolates the active region 3 from the surface defect region to reduce the surface nonradiative recombination rate, and pushes the heat-generating region of the pump light 8 into the device interior.

[0035] Furthermore, the bandgap energy of window layer 2 is greater than the photon energy of pump light 8, and window layer 2 is optically transparent to both pump light 8 and gain light. The thickness of window layer 2 is in the range of 50nm~2000nm.

[0036] The window layer 2 can be made of a wide-bandgap semiconductor material that matches the lattice of the active region 3. For example, when the active region 3 is an InGaAs / GaAs material system, the window layer 2 can be made of AlGaAs material. The window layer 2 is used to isolate the active region 3 from surface defects and atmospheric interface, significantly reducing the surface nonradiative recombination rate. On the other hand, through an appropriate longitudinal thickness, the main absorption and heat generation area of ​​the pump light 8 is moved from the surface to the inside of the device, thereby alleviating the front surface thermal lensing effect and reducing surface distortion.

[0037] The active region 3 is located below the window layer 2. The active region 3 can be composed of a pump light absorption layer and a quantum well layer. The pump light absorption layer is used to absorb the pump light 8 and form photogenerated carriers, thereby providing the necessary optical gain conditions for laser oscillation. The quantum well layer is located between the two pump light absorption layers. The potential barrier of the pump light absorption layer is higher than that of the quantum well layer, which can effectively inject photogenerated carriers into the quantum well layer to generate high material gain.

[0038] Furthermore, the photonic crystal layer 4 includes multiple holes arranged in a periodic pattern.

[0039] The photonic crystal layer 4 is located below the active region 3. The photonic crystal layer 4 is composed of periodically arranged holes. The arrangement of the holes includes, but is not limited to, single-period, double-period, and triple-period arrangements. The shape of the holes includes, but is not limited to, circles, ellipses, triangles, or rhombuses. The band-edge resonance effect of the photonic crystal layer 4 can periodically modulate the refractive index of the laser gain generated in the cavity, thereby achieving laser mode selection and resonant enhancement of laser gain. At the same time, the photonic crystal layer 4 constructs an additional high thermal conductivity channel in the lateral direction, which significantly reduces the longitudinal thermal resistance and provides a dissipation path for the heat generated in the active region 3. This effectively alleviates the thermal lensing effect under high-power pumping, which is also one of the important guarantees for maintaining high beam quality under large-aperture pumping.

[0040] Furthermore, the lattice constant, duty cycle, and effective refractive index of the photonic crystal layer 4 are designed such that the photonic bandgap of the photonic crystal layer 4 covers the frequency range corresponding to the higher-order transverse modes near the lasing laser wavelength, while maintaining the passband characteristics for the fundamental transverse mode. By adjusting the structural parameters of the photonic crystal layer 4, the intensity ratio of the first optical component to the second optical component can be controlled, thereby actively adjusting the light field intensity reaching the saturable absorber layer 5, avoiding oversaturation or thermal damage to the saturable absorber layer 5 due to excessively strong light field, while ensuring that the light received by the saturable absorber layer 5 has sufficient light intensity for mode-locking activation.

[0041] Therefore, the photonic crystal layer 4 not only determines the transverse mode composition, but also controls the intensity ratio of the downward-transmitted light component to the upward-transmitted light component, thereby precisely adjusting the light field intensity reaching the saturable absorber layer 5 to avoid oversaturation or thermal damage to the saturable absorber layer 5 due to excessive light intensity, while also avoiding mode-locking failure due to insufficient light intensity.

[0042] The saturable absorber layer 5 is located below the photonic crystal layer 4. The saturable absorber layer 5 is a material with nonlinear absorption characteristics. The saturable absorber layer 5 includes, but is not limited to, quantum wells, quantum dots, monolayer two-dimensional materials, and heterojunctions. The saturable absorber layer 5 exhibits a high-loss state under low light intensity and a low-loss state under high light intensity, performing periodic loss modulation on the intracavity optical field, thereby achieving gain screening and noise suppression of the lasing mode, and is used to generate passive mode-locking or Q-switching pulses, which is beneficial for shortening the laser settling time and improving the output signal-to-noise ratio and stability.

