Measurement of second-order response without perturbation

TL;DR

This paper experimentally confirms a method to measure second-order response functions of a colloidal particle in an anharmonic potential without applying external perturbations, matching traditional response measurements.

Contribution

It validates a recent theoretical approach for obtaining second-order susceptibilities solely from equilibrium trajectories, avoiding external perturbations.

Findings

Measured susceptibilities agree with those from external perturbation methods.

Confirmed the validity of a perturbation-free approach for second-order response measurement.

Demonstrated the method on colloidal particles in anharmonic potentials.

Abstract

We study the second order response functions of a colloidal particle being subjected to an anharmonic potential. Contrary to typical response measurements which require an external perturbation, here we experimentally confirm a recently developed approach where the system's susceptibilities up to second order are obtained from the particle's equilibrium trajectory [PCCP $\mathrm{{\bf 17}}$, 6653 (2015)]. The measured susceptibilities are in quantitative agreement with those obtained from the response to an external perturbation.