Liquid crystal polarization switch-based laser and preparation method thereof

Abstract

This invention relates to the field of semiconductor lasers, and discloses a laser based on a liquid crystal polarization switch and its fabrication method. The laser includes a VCSEL unit and a liquid crystal cavity unit disposed on the light-emitting surface of the VCSEL unit. The liquid crystal cavity unit includes: a circumferential sidewall disposed on the light-emitting surface and defining a liquid crystal cavity; liquid crystal contained within the liquid crystal cavity; and a multifunctional structure disposed on the side of the circumferential sidewall opposite to the VCSEL unit. The multifunctional structure is made of a conductive material and has multiple functions, including: selectively transmitting polarized light with a specific polarization direction, controlling the liquid crystal orientation, and serving as the first tuning electrode of the liquid crystal cavity unit. This invention simplifies the device structure, enabling the laser to have high integration and miniaturization characteristics, and also achieves high-quality single-mode polarized laser output with a high polarization suppression ratio.

Description

Technical Field

[0001] This invention relates to the field of semiconductor laser technology, and more specifically to a laser based on a liquid crystal polarization switch and its fabrication method. Background Technology

[0002] Vertical-cavity surface-emitting lasers (VCSELs) have become key light sources in data communication, sensing, and consumer electronics due to their superior characteristics such as low threshold current, circular far-field spot size, ease of two-dimensional integration, and low cost. However, the inherent uncontrollability and instability of the output polarization state of traditional VCSELs severely limit their application potential in higher-performance photonics systems. Polarization-controllable VCSELs have shown significant application value in optical interconnects, optical communication, three-dimensional sensing, polarization imaging, and optical coding. Current polarization-controlled VCSELs based on optical feedback, stress engineering, or elliptical aperture schemes generally suffer from problems such as a lack of static reconfigurability in controllability, complex fabrication processes, and the potential introduction of additional losses. In contrast, liquid crystal polarization-controlled VCSELs employ a hybrid integration approach of an external liquid crystal cavity and a VCSEL chip. Their outstanding advantage lies in the ability to dynamically and in real-time control the polarization state through voltage, while maintaining extremely high polarization suppression ratio and excellent operational stability.

[0003] The inventors of this application recognize that there is still room for further improvement in terms of structural simplification of existing liquid crystal polarization-controlled VCSELs. Summary of the Invention

[0004] The main objective of this invention is to provide a laser based on a liquid crystal polarization switch and its fabrication method, in order to solve the technical problem of how to provide a liquid crystal polarization control VCSEL with a simpler structure.

[0005] According to one aspect of the present invention, a laser based on a liquid crystal polarization switch is provided, comprising: a VCSEL unit and a liquid crystal cavity unit disposed on the light-emitting surface of the VCSEL unit. The liquid crystal cavity unit includes: a circumferential sidewall disposed on the light-emitting surface and defining a liquid crystal cavity; liquid crystal contained within the liquid crystal cavity; and a multifunctional structure disposed on the side of the circumferential sidewall opposite to the VCSEL unit. The multifunctional structure is made of a conductive material and has multiple functions, including: selectively transmitting polarized light with a specific polarization direction, controlling the orientation of the liquid crystal, and serving as a first tuning electrode of the liquid crystal cavity unit.

[0006] According to one embodiment of the present invention, the multifunctional structure is a subwavelength grating, and the working surface of the subwavelength grating faces the liquid crystal cavity.

[0007] According to one embodiment of the present invention, the liquid crystal cavity unit further includes an insulating protective layer covering the working surface of the subwavelength grating, the thickness of which is less than a predetermined value to maintain the trench structure of the working surface.

[0008] According to one embodiment of the present invention, the predetermined value is 1 / 10 of the period of the subwavelength grating.

[0009] According to one embodiment of the present invention, the material of the subwavelength grating includes GaAs; the material of the protective layer includes SiO2 and / or Al2O3.

[0010] According to one embodiment of the present invention, the liquid crystal cavity unit further includes a liquid crystal alignment layer disposed on the light-emitting surface and having stripes, wherein the direction of the trench of the subwavelength grating is perpendicular to the direction of the stripes of the liquid crystal alignment layer.

