THz frequency comb high-resolution heterodyne detection coupled with fast Fourier spectrometer

Terahertz (THz) frequency combs (THz-FCs) offer a new powerful route to high-resolution molecular spectroscopy, providing both a broad spectral coverage and inherently excellent frequency metrology and referencing. In this work, we demonstrate a THz spectroscopic system that exploits these advantages by combining a femtosecond-laser-based THz-FC with heterodyne detection coupled with a fast Fourier transform spectrometer (XFFTS) [1]. This configuration enables the rapid and simultaneous acquisition of more than 80 comb modes spanning a 7.5 GHz bandwidth. A complete spectrum can be recorded in under 12 minutes, achieving a uniform spectral resolution of 76 kHz, determined primarily by the native channel spacing of the XFFTS.

The setup addresses a fundamental challenge in THz spectroscopy, the inherent compromise between achieving high spectral resolution and maintaining large measurement bandwidth. By leveraging multiple, coherently spaced FC modes, our approach demonstrates that simultaneous multi-mode detection is a realistic and efficient solution. The current resolution and bandwidth are limited by the XFFTS baseband (0–2.5 GHz). This range can be extended by exploiting the second Nyquist band or by integrating multiple XFFTS units to increase instantaneous coverage without added acquisition time. The IF (intermediate frequency) bandwidth will ultimately be constrained by the heterodyne mixer, which has an electrical bandwidth up to 40 GHz so could allow an instantaneous spectral bandwidth of up to 80 GHz to be measured. With emerging new generation FFT spectrometers [2].

Our results highlight the significant potential of THz-FC-based spectroscopy for accelerating high-resolution molecular investigations. This approach provides the means to acquire detailed, accurate spectra at unprecedented speeds, enabling advanced studies in molecular physics, atmospheric science, and chemical analysis, including mixture identification and quantitative spectroscopy. The demonstrated system represents a promising step toward versatile, high-precision THz spectrometry for a wide range of scientific and technological applications.

 

Acknowledgments

The authors would also like to acknowledge the financial support of the French Agence Nationale de la Recherche via TIGER (ANR-21-CE30-0048) and HEROES (ANR-16-CE30 0020). Ardhendu Pal would like to acknowledge CPER WAVETECH PROGRAMME for his Postdoctoral Fellowship.

References

[1] F. Hindle, A. Khabbaz, A. Roucou, F.J. Lampin and G. Mouret 2025. Terahertz Frequency Comb High-Resolution Heterodyne Spectrometer. IEEE Transactions on Terahertz Science and Technology.

[2] www.mpifr-bonn.mpg.de/7081687/qffts4g