Optimal coherent control of CARS: signal enhancement and background elimination

TL;DR

This paper develops a method to optimize pulse shaping in CARS to maximize signal enhancement while minimizing background noise, using analytical and numerical techniques for specific pulse configurations.

Contribution

It introduces a pulse shaping strategy that optimally enhances resonant signals and suppresses background in CARS, including analytical solutions and numerical simulations.

Findings

Optimal probe pulse shaping improves signal-to-background ratio.

Superposition of linear and arctangent phases approximates optimal pulses.

Probe delay remains a quasi-optimal background suppression method.

Abstract

The ability to enhance resonant signals and eliminate the non-resonant background is analyzed for Coherent Anti-Stokes Raman Scattering (CARS). The analysis is done at a specific frequency as well as for broadband excitation using femtosecond pulse-shaping techniques. An appropriate objective functional is employed to balance resonant signal enhancement against non-resonant background suppression. Optimal enhancement of the signal and minimization of the background can be achieved by shaping the probe pulse alone while keeping the pump and Stokes pulses in transform-limited-form (TLF). In some cases analytical forms for the probe pulse can be found, and numerical simulations are carried out for other circumstances. It is found that a good approximate solution for the optimal pulse in the two-pulse CARS is a superposition of linear and arctangent type phases for the pump. The well-known probe delay method is shown to be a quasi-optimal scheme for background suppression. The results should provide a basis to improve the performance of CARS spectroscopy and microscopy.