[0011] According to one embodiment of the present invention, the refractive index anisotropy of the liquid crystal is greater than 0.223.

[0012] According to one embodiment of the present invention, the VCSEL cell includes an electrode close to the liquid crystal cavity cell, which also serves as a second tuning electrode of the liquid crystal cavity cell.

[0013] According to another aspect of the present invention, a method for fabricating a laser based on a liquid crystal polarization switch as described above is provided, comprising the following steps: fabricating a VCSEL unit; forming a circumferential sidewall defining a liquid crystal cavity on the light-emitting surface of the VCSEL unit; forming a multifunctional structure on a substrate; placing the substrate with the multifunctional structure on the side of the circumferential sidewall opposite to the VCSEL unit, such that the multifunctional structure faces the liquid crystal cavity; and injecting liquid crystal into the liquid crystal cavity.

[0014] According to one embodiment of the present invention, the multifunctional structure is a subwavelength grating, and the method further includes forming a protective layer covering the working surface of the subwavelength grating, wherein the protective layer is insulating and the thickness of the protective layer is less than a predetermined value to maintain the groove structure of the working surface.

[0015] In the technical solution of the present invention, a single multifunctional structure is used to provide multiple functions such as polarization selection, control of liquid crystal orientation, and serving as the first tuning electrode of the liquid crystal cavity unit. Compared with setting multiple structural layers that only provide a single function, the number of structural layers can be reduced, which is beneficial to simplifying the overall structure of the laser and making the laser have high integration and miniaturization characteristics. At the same time, the laser of the present invention can also achieve high-quality single-mode polarized laser output and exhibit a high polarization suppression ratio. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0017] Figure 1 A schematic diagram of a laser based on a liquid crystal polarization switch according to an embodiment of the present invention is shown; Figure 2 A cross-sectional schematic diagram of a laser based on a liquid crystal polarization switch according to an embodiment of the present invention is shown, wherein the protective layer is separated from the circumferential sidewall; Figure 3 A cross-sectional schematic diagram of a VCSEL unit according to an embodiment of the present invention is shown; Figure 4 A cross-sectional schematic diagram of a liquid crystal alignment layer formed on the light-emitting surface of a VCSEL cell according to an embodiment of the present invention is shown. Figure 5 A cross-sectional schematic diagram showing a circumferential sidewall formed on the light-emitting surface of a VCSEL cell according to an embodiment of the present invention is shown. Figure 6 A cross-sectional schematic diagram showing a subwavelength grating and a protective layer sequentially formed on a substrate according to an embodiment of the present invention is shown. Figure 7 A schematic cross-sectional view of a substrate having a subwavelength grating and a protective layer formed thereon attached to a circumferential sidewall is shown according to an embodiment of the present invention. Figure 8 A schematic cross-sectional view showing a substrate with a subwavelength grating and a protective layer formed thereon attached to a circumferential sidewall according to an embodiment of the present invention; Figure 9 A cross-sectional schematic diagram of liquid crystal injection into a liquid crystal cavity is shown according to an embodiment of the present invention; Figure 10 A schematic diagram showing the orientation of the trenches of the subwavelength grating and the orientation of the stripes of the liquid crystal alignment layer according to an embodiment of the present invention; Figure 11 A flowchart illustrating a method for fabricating a laser based on a liquid crystal polarization switch according to an embodiment of the present invention; Figure 12 A schematic diagram illustrating the switching of output light polarization state by a laser based on a liquid crystal polarization switch according to an embodiment of the present invention is shown. Figure 13 The diagram shows the test results of the polarization switching characteristics of a laser based on a liquid crystal polarization switch according to an embodiment of the present invention.

[0018] Explanation of reference numerals in the attached figures: 10. VCSEL unit; 11. First electrode; 12. Substrate; 13. First DBR; 14. Oxide confinement layer; 15. Active region; 16. Passivation layer; 17. Second electrode; 18. Second DBR; 20. Liquid crystal cavity unit; 21. Peripheral sidewall; 22. Liquid crystal; 23. Subwavelength grating; 24. Protective layer; 25. Liquid crystal alignment layer; 26. Substrate; 100. Laser based on liquid crystal polarization switch. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to specific examples and the accompanying drawings.

