Metrology of Complex Refractive Index for Solids in the Terahertz Regime Using Frequency Domain Spectroscopy

TL;DR

This paper introduces a new robust algorithm for extracting the complex refractive index of solids in the terahertz range using frequency domain spectroscopy, addressing systematic errors and improving measurement accuracy.

Contribution

A novel algorithm for optical property extraction that reduces systematic errors and enhances robustness in terahertz frequency measurements of solids.

Findings

The new algorithm outperforms existing methods in robustness to noise and non-idealities.

Different data processing techniques introduce distinct systematic errors.

Using multiple algorithms together improves measurement reliability.

Abstract

Frequency domain spectroscopy allows an experimenter to establish optical properties of solids in a wide frequency band including the technically challenging 10 THz region, and in other bands enables metrological comparison between competing techniques. We advance a method for extracting the optical properties of high-index solids using only transmission-mode frequency domain spectroscopy of plane-parallel Fabry-Perot optical flats. We show that different data processing techniques yield different kinds of systematic error, and that some commonly used techniques have inherent systematic errors which are underappreciated. We use model datasets to cross-compare algorithms in isolation from experimental errors, and propose a new algorithm which has qualitatively different systematic errors to its competitors. We show that our proposal is more robust to experimental non-idealities such as noise or apodization, and extract the complex refractive index spectrum of crystalline silicon as a practical example. Finally, we advance the idea that algorithms are complementary rather than competitive, and should be used together as part of a toolbox for better metrology.