Study of the coherent states in two-dimensional dye aggregates

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The purpose of this study was to gain the molecular picture of the two-dimensional excitonic states in dye aggregates. For this purpose, we have performed the following experiments :(1) We established the preparation techniques for J- and H-aggregates. For the former, adsorption from solution produced most desirable results whereas, for the latter, Langmuir-Blodgett technique worked best. (2) We made a cryostat equipped with a microscope which enabled us to do spectroscopy from 0.02 mm diameter area in a monolayr. (3) We performed absorption and emission spectroscopy from liquid He temperature to room temperature. (4) We studied harmonic generation from H-aggregate. (5) The fluorescence lifetime was measured with a streak camera. (6) A preliminary study was done using a pump-and-probe method.Our findings are as follows :(1) The spectroscopic data can be perfectly described by the Frenkel exciton picture modified with the static disorder and phonons. (2) The dimensionality does not appear qualitatively in the optical properties. (3) The fluorescence lifetime is extremely fast, which seems to be the manifestation of the large number of coherently excited molecules, a qualitative signature of two-dimensionality. (4) The local-field approximation well describes the second-harmonic generation process in H-aggregate, i.e.the coherency does not affect the harmonic generation.