2.1. SRS Imaging Setup

Our SRS imaging system consists of a dual output femtosecond (80 MHz repetition rate) laser (InSight DS+, MKS) with tunable (680 to 1300 nm) and fixed (1040 nm) outputs, a delay line to attain and adjust beam overlap, and a custom-built laser scanning microscope.²⁶ Spectral focusing is achieved using compact adjustable-dispersion TIH53 glass blocks²⁷ to chirp the femtosecond pulses into the picosecond domain, at 2.06 ps and 1.1 ps for pump and Stokes, respectively, and adjusting pulse overlap with the delay line, allowing for an ∼28cm−1 spectral resolution. We measured the stimulated Raman loss (SRL) signal using lock-in detection with the Stokes beam modulated by an acousto-optic modulator (1205C-2, Isomet) at 4.9 MHz. The pump beam intensity was detected by a 1×1cm photodiode (FDS1010, Thorlabs) biased to 60 V, filtered through a 2 to 7 MHz electronic bandpass filter, and delivered to a lock-in amplifier (MFLI, Zurich instruments) to extract the SRL signal. A pixel dwell time of 20μs was used for the SRS image. ScanImage²⁸ (Version 5.6, Vidrio Technologies) was used for control of the laser scanning and image acquisition.