Stimulated Raman spectroscopy for characterizing two-dimensional crystals

Two-dimensional dielectric materials such as hexagonal BN and mica give very weak signals in conventional spontaneous Raman scattering using visible excitation lasers. In this work, we report a micro-spectroscopy setup for stimulated Raman scattering (SRS), which gives a substantial increase in overall sensitivity. Pump and Stokes beams with temporal width of a few ps were prepared by spectrally slicing 200 fs pulses from an optical parametric oscillator. The pulse slicer acting as a narrow band-pass filter consisted of a grating (1200 grooves/mm), a cylindrical lens, a mirror, and a mechanical slit that were arranged in the 4f-configuration. Whereas the Stokes pulses were fixed at 1040 nm with a bandwidth of 0.7 nm, the pump pulses were varied from 734 nm to 998 nm for the Raman shift in the range of 400 ~ 4000 cm^-1. The pump beam intensity-modulated by interaction with the Stokes beam in thin hexagonal BN crystals was detected with a photodiode connected to a lock-in amplifier. The sensitivity of the SRS setup and its spatial resolution in raster-scan imaging will be presented.