Swept-frequency feedback interferometry using terahertz frequency QCLs: a method for imaging and materials analysis

Aleksandar D Rakić,A Giles Davies,Thomas Taimre,Paul Harrison,Alexander Valavanis,Dragan Indjin,Paul Dean,Suraj P Khanna,Karl Bertling,Edmund H Linfield,Yah Leng Lim,Zoran Ikonić,Mohammad Lachab,Stephen J Wilson

doi:10.1364/oe.21.022194

Abstract

The terahertz (THz) frequency quantum cascade laser (QCL) is a compact source of high-power radiation with a narrow intrinsic linewidth. As such, THz QCLs are extremely promising sources for applications including high-resolution spectroscopy, heterodyne detection, and coherent imaging. We exploit the remarkable phase-stability of THz QCLs to create a coherent swept-frequency delayed self-homodyning method for both imaging and materials analysis, using laser feedback interferometry. Using our scheme we obtain amplitude-like and phase-like images with minimal signal processing. We determine the physical relationship between the operating parameters of the laser under feedback and the complex refractive index of the target and demonstrate that this coherent detection method enables extraction of complex refractive indices with high accuracy. This establishes an ultimately compact and easy-to-implement THz imaging and materials analysis system, in which the local oscillator, mixer, and detector are all combined into a single laser.

Highlights

Significant scientific effort has been invested in the realization of terahertz (THz) frequency imaging [1, 2] and materials analysis systems [3] over the past two decades [4, 5]
Key to the success of THz time-domain spectroscopy (TDS) is its capability of measuring complex refractive indices of samples over bandwidths as large as 100 THz, due to its intrinsic ability to resolve the electric field amplitude of broadband THz pulses coherently and with subpicosecond resolution [6], as well as its insensitivity to thermal background radiation [7]
We propose a new method for coherent imaging and materials analysis using a THz quantum cascade laser (QCL) feedback interferometer in reflection mode

Summary

Introduction

Significant scientific effort has been invested in the realization of terahertz (THz) frequency imaging [1, 2] and materials analysis systems [3] over the past two decades [4, 5]. THz TDS systems in general have signal-to-noise ratios (SNRs) that are practically useful only below ∼3 THz [8] but have been reported with much higher bandwidths [9,10,11,12]. Their highest spectral resolution is typically reported between ∼5–7 GHz [13,14] (worse in high-bandwidth systems), and they are restricted to low THz powers on the order of 10–100 μW for commonly used optically-pumped photoconductive emitters.

Methods

Results

Conclusion