[0020] 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., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They 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. Therefore, they should not be construed as limitations on this invention.

[0021] It should be noted that all uses of "first" and "second" in the embodiments of the present invention are for the purpose of distinguishing two entities or parameters with the same name but different names. It is clear that "first" and "second" are only for the convenience of expression and should not be construed as limiting the embodiments of the present invention. Subsequent embodiments will not explain this in detail.

[0022] refer to Figure 1 and Figure 2 This invention proposes a laser 100 based on a liquid crystal polarization switch, comprising: a VCSEL unit 10 and a liquid crystal cavity unit 20 disposed on the light-emitting surface of the VCSEL unit 10. The liquid crystal cavity unit 20 includes: a circumferential sidewall 21 disposed on the light-emitting surface and defining a liquid crystal cavity; liquid crystal 22 housed within the liquid crystal cavity; and a multifunctional structure disposed on the side of the circumferential sidewall 21 opposite to the VCSEL unit 10. The multifunctional structure is made of a conductive material and has multiple functions: selectively transmitting polarized light with a specific polarization direction, controlling the orientation of the liquid crystal, and serving as the first tuning electrode of the liquid crystal cavity unit. It should be noted that, in the embodiments of this invention, "selectively transmitting polarized light with a specific polarization direction" means that only polarized light with a specific polarization direction is transmitted to a large extent, while a small amount of polarized light with other polarization directions may also be transmitted.

[0023] In the technical solution of the present invention, the multifunctional structure is a single structural layer. The single multifunctional structure provides multiple functions such as polarization selection, control of liquid crystal 22 orientation, and serving as the first tuning electrode of liquid crystal cavity unit 20. Compared with setting multiple structural layers that only provide a single function, the number of structural layers can be reduced, which is beneficial to simplify the overall structure of the laser, reduce the size of the laser, and simplify the manufacturing process. This makes the laser have high integration and miniaturization characteristics. At the same time, the laser 100 of the present invention can also achieve high-quality single-mode polarized laser output and exhibit a high polarization suppression ratio.

[0024] In some embodiments, the multifunctional structure is a subwavelength grating 23, the working surface of which (i.e., the surface with the groove structure) faces the liquid crystal cavity. The period of the subwavelength grating 23 is less than or equal to the incident light wavelength. The unique periodic groove structure of the subwavelength grating 23 can be used for polarization filtering of the incident light, and also for guiding and anchoring the orientation of liquid crystal molecules. Specifically, the parameters of the subwavelength grating 23 are configured to produce a significant difference in transmittance between the polarization components parallel to the grating grooves and the polarization components perpendicular to the grating grooves in the incident light at the target wavelength, thereby achieving single polarization state selective output. Simultaneously, guided by the groove structure of the subwavelength grating 23, the liquid crystal molecules can be oriented and aligned along the groove direction. Furthermore, the subwavelength grating 23 is made of a conductive material, and the grating itself forms a conductive network. Due to its conductivity, the subwavelength grating 23 can also be used as the first tuning electrode of the liquid crystal cavity. The subwavelength grating 23 of this invention integrates polarization selection, liquid crystal alignment, and electrode conductivity. Compared to having three separate structural layers—a polarization selection layer, a liquid crystal alignment layer, and electrodes—this invention uses a subwavelength grating 23 to provide all three functions simultaneously, reducing the number of structural layers, simplifying the device structure and fabrication process, and facilitating miniaturization. The subwavelength grating 23 can filter the polarization direction of the output light without the need for external optical components, thereby achieving an integrated polarization switch function.

