Fast, sensitive dual-comb CARS spectroscopy with a quasi-dual-comb laser

Dual-comb coherent anti-Stokes Raman scattering (DC-CARS) spectroscopy is an effective tool for high-speed acquisition of vibrational spectra in the fingerprint region. DC-CARS spectroscopy also provides high spectral resolution by virtue of its ability to cover a large time delay between the pump and probe pulses, from two optical frequency combs with slightly different, fixed pulse repetition rates. However, less than 1% of the incident pulse energy is used to acquire the CARS signal because the repetition interval of the laser pulses (<1 ns) is much longer than the coherence lifetime of molecular vibrations (~3 ps). This results in a low spectral acquisition rate and a low signal-to-noise ratio. Here, we introduce a novel method for DC-CARS spectroscopy with a nearly 100% energy efficiency using a “quasi”-dual-comb laser. Specifically, one of the repetition rates of the two lasers is rapidly modulated by controlling the pumping intensity of a Ti:sapphire laser, so that the group delay between two pulses is shorter than the coherence lifetime of molecular vibrations, while the group delay is monitored with two-color interferometry to calibrate the time-domain CARS signal. With this method, we realized a spectral acquisition rate of 100,000 spectra/s, which is 10x higher than conventional DC-CARS spectroscopy. Due to its ~100% energy efficiency, sensitivity at this record high acquisition rate is even higher than conventional DC-CARS spectroscopy operating at a slower acquisition rate of 10,000 spectra/s. Our method holds promise for diverse applications in fields ranging from materials science to life science, such as high-throughput screening, flow cytometry, and live-cell imaging.