[0043] Furthermore, the saturable absorber layer 5 includes at least one set of quantum well structures or quantum dot structures. The absorption wavelength of the saturable absorber layer 5 is slightly larger than the lasing laser wavelength, so as to provide absorption loss at low light intensity and absorption saturation at high light intensity, thereby achieving passive mode-locked output.

[0044] By placing the saturable absorber layer 5 between the photonic crystal layer 4 and the DBR layer 6, the laser mode in the saturable absorber layer 5 can be independently adjusted by the reflection characteristics of the photonic crystal layer 4 and the DBR layer 6. The modulation depth and saturation flux of the saturable absorber layer 5 can also be optimized by adjusting the number of quantum wells or quantum dots, the well width, and the position of the reflected standing wave peak relative to the DBR layer 6.

[0045] The saturable absorber layer 5 can be a semiconductor quantum well structure that is homogeneous with the active region 3 but thinner, such as an InGaAs quantum well with low saturation current, or it can be a low-dimensional material such as graphene, carbon nanotubes, or topological insulators. The band gap of the saturable absorber layer 5 is designed to be transparent to the pump light 8 or to have only weak absorption, while exhibiting significant saturable absorption characteristics for the gain light. Since the saturable absorber layer 5 is disposed below the photonic crystal layer 4, the optical field at its longitudinal position is mainly composed of light modulated by the photonic crystal layer 4. The intracavity light intensity level is moderate and far away from the extremely strong pump light 8 field region near the active region 3, thereby significantly reducing the thermal load generated by the saturable absorber due to the simultaneous absorption of strong pump light 8 and laser light, and avoiding thermal relaxation damage.

[0046] By adjusting the material composition, thickness, and optional ion implantation conditions of the saturable absorber layer 5, its recovery time can be matched with the round-trip time of the external cavity, thereby achieving stable passive mode-locking or passive Q-switching under low-loss conditions and outputting ultrashort pulses on the order of picoseconds or femtoseconds.

[0047] Furthermore, the photonic crystal layer 4 and the saturable absorber layer 5 form a cascaded mode modulation structure with a sequential action within the device. The intracavity optical field generated by the active region 3 first enters the photonic crystal layer 4. The photonic crystal layer 4 provides high mode coupling efficiency for the fundamental transverse modes that meet its phase matching condition, while introducing strong in-plane scattering, radiation leakage, or direction deflection for higher-order transverse modes that do not meet the phase matching condition. This results in the optical field reaching the saturable absorber layer 5 being dominated by the fundamental transverse mode, and the fundamental transverse mode having a higher local intensity at the saturable absorber layer 5. Therefore, the fundamental transverse mode preferentially saturates the saturable absorber layer 5, reducing its additional absorption loss, while the higher-order transverse modes maintain higher absorption loss due to their lower local intensity, creating a further widening effective loss difference between the fundamental and higher-order transverse modes. The optical field modulated by the saturable absorber layer 5 reaches the DBR layer 6 and is reflected. The reflected light passes through the saturable absorber layer 5 and then the photonic crystal layer 4 in the opposite direction, thus again being subjected to the combined effects of mode selection and nonlinear modulation. The aforementioned cascaded process of "photonic crystal mode screening - saturable absorber nonlinear modulation - DBR reflection feedback - re-mode screening" avoids the simple superposition of photonic crystal layer 4 and saturable absorber layer 5 as independent functional layers.

[0048] DBR layer 6 is located at the bottom. DBR layer 6 can be composed of alternating stacks of high refractive index medium and low refractive index medium. The center reflection wavelength of DBR layer 6 is the fundamental transverse mode laser wavelength. The reflectivity of DBR layer 6 to fundamental transverse mode laser is greater than 99.5%.

[0049] Furthermore, the reflectivity of the DBR layer 6 to the pump light 8 is less than 50%, which allows the residual pump light 8 that is not completely absorbed by the active region 3 to pass through the DBR layer 6 or be absorbed by it, thus preventing the residual pump light 8 from repeatedly reflecting between the anti-reflection micro / nano structure layer 1 and the DBR layer 6 to form standing waves, thereby suppressing non-uniform thermal deposition and material degradation caused by standing waves.

[0050] Furthermore, the reflectivity of DBR layer 6 for gain light is higher than 99.5%.

