Quantification of Ultrafast Nonlinear Photothermal and Photoacoustic Effects in Molecular Thin Films via Time-Domain Brillouin Scattering

TL;DR

This study introduces a novel time-domain Brillouin scattering method to quantify ultrafast nonlinear photothermal and photoacoustic effects in molecular thin films, aiding the development of NIR contrast agents for biomedical imaging.

Contribution

It presents an innovative experimental technique to measure nonlinear PT/PA effects in ultrathin films, specifically using femtosecond laser pulses and two-photon absorption.

Findings

Determined nonlinear absorption coefficients of a model organometallic compound.

Validated the use of time-domain Brillouin scattering for ultrafast nonlinear optical studies.

Provided insights for optimizing nonlinear PT/PA contrast agents.

Abstract

Improving the efficiency of photothermal (PT) therapies and photoacoustic (PA) imaging at the microscopic scale hinges on developing multiphoton-absorbing photothermal molecules or contrast agents that operate in the near-infrared (NIR) range. These advanced agents or molecules will enable excitation with NIR lasers, in an improved transparency range for biological tissues, while enabling minute, highly localized spatial control of the excitation area. However, progress in this field requires innovative experimental techniques to characterize photothermal and photoacoustic effects under multiphoton excitation. In this article, we showcase a study of a model organometallic molecular compound excited via two-photon absorption (2PA) using femtosecond laser pulses. Based on a time-domain Brillouin scattering technique, well adapted for investigating ultrafast nonlinear optical absorption processes in ultrathin films on substrates, we determine the effective nonlinear absorption coefficients of the compound directly linked to PT/PA. Our findings provide a practical approach for exploring and optimizing nonlinear PT/PA absorbers and contrast agents.