Liquid crystal tunable vertical cavity surface emitting laser and method of manufacturing the same

Abstract

The application relates to the field of semiconductor lasers, and discloses a liquid crystal tunable vertical cavity surface emitting laser and a preparation method thereof. The liquid crystal tunable vertical cavity surface emitting laser comprises a VCSEL unit, the VCSEL unit comprising a first DBR and a second DBR located above the first DBR; a circumferential sidewall formed on the upper side of the VCSEL unit, the circumferential sidewall defining a liquid crystal cavity; a mirror unit bonded to the upper side of the circumferential sidewall; and liquid crystal accommodated in the liquid crystal cavity; wherein the VCSEL unit further comprises a coupling layer located above the second DBR, the refractive index of the coupling layer and the refractive index of the high-refractive-index layer in the first DBR being in a first preset relationship, and / or the thickness of the coupling layer and the wavelength being in a second preset relationship. The application introduces a coupling layer structure to improve the tuning range and tuning efficiency, utilizes a polarization control layer to control the polarization direction of output light, and further studies a specific and feasible preparation method of an electrically pumped liquid crystal tunable VCSEL.

Description

Technical Field

[0001] This invention relates to the field of semiconductor laser technology, specifically to a liquid crystal tunable vertical cavity surface-emitting laser and its fabrication method. Background Technology

[0002] In recent years, with the rapid development of technologies such as virtual reality (VR), Internet of Things (IoT), LiDAR, 3D sensing, and biomedicine, the demand for lasers with high stability, low cost, easy integration, and tunable wavelength has been increasing, which has driven the rapid development of tunable semiconductor lasers. Different types of tunable semiconductor lasers are applied in different fields based on their respective advantages.

[0003] External cavity tunable semiconductor lasers play an important role in wavelength division multiplexing (WDM) due to their wide tuning range and high output power, but they still face challenges such as slow tuning speed, large size, and difficult packaging. Distributed feedback laser (DFB) arrays and distributed Bragg reflector (DBR) lasers are widely used in the C-band (1530-1565nm) and L-band (1565-1625nm) due to their fast tuning speed and high integration, but they also have limitations such as high power consumption and poor stability. To obtain low-power, high-modulation-rate, and low-cost light sources, tunable vertical-cavity surface-emitting lasers (VCSELs) have been extensively studied and have attracted much attention in short-range communication. Tunable VCSELs have advantages such as continuous wavelength tuning, wide wavelength coverage, high integration, and high modulation rate, and also have great potential in atomic clocks, OCT, and LiDAR.

[0004] With ongoing research, various tuning methods have been developed for tunable VCSELs, including thermal tuning, microelectromechanical systems (MEMS) tuning, liquid crystal tuning, and piezoelectric tuning. Among these, MEMS tunable VCSELs have been widely studied due to their high modulation rate and low power consumption. However, researchers have found that traditional cantilever beam structures are susceptible to temperature, mechanical vibration, and other external factors during tuning, leading to reduced stability and lifespan. Furthermore, the complex and costly fabrication process of MEMS cantilever beams is a significant factor to consider in applications. Thermal tuning, compared to other methods, has low tuning efficiency and a relatively small tuning range, severely limiting its applications. The characteristic of liquid crystals changing their optical properties under varying external electric fields offers a new direction for tunable VCSELs. Liquid crystal tuning can achieve a larger tuning range while maintaining stability and low cost, making it suitable for various applications and of great research significance.

[0005] The key aspects of fabricating tunable liquid crystal VCSELs include: (a) the fabrication of a high-quality liquid crystal cavity; (b) the extraction of liquid crystal tuning electrodes and VCSEL electrodes; and (c) the initial angle and type of liquid crystal molecules. Optically pumped VCSELs have demonstrated their potential for wide-range tunability, but this method often results in large size and high cost. Compared to optically pumped VCSELs, research on electrically pumped VCSELs is relatively limited, mainly because the fabrication of the liquid crystal cavity and the process compatibility issues arising from the extraction of pump electrodes remain unresolved. Therefore, feasible fabrication schemes for electrically pumped tunable liquid crystal VCSELs are still lacking, limiting the wavelength tuning range of these tunable VCSELs. Summary of the Invention

[0006] The main objective of this invention is to provide a liquid crystal tunable vertical cavity surface-emitting laser and its fabrication method, so as to solve the problem that the wavelength tuning range of liquid crystal tunable VCSELs is limited.

