INVESTIGATION OF EXCITONIC STATES CREATED BY TWO-PHOTON PAIRS IN PARAMETRIC DOWN CONVERSION.

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A nonlinear crystal pumped by laser light emits highly correlated photon pairs with parametric down conversion (PDC) processes. Excitonic state in a solid excited by these PDC photon pairs is thought to be different from those excited by normal two photons. We intended to study the phenomena in TlBr crystals doped with I^- ions.・We studied photo-luminescence due to the excitons in TlBr(I) under one photon excitation by a pulse Ti:S laser (455nm, 10ns) at different temperatures. Two broad emission bands were observed at 2.2 eV and 2.45 eV. Both emission bands decayed non-exponentially above 10 K while those decayed exponentially at 2 K. Analyzing their decay properties, these emission bands are ascribed to the two types of extrinsic self-trapping of the excitons.・We measured photo-luminescence of the excitons in TlBr(I) under normal two-photon excitation process. Two light sources were used : the fundamental of a Ti:S laser pumped by a pulsed Nd:YAG laser and that of a mode-locked Ti:S laser, whose pulse widths were 10ns and about 100fs, respectively. The results show little differences in the luminescence spectra under the one-photon excitation and the two-photon one.・We tried to observe the exciton luminescence under the excitation by entangled photon pairs of PDC light. The second harmonic of the ML Ti:S laser was fed to a nonlinear crystal BBO of Type II in the collinear configuration. The down converted photon pairs were found to be so small that we could not observe the exciton luminescence. Then the BBO crystal was excited by the second harmonic of the Ti:S laser whose pulse width was 10ns. The PDC light was seen to be stronger than that induced by the ML Ti:S laser. We will investigate the exciton luminescence in TlBr(I) under the excitation by entangled photon pairs of this PDC light. In addition we want to improve the output power of the ML Ti:S laser.