Mechanisms of Epi-Detected Stimulated Raman Scattering Microscopy

Epi-detection is indispensible for stimulated Raman scattering (SRS) imaging of opaque samples. We present an analysis of the mechanisms underlying the generation of epi-detected SRS signals. By study of forward- and epi-detected SRS signals from pharmaceutical films of controlled thickness and scatterer density, we show that the epi-detected SRS signal arises from the backscattering of the forward-propagating probe photons. Furthermore, we show that both forward- and epi-detected SRS signal intensity linearly depends on the local oscillator power at the detector at the same slope. Under the same excitation conditions, stimulated Raman loss produces a larger epi-detected signal than stimulated Raman gain due to the larger scattering cross section of light with a shorter wavelength.

I. Introduction

As a nonlinear vibrational imaging technique developed very recently [1]–[5], stimulated Raman scattering (SRS) microscopy probes the intensity gain in the Stokes beam or the intensity loss in the pump beam. The optical heterodyne detection used in this technique not only amplifies the signal but also renders the SRS intensity linearly proportional to the molecular concentration, thus making it sensitive to detect low-concentration molecules. Unlike the widely used coherent anti-Stokes Raman scattering microscopy [6]–[8] in which the signal contains a nonresonant electronic contribution and a Raman-resonance contribution, the SRS provides identical spectral information as spontaneous Raman scattering. The recent applications in vibrational mapping of biomass conversion, pharmaceutical tablets, and human skin show the potential of SRS microscopy for high-speed chemical imaging based on fingerprint Raman bands [5], [9], [10].