[0007] According to one aspect of the present invention, a liquid crystal tunable vertical-cavity surface-emitting laser is provided, comprising:

[0008] VCSEL unit, the VCSEL unit includes a first DBR and a second DBR located above the first DBR;

[0009] A circumferential sidewall is formed on the upper side of the VCSEL cell, and the circumferential sidewall defines the liquid crystal cavity;

[0010] The mirror unit is joined to the upper side of the circumferential sidewall; and

[0011] Liquid crystal contained within the liquid crystal cavity;

[0012] The VCSEL unit also includes a coupling layer located above the second DBR. The refractive index of the coupling layer is in a first preset relationship with the refractive index of the high refractive index layer in the first DBR, and / or the thickness of the coupling layer is in a second preset relationship with the wavelength.

[0013] According to one embodiment of the present invention, the first preset relationship includes the refractive index of the coupling layer being the square root of the refractive index of the high refractive index layer in the first DBR, and the second preset relationship includes the thickness of the coupling layer being one-quarter wavelength.

[0014] According to one embodiment of the present invention, the liquid crystal tunable vertical cavity surface-emitting laser further includes two polarization control layers located on the lower and upper sides of the liquid crystal cavity, respectively. The polarization control layers are configured to control the initial orientation of the liquid crystal molecules to control the polarization direction of the emitted light.

[0015] According to one embodiment of the present invention, two polarization control layers respectively have stripes extending along a first stripe direction and stripes extending along a second stripe direction, and a predetermined angle is formed between the first stripe direction and the second stripe direction.

[0016] According to one embodiment of the present invention, the circumferential sidewall is formed by spin-coating photoresist on the surface of the VCSEL cell and then performing photolithography and development.

[0017] According to one embodiment of the present invention, the VCSEL cell includes a second electrode located above the second DBR, the second electrode serving simultaneously as the injection electrode and the first liquid crystal tuning electrode of the VCSEL cell.

[0018] According to one embodiment of the present invention, the reflector unit includes a third DBR and a second liquid crystal tuning electrode, the second liquid crystal tuning electrode being located above the third DBR.

[0019] According to one embodiment of the present invention, the liquid crystal includes a nematic liquid crystal; and / or the birefringence difference of the liquid crystal is greater than 0.1 and less than 0.25.

[0020] According to one embodiment of the present invention, the VCSEL unit includes, from bottom to top: a first electrode, a substrate, a first DBR, an active region, an oxide confinement layer, a second DBR, a passivation layer, and a second electrode, with a coupling layer disposed in the area of ​​the second DBR not covered by the passivation layer; the mirror unit includes, from bottom to top: a third DBR, a second liquid crystal tuning electrode, and a substrate.

[0021] According to one embodiment of the present invention, the reflector unit is bonded to the circumferential sidewall by an adhesive, the adhesive being composed of at least flake copper-coated silver powder and a curing agent in a weight ratio of (8~12):1; the reflector unit is sealed to the circumferential sidewall by UV adhesive.

[0022] According to one embodiment of the present invention, the VCSEL cell type is any one of air column type, buried heterojunction type, oxidation confinement type, and proton injection type.

[0023] According to another aspect of the present invention, a method for fabricating a liquid crystal tunable vertical-cavity surface-emitting laser is provided, comprising:

[0024] A VCSEL unit is fabricated, comprising a first DBR, a second DBR located above the first DBR, and a coupling layer located above the second DBR. The refractive index of the coupling layer is the square root of the refractive index of the high-refractive-index layer in the first DBR, and the thickness is one-quarter wavelength.

[0025] A circumferential sidewall defining the liquid crystal cavity is formed on the upper side of the VCSEL cell;

[0026] Fabrication of the mirror unit;

[0027] The reflector unit is attached to the upper side of the circumferential sidewall, and liquid crystal is injected into the liquid crystal cavity.

[0028] According to one embodiment of the present invention, forming a circumferential sidewall includes:

[0029] Photoresist was spin-coated onto the VCSEL unit and photolithography was performed to obtain the circumferential sidewalls, wherein the height of the circumferential sidewalls was adjusted by adjusting the ratio of photoresist to diluent.

[0030] The circumferential sidewall is cured by the following curing steps performed in sequence: curing at 70~90℃ for 20~40 min, curing at 110~130℃ for 20~40 min, curing at 140~160℃ for 20~40 min, and curing at 190~210℃ for 0.8~1.2 h.

[0031] According to one embodiment of the present invention, the method further includes:

[0032] Polarization control layers are formed on the surfaces of the VCSEL unit and the mirror unit facing the liquid crystal cavity, respectively. According to the desired polarization direction of the emitted light, stripes extending along the first stripe direction and stripes extending along the second stripe direction are formed on the two polarization control layers, respectively, such that a corresponding angle is formed between the first stripe direction and the second stripe direction.

