Development of a new method of twe-photon absorption measurement and study of electronic structure of polydiacetylene crystals

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1. A sensitive and direct method Of two-photon absorption by utilizing two cw mode-locked dye-lasers was developed for spectroscopic use. It is found that the pump and probe method of the picosecond pulses with combined use of a high-frequency lock-in detection works well enough to observe the exciton lines in SnO_2 with the pump power Of 100 W.2. Feasibility of a similar, highly sensitive method with the use of a cw mode-locked Nd-YAG laser and a dye-laser was also investigated. As the result, the present system was found to be useful enough to determine the nature as well as location of the lowest direct band gap of TiO_2.3. We studied the two-photon absorption spectrum in a polydiacetylene crystal, TCDU-B to clarify the second-lowest, exciton, if any, of Ag symmetry. The spectrum was interpreted as arising mostly from imperfection, though in some crystals we observed a peak at 2.84 eV due presumably to the exciton.4. We found that an emission line could be observed in a PTS crystal when two-photon-excited with the energy 2.32 eV. This fact strongly suggests that the Ag exciton might exist.5. We succeeded in observing the two-photon Raman signal from a PTS crystal under the resonant condition, also supporting the above as well as providing information about ungerade phonons.