We derive the markovian description for the nonequilibrium brownian motion of a heated nanoparticle in a simple solvent with a temperature-dependent viscosity. Our analytical results for the generalized fluctuation-dissipation and Stokes-Einstein relations compare favorably with measurements of laser-heated gold nanoparticles and provide a practical rational basis for emerging photothermal tracer and nanoparticle trapping and tracking techniques.

We introduce a novel concept for optical correlation spectroscopy of non-fluorescent absorbing nanoparticles termed twin-focus photothermal correlation spectroscopy (Twin-PhoCS). This method exhibits a unique axial sensitivity, allowing for either the measurement of very slow axial dynamics or the determination of axial heterogeneities on sub-diffraction limited length-scales. To this end, the photothermal nano-lensing mechanism [Markus Selmke, Romy Schachoff, Marco Braun, and Frank Cichos, ACS Nano, 2012, 6(3), 2741] provides two sharply separated detection-subvolumes, comprising the twin-focus. The achieved axial sensitivity is superior to single detection-volume or dual-focus based fluorescence correlation spectroscopies. We demonstrate the sensitivity by measuring radiation-pressure induced flow velocities of diffusing 14 nm gold-colloids down to 10 nm ms 21 with the help of the provided analytical correlation functions.

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Photothermal correlation spectroscopy (PhoCS) is a powerful counterpart to fluorescence correlation spectroscopy (FCS). Using PhoCS it is possible to probe the dynamics of non-fluorescent and non-bleaching ultra-stable metal-nanoparticles in solution and biological specimen, where they can be used as tracers and markers. This paper complements the absorption correlation method by a histogram analysis framework, the photothermal signal distribution analysis (PhoSDA). It is hereby possible to extract individual absorbent tracer concentrations, size dispersions, heterogeneous populations and focal geometry parameters which are otherwise inaccessible by correlation analysis.