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.
Thin-film aluminum filters degrade in space with significant reduction of their Extreme Ultraviolet (EUV) transmission. This degradation was observed on the EUV Spectrophotometer (ESP) onboard the Solar Dynamics Observatory’s EUV Variability Experiment and the Solar EUV Monitor (SEM) onboard the Solar and Heliospheric Observatory. One of the possible causes for deterioration of such filters over time is contamination of their surfaces from plumes coming from periodic firing of their satellite’s Monomethylhydrazine (MMH) – Nitrogen Tetroxide (NTO) thrusters. When adsorbed by the filters, the contaminant molecules are exposed to solar irradiance and could lead to two possible compositions. First, they could get polymerized leading to a permanent hydrocarbon layer buildup on the filter’s surface. Second, they could accelerate and increase the depth of oxidation into filter’s bulk aluminum material. To study the phenomena we experimentally replicate contamination of such filters in a simulated environment by MMH-NTO plumes. We apply, Scanning Electron Microscopy and X-Ray photoelectron spectroscopy to characterize the physical and the chemical changes on these contaminated sample filter surfaces. In addition, we present our first analysis of the effects of additional protective layer coatings based on self-assembled carbon monolayers for aluminum filters. This coverage is expected to significantly decrease their susceptibility to contamination and reduce the overall degradation of filter-based EUV instruments over their mission life.