Liquid crystal tunable vertical cavity surface emitting laser and preparation method thereof

Abstract

The invention 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, and the VCSEL unit comprises a first DBR and a second DBR located above the first DBR; the circumferential side wall is formed on the upper side of the VCSEL unit, and a liquid crystal cavity is limited by the circumferential side wall; the reflecting mirror unit is connected to the upper side of the circumferential side wall; the liquid crystal is accommodated in the liquid crystal cavity; 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 are in a first preset relation, and / or the thickness of the coupling layer and the wavelength are in a second preset relation. The tuning range and the tuning efficiency are improved by introducing a coupling layer structure, the polarization direction of output light is controlled by utilizing a polarization control layer, and a specific and feasible preparation method of the electrically pumped liquid crystal tunable VCSEL is further researched.

Description

Technical Field

[0001] The invention relates to the technical field of semiconductor lasers, and in particular to a liquid crystal tunable vertical cavity surface emitting laser and a preparation method thereof. Background Art

[0002] In recent years, with the rapid development of technologies such as virtual reality (VR), Internet of Everything (IOT), lidar, 3D sensing, and biomedicine, the demand for high-stability, low-cost, easy-to-integrate, and wavelength-tunable lasers has increased day by day, which has promoted the rapid development of tunable semiconductor lasers. Different types of tunable semiconductor lasers are used in different fields based on their respective advantages.

[0003] External cavity tunable semiconductor lasers play an important role in the field of wavelength division multiplexing due to their advantages of 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 advantages such as fast tuning speed and high integration, but they also have limitations such as high power consumption and poor stability. In order to obtain low-power, high-modulation rate and low-cost light sources, tunable vertical cavity surface emitting lasers (VCSELs) have been widely studied and have attracted much attention in short-distance communications. Tunable VCSELs have the advantages of continuous wavelength tuning, wide wavelength coverage, high integration, and high modulation rate. They also have great potential in atomic clocks, OCT, and LIDAR.

[0004] With the continuous deepening of research, tunable VCSEL has developed a variety of tuning methods, the most common of which are thermal tuning, micro-electromechanical system (MEMS) tuning, liquid crystal tuning, piezoelectric tuning, etc. Among them, the typical MEMS tunable VCSEL has been widely studied due to its high modulation rate and low power consumption. However, researchers have found that the traditional cantilever beam structure is more susceptible to temperature, mechanical vibration and other external factors during the tuning process, which leads to reduced stability and service life. At the same time, the preparation process of MEMS cantilever is complex and costly, which has become a factor that must be considered in applications. Compared with other tuning methods, the thermal tuning method has low tuning efficiency and a relatively small tuning range, and its application is very limited. The characteristic that the optical properties of liquid crystal change with the application of a changing electric field provides a new direction for tunable VCSEL. The liquid crystal tuning method can obtain a larger tuning range while having stability and low cost. It is suitable for a variety of applications and has great research significance.

[0005] The key to the preparation of liquid crystal tunable VCSELs includes: (a) the preparation of high-quality liquid crystal cavities; (b) the extraction of liquid crystal tuning electrodes and VCSEL electrodes; (c) the initial angle and model of liquid crystal molecules. The optical pumping method has demonstrated the potential of liquid crystal tunable VCSELs for wide tunability, but the optical pumping method often brings problems of large size and high cost. Compared with the optically pumped liquid crystal tunable VCSELs, the electrically pumped method has been less studied, mainly because the difficulties in preparing the liquid crystal cavity of the electrically pumped liquid crystal tunable VCSELs and the process compatibility issues caused by the extraction of the pumping electrodes have not been resolved. Therefore, there is still a lack of some feasible preparation schemes for electrically pumped liquid crystal tunable VCSELs, and the wavelength tuning range of liquid crystal tunable VCSELs is limited. Summary of the invention

[0006] The main purpose of the present invention is to provide a liquid crystal tunable vertical cavity surface emitting laser and a preparation method thereof, so as to solve the problem that the wavelength tuning range of the liquid crystal tunable VCSEL is limited.

