Time-resolved photon counting Fourier-transform micro-spectroscopy enables simultaneous Raman and fluorescence lifetime imaging

To address the current limitations of time-gated Raman spectroscopy, specifically its narrow spectral range and low spectral resolution, and simultaneously acquire Raman and fluorescence life-time images, we have developed a Fourier-transform photon counting spectroscopy platform. A Mach-Zehnder interferometer employing a high accuracy linear motor stage was combined with photon-counting avalanche diodes and time-tagged acquisition, allowing to sort photons into a matrix of stage positions determined using their coarse arrival time with 50 ns steps of the excitation laser repetition period, and a fine arrival time of 80 ps resolution relative to the excitation pulse of 100 ps duration. The instrument achieves a time resolution of 547 ps, a wide spectral range of -1000 to 10,000 cm^-1 Raman shift from the excitation at 532 nm wavelength, and a high spectral resolution of 0.05 cm^-1. For experimental validation, we used fluorescently coated silicon wafers and fluorescent plastic microspheres. Raman signal was observed during the laser excitation pulse within the time-resolution, while fluorescence signals dominate afterwards. The results confirm that the instrument can effectively separate Raman and fluorescence signals.