# Generalized Brewster-angle effect in thin-film optical absorbers and its application for graphene hydrogen sensing

## TL;DR

This paper experimentally demonstrates a generalized Brewster-angle effect in thin-film absorbers at visible wavelengths, enabling highly sensitive graphene-based hydrogen sensing through phase-sensitive light-matter interactions.

## Contribution

It introduces a theoretical framework and experimental realization of GBA in thin films, expanding applications to ultra-sensitive gas sensing.

## Key findings

- Achieved polarization by reflection for both p- and s-polarized light at different angles and wavelengths.
- Demonstrated hydrogen sensing with ~1 fg/mm2 sensitivity using graphene on a thin-film absorber.
- Highlighted potential for advanced gas and biosensing applications.

## Abstract

Generalized Brewster angle (GBA) is the incidence angle at which polarization by reflection for p- and s-polarized light takes place. Realizing s-polarization Brewster effect requires a material with magnetic response which is challenging at optical frequencies since the magnetic response of materials at these frequencies is extremely weak. Here, we experimentally realize GBA effect in the visible using a thin-film absorber system consisting of a dielectric film on an absorbing substrate. Polarization by reflection is realized for both p- and s- polarized light at different angles of incidence and multiple wavelengths. We provide a theoretical framework for the generalized Brewster effect in thin-film light absorbers. We demonstrate hydrogen gas sensing using a single layer graphene film transferred on a thin-film absorber at the GBA with ~1 fg/mm2 aerial mass sensitivity. The ultrahigh sensitivity stems from the strong phase sensitivity near point of darkness, particularly at the GBA, and the strong light-matter interaction in planar nanocavities. These findings depart from the traditional domain of thin-films as mere interference optical coatings and highlight its many potential applications including gas sensing and biosensing.

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Source: https://tomesphere.com/paper/1904.10075