[0033] According to one embodiment of the present invention, a polarization control layer is formed by spin-coating a diluted polymer, wherein the thickness of the polarization control layer is adjusted by adjusting the dilution ratio.

[0034] According to one embodiment of the present invention, the reflector unit is joined to the upper side of the circumferential sidewall, and liquid crystal is injected into the liquid crystal cavity, comprising:

[0035] The mirror unit is bonded to the upper side of the circumferential sidewall with adhesive, and a gap is left between the mirror unit and the circumferential sidewall on at least one side.

[0036] Liquid crystal is injected into the liquid crystal cavity through the gap, and the gap is sealed with UV glue.

[0037] In the technical solution of the present invention, by introducing a coupling layer whose refractive index is in a first preset relationship with the refractive index of the high refractive index layer in the first DBR and whose thickness is in a second preset relationship with the wavelength, more energy in the semiconductor cavity can enter the liquid crystal cavity, thereby improving the tuning range and tuning efficiency of the device. Attached Figure Description

[0038] 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.

[0039] Figure 1 A perspective view of a liquid crystal tunable vertical cavity surface-emitting laser according to an embodiment of the present invention is shown;

[0040] Figure 2 A cross-sectional view of a liquid crystal tunable vertical cavity surface-emitting laser according to an embodiment of the present invention is shown;

[0041] Figure 3 A cross-sectional view of a VCSEL cell according to an embodiment of the present invention is shown;

[0042] Figure 4 A cross-sectional view of a VCSEL cell and its circumferential sidewalls according to an embodiment of the present invention is shown;

[0043] Figure 5 A cross-sectional view of a mirror unit according to an embodiment of the present invention is shown;

[0044] Figure 6 A cross-sectional view of a polarization control layer formed on the surface of a mirror unit and a VCSEL unit according to an embodiment of the present invention is shown.

[0045] Figure 7 A schematic cross-sectional view of the mirror unit after it is joined to the circumferential sidewall according to an embodiment of the present invention is shown;

[0046] Figure 8 A liquid crystal tunable vertical cavity surface-emitting laser fabricated according to an embodiment of the present invention is shown, wherein the liquid crystal cavity contains liquid crystal.

[0047] Figure 9 A schematic diagram of two polarization control layers according to an embodiment of the present invention is shown;

[0048] Figure 10 A flowchart illustrating a method for fabricating a liquid crystal tunable vertical-cavity surface-emitting laser according to an embodiment of the present invention is shown.

[0049] Figure 11 The difference in electric field mode distribution and tuning range with and without a coupling layer is shown;

[0050] Figure 12 The diagram shows the wavelength tuning test results of a liquid crystal tunable vertical cavity surface-emitting laser according to an embodiment of the present invention.

[0051] Explanation of reference numerals in the attached figures:

[0052] 10. VCSEL unit; 11. First electrode; 12. Substrate; 13. First DBR; 14. Active region; 15. Oxide confinement layer; 16. Second DBR; 17. Coupling layer; 18. Passivation layer; 19. Second electrode; 20. Peripheral sidewall; 21. Polarization control layer; 22. Liquid crystal cavity; 23. Polarization control layer; 30. Mirror unit; 31. Substrate; 32. Second liquid crystal tuning electrode; 33. Third DBR; 40. Liquid crystal; 100. Liquid crystal tunable vertical cavity surface-emitting laser. Detailed Implementation

[0053] 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.

[0054] 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.

[0055] refer to Figure 1 and Figure 2 This invention proposes a liquid crystal tunable vertical cavity surface-emitting laser 100, comprising: a VCSEL (vertical cavity surface-emitting laser) unit 10, the VCSEL unit 10 including a first DBR (distributed Bragg mirror) 13 and a second DBR 16 located above the first DBR 13; a circumferential sidewall 20 formed on the upper side of the VCSEL unit 10, the circumferential sidewall 20 defining a liquid crystal cavity 22; a mirror unit 30, the mirror unit 30 being joined to the upper side of the circumferential sidewall 20; and liquid crystal 40 housed within the liquid crystal cavity 22; wherein, the VCSEL unit 10 further includes a coupling layer 17 located above the second DBR 16, the refractive index of the coupling layer 17 having a first preset relationship with the refractive index of the high refractive index layer in the first DBR 13 (DBRs are typically formed by alternating stacks of materials with different reflectivities, including a high refractive index layer and a low refractive index layer), and / or the thickness of the coupling layer 17 having a second preset relationship with the wavelength.

