High‐sensitivity impulsive stimulated Raman spectrometer with fast data acquisition

Impulsive stimulated Raman scattering (ISRS) is a nonlinear coherent technique for the measurement of the vibrational spectrum of samples within the bandwidth of the excitation laser pulse, which, for pulse durations of several tens of femtoseconds, translates into easy access to the low-wavenumber Raman spectrum (below few hundreds of cm(-1)). Recently, a degenerate ISRS Fourier-transform Michelson spectrometer for measurements in bulk, transparent liquids operating with a single, nonamplified femtosecond laser source, was demonstrated, although with low measurement speed, exploiting a position-sensitive detector to measure the time evolution of the tiny spectral asymmetries of the probe beam produced by stimulated Raman gain and loss. Building on this innovation of the detection method, in this work, we show that using a different data acquisition concept, based on the fast modulation of the delay in one of the spectrometer arms, it is possible to measure scattering and absorbing samples in a much shorter time, approaching the real-time visualization of the Raman spectrum of the sample under analysis. Furthermore, the setup described here does not suffer from creation of microbubbles, heating effects or thermal instabilities produced in the sample by increasingly high excitation fluences, and is therefore suitable to be implemented in vibrational microscopy applications with tight beam focusing. Thus, the present work is a contribution toward practical implementation of low-wavenumber vibrational imaging, which is of importance for studies of intramolecular vibrational modes of large molecules or of heavy functional groups or for the study of intermolecular force constants in biological and nanostructured materials.