[0051] Furthermore, the gain chip also includes: a first buffer layer located between the photonic crystal layer 4 and the saturable absorber layer 5; and a second buffer layer located between the saturable absorber layer 5 and the DBR layer 6. The materials of the first and second buffer layers are wide-bandgap semiconductors that are homologous to and lattice-matched with the active region 3, used to alleviate heterojunction stress, block defect propagation, and improve longitudinal thermal conductivity, thereby enhancing the overall reliability of the device.

[0052] Further reference Figure 2 In the photonic crystal layer 4, the holes are arranged in rows along the first direction, and multiple rows of holes are arranged along the second direction. In the antireflection micro / nano structure layer 1, each grating strip extends along the second direction. The first direction can be perpendicular to the second direction, and the plane containing the first and second directions is perpendicular to the thickness direction of each film layer in the self-locked photonic crystal surface-emitting laser. Furthermore, the self-mode-locked photonic crystal surface-emitting laser also includes: an external output mirror 7, which is located on the side of the antireflection micro / nano structure layer 1 away from the window layer 2, and the external output mirror 7 is spaced apart from the gain chip; wherein, the outgoing light output from the gain chip, which includes a first light component and a second light component, is incident on the external output mirror 7, and the external output mirror 7 reflects part of the light back into the gain chip to maintain oscillation, while the other part of the light passes through the external output mirror 7 as laser output.

[0053] DBR layer 6 is used to form a stable plano-concave resonant cavity with the external output mirror 7, enhancing the resonance of the optical field within the cavity.

[0054] In a specific example, the self-mode-locked photonic crystal surface-emitting laser is an optically pumped photonic crystal self-mode-locked photonic crystal surface-emitting laser based on a GaAs / AlGaAs material system, with an emission wavelength of approximately 980 nm. The DBR layer 6 can include 30 pairs of Al... 0.9 Ga 0.1As / GaAs, GaAs has a refractive index of approximately 3.5 and a physical thickness of approximately 70 nm, while Al... 0.9 Ga 0.1 The refractive index of A is approximately 3.0, and its physical thickness is approximately 81 nm. Both satisfy the quarter-wavelength condition. The DBR layer 6 has a reflectivity greater than 99.5% for oscillating light at 980 nm, providing extremely strong vertical light feedback. The saturable absorber layer 5 may include 5 pairs of In... 0.1 Ga 0.9 As / GaAs, nonradiative recombination centers are introduced into this layer through proton injection, significantly reducing its saturation intensity. This is used to achieve passive mode-locking and absorb residual pump light 8, minimizing the thermal effects caused by pump light 8 entering the DBR layer 6. The photonic crystal layer 4 is approximately 200 nm thick and has multiple cylindrical holes arranged in a two-dimensional periodic pattern. The hole lattice period is 300 nm, and the hole radius can be 60 nm, satisfying the Bragg diffraction condition. This provides strong in-plane distributed feedback for the 980 nm laser, ensuring that the device can maintain single transverse mode output even in large areas. In addition, additional heat diffusion channels are constructed laterally. The combination of the semiconductor framework and periodic air holes effectively reduces longitudinal thermal resistance, which helps alleviate the heat accumulation effect under high-power pump conditions, thereby suppressing wavefront distortion caused by thermal lenses. The active region 3 may include 3 pairs of In 0.1 Ga 0.9 In As / GaAs, the total optical thickness of the active region 3 is precisely designed to be a half-integer multiple of the external pump light wavelength of 808 nm, ensuring that the antinode of the pump light 8 falls exactly within the quantum well region, maximizing stimulated emission gain; the window layer 2 can be undoped Al. 0.3 Ga 0.7 The As layer, window layer 2, has a thickness of 150 nm and a bandgap width of approximately 1.8 eV, which is larger than the bandgap width of the quantum well (approximately 1.3 eV), with a difference exceeding 100 meV. It is transparent to both 808 nm pump light and 980 nm laser light. The thickness design of window layer 2 cleverly pushes the first antinode of the intracavity oscillating light field away from the top surface of the device, effectively avoiding nonradiative recombination loss caused by surface defects. The antireflection micro / nano structure layer 1 is a subwavelength grating structure with a grating period of 200 nm, a duty cycle of 0.5, and an etching depth of 100 nm. Macroscopically, the antireflection micro / nano structure layer 1 exhibits an equivalent refractive index that gradually changes from 1.0 (air) at the top to 3.5 (GaAs) at the bottom, thereby reducing the reflectivity of the 808 nm pump light 8 at the air-semiconductor interface to below 1%, greatly improving the coupling efficiency of optical pumping.

