Novel time-resolved CARS implementation for application in microscopy

Abstract

Vibrational dephasing times for benzene and carbon disulfide are measured using a custom single-beam Coherent Anti-stokes Raman Spectroscopy (CARS) setup. A femtosecond oscillator is used to pump a Polarization Maintaining All Normal Dispersion Photonic Crystal Fibre (PM-ANDi-PCF) to generate a broad band supercontinuum, covering a spectral region from 680 – 900 nm. The dispersion properties of the PM-ANDi-PCF ensures the supercontinuum is stable and there exists a fixed phase relationship between the spectral components of the supercontinuum. This enables its temporal compression using i2PIE, implemented using a liquid crystal spatial light modulator (SLM) in a 4f geometry. This SLM is also used to shape the pulse spectrally and temporally. With this setup we could demonstrate time-resolved CARS, measuring the vibrational relaxation times of a CS2/benzene mixture, and eliminate the non-resonant background completely. The main advantage of this setup is the fact that it is a single beam technique, eliminating the requirement for aligning the overlap of the pump and probe, both spatially and temporally, in the focal plane of the microscope. The strengths and limitations of the technique are highlighted and the route to time-resolved/background free vibrational microscopy is proposed.