High Spectral Resolution Chirp Modulation Stimulated Raman Scattering Microscopy

Stimulated Raman scattering (SRS) microscopy enables rapid, chemical-specific, label-free imaging and is a powerful technique for bioimaging and material characterization. In spectral focusing SRS, the two input laser pulses are linearly chirped and delay-scanned, providing rapid tunability while preserving Raman spectral resolution. Conventional linear modulation-transfer detection schemes commonly used in SRS, however, can be challenged by non-Raman backgrounds. Leveraging the quadratic phase control inherent to spectral focusing SRS enables Chirp modulation (CM)-SRS, a modulation transfer technique based upon modulating only the sign of the linear chirp. CM-SRS cancels all non-Raman signals while retaining a quantitative, highly sensitive Raman response. Motivated by these advantages, we designed and implemented an updated CM-SRS setup optimized for maximal background subtraction. Achieving both high spectral resolution and quality chirp matching in spectral focusing CM-SRS requires fine control over the pulse chirps at the microscope sample plane. To this end, we developed Fringe-averaged collinear Frequency-resolved optical gating (FRACOL-FROG), a robust method for in situ characterization of the input laser fields. Together, these advances position our CM-SRS microscopy system to deliver high-resolution, background-free, chemical-specific, and high-sensitivity measurements across a range of important samples previously challenged by background signals.