Photonic time stretch for broadband stimulated Raman scattering

Abstract

Raman spectroscopy allows for label-free identification of molecules, retrieving their characteristic vibrational fingerprints. It finds applications in many disciplines, such as biomedical sciences [1]. The cross section of spontaneous Raman scattering is inherently very low, thus hampering its use for high-speed microscopy applications. Coherent Raman scattering (CRS) boosts the signal by many orders of magnitude, allowing three-dimensional imaging up to the video rate [2]. Stimulated Raman Scattering (SRS) is a CRS technique requiring two ultrashort laser pulses (called pump and Stokes). In its standard implementation, it probes a single vibrational line, thus providing limited information on the target's chemical composition. Combining a narrowband (picosecond) pump with a broadband (femtosecond) Stokes, on the other hand, it is possible to retrieve the full Raman spectrum at once. In this configuration, called broadband or multiplex SRS, however, it is technically challenging to push the acquisition speed below a millisecond per pixel (or voxel). Therefore, in recent years, there is a great effort in speeding up the acquisition time of broadband SRS [3].