[0025] refer to Figure 2 and Figure 10In some embodiments, the liquid crystal cavity unit 20 further includes a liquid crystal alignment layer 25 with stripes disposed on the light-emitting surface of the VCSEL unit 10. The direction of the trenches of the subwavelength grating 23 (e.g., the y-direction) is perpendicular to the direction of the stripes of the liquid crystal alignment layer 25 (e.g., the x-direction). The subwavelength grating 23 and the liquid crystal alignment layer 25 constitute the upper alignment layer and the lower alignment layer of the liquid crystal cavity unit 20, respectively, with an alignment angle of 90° between them. Through this orthogonal alignment structure, the liquid crystal alignment within the liquid crystal cavity can be gradually twisted, guiding the liquid crystal molecules to undergo directional rotation at a specific angle, thereby achieving a 90° twisted output of the incident light polarization direction. The liquid crystal alignment layer 25 can be prepared using a photoresist material, and its film thickness can be controlled by adjusting the dilution ratio of the photoresist. The liquid crystal alignment layer 25, as the lower alignment layer, can be spin-coated onto the surface of the VCSEL unit 10, and its alignment stripe direction can be determined by a rubbing process. The alignment direction of the upper alignment layer is determined by the trench direction of the subwavelength grating 23.

[0026] The average refractive index of a liquid crystal cavity is modulated by the voltage applied across the cavity. When the voltage applied across the cavity changes, the equivalent refractive index of the liquid crystal layer changes accordingly, thereby modulating the phase delay of the transmitted light and ultimately achieving dynamic control of the polarization state of the output light. Specifically, when no voltage is applied to the liquid crystal cavity, the incident light → due to the birefringence of the liquid crystal, a phase delay is induced → the polarization state is rotated, i.e., x-polarized light rotates to the y-polarization direction (similarly, y-polarized light rotates to the x-polarization direction) and exits. Applying a voltage to the liquid crystal → causes a change in the equivalent refractive index / birefringence, which leads to the phase delay gradually disappearing → the original polarization state is output, i.e., y-polarized light exits in the y-polarization direction (similarly, x-polarized light exits in the x-polarization direction).

[0027] In some embodiments of the present invention, the VCSEL unit 10 is configured to output strong x-polarized light and extremely weak y-polarized light, wherein the x-polarized light and y-polarized light are perpendicular to each other. When no voltage is applied to the liquid crystal cavity, the upper and lower alignment layers forming the orthogonal alignment structure cause the liquid crystal to be twisted by 90°. The 90° twisted liquid crystal can rotate the incident polarized light by 90°, realizing the optical rotation function, that is, the x-polarized light rotates to the y direction, and the y-polarized light rotates to the x direction. Since the trench direction of the subwavelength grating 23 is parallel to the y direction, the strong x-polarized light rotates to the y direction and can pass through the subwavelength grating 23 and be emitted, presenting a bright state. It should be noted that the polarized light output at this time is x-polarized light with its polarization orientation rotated to the y direction, and it still carries the mode information of x-polarized light. As the voltage increases, the liquid crystal gradually presents an upright state instead of a twisted state and thus loses the optical rotation function. When the voltage applied to the liquid crystal cavity reaches the threshold voltage, the liquid crystal is completely upright, at which point the optical rotation function completely disappears, and the polarized light does not undergo a polarization state conversion after passing through the liquid crystal. X-polarized light is emitted in the x direction and y-polarized light is emitted in the y direction. In this case, the extremely weak y-polarized light can be emitted from the subwavelength grating 23, but because the beam is very weak, it is almost invisible and appears dark.

[0028] Based on the voltage-controlled liquid crystal cavity to regulate the polarization state of the output light, a subwavelength grating 23 is further used to selectively transmit the polarization state of the output light, thereby synergistically realizing the function of an electrically tunable optical switch. This invention can stably output high-quality single-mode laser with a polarization suppression ratio of 21.5 dB, and can directly achieve active control of the output polarization state without requiring an external optical path.

[0029] In some embodiments, the subwavelength grating 23 is made of GaAs. The subwavelength grating 23 serves as an electrode in the liquid crystal cavity to apply voltage; theoretically, any conductor can be used as an electrode. Considering that using metallic materials would cause light absorption, this invention uses GaAs to fabricate the subwavelength grating 23.

