Intrinsic backward emission of coherent Raman photons from homogeneous media

Abstract

In homogeneous materials, scattered light propagates predominantly in the forward direction; thus, collecting signal on the illumination side often relies on interfaces, disorder or multiple scattering. Nonlinear wave mixing can relax this constraint by combining the directions of the driving fields. If the generated wavelength is longer than the driving wavelengths, intrinsic backward emission can become efficient. Here we exploit this regime using coherent Stokes Raman scattering to generate signal directly into the backward hemisphere. Simulations pinpoint high-numerical aperture geometries and polarization states that maximize backward emission, and experiments confirm backward signals from liquids and tissues with detected levels approaching the strength of forward-detected coherent Raman signals. Because the generated signal originates at the focus and remains refocusable, it enables coherent Raman microscopy with same-side confocal collection compatible with fibre coupling and spectrometers when transmission access is impractical. The approach is particularly relevant and beneficial for samples on opaque substrates, thick specimens, tissue biopsies and fibre- or spectrometer-based implementations.