[0056] This invention introduces a coupling layer whose refractive index has a first preset relationship with the refractive index of the high refractive index layer in the first DBR and whose thickness has a second preset relationship with the wavelength, thereby allowing more energy in the semiconductor cavity to enter the liquid crystal cavity, thus improving the tuning range and tuning efficiency of the device.

[0057] According to several embodiments of the present invention, the aforementioned first preset relationship may include the refractive index of the coupling layer 17 being the square root of the refractive index of the high-refractive-index layer in the first DBR 13, and the second preset relationship may include the thickness of the coupling layer 17 being a quarter wavelength. In this case, the phase distribution of the intracavity optical field is improved, and the amount of energy entering the liquid crystal cavity from the semiconductor cavity is further increased, thereby further improving the tuning range and tuning efficiency of the device.

[0058] In embodiments of the present invention, the vertical direction may correspond to the stacking direction or growth direction of the VCSEL unit 10. The circumferential sidewall 20 may have a structure that surrounds 360 degrees circumferentially, and its cross-section is not limited to a circle, rectangle or other regular or irregular shape.

[0059] In embodiments of the present invention, any suitable VCSEL cell from the prior art can be used. For example, the type of VCSEL cell 10 can be any one of air column type, buried heterojunction type, oxidation-confined type, and proton-injected type.

[0060] This invention introduces a coupling layer 17 in the VCSEL cell 10 to improve the tuning range and tuning efficiency. In some embodiments, the VCSEL cell 10 may include, from bottom to top: a first electrode 11, a substrate 12, a first DBR 13, an active region 14, an oxide confinement layer 15, a second DBR 16, a passivation layer 18, and a second electrode 19, wherein the coupling layer 17 is disposed in the region of the second DBR 16 not covered by the passivation layer 18. The second electrode 19 serves simultaneously as the injection electrode of the VCSEL cell 10 (for providing injection current to the VCSEL cell 10) and the first liquid crystal tuning electrode. The mirror unit 30 may include a substrate 31, a second liquid crystal tuning electrode 32, and a third DBR 33. Thus, the overall structure of the laser 100 adopts a three-electrode design, which can reduce the complexity of the manufacturing process.

[0061] The voltage across the liquid crystal cavity 22 is provided by the second liquid crystal tuning electrode 32 in the mirror unit 30 and the second electrode 19 of the VCSEL unit 10, respectively. By controlling the voltage applied across the liquid crystal cavity 22, the average refractive index of the liquid crystal layer is changed, thereby altering the optical length of the laser cavity structure to control the resonant wavelength of the laser 100 output. As the voltage applied to the second liquid crystal tuning electrode 32 increases, the refractive index of the liquid crystal cavity 22 decreases, and the resonant wavelength shifts towards a lower wavelength. In this invention, since the entire liquid crystal tunable VCSEL has no additional moving parts and is excited by electric pumping, it offers advantages in miniaturization and high integration.

[0062] Since the lasing wavelength of VCSEL is very sensitive to changes in the effective cavity length, and the refractive index of liquid crystal is generally between 1.2 and 1.8, for a circumferential sidewall 20 with a height of a few micrometers, a small change in the refractive index of liquid crystal is enough to cause a change in the resonant wavelength of tens of nanometers. At the same time, liquid crystal molecules have a fast response speed to voltage, so the tunable VCSEL of the present invention has a fast wavelength response capability.

[0063] In some embodiments of the present invention, the liquid crystal tunable vertical-cavity surface-emitting laser 100 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 19 may use material systems including but not limited to Ti / Au and Au / Ge / Ni. The third DBR 33 serves as the upper DBR of the electrically pumped liquid crystal tunable VCSEL, used to form optical feedback. It not only serves as the upper DBR but also as the upper blocking layer of the liquid crystal cavity 22, and may include a multilayer dielectric material system with low stress and high refractive index contrast, such as SiO2 / TiO2 or SiO2 / Ta2O5. In one embodiment, TiO2 and SiO2 are grown alternately to achieve distributed feedback, with TiO2 as the first and last layers, depending on the type of the active cavity of the VCSEL. The passivation layer 18 includes SiO2. The second liquid crystal tuning electrode 32 may be an ITO electrode.

[0064] In some embodiments, the reflector unit 30 includes, from bottom to top: a third DBR 33, a second liquid crystal tuning electrode 32, and a substrate 31. The second liquid crystal tuning electrode 32 is located above the third DBR 33, that is, on the side of the third DBR 33 facing away from the liquid crystal cavity 22. The inventors of this application have discovered that if the second liquid crystal tuning electrode 32 is located below the third DBR 33, i.e., inside the resonant cavity, it will generate a large scattering loss, causing the laser to fail to emit laser light normally or to have very low power. However, placing the second liquid crystal tuning electrode 32 above the third DBR, i.e., outside the resonant cavity, can still achieve the function of tuning the liquid crystal without causing excessive loss.