[0030] In some embodiments, the liquid crystal cavity unit 20 further includes a protective layer 24 that covers the working surface of the sub-wavelength grating 23 and has insulating properties. The thickness of the protective layer 24 is less than a predetermined value to maintain the groove structure of the working surface, that is, the protective layer 24 is relatively thin so as not to affect the grating topography. The predetermined value can be 1 / 10 of the period of the sub-wavelength grating 23. That is to say, the thickness H of the protective layer 24 and the period P of the sub-wavelength grating 23 satisfy the following relationship: H < P / 10. The material of the protective layer 24 is an insulating material. For example, SiO2 and / or Al2O3 can be selected. The protective layer 24 can physically isolate the liquid crystal from the grating without changing the periodic topological structure of the grating, thereby playing a protective role. In some cases, the grating is made of GaAs material. GaAs is prone to oxidation and may undergo ion migration. Long-term contact with the liquid crystal will affect the device reliability. SiO2 and / or Al2O3 do not chemically react with the liquid crystal, nor will they diffuse ions to pollute the liquid crystal. They can maintain the interface stability for a long time and can effectively isolate the electrodes to prevent leakage.

[0031] In some embodiments, the refractive index anisotropy (birefringence) of the liquid crystal 22 is greater than 0.223, and the liquid crystal 22 can be a positive dielectric anisotropy liquid crystal. By selecting a liquid crystal material with a refractive index anisotropy greater than 0.223 and adjusting process parameters such as the thickness of the circumferential sidewall 21, effective control of the polarization state of the emitted light can be achieved within a wide wavelength range. At the same time, the liquid crystal 22 has good electric field response characteristics and a relatively fast response speed, enabling the laser 100 of the present invention to exhibit excellent dynamic response performance during the electro-tuning polarization switching process.

[0032] In some embodiments, the VCSEL unit 10 includes an electrode (the second electrode 17) close to the liquid crystal cavity unit, and the second electrode 17 is simultaneously used as the second tuning electrode of the liquid crystal cavity unit 20. The injection electrode of the VCSEL unit 10 is multiplexed as the second tuning electrode of the liquid crystal cavity. The device adopts a three-electrode integrated structure, which can effectively simplify the structure and manufacturing process. The voltages at both ends of the liquid crystal cavity are provided by the sub-wavelength grating 23 and the second electrode 17 respectively.

[0033] The VCSEL unit 10, from bottom to top, may include: a first electrode 11 sputtered on the back side of a substrate 12 (e.g., a GaAs substrate), the substrate 12, a first DBR 13, an active region 15, an oxide confinement layer 14, a second DBR 18, a passivation layer 16, and a second electrode 17. The second electrode 17 is used for current injection into the laser and as a second tuning electrode for the liquid crystal cavity. The aperture diameter of the passivation layer 16 is smaller than the mesa diameter of the VCSEL unit 10, and the aperture diameter of the second electrode 17 is smaller than the aperture diameter of the passivation layer 16. The VCSEL unit 10 may use material systems including, but not limited to, GaAs, with wavelengths including, but not limited to, 1060 nm. The first electrode 11 and the second electrode 17 may use material systems including, but not limited to, Ti / Au. The material of the passivation layer 17 may include SiO2. The VCSEL unit 10 may be one of the following: air-pillar type, buried heterojunction type, oxide-confinement type, or proton-injection type.

[0034] The liquid crystal cavity unit 20, from bottom to top, may include: a liquid crystal alignment layer 25, circumferential sidewalls 21, a protective layer 24, a multifunctional structure (e.g., a subwavelength grating 23), and a substrate 26 (e.g., a SiO2 glass substrate). The protective layer 24 is used to physically separate the liquid crystal and the multifunctional structure, improving reliability. The multifunctional structure is used for initial angle control of the liquid crystal 22, serves as the first tuning electrode of the liquid crystal cavity unit 20, and acts as a polarization selection layer for filtering the polarization state of the final output light.

