Near-zero-index behavior in indium antimonide revealed by phase-corrected terahertz reflection spectroscopy

TL;DR

This paper introduces a phase correction method for broadband terahertz spectroscopy that accurately measures complex refractive indices, revealing near-zero index behavior in indium antimonide near its plasma frequency.

Contribution

The authors developed an analytical phase correction technique based on Kramers-Kronig relations for terahertz reflection spectroscopy, enabling precise refractive index measurements without intensive computation.

Findings

Validated method with silicon, achieving <2.5% accuracy

Observed near-zero index in InSb between 1-2 THz

Detected group velocity as low as 0.08c in InSb

Abstract

We developed a phase correction method for broadband terahertz time-domain spectroscopy in reflection geometry, which allows us to obtain quantitative and accurate values for the complex refractive index of materials. The process is analytical, based on the Kramers-Kronig relations, and does not require any computationally intensive algorithms. We validate it by extracting the refractive index of silicon, obtaining the nominal value with an accuracy better than 2.5\% over the 0.25--3.5 THz range, and better than 0.6\% in the 1--2 THz range. We use the method to experimentally observe that an undoped InSb crystal shows a refractive index of $n<1$ between 1 and 2 THz, in proximity to the plasma frequency of the material where, the amplitude of the group velocity goes as low as 0.08$c$.