  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.