[0035] According to another aspect of the present invention, a method for fabricating a liquid crystal polarization switch-based laser 100 as described above is provided, comprising the following steps: Fabrication of VCSEL unit 10; A circumferential sidewall 21 defining a liquid crystal cavity is formed on the light-emitting surface of the VCSEL unit 10; A multifunctional structure is formed on substrate 26; A substrate 26 with a multifunctional structure is disposed on the side of the circumferential sidewall 21 opposite to the VCSEL unit 10, and the multifunctional structure faces the liquid crystal cavity. Liquid crystal is injected into the liquid crystal cavity.

[0036] The circumferential sidewall 21 can be fabricated using photolithography. First, photoresist is spin-coated onto the surface of the VCSEL unit 10, and then the circumferential sidewall 21 defining the liquid crystal cavity is formed through exposure and development. The cavity thickness is determined by the thickness of the spin-coated photoresist. The cavity thickness of the liquid crystal cavity can be precisely controlled by adjusting the fabrication process parameters of the circumferential sidewall 21, thereby achieving high compatibility and efficient adaptation between the liquid crystal cavity unit 20 and different lasers in a wide wavelength range.

[0037] In some embodiments, the multifunctional structure is a subwavelength grating 23, and the fabrication method further includes forming a protective layer 24 covering the working surface of the subwavelength grating 23, wherein the protective layer 24 is insulating, and the thickness of the protective layer 24 is less than a predetermined value to maintain the trench structure of the working surface of the subwavelength grating 23.

[0038] Figures 3 to 9 The diagram shows cross-sectional schematics of each stage in the laser fabrication method of the present invention. Figure 11 The overall flowchart is shown. (Reference) Figures 3 to 9 and Figure 11 The preparation method may include the following steps: Step S1: The first DBR 13, the active region 15, the oxide confinement layer 14, and the second DBR 18 are sequentially epitaxially grown on the substrate 12 by metal-organic chemical vapor deposition or molecular beam epitaxy to obtain an epitaxial wafer.

[0039] Step S2: After the photolithography process, a circular mesa structure for the light source is prepared by chemical etching. The etching depth is controlled to be deeper than the position of the oxide confinement layer 14 to expose it laterally, thus creating conditions for the formation of the current-limiting aperture in the subsequent wet oxidation process.

[0040] Step S3: The chip with the mesa etched is placed in an oxidation furnace, and nitrogen saturated with water vapor is introduced as the oxidation atmosphere. Under heating conditions, the aluminum component in the oxide confinement layer 14 reacts with water to generate the corresponding oxide. By precisely controlling key process parameters such as oxidation temperature, gas flow rate, and oxidation time, oxide confinement holes with dimensions meeting design requirements are formed to effectively confine the injected current and lateral optical field mode.

[0041] Step S4: After the oxidation process is completed, a passivation layer 16 is grown on the device surface using metal-organic chemical vapor deposition (MOCVD) technology for device surface protection and current leakage suppression. The thickness of the passivation layer 16 needs to be strictly controlled: if it is too thin, it is easy to form pinhole defects, leading to leakage current paths; if it is too thick, it will introduce excessive internal stress, which may cause damage to the device structure.

[0042] Step S5: A circular electrode pattern is overlaid using photolithography, and the passivation layer 16 in the electrode area is removed by etching to form a current injection window. Subsequently, a Ti / Au composite layer is sputtered as the second electrode 17, and the light-emitting aperture of the second electrode 17 is formed by etching or lift-off processes. The diameter of the light-emitting aperture of the passivation layer 16 must be smaller than the diameter of the mesa of the light source portion, and the diameter of the light-emitting aperture of the second electrode 17 must be further smaller than the diameter of the light-emitting aperture of the passivation layer 16 to achieve precise alignment of current limiting and the optical window.

[0043] Step S6: The device is thinned using a grinding mill, followed by sputtering of the first electrode 11 to complete the fabrication of the VCSEL unit 10. Figure 3 As shown.

[0044] Step S7: After protecting the two end electrodes on the surface of the prepared VCSEL unit 10, spin-coat a diluted polyimide solution and cure it according to the polyimide curing instructions. After curing, perform rubbing alignment to confirm the initial stripe direction, forming a liquid crystal alignment layer 25, as shown. Figure 4 As shown.

