Bridging Mid-IR and Terahertz Domains in a Single High-Resolution Dual-Comb Spectroscopy Measurement

Abstract

Dual-comb spectroscopy (DCS) utilizes a pair of broadband mutually coherent laser frequency combs to enable high-resolution, high-accuracy spectroscopic measurements with atomic-clock-level frequency referencing, and rapid, multiplexed acquisition without moving parts. It has traditionally been confined to specific domains: terahertz, infrared, visible, and ultraviolet, each requiring distinct comb sources and detection mechanisms tailored to the nature of the spectroscopic target. Yet, similar techniques may be implemented in the terahertz (THz) and mid-infrared (MIR) regions, such as optical rectification for comb generation and electro-optic sampling for detection, both using crystals with quadratic nonlinearity. However, in the Reststrahlen band near phonon resonances in these crystals, typically between 5 and 10 THz, both linear and nonlinear susceptibilities experience abnormally high dispersion, and light propagation is strongly suppressed. This confines DCS operation to spectral regions either below or above the Reststrahlen band and effectively separating the THz and MIR domains. Here we demonstrate high-resolution DCS performed simultaneously over two broad spectral bands, each spanning an octave or more. The measurements cover both the MIR (350-1150 cm$^{-1}$; 8.7-28.5 $μ$m; 10.5-34.5 THz) and the THz region (80-160 cm$^{-1}$; 62.5-125 $μ$m; 2.4-4.8 THz), effectively bridging these traditionally separate regions within a single acquisition. This enables direct cross-referencing of molecular absorption line strengths across widely separated spectral domains. As a proof of concept, we demonstrate the simultaneous acquisition of ro-vibrational and pure rotational absorption spectra of ammonia (NH$_3$) with a spectral resolution of 7.3 MHz (0.00024 cm$^{-1}$), sufficient to fully resolve Doppler-broadened line shapes across the entire measured spectral range.