[0055] In the self-mode-locked photonic crystal surface-emitting laser provided in this embodiment, the pump light 8 is incident from the top of the gain chip, passes through the anti-reflection micro / nano structure layer 1 and the window layer 2, and is efficiently absorbed by the active region 3, generating population inversion. The generated laser is subjected to high reflection feedback from the bottom DBR layer 6 in the vertical direction and two-dimensional Bragg scattering feedback from the photonic crystal layer 4 in the horizontal direction, forming a stable two-dimensional external cavity resonance. The top anti-reflection micro / nano structure layer 1 ensures efficient coupling and injection of the pump light 8, avoiding power dissipation caused by surface reflection. The photonic crystal layer 4 selectively filters out intracavity oscillation modes and suppresses higher-order mode oscillations. The anti-reflection micro / nano structure layer 1 and the photonic crystal layer 4 work together to efficiently concentrate the pump energy for the fundamental transverse mode gain, thereby significantly improving the optical-to-optical conversion efficiency. Meanwhile, the photonic crystal layer 4 actively controls the light field intensity reaching the saturable absorber layer 5 within a suitable saturation flux range. Because it pre-filters out higher-order transverse modes, the light field mode reaching the saturable absorber layer 5 is pure. As a result, the saturable absorber layer 5 only needs to perform nonlinear loss modulation on the fundamental transverse mode, thereby effectively reducing the additional intracavity loss introduced by mode-locking and reducing the thermal load of the saturable absorber. Under the synergistic effect of the above structures, this self-mode-locked photonic crystal surface-emitting laser can achieve low-threshold, self-starting, stable passive mode-locking under continuous pumping conditions, outputting ultrashort pulse lasers with high repetition frequency stability and low time jitter. Furthermore, due to the purity of the intracavity transverse modes and the effective suppression of thermal effects, its beam quality factor will approach the diffraction limit, and it can still maintain excellent beam quality under high-power operation. Furthermore, since the photonic crystal layer 4 is located above the saturable absorber layer 5, the fundamental transverse mode selected by the photonic crystal layer 4 preferentially saturates the saturable absorber layer 5, while the higher-order transverse modes suffer higher absorption losses due to the lower local light intensity. As a result, the saturable absorber layer 5 generates differentiated nonlinear losses for the fundamental transverse mode and the higher-order transverse mode, further increasing the effective loss difference between the two, which is beneficial to improving the stability of single transverse mode output.

[0056] Figure 3 The diagram illustrates the cascaded process of the intracavity optical field following the sequence of "photonic crystal mode screening—saturable absorber nonlinear modulation—DBR reflection feedback—re-mode screening." The diagram uses different line shapes to indicate the process by which the fundamental transverse mode 12 passes through the photonic crystal layer 4 with low loss, and the process by which the higher-order transverse mode 13 is suppressed at the photonic crystal layer 4 due to in-plane scattering or radiation leakage. It also shows the local light intensity difference between the fundamental transverse mode 12 and the higher-order transverse mode 13 at the saturable absorber layer 5 and the corresponding differential nonlinear loss modulation effect.

[0057] In the self-mode-locked photonic crystal surface-emitting laser provided by this invention, the parameters of the anti-reflection micro / nano structure layer 1 determine the reflection and coupling characteristics of the pump light 8 at the incident surface. The lattice constant, duty cycle, and etching depth of the photonic crystal layer 4 determine the transverse mode selectivity, polarization selectivity, and the ratio of upward and downward propagating light components. The anti-reflection micro / nano structure layer 1 is designed only for the center wavelength of the pump light 8 and does not participate in the optical feedback and mode control of the gain light. The thickness of the window layer 2 affects the position of the standing wave peak of the pump light 8 at the active region 3. The spacing between the saturable absorber layer 5 and the DBR layer 6 determines the phase condition of the standing wave field in the saturable absorber layer 5. By jointly optimizing the above parameters, the standing wave field distributions in the gain region, saturable absorber region, and mode-selection region can be independently or semi-independently controlled, thereby achieving precise matching of the saturated flux of gain and absorption, ensuring stable passive mode-locking under low additional loss conditions, while also taking into account high-efficiency pump absorption, excellent beam quality, and long-term operational stability.