[0045] Step S8: On the surface of the cured liquid crystal alignment layer 25, photoresist is spin-coated, and after photolithography patterning and curing, circumferential sidewalls 21 are formed, as shown below. Figure 5 As shown.

[0046] Step S9: A GaAs layer is formed on the substrate 26, and a trench structure of a grating is formed on the GaAs layer by electron beam exposure to prepare a subwavelength grating 23, which serves as the first tuning electrode of the liquid crystal cavity, the liquid crystal molecule alignment layer, and the polarization state screening layer.

[0047] Step S10: A protective layer 24 is prepared on the surface of the fabricated subwavelength grating 23 by atomic layer deposition to achieve physical isolation between the liquid crystal and the grating, such as... Figure 6 As shown.

[0048] Step S11: The substrate 26, on which the subwavelength grating 23 and the protective layer 24 are formed, is inverted and bonded to the circumferential sidewall 21 to complete the fabrication of the liquid crystal cavity unit 20. Figure 7 and Figure 8 As shown.

[0049] Step S12: Under vacuum conditions, liquid crystal 22 (nematic liquid crystal) is poured in and sealed with ultraviolet-curable adhesive (UV adhesive) to complete the fabrication of laser 100. Figure 9 As shown.

[0050] In step S2, the composition of the epitaxial wafer etching solution is CH3OH: H3PO4: H2O2: H2O = (1.5~3):(0.5~1.5): (0.5~1.5): (4.5~5.5), by volume.

[0051] In step S5, the composition of the passivation layer corrosion solution is HF: NH4F: H2O = (2~4) mL: (5~7) mg: (9~11) mL.

[0052] The curing conditions in steps S7 and S8 can be a temperature of 200℃ and a curing time of 30 minutes.

[0053] In step S8, the circumferential sidewall 21 can be made of photoresist of types including but not limited to 5214, or polyimide including but not limited to JSR-5100. By adjusting the ratio of polyimide to diluent, support wall structures of different thicknesses can be achieved. The diluent can be an organic solvent, such as carbon tetrachloride or chlorobenzene. The feature size of the circumferential sidewall 21 can be on the order of hundreds of micrometers, making it suitable for high-density optoelectronic integration applications.

[0054] In step S11, liquid crystal can be infused in a vacuum environment through capillary action, and the ultraviolet curing adhesive includes epoxy resin.

[0055] Figure 12 The polarization control characteristics of a laser 100 in one embodiment of the present invention are shown. When the injection current is 2 mA and no voltage is applied to the liquid crystal, the output light is x-polarized light with its polarization orientation rotated to the y-direction; after applying voltage, the output light switches to y-polarized light. Dynamic switching between the two polarizations can be achieved through electro-controlled liquid crystal, with a polarization suppression ratio reaching 21.5 dB.

[0056] Figure 13 The results of the polarization switching characteristics test of the laser 100 in one embodiment of the present invention are shown. As the applied voltage changes, the equivalent refractive index of the liquid crystal cavity unit changes accordingly, thereby realizing continuous control of the polarization state of the output light; combined with the selective transmission effect of the polarization selection layer, a significant switching effect between "bright state" (high transmission) and "dark state" (low transmission) is finally exhibited at the output end, verifying the electro-optical switching function of the device.

[0057] In summary, the liquid crystal polarization-controlled VCSEL structure provided by this invention achieves electrically controlled modulation of the output polarization state by vertically integrating a liquid crystal cavity with a twisted orientation on the VCSEL surface. By adjusting the applied voltage, the equivalent refractive index of the liquid crystal cavity can be changed, thereby controlling the phase delay introduced by the cavity to the transmitted light, ultimately achieving dynamic switching of the output polarization state. Simultaneously, a multifunctional structure serving as a polarization selection layer is integrated on top of the liquid crystal cavity, enabling selective transmission of specific polarization states of the output light, thus working in conjunction with the electrically controlled liquid crystal cell to achieve a polarization switching function. In addition to providing polarization selection, the multifunctional structure also provides liquid crystal alignment and liquid crystal cavity electrode functions, simplifying the structure and fabrication process.