[0007] According to one aspect of the present invention, a liquid crystal tunable vertical cavity surface emitting laser is provided, comprising: A VCSEL unit, the VCSEL unit comprising a first DBR and a second DBR located above the first DBR; A circumferential sidewall formed on an upper side of the VCSEL unit, the circumferential sidewall defining a liquid crystal cavity; a reflector unit, the reflector unit being joined to the upper side of the circumferential side wall; and Liquid crystal contained in the liquid crystal cavity; The VCSEL unit further 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.

[0008] According to one embodiment of the present invention, the first preset relationship includes that 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 second preset relationship includes that the thickness of the coupling layer is one quarter of the wavelength.

[0009] According to one embodiment of the present invention, the liquid crystal tunable vertical cavity surface emitting laser further comprises two polarization control layers respectively located at the lower side and the upper side of the liquid crystal cavity, and 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.

[0010] According to an embodiment of the present invention, the 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.

[0011] According to one embodiment of the present invention, the circumferential sidewall is formed by spin coating photoresist on the surface of the VCSEL unit and performing photolithography development.

[0012] According to one embodiment of the present invention, the VCSEL unit comprises a second electrode located above the second DBR, the second electrode simultaneously serving as an injection electrode of the VCSEL unit and a first liquid crystal tuning electrode.

[0013] According to one embodiment of the present invention, the reflector unit includes a third DBR and a second liquid crystal tuning electrode, and the second liquid crystal tuning electrode is located above the third DBR.

[0014] According to an embodiment of the present invention, the liquid crystal includes nematic liquid crystal; and / or the birefringence difference of the liquid crystal is greater than 0.1 and less than 0.25.

[0015] 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 area, an oxidation restriction layer, a second DBR, a passivation layer, and a second electrode, and the coupling layer is arranged in the area of ​​the second DBR not covered by the passivation layer; the reflector unit includes from bottom to top: a third DBR, a second liquid crystal tuning electrode, and a substrate.

[0016] According to one embodiment of the present invention, the reflector unit is bonded to the circumferential side wall by an adhesive, and the adhesive is composed of at least flaky copper-coated silver powder and a curing agent in a weight ratio of (8~12):1; the reflector unit is sealed and joined to the circumferential side wall by UV glue.

[0017] According to an embodiment of the present invention, the type of the VCSEL unit is any one of an air column type, a buried heterojunction type, an oxidation confinement type, and a proton implantation type.

[0018] According to another aspect of the present invention, a method for preparing a liquid crystal tunable vertical cavity surface emitting laser is provided, comprising: Prepare a VCSEL unit, the VCSEL unit includes 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 of the wavelength; forming a circumferential side wall defining a liquid crystal cavity on an upper side of the VCSEL unit; preparing a reflector unit; The reflector unit is joined to the upper side of the circumferential side wall, and liquid crystal is injected into the liquid crystal cavity.

[0019] According to one embodiment of the present invention, forming the circumferential sidewall includes: Spin-coating photoresist on the VCSEL unit and performing photolithography to obtain a circumferential sidewall, wherein the height of the circumferential sidewall is adjusted by adjusting the ratio of the photoresist to the diluent; The circumferential side wall is cured, including the following curing steps performed in sequence: 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.

[0020] According to one embodiment of the present invention, the method further includes: Polarization control layers are respectively formed on the surfaces of the VCSEL unit and the reflector unit facing the liquid crystal cavity, and stripes extending along a first stripe direction and stripes extending along a second stripe direction are respectively formed on the two polarization control layers according to a desired polarization direction of the output light, wherein a corresponding angle is formed between the first stripe direction and the second stripe direction.

[0021] According to one embodiment of the present invention, the polarization control layer is formed by spin coating the diluted polymer, wherein the thickness of the polarization control layer is adjusted by adjusting the dilution ratio.

