Study On Si-Based Zero-Contrast Grating Reflector For VCSEL

In recent years,the demand for vertical-cavity surface-emitting laser（VCSEL）as the main coherent light source has been increasing rapidly due to the explosive growth of markets such as 3D sensing,Internet of Things,Li DAR,and data communication.Developing high-efficiency,low-cost,and high-stability VCSEL devices has become an urgent problem for various research institutions and manufacturers.Typical VCSELs have an active region sandwiched between two distributed Bragg reflectors（DBRs）.However,too many layers of DBRs will increase the device’s series resistance and hinder its miniaturization.Subwavelength gratings with simple layer structures have broadband high reflectivity and can replace some or all of the DBR mirrors in VCSEL design,effectively simplifying the VCSEL structure,reducing the difficulty and cost of VCSEL manufacturing,and improving device performance.Therefore,this paper mainly focuses on the study of zero-contrast subwavelength gratings（ZCG）reflectors for VCSEL.Simulation study was conducted on Si-based ZCG reflectors for 940nm VCSEL.（1）The optimization of structures with and without low refractive index layers were performed using rigorous coupled-wave theory,and the optimization range of each structural parameter for achieving broadband high reflectivity under different polarization states was discussed and analyzed.The optimized TM-ZCG with low refractive index layer had a high reflectivity band with R≥99.5%ranging from 850nm to 1130nm,Δ/₀=29.8%,while the TE-ZCG had a high reflectivity band with R≥99.5%ranging from 880nm to 1113nm,Δ/₀=24.8%.The ZCG without low refractive index layer had a high reflectivity band width of 85nm and 74nm under TM and TE polarization,respectively.（2）Taking the Si-based ZCG with low refractive index layer as an example,the characteristic differences between TM-ZCG and TE-ZCG reflectors,as well as the effect of grating morphology on ZCG reflectivity performance were compared and analyzed from the perspective of device fabrication.（3）To address the issue of the difficulty in accurately controlling the grating thickness and duty cycle during actual fabrication,a method of dynamically selecting grating parameters was proposed,which enabled ZCG to achieve high reflectivity while having a larger fabrication tolerance.（4）Combining FDTD method,the optical field distribution characteristics of ZCG and the diffraction efficiency of different diffraction orders were studied,and the theoretical mechanism of ZCG having broadband high reflectivity was discussed.On the basis of Si-based ZCG design,a simple-structured Ga As-based ZCG reflector was proposed.（1）Using the rigorous coupled-wave theory,the relationship between various grating parameters,wavelength,and reflectivity was analyzed in detail,and the optimal values of grating parameters and manufacturing tolerances were given.The optimized TM-ZCG had a high reflection band with reflectivity R≥99.5%at 910-983nm,and the optimized TE-ZCG had a high reflection band with reflectivity R≥99.5%at 906-977nm.（2）The diffraction characteristics of three typical sub-wavelength gratings were thoroughly studied,and the reasons for the significant differences in the reflection band width of different structures were analyzed.（3）The polarization properties of Si-based ZCG and Ga As-based ZCG reflectors were analyzed,with the Ga As-based ZCG showing better polarization selectivity.（4）Considering practical limitations,the influence of grating size and the limiting aperture size of oxidation on VCSEL performance was explored.Both designed ZCG reflectors with different material systems could fully meet the requirements of the VCSEL resonant cavity mirror,and the research results provided theoretical and technical support for the application of ZCG reflectors in VCSELs.