Noise auto-correlation spectroscopy with coherent Raman scattering

TL;DR

This paper introduces a novel coherent Raman spectroscopy method that uses deliberately randomized noise in laser pulses to enhance spectral resolution and robustness, especially useful in scattering media.

Contribution

The authors present a new noise auto-correlation spectroscopy technique that improves spectral resolution and robustness without complex pulse shaping or scanning.

Findings

Spectral resolution is independent of pulse bandwidth.

The method is robust against medium dispersion and scattering.

It enables efficient molecular vibration probing with noisy broadband pulses.

Abstract

Ultrafast lasers have become one of the most powerful tools in coherent nonlinear optical spectroscopy. Short pulses enable direct observation of fast molecular dynamics, whereas broad spectral bandwidth offers ways of controlling nonlinear optical processes by means of quantum interferences. Special care is usually taken to preserve the coherence of laser pulses as it determines the accuracy of a spectroscopic measurement. Here we present a new approach to coherent Raman spectroscopy based on deliberately introduced noise, which increases the spectral resolution, robustness and efficiency. We probe laser induced molecular vibrations using a broadband laser pulse with intentionally randomized amplitude and phase. The vibrational resonances result in and are identified through the appearance of intensity correlations in the noisy spectrum of coherently scattered photons. Spectral resolution is neither limited by the pulse bandwidth, nor sensitive to the quality of the temporal and spectral profile of the pulses. This is particularly attractive for the applications in microscopy, biological imaging and remote sensing, where dispersion and scattering properties of the medium often undermine the applicability of ultrafast lasers. The proposed method combines the efficiency and resolution of a coherent process with the robustness of incoherent light. As we demonstrate here, it can be implemented by simply destroying the coherence of a laser pulse, and without any elaborate temporal scanning or spectral shaping commonly required by the frequency-resolved spectroscopic methods with ultrashort pulses.