[0022] According to one embodiment of the present invention, the reflector unit is bonded to the upper side of the circumferential side wall, and liquid crystal is injected into the liquid crystal cavity, including: Adhere the reflector unit to the upper side of the circumferential side wall by adhesive, and reserve a gap between the reflector unit and the circumferential side wall at least on one side; Liquid crystal is injected into the liquid crystal cavity through the gap, and the gap is sealed with UV glue.

[0023] 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 enters the liquid crystal cavity, thereby improving the tuning range and tuning efficiency of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0025] Figure 1 A perspective view showing a liquid crystal tunable vertical cavity surface emitting laser according to an embodiment of the present invention; Figure 2 A cross-sectional view showing a liquid crystal tunable vertical cavity surface emitting laser according to an embodiment of the present invention; Figure 3 shows a cross-sectional view of a VCSEL unit according to an embodiment of the present invention; Figure 4 A cross-sectional view showing a VCSEL unit and a circumferential sidewall according to an embodiment of the present invention; Figure 5 shows a cross-sectional view of a reflector unit according to an embodiment of the present invention; Figure 6 A cross-sectional view showing a polarization control layer formed on a surface of a reflector unit and a surface of a VCSEL unit according to an embodiment of the present invention; Figure 7 A schematic cross-sectional view showing a reflector unit according to an embodiment of the present invention after being joined to a circumferential side wall; Figure 8 The figure shows a liquid crystal tunable vertical cavity surface emitting laser manufactured according to an embodiment of the present invention, wherein liquid crystal is contained in the liquid crystal cavity; Fig. 9 A schematic diagram showing two polarization control layers according to an embodiment of the present invention; Fig.10 A flow chart showing a method for preparing a liquid crystal tunable vertical cavity surface emitting laser according to an embodiment of the present invention; Fig.11 The difference in the electric field mode distribution and tuning range with and without the coupling layer is shown; Fig.12 A graph showing wavelength tuning test results of a liquid crystal tunable vertical cavity surface emitting laser according to an embodiment of the present invention is shown.

[0026] Description of reference numerals: 10. VCSEL unit; 11. first electrode; 12. substrate; 13. first DBR; 14. active region; 15. oxidation restriction layer; 16. second DBR; 17. coupling layer; 18. passivation layer; 19. second electrode; 20. circumferential side wall; 21. polarization control layer; 22. liquid crystal cavity; 23. polarization control layer; 30. reflector 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 DESCRIPTION

[0027] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the embodiments of the present invention are further described in detail below in combination with specific embodiments and with reference to the accompanying drawings.

[0028] It should be noted that all expressions using "first" and "second" in the embodiments of the present invention are for distinguishing two non-identical entities with the same name or non-identical parameters. It can be seen that "first" and "second" are only for the convenience of expression and should not be understood as limitations on the embodiments of the present invention. The subsequent embodiments will not explain this one by one.

[0029] refer to Figure 1 and Figure 2 The present 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 comprising a first DBR (distributed Bragg reflector) 13 and a second DBR 16 located above the first DBR 13; a circumferential side wall 20 formed on the upper side of the VCSEL unit 10, the circumferential side wall 20 defining a liquid crystal cavity 22; a reflector unit 30, the reflector unit 30 being bonded to the upper side of the circumferential side wall 20; and a liquid crystal 40 accommodated in the liquid crystal cavity 22; wherein the VCSEL unit 10 further comprises 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 a high refractive index layer (DBR is usually formed by alternately stacking materials with different reflectivities, including a high refractive index layer and a low refractive index layer) in the first DBR 13, and / or the thickness of the coupling layer 17 is in a second preset relationship with the wavelength.

[0030] The present invention introduces 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, so that more energy in the semiconductor cavity enters the liquid crystal cavity, thereby improving the tuning range and tuning efficiency of the device.

[0031] According to some embodiments of the present invention, the first preset relationship may include that 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 second preset relationship may include that the thickness of the coupling layer 17 is one quarter of the wavelength. In this case, the phase distribution of the light field in the cavity is improved, and the amount of energy in the semiconductor cavity entering the liquid crystal cavity is further increased, thereby further improving the tuning range and tuning efficiency of the device.

