Broadband terahertz comb with sub-Hz comb linewidth

TL;DR

This paper presents a broadband, ultra-narrow linewidth terahertz frequency comb generated via a noise-suppressed electro-optical method, enabling high-resolution spectroscopy and precise metrology without active stabilization.

Contribution

It introduces a novel noise-suppression technique for EO combs that produces a broad, stable THz comb with sub-Hz linewidths and wide tunability, advancing THz spectroscopy capabilities.

Findings

Broad spectral coverage from 0.05 to 4 THz

Comb linewidths as narrow as 0.3 Hz

Frequency stability of 8.6×10^-14 at 1 second

Abstract

Terahertz (THz) frequency combs are increasingly essential for spectroscopy, metrology, and quantum science. However, generating a dense array of evenly spaced ultra-narrow THz comb lines is challenging. Here, we demonstrate broadband THz comb generation using a photoconductive antenna that transfers a noise-suppressed near-infrared electro-optical (EO) comb into the THz domain. Our noise-suppression strategy, leveraging soliton self-frequency shift and spectral filtering, effectively suppresses EO comb phase noise without requiring active stabilization. The resulting THz comb exhibits broad spectral coverage (0.05-4 THz), narrow comb linewidths (0.3 Hz at the Fourier-transform limit), and excellent frequency stability (8.6*10^-14 at 1-second integration). We further demonstrate asynchronous THz time-domain spectroscopy, resolving ~36,000 comb lines with 50 MHz spacing. Crucially, the inherent frequency agility of the EO comb enables rapid and wide-range tuning of the THz comb line spacing. These attributes position our THz comb as a versatile tool for high-resolution molecular spectroscopy and precision THz metrology.