lASER FORCE SPECTROSCOPY OF SINGLE NANO-ASSEMBLIES

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A novel method of dynamic absorption spectroscopy with using laser radiation pressure has been developed for analyzing photophysical phenomena of single nano-assemblies with high sensitivity and high precision. This method is based on a fact that the radiation pressure induced by irradiation of laser light depends on the refractive index and the absorption coefficient (complex refractive index) of the assembly. That is an original idea that absorption is measured through conversion into an optical force, instead of the change in the transmission light intensity. Thus, there is no influence of scattering and stray lights, and high sensitivity can be obtained for linear absorption measurement, those are advantages to the conventional spectroscopy techniques. In the method, our original technique that makes it possible to directly measure an extremely weak force exerted on a Brownian nano-assembly by statistical mechanics is used. This technique can realize femto-Newton order force observation so that assemblies with absorption cross sections of square Angstrom could be analyzed. We have developed a laser force spectroscopy system with a laser-diode-pumped Nd : YAG laser and originally designed microscope optics. Theoretical analyses and experimental date obtained for dye-doped polymer latexes demonstrated the effectiveness of the method, and we discussed on the expectation of sensitivity and precision in the present technique.