Broadband pump-probe spectroscopy at 20-MHz modulation frequency with fourier-transform detection

Fourier-transform (FT) spectrometers [1] are widely employed in the infrared spectral region (FTIRs) due to the lack of cheap multiplex detectors. They use an interferometer (usually a Michelson) to create two time-delayed replicas of the incoming light. A single-pixel detector then records an interferogram (i.e. the overall incident energy as a function of the optical path difference)· whose FT directly provides the light spectrum. The extension of FT spectrometers to the visible/UV wavelengths calls for complicated setups to guarantee a sufficient positioning accuracy of the interferometer (typically ∼λ/100). To this purpose, we have recently introduced a passive common-path ultra-stable interferometer based on birefringence (called TWINS) with exceptional (<λ/300) path-length stability and reproducibility. We have successfully employed it for linear spectroscopy, realizing a visible/near-IR spectrometer and spectrophotometer over one octave of bandwidth [2] and for non-linear spectroscopy, performing Stimulated Raman Scattering (SRS) spectroscopy and microscopy [3].