Broadband Up‐Conversion Mid‐Infrared Time‐Stretch Spectroscopy

Abstract Time‐stretch spectroscopy is a powerful tool for studying transient and nonrepetitive physical events. Its extension to the fundamental molecular fingerprint region is promising for chemical and biological research but encounters difficulties in mid‐infrared photodetection and strong attenuation of aqueous environments. Here, mid‐infrared time‐stretch spectroscopy via coincidently‐pumped nonlinear frequency conversion for efficient mid‐infrared generation and detection is demonstrated. Broadband (≈200 cm −1 ) mid‐infrared spectra at 77.5 Mspectra s −1 with high signal‐to‐noise ratio are recorded. The single‐shot signal‐to‐noise ratio (SNR) measured within 2 ns exceeds 200. The method reveals fast spectral dynamics in a laser‐induced liquid evaporation process with a temporal resolution of 12.9 ns. Furthermore, probing transient spectra during evaporation opens up a new opportunity for interrogating chemicals inside or behind a thick water layer. Integrating high speed, broad bandwidth, high SNR, and the potential for operating under an aqueous environment, this method will benefit many applications including molecular dynamics, chemical reactions, and biological diagnostics.