[0065] In some embodiments, the circumferential sidewall 20 is formed by spin-coating photoresist onto the surface of the VCSEL unit 10 and then performing photolithography. The thickness of the circumferential sidewall 20 can be varied by the photoresist diluent. By changing the ratio of photoresist to diluent, photoresist layers of different thicknesses can be obtained, thereby obtaining circumferential sidewalls 20 with different heights (i.e., dimensions in the vertical direction). Therefore, the circumferential sidewall 20 is highly flexible and controllable, and highly compatible with semiconductor processes. The fabrication of the circumferential sidewall 20 can be flexibly changed according to the design without involving epitaxial technology, resulting in higher operability. The size of the circumferential sidewall 20 can be at the hundred-micrometer level, suitable for high-density integration. In one embodiment, the material of the circumferential sidewall 20 may include, but is not limited to, photoresist of type 5214. In another embodiment, the material of the circumferential sidewall 20 may include, but is not limited to, polyimide of type JSR-5100, and the diluent includes organic solvents, such as, but not limited to, carbon tetrachloride and chlorobenzene.

[0066] In some embodiments, the reflector unit 30 is bonded to the circumferential sidewall 20 by an adhesive, the adhesive being composed of at least flake copper-coated silver powder and a curing agent in a weight ratio of (8-12):1, and the curing agent may include epoxy resin, with a weight ratio of, for example, 10:1. The reflector unit 30 is sealed to the circumferential sidewall 20 by a UV adhesive (ultraviolet curing adhesive), the UV adhesive including epoxy resin.

[0067] In some embodiments, the liquid crystal tunable vertical-cavity surface-emitting laser 100 further includes a polarization control layer 21 and a polarization control layer 23 located on the lower and upper sides of the liquid crystal cavity 22, respectively. The polarization control layers 21 and 23 are configured to control the initial orientation of the liquid crystal molecules to control the polarization direction of the emitted light. The polarization control layers 21 and 23 are formed on the surfaces of the VCSEL unit 10 and the mirror unit 30 facing the liquid crystal cavity 22, respectively, and can seal the upper and lower sides of the liquid crystal cavity 22. The polarization control layers 21 and 23 contact the liquid crystal and adjust the initial angle of the liquid crystal molecules.

[0068] refer to Figure 9In some embodiments, polarization control layer 21 and polarization control layer 23 have stripes extending along a first fringe direction L1 and a second fringe direction L2, respectively, forming a predetermined angle θ between the first fringe direction L1 and the second fringe direction L2. The angle θ can be arbitrary, thereby controlling any polarized emission direction. The corresponding angle θ can be set according to the desired polarization direction. In some embodiments, liquid crystal 40 includes nematic liquid crystal, which is composed of rod-shaped molecules with a large aspect ratio. A portion of the liquid crystal molecules in the liquid crystal cavity 22 are oriented along a first direction under the control of polarization control layer 21, while another portion of the liquid crystal molecules are oriented along a second direction under the control of polarization control layer 23. The polarization direction of the emitted light is determined by the combined effect of the different orientations. In some embodiments, the birefringence difference of liquid crystal 40 is greater than 0.1 and less than 0.25, and a larger birefringence difference can meet the requirements of the tuning range. Liquid crystal 40 can be a positive dielectric anisotropic liquid crystal or a negative dielectric anisotropic liquid crystal.

[0069] Polarization control layers 21 and 23 can be prepared using diluted polymers (e.g., photoresist), and their thickness can be determined by different dilution ratios. The photoresist may include polyimide, and the diluent may include an organic solvent. Stripes are formed and their orientation is determined by friction.

[0070] refer to Figure 10 The present invention also proposes a method for fabricating a liquid crystal tunable vertical cavity surface-emitting laser 100, comprising the following steps:

[0071] S10, fabricate VCSEL unit 10. VCSEL unit 10 includes a first DBR 13, a second DBR 16 located above the first DBR 13, and a coupling layer 17 located above the second DBR 16. The refractive index of the coupling layer 17 is the square root of the refractive index of the high refractive index layer in the first DBR 13, and the thickness is one-quarter wavelength.

[0072] S20, a circumferential sidewall 20 defining a liquid crystal cavity 22 is formed on the upper side of the VCSEL unit 10;

[0073] S30, fabrication of mirror unit 30;

[0074] S40, the reflector unit 30 is joined to the upper side of the circumferential sidewall 20, and liquid crystal 40 is injected into the liquid crystal cavity 22.