[0032] In the embodiment of the present invention, the up and down direction may correspond to the stacking direction or growth direction of the VCSEL unit 10. The circumferential sidewall 20 may have a structure surrounding 360 degrees along the circumference, and its cross section is not limited to circular, rectangular or other regular or irregular shapes.

[0033] In the embodiments of the present invention, any suitable VCSEL unit in the prior art may be used. For example, the type of the VCSEL unit 10 may be any one of an air column type, a buried heterojunction type, an oxidation confinement type, and a proton implantation type.

[0034] The present invention introduces a coupling layer 17 into the VCSEL unit 10 to improve the tuning range and tuning efficiency. In some embodiments, the VCSEL unit 10 may include, from bottom to top: a first electrode 11, a substrate 12, a first DBR 13, an active region 14, an oxidation restriction layer 15, a second DBR 16, a passivation layer 18, and a second electrode 19, wherein the coupling layer 17 is disposed in an area of ​​the second DBR 16 that is not covered by the passivation layer 18. The second electrode 19 is simultaneously used as an injection electrode of the VCSEL unit 10 (for providing an injection current to the VCSEL unit 10) and a first liquid crystal tuning electrode. The reflector 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 process complexity, The voltage across the liquid crystal cavity 22 is provided by the second liquid crystal tuning electrode 32 in the reflector unit 30 and the second electrode 19 of the VCSEL unit 10, respectively. The voltage applied to the liquid crystal cavity 22 is controlled to change the average refractive index of the liquid crystal layer, thereby changing the optical length of the structure in the laser cavity to achieve the control of the resonant wavelength output by the laser 100. 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 moves toward a lower wavelength. In the solution of the present invention, since the entire liquid crystal tunable VCSEL has no additional moving parts and the excitation method adopts electric pumping, it has the advantages of miniaturization and high integration.

[0035] Since the lasing wavelength of the VCSEL is very sensitive to changes in the effective cavity length, and the refractive index of the liquid crystal is generally between 1.2 and 1.8, for the circumferential side wall 20 with a height of several microns, a slight change in the refractive index of the liquid crystal is sufficient to cause the resonant wavelength to change by tens of nanometers. At the same time, the liquid crystal molecules have a faster response speed to voltage, so that the tunable VCSEL of the present invention has a faster wavelength response capability.

[0036] In some embodiments of the present invention, the liquid crystal tunable vertical cavity surface emitting laser 100 can use a material system including but not limited to GaAs, and its wavelength is not limited to 1060nm. The first electrode 11 and the second electrode 19 can include but are not limited to Ti / Au, Au / Ge / Ni material systems. The third DBR 33 is used as the upper DBR of the electrically pumped liquid crystal tunable VCSEL to form optical feedback. It is not only used as the upper DBR, but also used as the upper barrier layer of the liquid crystal cavity 22. It can include a multilayer dielectric material system with low stress and high refractive index contrast, such as SiO2 / TiO2, SiO2 / Ta2O5 material system. In one embodiment, TiO2 and SiO2 are grown alternately to achieve distributed feedback, with TiO2 as the first layer and the last layer, depending on the active region cavity type of the VCSEL. The passivation layer 18 includes SiO2. The second liquid crystal tuning electrode 32 can be an ITO electrode.

[0037] 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, and the second liquid crystal tuning electrode 32 is located above the third DBR 33, that is, the second liquid crystal tuning electrode 32 is located on the side of the third DBR 33 away from the liquid crystal cavity 22. The inventors of the present application have found that if the second liquid crystal tuning electrode 32 is located below the third DBR 33, that is, inside the resonant cavity, a large scattering loss will be generated, resulting in the laser being unable to emit laser light normally or with very low power. However, placing the second liquid crystal tuning electrode 32 above the third DBR, that is, outside the resonant cavity, can still play the role of tuning the liquid crystal without causing excessive loss.

