  The photothermal heterodyne imaging technique enabled studies of individual
weakly absorbing nano-objects in various environments. It uses a photo-induced
change in the refractive index of the environment. Taking advantage of the
dramatic index of refraction change occurring around a thermotropic liquid
crystalline phase transition, we demonstrate a 40-fold signal-to-noise ratio
enhancement for gold nanoparticles imaged in 4-Cyano-4'-pentylbiphenyl (5CB)
liquid crystals over those in a water environment. We studied the photothermal
signal as a function of probe laser polarization, heating power, and sample
temperature quantifying the optimal enhancement. This study established
photothermal microscopy as a valuable technique for inducing and/or detecting
local phase transitions at the nanometer scales.
