# Probing the fluctuations of the optical properties in time-resolved spectroscopy

## TL;DR

This paper introduces a statistical method in time-resolved spectroscopy to measure the dynamic noise of electronic currents in nonequilibrium materials, revealing insights into thermalization and fluctuation-dissipation relations.

## Contribution

It proposes a novel framework for analyzing optical pulse fluctuations to extract dynamical information about electronic and other degrees of freedom in nonequilibrium states.

## Key findings

- Allows measurement of current noise dynamics in ultrafast experiments
- Reveals how electronic systems obey fluctuation-dissipation theorem during thermalization
- Provides a general approach extendable to magnetic and vibrational excitations

## Abstract

We show that, in optical pump-probe experiments on bulk samples, the statistical distribution of the intensity of ultrashort light pulses after the interaction with a nonequilibrium complex material can be used to measure the time-dependent noise of the current in the system. We illustrate the general arguments for a photo-excited Peierls material. The transient noise spectroscopy allows to measure to what extent electronic degrees of freedom dynamically obey the fluctuation-dissipation theorem, and how well they thermalize during the coherent lattice vibrations. The proposed statistical measurement developed here provides a new general framework to retrieve dynamical information on the excited distributions in nonequilibrium experiments which could be extended to other degrees of freedom of magnetic or vibrational origin.