[0038] In some embodiments, the circumferential sidewall 20 is formed by spin coating a photoresist on the surface of the VCSEL unit 10 and performing photolithography development. The thickness of the circumferential sidewall 20 can be changed by a photoresist diluent, and a photoresist layer of different thicknesses can be obtained by changing the ratio of the photoresist and the diluent, thereby obtaining a circumferential sidewall 20 of different heights (i.e., the size in the up and down directions). Thus, the circumferential sidewall 20 has the characteristics of being highly flexible and controllable and highly compatible with semiconductor processes. The preparation of the circumferential sidewall 20 can be flexibly changed according to the design without involving epitaxial technology, and has higher operability. The size of the circumferential sidewall 20 can be hundreds of microns, which is suitable for high-density integration. In one embodiment, the material of the circumferential sidewall 20 can include but is not limited to a 5214 model photoresist. In another embodiment, the material of the circumferential sidewall 20 can include but is not limited to a polyimide of model JSR-5100, and the diluent includes an organic solvent, for example, one of carbon tetrachloride and chlorobenzene can be used but is not limited to.

[0039] In some embodiments, the reflector unit 30 is bonded to the circumferential side wall 20 by an adhesive, the adhesive is at least composed of flaky copper-coated silver powder and a curing agent in a weight ratio of (8-12): 1, the curing agent may include epoxy resin, and the weight ratio may be, for example, 10: 1. The reflector unit 30 is sealed and joined to the circumferential side wall 20 by UV glue (ultraviolet curing glue), and the UV glue may include epoxy resin.

[0040] 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 respectively located at the lower side and the upper side of the liquid crystal cavity 22, and the polarization control layer 21 and the polarization control layer 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 layer 21 and the polarization control layer 23 are respectively formed on the surfaces of the VCSEL unit 10 and the reflector unit 30 facing the liquid crystal cavity 22, and can close the upper and lower sides of the liquid crystal cavity 22. The polarization control layer 21 and the polarization control layer 23 are in contact with the liquid crystal and adjust the initial angle of the liquid crystal molecules.

[0041] refer to Fig. 9 In some embodiments, the polarization control layer 21 and the polarization control layer 23 respectively have stripes extending along the first stripe direction L1 and stripes extending along the second stripe direction L2, and a predetermined angle θ is formed between the first stripe direction L1 and the second stripe 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, the liquid crystal 40 includes a nematic liquid crystal, which is composed of rod-shaped molecules with a large aspect ratio. A part of the liquid crystal molecules in the liquid crystal cavity 22 is oriented in the first direction under the regulation of the polarization control layer 21, and another part of the liquid crystal molecules is oriented in the second direction under the regulation of the polarization control layer 23, and the polarization direction of the emitted light is determined under the comprehensive effect of different orientations. In some embodiments, the birefringence difference of the 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. The liquid crystal 40 can use a positive dielectric anisotropic liquid crystal or a negative dielectric anisotropic liquid crystal.

[0042] The polarization control layer 21 and the polarization control layer 23 can be prepared by using a diluted polymer (such as a photoresist), and the thickness thereof can be determined by different dilution ratios. The photoresist can include polyimide, and the diluent can include an organic solvent. The stripes are formed by friction and the direction of the stripes is determined.

[0043] refer to Fig.10 The present invention also proposes a method for preparing a liquid crystal tunable vertical cavity surface emitting laser 100, comprising the following steps: S10, preparing a VCSEL unit 10, the VCSEL unit 10 comprising 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 being the square root of the refractive index of the high refractive index layer in the first DBR 13, and the thickness being a quarter wavelength; S20, forming a circumferential side wall 20 defining a liquid crystal cavity 22 on an upper side of the VCSEL unit 10; S30, preparing a reflector unit 30; S40 , bonding the reflector unit 30 to the upper side of the circumferential side wall 20 , and injecting liquid crystal 40 into the liquid crystal cavity 22 .

