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O: Fachverband Oberflächenphysik

O 36: Poster Session II (Organic films and electronics, photoorganics; Nanostructures; Plasmonics and nanooptics, Surface chemical reactions and heterogeneous catalysis, Surface dynamics )

O 36.20: Poster

Tuesday, March 12, 2013, 18:15–21:45, Poster B2

Photothermal Microscopy in Liquid Crystals — •André Heber, Markus Selmke, Marco Braun, and Frank Cichos — Molecular Nanophotonics Group, University of Leipzig, Linnéstraße 5, D - 04103 Leipzig

A weakly fluorescent nano-particle that resonantly absorbs light dissipates its excitation energy into the environment as heat. The resulting temperature profile creates a refractive index profile around the particle which affects the scattering of a second off-resonant laser beam. This principle called photothermal contrast is used for correlation spectroscopy, imaging and absorption spectroscopy of absorbing nano-particles such as metallic nano-particles, quantum dots and single molecules.

Here we employ the technique to study single gold nano-particles in the uniaxial liquid crystal 8CB. It is anisotropic and has phase transitions at ambient temperature which are connected to large jumps in the refractive index. This can enhance the photothermal signal. But the signal depends sensitively on frequency and sample temperature due to the phase transition. The signal anisotropy can be used to measure the local orientation of nematic domains. We analyse the photothermal signal and model it analytically. The results show that photothermal microscopy is an ideal tool to study microscopic phase transitions.