Stimulated Raman spectroscopy for two-dimensional dielectric crystals

Spontaneous Raman spectroscopy of two-dimensional (2D) dielectric crystals is often challenged by weak signals because the crystals have a small Raman cross-section. In this work, we report a micro-spectroscopy setup for stimulated Raman scattering (SRS) of dielectric 2D crystals. Coherent nonlinear scattering gives SRS a substantial increase in overall sensitivity in addition to avoiding non-resonant background signals, unlike coherent anti-stokes Raman scattering. The spatial and temporal overlap of ps pump and Stokes beams were optimized for graphene using transient reflectance signals epi-collected with a microscope objective. The Raman loss or change in the pump beam was lock-in-detected with a fast photodiode while the Stokes beam was modulated at 20 MHz. SRS was verified with respect to the spontaneous Raman spectrum of hexagonal BN and further optimized to improve sensitivity. As quasi-2D model systems, thin crystals of hexagonal BN and calcium hydroxide were prepared and probed in ~1370 cm^-1 and 3650 cm^-1, respectively. We will discuss the results in comparison with their spontaneous Raman spectra and instrumental refinements to reach a 2D limit.