[0044] In some embodiments, the preparation of the VCSEL unit 10 includes: growing a first DBR 13, an active region 14, an oxidation restriction layer 15, a second DBR 16, and a coupling layer 17 on a substrate 12 in sequence to obtain an epitaxial wafer, and performing photolithography and etching on the epitaxial wafer to form a 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 type of etching and the etching rate.

[0045] In some embodiments, the preparation of the VCSEL unit 10 further includes: growing a passivation layer 18 on the epitaxial wafer, and etching the passivation layer 18 to form a passivation layer light exit hole; wherein the etching solution used to corrode the passivation layer 18 contains HF, NH4F, and H2O, and the amount ratio of HF, NH4F, and H2O is (2~4mL): (5~7mg): (9~11mL). Under this ratio, the etching rate is relatively stable, easy to control, and suitable for etching the SiO2 passivation layer. Of course, the present invention is not limited to this, and a suitable etching solution can be selected according to the type of corrosion and the corrosion rate.

[0046] In some embodiments, forming the circumferential sidewall 20 includes: spin coating photoresist on the VCSEL unit 10 and photolithography to obtain the circumferential sidewall 20, wherein the thickness of the photoresist is adjusted by adjusting the ratio of the photoresist and the diluent, thereby adjusting the height of the circumferential sidewall 20; curing the circumferential sidewall 20, including the following curing steps performed in sequence: 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.

[0047] There is still a lack of feasible solutions for the preparation of liquid crystal cavities in the prior art. This application studies and obtains a feasible method for preparing liquid crystal cavities including the above-mentioned curing parameters. Since the liquid crystal cavity has very high requirements for flatness, the curing conditions need to be strictly controlled. The above-mentioned curing step is beneficial to the flatness of the photoresist surface.

[0048] 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 reflector unit 30 facing the liquid crystal cavity 22, respectively, and forming stripes extending along a first stripe direction L1 and stripes extending along a second stripe direction L2 on the polarization control layer 21 and the polarization control layer 23 according to a desired polarization direction of the output light, respectively, wherein a corresponding angle θ is formed between the first stripe direction L1 and the second stripe direction L2.

[0049] In some embodiments, the polarization control layer 21 and the polarization control layer 23 are formed by spin coating the diluted polymer, wherein the thickness of the polarization control layer 21 and the polarization control layer 23 is adjusted by adjusting the dilution ratio.

[0050] In some embodiments, the reflector unit 30 is bonded to the upper side of the circumferential side wall 20, and liquid crystal 40 is injected into the liquid crystal cavity 22, including: bonding the reflector unit 30 to the upper side of the circumferential side wall 20 by adhesive, and leaving a gap between the reflector unit 30 and the circumferential side wall 20 at least on one side; injecting liquid crystal 40 into the liquid crystal cavity 22 through the gap (the liquid crystal can be injected using the capillary principle under vacuum conditions), and sealing the gap with UV glue. Considering that the liquid crystal is liquid-like and has fluidity, and the size of 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 difficulty of bonding. Therefore, the present application first bonds and then adds the liquid crystal to reduce the difficulty of preparation.

[0051] Figures 3 to 8 1 is a cross-sectional view showing various stages of a method for preparing a liquid crystal tunable vertical cavity surface emitting laser 100. Figure 3 The cross-sectional view of the VCSEL unit 10 is shown, in which the light-emitting mesa etching, the oxidation of the oxidation restriction layer, the growth of the passivation layer and the etching of the light-emitting hole, the sputtering of the Ti / Au second electrode and the stripping of the light-emitting hole and the sputtering of the back first electrode have been completed. Figure 4 A cross-sectional view of a VCSEL cell and the circumferential sidewall after photolithography is shown. Figure 5 A cross-sectional view of a reflector unit is shown, in which the ITO electrode and the third DBR have been prepared, and the third DBR has been etched to expose the ITO portion. Figure 6 A schematic cross-sectional view showing the polarization control layer formed on the surface of the reflector unit and the surface of the VCSEL unit. Figure 7 A schematic cross-sectional view showing the bonding of the reflector unit and the VCSEL unit. Figure 8 The manufactured liquid crystal tunable vertical cavity surface emitting laser 100 is shown, wherein the liquid crystal cavity contains liquid crystal.

