Strong Long-Wave Infrared Optical Response in a Topological Semiconductor with a Mexican Hat Band Structure

TL;DR

This paper demonstrates that a topological semiconductor with a Mexican hat band structure exhibits significantly enhanced infrared optical response, promising improved FIR/MIR photodetectors.

Contribution

It provides experimental validation of the large optical response in a topological semiconductor with a Mexican hat band structure, previously predicted theoretically.

Findings

Optical response >10x larger than traditional materials

Experimental validation of Mexican hat band structure effects

Enhanced extinction coefficient and refractive index near absorption edge

Abstract

Light sources and photodetectors operating in the far- to mid-infrared (FIR/MIR) band ($8$-$12~\rm \mu m$, $0.1$-$0.15~\rm eV$) remain relatively poorly developed compared to their counterparts operating in the visible and near-infrared ranges, despite extensive application potential for thermal imaging, standoff sensing, and other technologies. This is attributable in part to the lack of narrow-gap materials ($<0.1~\rm eV$) with high optical gain and absorption. In this work, a narrow-gap semiconductor, $\rm Pb_{0.7}Sn_{0.3}Se$, is demonstrated to exhibit an optical response $>10\times$ larger than that of $\rm Hg_{x}Cd_{1-x}Te$ (MCT), the dominant material for FIR/MIR photodetectors. A previous theoretical investigation indicated that chalcogen $p$ and metal $d$ band inversion in this material creates a Mexican hat band structure (MHBS), which results in a dramatic increase in the joint density of states at the optical transition edge compared to typical semiconductors. This prediction is experimentally validated here using single-crystal specimens of $\rm Pb_{0.7}Sn_{0.3}Se$ measured using temperature-dependent spectroscopic ellipsometry over a wavelength range of $1.7$-$20~\rm \mu m$ ($0.73$-$0.062~\rm eV$). These measurements demonstrate a large enhancement in extinction coefficient and refractive index characteristic of a MHBS in the vicinity of the absorption edge, in agreement with theoretical predictions. The realization of topological semiconductors with a MHBS is expected to lead to high-efficiency detectors operating in the FIR/MID range.