Ultrabroadband time-domain Raman spectroscopy using synchronized mode-locked lasers

Abstract

High-speed Raman spectroscopy has enabled label-free characterization of molecules in cells and materials in a space- and time-resolved manner. Among these, time-domain Raman spectroscopy (TDRS) techniques, such as Fourier-transform coherent anti-Stokes Raman scattering (FT-CARS) and impulsive stimulated Raman scattering (ISRS) spectroscopies, have unique capabilities such as high spectral acquisition rates, broadband spectral sensitivity in the fingerprint region, and nonresonant-background-free spectral acquisition. With a few exceptions, most TDRS studies have focused only on the fingerprint region (200 – 1800 cm-1) because the ultrashort pulses typically used for ultrabroadband (200 – 3200 cm-1) spectral acquisitions are difficult to generate and handle. For example, detecting Raman peaks above 3000 cm-1 necessitates a pulse duration of < 10 fs, which demands an expensive laser source and careful dispersion control. Furthermore, with sub-10-fs pulses, Raman detection sensitivity in the fingerprint is compromised because the spectral power density is diluted in the spectrally broad ultrashort pulse. The present research demonstrates FT-CARS spectroscopy covering both the fingerprint and CH-stretching regions by employing synchronized mode-locked Ti:Sapphire and Yb-doped fiber lasers as the light source. With this method, we show that ultra-broadband FT-CARS spectra can be obtained without using sub-10-fs pulses, which significantly mitigates experimental complexity. More importantly, ultra-broadband Raman detection can be achieved in this scheme without compromising the sensitivity in the fingerprint region, unlike previous ultrashort-pulse approaches. The present method will significantly broaden the application range of TDRS for biomedical and material science research.