Photothermal
perturbation
of the cell membrane is typically achieved
using transducers that convert light into thermal energy, eventually
heating the cell membrane. In turn, this leads to the modulation of
the membrane electrical capacitance that is assigned to a geometrical
modification of the membrane structure. However, the nature of such
a change is not understood. In this work, we employ an all-optical
spectroscopic approach, based on the use of fluorescent probes, to
monitor the membrane polarity, viscosity, and order directly in living
cells under thermal excitation transduced by a photoexcited polymer
film. We report two major results. First, we show that rising temperature
does not just change the geometry of the membrane but indeed it affects
the membrane dielectric characteristics by water penetration. Second,
we find an additional effect, which is peculiar for the photoexcited
semiconducting polymer film, that contributes to the system perturbation
and that we tentatively assigned to the photoinduced polarization
of the polymer interface.