Mid-infrared cross-comb spectroscopy

Dual-comb spectroscopy has been proven beneficial in molecular characterization but remains challenging in the mid-infrared region due to difficulties in sources and efficient photodetection. Here we introduce cross-comb spectroscopy, in which a mid-infrared comb is upconverted via sum-frequency generation with a near-infrared comb of a shifted repetition rate and then interfered with a spectral extension of the near-infrared comb. We measure CO2 absorption around 4.25 mu m with a 1-mu m photodetector, exhibiting a 233-cm(-1) instantaneous bandwidth, 28000 comb lines, a single-shot signal-to-noise ratio of 167 and a figure of merit of 2.4 x 10(6) Hz(1/2). We show that cross-comb spectroscopy can have superior signal-to-noise ratio, sensitivity, dynamic range, and detection efficiency compared to other dual-comb-based methods and mitigate the limits of the excitation background and detector saturation. This approach offers an adaptable and powerful spectroscopic method outside the well-developed near-IR region and opens new avenues to high-performance frequency-comb-based sensing with wavelength flexibility. The authors introduce and demonstrate cross-comb spectroscopy in the mid-infrared as a variant of dual-comb spectroscopy. It provides enhanced performance and allows mid-infrared spectral information to be obtained by near-infrared detection.