Nonlinear optical absorption in nanoscale films revealed through ultrafast acoustics

TL;DR

This paper introduces a new ultrafast acoustics method to measure nonlinear optical absorption in nanoscale films, demonstrated on quinacridone, revealing multi-photon absorption processes and enabling characterization of challenging materials.

Contribution

The study presents a novel spinning pump-probe photoacoustic technique for quantifying nonlinear optical absorption in thin films, especially useful for nanoscale and opaque materials.

Findings

Quinacridone exhibits quadratic and cubic laser fluence dependence.

The technique can detect two- and three-photon absorption processes.

Applicable to materials difficult to characterize with traditional methods.

Abstract

Herein we describe a novel spinning pump-probe photoacoustic technique developed to study nonlinear absorption in thin films. As a test case, an organic polycrystalline thin film of quinacridone, a well-known pigment, with a thickness in the tens of nanometers range, is excited by a femtosecond laser pulse which generates a time-domain Brillouin scattering signal. This signal is directly related to the strain wave launched from the film into the substrate and can be used to quantitatively extract the nonlinear optical absorption properties of the film itself. Quinacridone exhibits both quadratic and cubic laser fluence dependence regimes which we show to correspond to two- and three-photon absorption processes. This technique can be broadly applied to materials that are difficult or impossible to characterize with conventional transmittance-based measurements including materials at the nanoscale, prone to laser damage, with very weak nonlinear properties, opaque or highly scattering.