Stealth technology-based Terahertz frequency-domain ellipsometry instrumentation

TL;DR

This paper introduces a novel terahertz frequency-domain ellipsometer utilizing stealth technology to suppress standing waves, enabling precise measurements of dielectric properties and free charge carriers in various materials.

Contribution

The paper presents a new THz ellipsometer design with stealth technology features, improving measurement accuracy and enabling versatile in-situ and resonant cavity experiments.

Findings

Successful dielectric constant measurements of silicon and sapphire.

Determination of free charge carrier properties in 2D electron gases.

Enhanced accuracy in optical Hall effect measurements.

Abstract

We present a terahertz (THz) frequency-domain spectroscopic (FDS) ellipsometer design which suppresses formation of standing waves by use of stealth technology approaches. The strategy to suppress standing waves consists of three elements \textit{geometry}, \textit{coating} and \textit{modulation}. The instrument is based on the rotating analyzer ellipsometer principle and can incorporate various sample compartments, such as a superconducting magnet, \textit{in-situ} gas cells or resonant sample cavities, for example. A backward wave oscillator and three detectors are employed, which permit operation in the spectral range of 0.1--1~THz (3.3--33~cm$^{-1}$ or 0.4--4~meV). The THz frequency-domain ellipsometer allows for standard and generalized ellipsometry at variable angles of incidence in both reflection and transmission configurations. The methods used to suppress standing waves and strategies for an accurate frequency calibration are presented. Experimental results from dielectric constant determination in anisotropic materials, and free charge carrier determination in optical Hall effect, resonant-cavity enhanced optical Hall effect and \textit{in-situ} optical Hall effect experiments are discussed. Examples include silicon and sapphire optical constants, free charge carrier properties of two dimensional electron gas in group-III nitride high electron mobility transistor structure, and ambient effects on free electron mobility and density in epitaxial graphene.