# Composite Reflective/Absorptive IR-Blocking Filters Embedded in Metamaterial Antireflection Coated Silicon

## TL;DR

This paper introduces a novel IR-blocking filter combining patterned frequency selective structures and a thin absorptive composite on silicon, significantly reducing IR thermal load while maintaining high in-band transmission for cryogenic applications.

## Contribution

It presents a new IR filter design integrating metamaterial antireflection coatings with composite absorptive layers on silicon, enhancing thermal management and optical performance.

## Key findings

- Reflects ~50% of 300 K blackbody IR radiation
- Blocks >99.8% of total IR power with negligible thermal gradients
- In-band reflection reduced below 1% with <1% absorption below 750 GHz

## Abstract

Infrared (IR) blocking filters are crucial for controlling the radiative loading on cryogenic systems and for optimizing the sensitivity of bolometric detectors in the far-IR. We present a new IR filter approach based on a combination of patterned frequency selective structures on silicon and a thin (50 $\mu \textrm{m}$ thick) absorptive composite based on powdered reststrahlen absorbing materials. For a 300 K blackbody, this combination reflects $\sim$50\% of the incoming light and blocks \textgreater 99.8\% of the total power with negligible thermal gradients and excellent low frequency transmission. This allows for a reduction in the IR thermal loading to negligible levels in a single cold filter. These composite filters are fabricated on silicon substrates which provide excellent thermal transport laterally through the filter and ensure that the entire area of the absorptive filter stays near the bath temperature. A metamaterial antireflection coating cut into these substrates reduces in-band reflections to below 1\%, and the in-band absorption of the powder mix is below 1\% for signal bands below 750 GHz. This type of filter can be directly incorporated into silicon refractive optical elements.