Ultra-precise determination of thicknesses and refractive indices of optically thick dispersive materials by dual-comb spectroscopy

TL;DR

This paper introduces a dual-comb spectroscopy method that accurately measures the thickness and refractive index of optically thick dispersive materials with high precision and wide dynamic range, overcoming previous phase ambiguity issues.

Contribution

It presents a novel approach using dual-comb spectroscopy to determine absolute phase-shifts, enabling precise, simultaneous measurement of thickness and refractive index without prior knowledge.

Findings

Achieved five and a half digit precision in measurements.

Demonstrated measurement of silicon wafer at 193.414 THz.

Method overcomes phase ambiguity in optical measurements.

Abstract

Precise measurements of the geometrical thickness of a sample and its refractive index are important for materials science, engineering, and medical diagnosis. Among the possible non-contact evaluation methods, optical interferometric techniques possess the potential of providing superior resolution. However, in the optical frequency region, the ambiguity in the absolute phase-shift makes it difficult to measure these parameters of optically thick dispersive materials with sufficient resolution. Here, we demonstrate that dual frequency-comb spectroscopy can be used to precisely determine the absolute sample-induced phase-shift by analyzing the data smoothness. This method enables simultaneous determination of the geometrical thickness and the refractive index of a planar sample with a precision of five and a half digits and an ultra-wide dynamic range. The thickness and the refractive index at 193.414 THz of a silicon wafer determined by this method are 0.52047(3) mm and 3.4756(3), respectively, without any prior knowledge of the refractive index.