Time-correlated Raman and fluorescence lifetime spectroscopy at megahertz repetition rates

Raman spectroscopy is a hugely informative tool with a plethora of applications from biomedicine to analytical chemistry. Potentially, the technique could improve liver transplantation success rates through investigating Raman signals associated with metabolic changes prior to transplant rejection. However, studying biological systems is challenging since background fluorescence dominates the weak Raman signal. Thus, there is a need to improve signal-to-noise and Raman-to-fluorescence ratios and drive down spectral acquisition times. Pulsed lasers combined with time-resolving single photon avalanche diode (SPAD) detection systems have been shown to enhance Raman and fluorescence discrimination. We report significant advances in time-correlated single photon counting (TCSPC) Raman spectroscopy using a laser exhibiting up to 200 W peak power and 40 MHz repetition rates in combination with a 512 spectral channel, 16.5 gigaevent/s throughput SPAD histogramming line sensor. Using a diamond sample, we report 0.4 MHz Raman count rates, millisecond spectral acquisition times, and signal-to-noise ratios of over 200. We demonstrate simultaneous, single-exposure acquisition of Raman and fluorescence signals in sesame oil. Time-based Raman-fluorescence discrimination techniques are subject to fluorescence signal tail influences from previous pulses, and data obtained with laser periods of 25 ns and 50 ns are presented. We achieved optimised Raman-to-fluorescence ratios through adjustment of histogram bin positions in 63 ps increments. Achieving high count rates while discriminating fluorescence from Raman signals unlocks the potential of combined Raman/fluorescence lifetime spectroscopy for biomedical imaging applications.