[0075] In some embodiments, fabricating the VCSEL unit 10 includes: sequentially growing a first DBR 13, an active region 14, an oxide confinement layer 15, a second DBR 16, and a coupling layer 17 on a substrate 12 to obtain an epitaxial wafer, and performing photolithography and etching on the epitaxial wafer to form mesa; wherein the etching solution used to etch the epitaxial wafer comprises CH3OH, H3PO4, H2O2, and H2O, and the volume ratio of CH3OH, H3PO4, H2O2, and H2O is (1.5~3):(0.5~1.5):(0.5~1.5):(4.5~5.5). Under this ratio, the etching rate is relatively stable and easy to control. Of course, the present invention is not limited to this, and a suitable etching solution can be selected according to the etching type and etching rate.

[0076] In some embodiments, the preparation of the VCSEL unit 10 further includes: growing a passivation layer 18 on an epitaxial wafer and etching the passivation layer 18 to form a passivation layer light-emitting aperture; wherein the etching solution used to etch the passivation layer 18 comprises HF, NH4F, and H2O, and the ratio of HF, NH4F, and H2O is (2~4 mL):(5~7 mg):(9~11 mL). This ratio provides a relatively stable etching rate, is easy to control, and is suitable for etching SiO2 passivation layers. Of course, the present invention is not limited to this, and a suitable etching solution can be selected according to the etching type and etching rate.

[0077] In some embodiments, forming the circumferential sidewall 20 includes: spin-coating photoresist onto the VCSEL unit 10 and photolithographically forming the circumferential sidewall 20, wherein the thickness of the photoresist is adjusted by adjusting the ratio of photoresist to diluent, thereby adjusting the height of the circumferential sidewall 20; and curing the circumferential sidewall 20, including the following curing steps performed sequentially: curing at 70~90°C for 20~40 min, curing at 110~130°C for 20~40 min, curing at 140~160°C for 20~40 min, and curing at 190~210°C for 0.8~1.2 h. For example, curing at 70~90°C for 30 min, curing at 110~130°C for 30 min, curing at 140~160°C for 30 min, and curing at 190~210°C for 1 h.

[0078] Currently, there is a lack of feasible methods for fabricating liquid crystal cavities in the prior art. This application presents a practical method for fabricating liquid crystal cavities that incorporates the aforementioned curing parameters. Since liquid crystal cavities require high flatness, curing conditions need to be strictly controlled. The aforementioned curing steps are beneficial for the flatness of the photoresist surface.

[0079] In some embodiments, the method further includes: forming a polarization control layer 21 and a polarization control layer 23 on the surfaces of the VCSEL unit 10 and the mirror unit 30 facing the liquid crystal cavity 22, respectively, and forming a stripe extending along a first fringe direction L1 and a stripe extending along a second fringe direction L2 on the polarization control layer 21 and the polarization control layer 23 according to the desired polarization direction of the emitted light, wherein a corresponding included angle θ is formed between the first fringe direction L1 and the second fringe direction L2.

[0080] In some embodiments, a diluted polymer is spin-coated to form a polarization control layer 21 and a polarization control layer 23, wherein the thickness of the polarization control layer 21 and the polarization control layer 23 is adjusted by adjusting the dilution ratio.

[0081] In some embodiments, the reflector unit 30 is bonded to the upper side of the circumferential sidewall 20, and liquid crystal 40 is injected into the liquid crystal cavity 22. This includes: bonding the reflector unit 30 to the upper side of the circumferential sidewall 20 with an adhesive, such that a gap is left between the reflector unit 30 and the circumferential sidewall 20 on at least one side; injecting liquid crystal 40 into the liquid crystal cavity 22 through the gap (the liquid crystal can be injected using capillary action under vacuum conditions), and sealing the gap with UV adhesive. Considering that liquid crystal is liquid-like and has fluidity, and the liquid crystal cavity is very small, with a diameter of about 200~400μm, if the liquid crystal is added first and then the reflector unit is bonded, it will face great difficulties. On the one hand, it is extremely difficult to restrict the liquid crystal, and on the other hand, it will greatly increase the bonding difficulty. Therefore, this application first performs bonding and then adds liquid crystal to reduce the manufacturing difficulty.