[0058] The technical solution proposed in this invention enables efficient and flexible control of the polarization state of VCSEL output light using liquid crystal cells. This not only allows for precise control of the polarization direction of the emitted light, meeting the stringent requirements of polarization switching, but also provides a high polarization suppression ratio, significantly improving the system's optical performance. Furthermore, by adjusting the liquid crystal cavity process parameters, this structure can be widely adapted to VCSEL devices in different wavelength bands, exhibiting excellent versatility and band expansion capabilities. In addition, the monolithic integration method of liquid crystal and VCSEL employed in this invention supports a highly integrated device architecture, combining the advantages of low fabrication cost, high operational stability, and strong process compatibility, providing a feasible path for the practical application of high-performance polarization-controllable semiconductor lasers.

[0059] The solution of this invention does not require modification of the active region structure of the VCSEL, maintaining its intrinsic light-emitting characteristics and possessing good process compatibility. It provides a flexible and reliable polarization management solution for cutting-edge fields such as optical communication, optical computing, and high-precision optical sensing. The device employs an electrically pumped mechanism, eliminating the need for an external excitation light source, significantly reducing device size and providing an effective path for high-density optoelectronic integration. Furthermore, the device fabrication process is simple and feasible, significantly reducing process complexity and manufacturing costs while ensuring stable output of high-quality single-mode laser light and a high polarization suppression ratio.

[0060] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of the different aspects of the invention as described above exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A laser based on a liquid crystal polarization switch, characterized in that, include: VCSEL unit; A liquid crystal cavity unit, disposed on the light-emitting surface of the VCSEL unit, includes: A circumferential sidewall is disposed on the light-emitting surface and defines the liquid crystal cavity; Liquid crystal is housed within the liquid crystal cavity; A multifunctional structure is disposed on the side of the circumferential sidewall opposite to the VCSEL unit. The multifunctional structure is made of a conductive material and has the following multiple functions: selectively transmitting polarized light with a specific polarization direction, controlling the liquid crystal orientation, and serving as the first tuning electrode of the liquid crystal cavity unit.

2. The laser according to claim 1, characterized in that, The multifunctional structure is a subwavelength grating, and the working surface of the subwavelength grating faces the liquid crystal cavity.

3. The laser according to claim 2, characterized in that, The liquid crystal cavity unit further includes an insulating protective layer covering the working surface of the subwavelength grating, the thickness of which is less than a predetermined value to maintain the groove structure of the working surface.

4. The laser according to claim 3, characterized in that, The predetermined value is 1 / 10 of the period of the subwavelength grating.

5. The laser according to claim 3, characterized in that, The material of the subwavelength grating includes GaAs; the material of the protective layer includes SiO2 and / or Al2O3.

6. The laser according to claim 2, characterized in that, The liquid crystal cavity unit further includes a liquid crystal alignment layer disposed on the light-emitting surface and having stripes, wherein the direction of the trench of the subwavelength grating is perpendicular to the direction of the stripes of the liquid crystal alignment layer.

7. The laser according to claim 1, characterized in that, The refractive index anisotropy of the liquid crystal is greater than 0.

223.

8. The laser according to claim 1, characterized in that, The VCSEL unit includes an electrode located near the liquid crystal cavity unit, which also serves as a second tuning electrode for the liquid crystal cavity unit.

9. A method for fabricating a laser based on a liquid crystal polarization switch as described in any one of claims 1-8, characterized in that, Includes the following steps: Fabricate the VCSEL unit; The circumferential sidewall defining the liquid crystal cavity is formed on the light-emitting surface of the VCSEL cell; The multifunctional structure is formed on the substrate; The substrate on which the multifunctional structure is formed is disposed on the side of the circumferential sidewall opposite to the VCSEL cell, such that the multifunctional structure faces the liquid crystal cavity; Liquid crystal is injected into the liquid crystal cavity.

10. The preparation method according to claim 9, characterized in that, The multifunctional structure is a subwavelength grating, and the method further includes forming a protective layer covering the working surface of the subwavelength grating, wherein the protective layer is insulating and the thickness of the protective layer is less than a predetermined value to maintain the groove structure of the working surface.

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