Giant two-photon absorption spectrum for the direct generation of excitonic molecules is measured with using a frequency tunable dye laser, and absorption peak is found at 3.1870 eV. Emission spectrum with excitation at the giant two-photon absorption band depends on the spectral width,
ΔΩ1, of excitation light. In the case of
ΔΩ1=0.25 meV, the two-photon resonance Raman scattering is found to be predominant. The Raman process involves an excitonic molecule and a longitudinal exciton as the intermediate and final states, respectively. On the other hand, for the case of
ΔΩ1=2.3 meV, the emission spectrum shows a very sharp line at 3.1649 eV, which has been previously ascribed to be due to the Bose-Einstein condensation of excitonic molecules. Discussions are made on the dependence of emission spectrum upon the energy band-width of excitation light.
A calculation is made of the stimulated emission threshold of organic dye solutions as a function of the diffusion coefficient, intensity of induced singlet–singlet or triplet–triplet absorption at the stimulated emission wavelength, steepness of the leading edge of pump pulses, and other parameters. It is shown that, if the dipole moment of a fluorescence transition is perpendicular to the dipole moments of associated induced singlet–singlet or triplet–triplet absorption transitions, the laser threshold of high–viscosity solutions is considerably lower than that of low-viscosity systems. The maximum value of the ratio of the cross sections of the singlet-singlet absorption and stimulated emission, at which a given molecule ceases to emit stimulated radiation, increases from 1 to 3 when the rotational diffusion coefficient decreases from ∞ to 0.
The problem about two-photon induced fluorescence of organic dye active molecules located in spherical transparent microparticles at the ultrashort laser pulses influence was theoretically investigated. The system of kinetic equations for laser generation was written taking into account stimulation transitions. Generation regimes of stimulated radiation were found. Resonator properties of spherical microparticles were theoretically investigated using the method of expanding optical fields into series of eigenfunctions. The breakdown of the beginning of stimulated radiation in the microparticle was determined. It was shown that stimulated radiation intensity depends on degree of its spatial overlap with all other modes of the pump radiation. The interpretation of experimental data of two-photon induced fluorescence in the particles with organic dye was carried out.