Unambiguous determination of optical constants and thickness of ultrathin films by using optical anisotropic substrates

TL;DR

This paper presents a novel ellipsometry method using anisotropic substrates to accurately determine optical constants and thickness of ultrathin films below 1 nm, overcoming previous limitations.

Contribution

The study introduces a new approach employing anisotropic substrates in ellipsometry to unambiguously measure optical properties of ultrathin films under 1 nm.

Findings

Allows simultaneous measurement of n, k, and d for films <1 nm

Uses anisotropic substrates to decouple optical parameters

Validated by extensive simulations

Abstract

The unambiguous and universal determination of optical constants such as complex refractive indices (n, k) and thickness (d) in one measurement is typically confined for films with a thickness of more than 10~nm that hampers its application for ultra-thin films such as two-dimensional materials. We demonstrate that the commonly accepted limit of n, k, d coupling is overcome in ellipsometry by utilizing anisotropic substrates. In this way, ellipsometry allows to determine n, k and d simultaneously for thicknesses lower than 1~nm with high accuracy. Extensive simulations proof the potential of using anisotropic substrates for decoupling. The simultaneous determination of n, k and d is of great interest in several fields, two-dimensional materials, their heterostructures, ultra thin films for application in quantum technology, plasmonics and nano-photonics.