In the past decade, subwavelength high contrast gratings (HCGs) have been developed and studied, which has led to many applications. The broadband reflectance in HCGs mainly comes from the contrast between the grating material and its surrounding environment, so high-index and low-loss materials are required for making HCGs. Compared with infrared (IR) ranges, building HCGs in visible or near-IR wavelength ranges is much harder due to the limitation of optical materials.
In order to overcome the challenge of materials in making HCGs in visible to near-IR ranges, hybrid HCGs are proposed. The design of hybrid HCGs is a combination of low-loss and low-index materials and high-loss and high-index materials. In order to reduce the optical loss due to the incorporation of high-loss material, optical modes must be manipulated to be confined in the low-loss region.
In our work, the structure and parameters for hybrid HCGs are optimized based on numerical study (both FDTD and RCWA). As a proof-of-principle demonstration, hybrid HCGs composed of amorphous silicon, silicon nitride and silicon dioxide are optimized. The optimal structure has a broadband reflectance (>90%) in visible to near-IR ranges. The design demonstrates a great fabrication tolerance to line width, dielectric thicknesses and sidewall verticality. The hybrid HCGs are patterned by nanoimprint lithography. Reactive ion etching at cryogenic temperature is optimized for the best etching profile. More details on design, fabrication and measurement will be presented at the conference.