[0082] Figures 3 to 8 Cross-sectional views are shown at various stages of the fabrication method of the liquid crystal tunable vertical-cavity surface-emitting laser 100. Among them, Figure 3 A cross-sectional view of VCSEL cell 10 is shown, in which the etching of the light-emitting mesa, oxidation of the oxide confinement layer, growth of the passivation layer and etching of the light-emitting aperture, sputtering of the Ti / Au second electrode and stripping of the light-emitting aperture, and sputtering of the back first electrode have been completed. Figure 4 The image shows a cross-sectional view of the VCSEL cell and the circumferential sidewall after photolithography. Figure 5 The diagram shows a cross-sectional view of the mirror unit, in which the ITO electrode and the third DBR have been fabricated, and the third DBR has been etched to expose the ITO portion. Figure 6 A schematic cross-sectional view of the polarization control layer formed on the surface of the mirror unit and the VCSEL unit is shown. Figure 7 A schematic diagram of the cross-section of the reflector unit and the VCSEL unit after they are bonded together is shown. Figure 8 The fabricated liquid crystal tunable vertical cavity surface-emitting laser 100 is shown, wherein the liquid crystal cavity contains liquid crystal.

[0083] refer to Figures 3 to 8 In one specific embodiment, the fabrication method of the liquid crystal tunable vertical cavity surface-emitting laser includes the following steps:

[0084] Step S1: The first DBR, active region, oxide confinement layer, second DBR and coupling layer are epitaxially grown sequentially on the GaAs substrate by metal-organic chemical vapor deposition or molecular beam epitaxy to obtain an epitaxial wafer;

[0085] Step S2: After photolithography, etching is used to form the mesa of the light source area, and wet nitrogen oxidation is used to create oxide confinement holes to confine the current and the transverse light field mode;

[0086] Step S3: Grow a SiO2 passivation layer by MOCVD, and etch it to form a passivation layer light-emitting hole; sputter the second electrode, and etch or peel it to form a second electrode light-emitting hole; wherein the diameter of the passivation layer light-emitting hole is smaller than the mesa diameter of the light source part, and the diameter of the second electrode light-emitting hole is smaller than the diameter of the passivation layer light-emitting hole.

[0087] Step S4: Thin the wafer using a grinding mill, spin-coat polyimide and photolithography to form the circumferential sidewalls;

[0088] Step S5: Cure the circumferential sidewalls, following the curing parameters described below;

[0089] Step S6: Spin-coat diluted polyimide or PMMA and other polymers as a lower polarization control layer, cure the lower polarization control layer, and complete the fabrication of the vertical cavity surface-emitting laser unit;

[0090] Step S7: Sputter ITO onto the SiO2 substrate as the upper tuning electrode for the liquid crystal;

[0091] Step S8: Grow a third DBR on ITO and etch a portion of the third DBR into ITO for use as a tuning electrode lead-out on the liquid crystal.

[0092] Step S9: Spin-coat diluted polyimide or PMMA and other polymers as the upper polarization control layer, cure the upper polarization control layer, and complete the fabrication of the mirror unit;

[0093] Step S10: Rub the upper and lower polarization control layers of the liquid crystal molecules to determine the direction of the initial stripes;

[0094] Step S11: Invert the mirror unit onto the vertical cavity surface-emitting laser unit and bond it together to complete the fabrication of the liquid crystal cavity unit;

[0095] Step S12: Under vacuum conditions, inject nematic liquid crystal and seal it with UV adhesive.

[0096] Figure 11 This illustrates the difference in electric field mode distribution and tuning range with and without a coupling layer. (Refer to...) Figure 11The wavelength range is wider with a coupling layer than without a coupling layer. Figure 12 The image shows the wavelength tuning test results of a polarization-controlled tunable liquid crystal VCSEL with a coupled cavity provided in an embodiment of the present invention. As the voltage increases, the refractive index of the liquid crystal cavity unit decreases, and the resonant wavelength shifts towards lower wavelengths. (Reference) Figure 12 The resonant wavelength is 1075nm at 2.2V and 1044nm at 15V, achieving a wide range of 31nm.

[0097] In summary, this invention provides a reliable polarization-controllable liquid crystal tunable VCSEL structure and fabrication scheme with a coupled cavity. Through this invention, a wide-range electrically pumped liquid crystal tunable VCSEL can be realized, possessing a large tuning range while maintaining good single-mode characteristics. The polarization direction of the emitted light is controllable. The electrically pumping method offers the possibility of high integration, while the liquid crystal tuning method brings advantages of low cost and high stability. The liquid crystal tunable vertical-cavity surface-emitting laser of this invention has the advantages of miniaturization, low cost, high stability, and wide range. The fabrication of the liquid crystal cavity unit provided by this invention is simple and feasible, greatly reducing process complexity and cost, and can stably output high-quality single-mode laser with a tuning range of over 30 nm.