[0058] The antireflection micro / nano structure layer 1 located on the surface improves the utilization efficiency of the pump light 8, the mode screening of the internal photonic crystal layer 4 improves the mode purity of the intracavity optical field, and the heat dissipation function of the photonic crystal layer 4 and its reasonable distribution of the optical field intensity of the saturable absorber layer 5 effectively reduce the thermal effect and nonlinear loss introduced by mode locking. The two work together to ensure that the beam quality can still be maintained close to the diffraction limit under high power pulse operation.

[0059] Another aspect of this invention provides a method for fabricating a self-mode-locked photonic crystal surface-emitting laser (SMCS). The method comprises: forming a gain chip, including: sequentially epitaxially growing a DBR layer 6 and a saturable absorber layer 5 on a substrate; epitaxially growing a photonic crystal initial layer on the saturable absorber layer 5; etching the photonic crystal initial layer to form a plurality of periodically arranged holes, obtaining a photonic crystal layer 4; sequentially epitaxially growing an active region 3 and a window layer 2 on the photonic crystal layer 4 through secondary epitaxy, and retaining air or filling the holes with a low-refractive-index medium; forming an anti-reflection micro / nano structure layer 1 on the side of the window layer 2 away from the active region 3, the anti-reflection micro / nano structure layer 1 being used to reduce Fresnel reflection of the pump light 8 at its incident surface.

[0060] Among them, the antireflection micro / nano structure layer 1, window layer 2, active region 3, photonic crystal layer 4, saturable absorber layer 5 and DBR layer 6 can be prepared by metal-organic chemical vapor deposition or molecular beam epitaxy. The antireflection micro / nano structure layer 1 and photonic crystal layer 4 can be prepared by electron beam lithography, nanoimprinting or holographic lithography combined with inductively coupled plasma etching. Since the photonic crystal layer 4 is located between the active region 3 and the saturable absorber layer 5, the photonic crystal layer 4 can be fabricated as follows: the DBR layer 6, the saturable absorber layer 5, and the initial photonic crystal layer for forming the photonic crystal layer 4 are epitaxially grown sequentially on the substrate; the initial photonic crystal layer is subjected to electron beam lithography, nanoimprinting, or holographic lithography combined with inductively coupled plasma etching to form multiple periodically arranged holes; then a second epitaxy is performed to grow the active region 3 and the window layer 2 sequentially on the photonic crystal layer 4, and air is retained or a low refractive index medium is filled in the holes; finally, the antireflection micro / nano structure layer 1 is etched on the side of the window layer 2 away from the active region 3.

[0061] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0062] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A self-mode-locked photonic crystal surface-emitting laser, characterized in that, include: The gain chip includes a DBR layer, a saturable absorber layer, a photonic crystal layer, an active region, a window layer, and an anti-reflection micro / nano structure layer stacked sequentially along the thickness direction. Pump light is incident on the antireflection micro / nano structure layer, and then passes through the window layer to be incident on the active region to generate gain light. The antireflection micro / nano structure layer is used to reduce Fresnel reflection of the pump light at its incident surface to improve the absorption efficiency of the pump light. The gain light is incident downward on the photonic crystal layer to form an upward-transmitting first light component and a downward-transmitting second light component. The second light component passes through the saturable absorber layer and reaches the DBR layer. After being reflected by the DBR layer, it is transmitted upward and passes through the photonic crystal layer. The first light component and the second light component merge and sequentially pass through the active region, the window layer, and the antireflection micro / nano structure layer to transmit upward. The photonic crystal layer is used to filter out higher-order transverse modes, pass through the basic transverse mode, and control the intensity ratio of the first optical component and the second optical component. The saturable absorber layer is used to passively mode-lock or passively Q-switch the fundamental transverse modes after they have been filtered by the photonic crystal layer. The photonic crystal layer enables the base transverse mode, after being filtered by it, to have a higher local light intensity at the saturable absorber layer than the higher-order transverse mode, so that the additional absorption loss of the saturable absorber layer on the base transverse mode is less than the additional absorption loss on the higher-order transverse mode.

