Broadband terahertz comb with sub-Hz comb linewidth

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.

💡 Research Summary

The authors present a novel approach to generating a broadband, ultra‑stable terahertz (THz) frequency comb with sub‑hertz linewidths, addressing long‑standing challenges in THz metrology and high‑resolution spectroscopy. The core of the system is a near‑infrared electro‑optic (EO) frequency comb derived from a cavity‑stabilized 1550 nm continuous‑wave laser (<1 Hz linewidth). A Fabry‑Pérot EO modulator creates a 5 GHz spaced comb, which is then down‑sampled to a 50 MHz repetition rate using an intensity modulator driven by a picosecond pulse generator operating at one‑hundredth of the original rate. This low repetition frequency matches the excitation bandwidth of a photoconductive antenna (PCA) and provides sufficient pulse energy for subsequent nonlinear processes.

During erbium‑doped fiber amplification, amplified spontaneous emission (ASE) would normally overlap the EO comb spectrum, degrading the THz comb’s coherence. To suppress this, the authors exploit soliton self‑frequency shift (SSFS) in a 2‑m polarization‑maintaining fiber. High‑energy EO pulses undergo Raman‑induced red‑shift to ~1575 nm, moving them out of the ASE band (1520‑1570 nm). A subsequent band‑pass filter isolates the shifted, low‑noise pulses, effectively eliminating ASE without active phase‑lock loops. Measured phase‑noise spectra show a reduction of 10–20 dB across 10 Hz–1 MHz, reaching –136 dBc/Hz at 1 kHz. The resulting EO comb exhibits a fractional frequency instability of 10⁻¹⁵ at 1 s, improving to 5.6 × 10⁻¹⁷ at 256 s.

The cleaned EO comb is further spectrally broadened in an EDF‑A and compressed to 65 fs pulses. When coupled into a PCA, it generates a THz comb spanning 0.05–4 THz. Because the comb has zero offset frequency, each line is simply fₘ = m·fᵣ with fᵣ = 50 MHz, yielding about 36 000 resolved lines up to 1.8 THz. Heterodyne beat measurements against an ultra‑stable CW THz source (e.g., 96 GHz) reveal a Fourier‑transform‑limited linewidth of 0.3 Hz and a phase‑noise floor of –80 dBc/Hz at 1 kHz offset. Allan deviation analysis gives a fractional frequency instability of 8.6 × 10⁻¹⁴ at 1 s and 5.1 × 10⁻¹⁵ at 265 s, three orders of magnitude better than previously reported PCA‑based broadband THz combs.

The authors also demonstrate asynchronous THz time‑domain spectroscopy (TDS), an analog of dual‑comb spectroscopy. Two EO combs are generated from the same CW laser; the first drives the PCA to emit the THz comb, while the second, with a slightly offset repetition rate (Δfᵣ = 20 Hz), samples the THz field. This configuration yields a full 0.05–4 THz spectrum in a single 1 ms acquisition, corresponding to a 20 Hz refresh rate. Because the two combs share the same optical reference, no active phase correction is required, allowing coherent averaging directly in the time domain. After 200‑fold averaging (total 100 s), the spectrum shows ~36 000 discrete lines with uniform 50 MHz spacing, each line exhibiting the expected 2 Hz RF (5 MHz THz) linewidth.

A key advantage of the EO‑based architecture is rapid, linear, and wide‑range tunability of the repetition frequency, unrestricted by a mode‑locked cavity. The authors demonstrate stepwise tuning of fᵣ in 500 Hz increments, resulting in ~5 MHz shifts of individual THz lines around 0.5 THz, with a linearity R² > 0.999. This agility enables spectral interleaving, Doppler‑free spectroscopy, and flexible gas‑sensing schemes that are difficult with traditional mode‑locked THz combs.

In summary, the work introduces (1) a passive SSFS‑based noise‑suppression technique that yields an ultra‑low‑phase‑noise EO comb, (2) a PCA‑driven broadband THz comb covering 0.05–4 THz with 50 MHz spacing and sub‑hertz linewidths, (3) exceptional frequency stability (10⁻¹⁴ at 1 s), (4) high‑speed asynchronous THz‑TDS capable of millisecond‑scale full‑band acquisition, and (5) fast, linear tunability of comb lines without cavity constraints. These capabilities position the demonstrated THz comb as a powerful tool for next‑generation THz metrology, precision molecular spectroscopy, and quantum‑science applications.