[0052] refer to Figures 3 to 8 In a specific embodiment, the method for preparing a liquid crystal tunable vertical cavity surface emitting laser comprises the following steps: Step S1: epitaxially growing a first DBR, an active region, an oxidation restriction layer, a second DBR, and a coupling layer in sequence on a GaAs substrate by metal organic chemical vapor deposition or molecular beam epitaxy to obtain an epitaxial wafer; Step S2: etching after photolithography to form a light source part mesa, wet nitrogen oxidation to form an oxidation limiting hole for limiting current and lateral light field mode; Step S3: growing a SiO2 passivation layer by MOCVD, and corroding to form a passivation layer light exit hole; sputtering a second electrode, and corroding or peeling to form a second electrode light exit hole; wherein the diameter of the passivation layer light exit hole is smaller than the table diameter of the light source part, and the diameter of the second electrode light exit hole is smaller than the diameter of the passivation layer light exit hole; Step S4: thinning the film using a grinding machine, spin coating polyimide and photolithography to form a circumferential side wall; Step S5: curing the circumferential side wall, wherein the curing conditions comply with the curing parameters described below; Step S6: spin coating a diluted polymer such as polyimide or PMMA as a lower polarization control layer, and curing the lower polarization control layer to complete the preparation of the vertical cavity surface emitting laser unit; Step S7: sputtering ITO on the SiO2 substrate as a tuning electrode on the liquid crystal; Step S8: growing a third DBR on the ITO, and etching a portion of the third DBR to the ITO for leading out the tuning electrode on the liquid crystal; Step S9: spin coating diluted polyimide or PMMA or other polymers as an upper polarization control layer, curing the upper polarization control layer, and completing the preparation of the reflector unit; Step S10: Rubbing the upper and lower polarization control layers of the liquid crystal molecules to determine the direction of the initial stripes; Step S11: placing the reflector unit upside down on the vertical cavity surface emitting laser unit and bonding them together to complete the preparation of the liquid crystal cavity unit; Step S12: In a vacuum condition, nematic liquid crystal is poured and sealed with UV glue.

[0053] Fig.11 The difference between the electric field mode distribution and tuning range with and without the coupling layer is shown in Figure 2. Fig.11 , the coupling layer has a wider wavelength range than the one without coupling layer. Fig.12 The wavelength tuning test result diagram of the polarization-controllable liquid crystal tunable VCSEL with a coupled cavity provided by an embodiment of the present invention, wherein as the voltage increases, the refractive index of the liquid crystal cavity unit decreases and the resonant wavelength moves toward a lower wavelength. Fig.12, the resonant wavelength is 1075nm at 2.2V, and the resonant wavelength is 1044nm at 15V, reaching a wide range of 31nm.

[0054] In summary, the present invention provides a reliable polarization-controllable liquid crystal tunable VCSEL structure and preparation scheme containing a coupled cavity. Through the technical scheme of the present invention, a wide-range electrically pumped liquid crystal tunable VCSEL can be realized, which has a large tuning range and good single-mode characteristics. The polarization direction of the emitted light is controllable, and the electric pumping method will provide the possibility of high integration, and the liquid crystal tuning method brings the advantages of low cost and high stability. The liquid crystal tunable vertical cavity surface emitting laser of the present invention has the advantages of miniaturization, low cost, high stability and wide range. The preparation of the liquid crystal cavity unit provided by the present invention has the advantages of simplicity and feasibility, greatly reducing the process complexity and cost, and can stably output high-quality single-mode lasers with a tuning range of more than 30nm.

