Self-suppression of the Giant CARS Background for Detection of Buried Interface with Sub-monolayer Sensitivity

TL;DR

This paper introduces a novel CARS technique that significantly suppresses bulk background signals, enabling detection of buried interfaces with sub-monolayer sensitivity and high signal-to-noise ratio.

Contribution

The authors develop a self-suppression scheme for CARS that reduces bulk background by over 10^5 times using interference near the Brewster angle, allowing interface analysis with unprecedented sensitivity.

Findings

Achieved at least 10^5-fold suppression of bulk background in CARS.

Enabled vibrational spectrum resolution of sub-monolayer interfacial species.

Potential to improve nonlinear optical spectroscopy signal-to-background ratio.

Abstract

The past decades have witnessed marked progresses on the research of interfacial science in complex systems promoted by the advances in novel experimental techniques. Despite its success in many fields, implementation of coherent anti-Stokes Raman spectroscopy (CARS) for tackling the problems at interfaces was hindered by the huge resonant and non-resonant background from the bulk. Here we have developed a novel CARS scheme that is capable of probing a buried interface via suppression of the non-resonant and resonant bulk contribution by at least $10^5$ times. The method utilizes self-destructive interference between the forward and backward CARS generated in the bulk near the Brewster angle. As a result, we are able to resolve the vibrational spectrum of sub-monolayer interfacial species immersed in the surrounding media with huge CARS responses. We expect our approach not only opens up the opportunity for interrogation of the interfaces that involve apolar molecules, but also benefits other nonlinear optical spectroscopic techniques in promoting signal-to-background noise ratio.