During the past decades, breakthroughs in nanophotonics and nanofabrication technologies have vigorously promoted the development of optical metastructures. With the help of precise design on metastructures, incident light can be effectively manipulated. However, the difficulty in finding high-index and low-loss dielectrics in visible range limits the application of all-dielectric metastructures for visible wavelengths. Besides, the edge and surface roughness of fabricated metastructure also have more significant effects on its performance.
Here, we report the design of high contrast all-dielectric metastructure for visible range applications using the switchable all-dielectric metastructure an example. The physics behind the high contrast all dielectric metastructure is studied and analyzed. Based on this, the effect of edge and surface roughness on fabricated high contrast all-dielectric metastructure is explained. A method that can optimize the metastructure performance effectively is also proposed.
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.
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