2. The self-mode-locked photonic crystal surface-emitting laser according to claim 1, characterized in that, The antireflective micro / nano structure layer comprises multiple micro / nano units arranged periodically or quasi-periodicly. The Fresnel reflectivity of the antireflective micro / nano structure layer to the pump light is less than 1%, and the equivalent refractive index of the antireflective micro / nano structure layer gradually increases from top to bottom.

3. The self-mode-locked photonic crystal surface-emitting laser according to claim 2, characterized in that, The antireflection micro / nano structure layer is a two-dimensional subwavelength grating with a duty cycle ranging from 0.3 to 0.7 and a grating depth d. g Satisfy: 0.1λ p ≤d g ≤0.3λ p The grating period P of the two-dimensional subwavelength grating satisfies: P < λ p / n, where λ p Where λ is the center wavelength of the pump light, and n is the refractive index of the window layer for the pump light; The grating period of the two-dimensional subwavelength grating is 0.15λ. p up to 0.28λ p Within the range.

4. The self-mode-locked photonic crystal surface-emitting laser according to claim 2, characterized in that, The antireflection micro / nano structure layer is a one-dimensional subwavelength grating or an asymmetric metasurface structure. The antireflection micro / nano structure layer has different equivalent refractive indices for pump light polarized in different directions, so as to have different reflectivities for pump light polarized in different directions.

5. The self-mode-locked photonic crystal surface-emitting laser according to claim 1, characterized in that, The photonic crystal layer includes a plurality of periodically arranged holes and / or a plurality of periodically arranged columnar structures. The lattice constant, duty cycle and etching depth of the photonic crystal layer are configured such that the photonic bandgap of the photonic crystal layer covers the frequency range corresponding to the higher-order transverse modes, and the frequency corresponding to the fundamental transverse mode is located within the passband of the photonic crystal layer.

6. The self-mode-locked photonic crystal surface-emitting laser according to claim 1, characterized in that, The saturable absorber layer includes a quantum well structure, a quantum dot structure, graphene, carbon nanotubes, or a topological insulator. The absorption wavelength of the saturable absorber layer is greater than the lasing laser wavelength, and the position of the saturable absorber layer relative to the reflected standing wave field of the DBR layer is configured to adjust the modulation depth and saturation flux of the saturable absorber layer.

7. The self-mode-locked photonic crystal surface-emitting laser according to claim 1, characterized in that, The bandgap energy of the window layer is greater than the photon energy of the pump light, and the thickness of the window layer is in the range of 50nm~2000nm; The gain chip further includes: a first buffer layer located between the photonic crystal layer and the saturable absorber layer; and a second buffer layer located between the saturable absorber layer and the DBR layer.

8. The self-mode-locked photonic crystal surface-emitting laser according to claim 1, characterized in that, The DBR layer has a reflectivity of less than 50% for pump light and a reflectivity of more than 99.5% for gain light.

9. The self-mode-locked photonic crystal surface-emitting laser according to claim 1, characterized in that, The self-mode-locked photonic crystal surface-emitting laser also includes: An external output mirror is located on the side of the antireflection micro / nano structure layer away from the window layer, and the external output mirror is spaced apart from the gain chip. The output light from the gain chip, which includes a first light component and a second light component, is incident on the external output mirror. The external output mirror reflects part of the light back into the gain chip to maintain oscillation, while the other part of the light passes through the external output mirror as laser output.

10. A method for fabricating a self-mode-locked photonic crystal surface-emitting laser, characterized in that, The method for fabricating the self-mode-locked photonic crystal surface-emitting laser according to any one of claims 1 to 9 comprises: Forming a gain chip includes: epitaxially growing a DBR layer and a saturable absorber layer sequentially on a substrate; epitaxially growing a photonic crystal initial layer on the saturable absorber layer, etching the photonic crystal initial layer to form a plurality of periodically arranged holes to obtain a photonic crystal layer; epitaxially growing an active region and a window layer sequentially on the photonic crystal layer through secondary epitaxy, and retaining air or filling the holes in the photonic crystal layer with a low refractive index medium; forming an antireflection micro / nano structure layer on the side of the window layer away from the active region.

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