[0098] 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 liquid crystal tunable vertical cavity surface-emitting laser (100), characterized in that, include: VCSEL unit (10), the VCSEL unit (10) includes a first DBR (13) and a second DBR (16) located above the first DBR (13). A circumferential sidewall (20) is formed on the upper side of the VCSEL unit (10), and the circumferential sidewall (20) defines a liquid crystal cavity (22). The mirror unit (30) is joined to the upper side of the circumferential sidewall (20); and Liquid crystal (40) housed within the liquid crystal cavity (22); The VCSEL unit (10) further includes a coupling layer (17) located above the second DBR (16). The refractive index of the coupling layer (17) is in a first preset relationship with the refractive index of the high refractive index layer in the first DBR (13), and the thickness of the coupling layer (17) is in a second preset relationship with the wavelength. The first preset relationship includes the refractive index of the coupling layer (17) being the square root of the refractive index of the high refractive index layer in the first DBR (13), and the second preset relationship includes the thickness of the coupling layer (17) being one-quarter wavelength.

2. The laser (100) according to claim 1, characterized in that, It also includes two polarization control layers (21, 23) located on the lower and upper sides of the liquid crystal cavity (22), respectively. The polarization control layers (21, 23) are configured to control the initial orientation of the liquid crystal molecules to control the polarization direction of the emitted light. The two polarization control layers (21, 23) have stripes extending along a first stripe direction (L1) and stripes extending along a second stripe direction (L2), respectively, and a predetermined angle (θ) is formed between the first stripe direction (L1) and the second stripe direction (L2).

3. The laser (100) according to claim 1, characterized in that, The circumferential sidewall (20) is formed by spin-coating photoresist onto the surface of the VCSEL unit (10) and then performing photolithography.

4. The laser (100) according to claim 1, characterized in that, The reflector unit (30) includes, from bottom to top: a third DBR (33), a second liquid crystal tuning electrode (32), and a substrate (31).

5. The laser (100) according to claim 1, characterized in that, The reflector unit (30) is bonded to the circumferential sidewall (20) by an adhesive, which is composed of at least flake copper-coated silver powder and a curing agent in a weight ratio of (8~12):1; the reflector unit (30) is sealed to the circumferential sidewall (20) by UV adhesive.

6. A method for fabricating a liquid crystal tunable vertical cavity surface-emitting laser (100), characterized in that, include: A VCSEL unit (10) is fabricated, the VCSEL unit (10) includes a first DBR (13), a second DBR (16) located above the first DBR (13) and a coupling layer (17) located above the second DBR (16), the refractive index of the coupling layer (17) is the square root of the refractive index of the high refractive index layer in the first DBR (13), and the thickness is one-quarter wavelength; A circumferential sidewall (20) defining a liquid crystal cavity (22) is formed on the upper side of the VCSEL unit (10). Fabricate the mirror unit (30); The reflector unit (30) is attached to the upper side of the circumferential sidewall (20), and liquid crystal (40) is injected into the liquid crystal cavity (22).

7. The method according to claim 6, characterized in that, Forming the circumferential sidewall (20) includes: The circumferential sidewall (20) is obtained by spin-coating photoresist onto the VCSEL unit (10) and photolithography, wherein the height of the circumferential sidewall (20) is adjusted by adjusting the ratio of photoresist to diluent; The circumferential sidewall (20) is cured by the following curing steps performed in sequence: curing at 70~90℃ for 20~40 min, curing at 110~130℃ for 20~40 min, curing at 140~160℃ for 20~40 min, and curing at 190~210℃ for 0.8~1.2 h.

8. The method according to claim 6, characterized in that, Also includes: Polarization control layers (21, 23) are formed on the surfaces of the VCSEL unit (10) and the mirror unit (30) facing the liquid crystal cavity (22), respectively. According to the desired polarization direction of the emitted light, stripes extending along the first fringe direction (L1) and stripes extending along the second fringe direction (L2) are formed on the two polarization control layers (21, 23), respectively, such that a corresponding angle (θ) is formed between the first fringe direction (L1) and the second fringe direction (L2).

9. The method according to claim 6, characterized in that, The process of attaching the reflector unit (30) to the upper side of the circumferential sidewall (20) and injecting liquid crystal (40) into the liquid crystal cavity (22) includes: The reflector unit (30) is bonded to the upper side of the circumferential sidewall (20) with an adhesive, and a gap is reserved between the reflector unit (30) and the circumferential sidewall (20) on at least one side. Liquid crystal (40) is injected into the liquid crystal cavity (22) through the gap, and the gap is sealed with UV adhesive.

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