[0055] A person skilled in the art should understand that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope of the disclosure of the embodiments of the present invention (including the claims) is limited to these examples; under the concept of the embodiments of the present invention, the technical features in the above embodiments or different embodiments can also be combined, and there are many other changes in different aspects of the embodiments of the present invention as described above, which are not provided in detail for the sake of simplicity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the embodiments of the present invention should be included in the protection scope of the embodiments of the present invention.

Claims

1. A liquid crystal tunable vertical cavity surface emitting laser (100), characterized in that: include: A VCSEL unit (10), the VCSEL unit (10) comprising a first DBR (13) and a second DBR (16) located above the first DBR (13); A circumferential side wall (20) formed on the upper side of the VCSEL unit (10), the circumferential side wall (20) defining a liquid crystal cavity (22); a reflector unit (30), the reflector unit (30) being joined to the upper side of the circumferential side wall (20); and Liquid crystal (40) contained in the liquid crystal cavity (22); The VCSEL unit (10) further comprises a coupling layer (17) located above the second DBR (16), the refractive index of the coupling layer (17) being in a first preset relationship with the refractive index of the high refractive index layer in the first DBR (13), and / or the thickness of the coupling layer (17) being in a second preset relationship with the wavelength.

2. The laser (100) according to claim 1, characterized in that The first preset relationship includes that 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 second preset relationship includes that the thickness of the coupling layer (17) is one quarter of a wavelength.

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

4. The laser (100) according to claim 1, characterized in that The circumferential side wall (20) is formed by spin coating photoresist on the surface of the VCSEL unit (10) and performing photolithography development.

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

6. The laser (100) according to claim 1, characterized in that The reflector unit (30) is bonded to the circumferential side wall (20) by means of an adhesive, wherein the adhesive is composed of at least flaky copper-coated silver powder and a curing agent in a weight ratio of (8-12):1; the reflector unit (30) is sealed and joined to the circumferential side wall (20) by means of UV glue.

7. A method for preparing a liquid crystal tunable vertical cavity surface emitting laser (100), characterized in that: include: A VCSEL unit (10) is prepared, wherein the VCSEL unit (10) comprises 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), wherein 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 of the wavelength; A circumferential side wall (20) defining a liquid crystal cavity (22) is formed on the upper side of the VCSEL unit (10); preparing a reflector unit (30); The reflector unit (30) is joined to the upper side of the circumferential side wall (20), and liquid crystal (40) is injected into the liquid crystal cavity (22).

8. The method according to claim 7, characterized in that Forming the circumferential side wall (20) comprises: Spin coating photoresist on the VCSEL unit (10) and performing photolithography to obtain the circumferential sidewall (20), wherein the height of the circumferential sidewall (20) is adjusted by adjusting the ratio of the photoresist to the diluent; The circumferential side wall (20) is cured, comprising the following curing steps performed in sequence: curing at 70-90°C for 20-40 minutes, curing at 110-130°C for 20-40 minutes, curing at 140-160°C for 20-40 minutes, and curing at 190-210°C for 0.8-1.2 hours.

9. The method according to claim 7, characterized in that: Also includes: Polarization control layers (21, 23) are respectively formed on the surfaces of the VCSEL unit (10) and the reflector unit (30) facing the liquid crystal cavity (22), and stripes extending along a first stripe direction (L1) and stripes extending along a second stripe direction (L2) are respectively formed on the two polarization control layers (21, 23) according to a desired polarization direction of the output light, wherein a corresponding angle (θ) is formed between the first stripe direction (L1) and the second stripe direction (L2).

10. The method according to claim 7, characterized in that The reflector unit (30) is joined to the upper side of the circumferential side wall (20), and liquid crystal (40) is injected into the liquid crystal cavity (22), comprising: The reflector unit (30) is bonded to the upper side of the circumferential side wall (20) by means of an adhesive, and a gap is reserved between the reflector unit (30) and the circumferential side wall (20) at least on one side; Liquid crystal (40) is injected into the liquid crystal cavity (22) through the gap, and the gap is sealed with UV glue.

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