Vibrational phase imaging in wide-field CARS for nonresonant background suppression

Self-Suppression of the Giant Coherent Anti-Stokes Raman Scattering Background for Detection of Buried Interfaces with Submonolayer Sensitivity

Despite its success in many fields, the implementation of coherent anti-Stokes Raman spectroscopy (CARS) in tackling the problems at interfaces was hindered by the enormous resonant and nonresonant background from the bulk. In this work, we have developed a novel CARS scheme that can probe a buried interface via ≥10⁵-fold suppression of the nonresonant and resonant bulk contribution. The method utilizes self-destructive interference between the forward and backward CARS generated in the bulk near the Brewster angle. As a result, we can resolve the vibrational spectrum of submonolayer interfacial polar/apolar species immersed in the surrounding medium with huge CARS responses. We expect that our approach opens up the opportunity to interrogate the interfaces involving apolar molecules and benefits other nonlinear optical spectroscopic techniques, e.g., sum-frequency spectroscopy and four-wave mixing spectroscopy in general, to promote the signal-to-background noise ratio.

Coherent anti-Stokes Raman Fourier ptychography

We present a theoretical and numerical study of coherent anti-Stokes Raman scattering Fourier ptychography microscopy (CARS-FPM), a scheme that has not been considered so far in the previously reported CARS wide-field imaging schemes. In this approach, the distribution of the Raman scatterer density of the sample is reconstructed numerically from CARS images obtained under various angles of incidences of the pump or Stokes beam. Our inversion procedure is based on an accurate vectorial model linking the CARS image to the sample and yields both the real and imaginary parts of the susceptibility, the latter giving access to the Raman information, with an improved resolution.

Three-dimensional wide-field pump-probe structured illumination microscopy

We propose a new structured illumination scheme for achieving depth resolved wide-field pump-probe microscopy with sub-diffraction limit resolution. By acquiring coherent pump-probe images using a set of 3D structured light illumination patterns, a 3D super-resolution pump-probe image can be reconstructed. We derive the theoretical framework to describe the coherent image formation and reconstruction scheme for this structured illumination pump-probe imaging system and carry out numerical simulations to investigate its imaging performance. The results demonstrate a lateral resolution improvement by a factor of three and providing 0.5 µm level axial optical sectioning.