NONLINEAR SPECTROSCOPY OF Y-BA-CU-O NONEQUILIBRIUM STATES EXCITED BY PICOSECOND OPTICAL-PUMPING

Nonequilibrium states of HTSC Y-Ba-Cu-O films with a thickness of 300 nm at the temperatures THETA = 80-85 K below the phase transition poin t THETA(c) congruent-to 91 K have been investigated by a biharmonic pu mping (BP) technique. It has been found that as a result of optical ex citation by picosecond laser pulses with the energy density of 10(-4)- 10(-3) J/cm2 at the wavelengths of 1.064, 0.532 and 0.620 mum, thermod ynamic equilibrium is attained only after a time delay of more than 1 ns. Both for the above-specified case and for the case with the equili brium superconductive phase at THETA < THETA(c), we revealed the same specific dip in the self-diffraction efficiency as a function of the B P frequency detuning OMEGA in the spectral range -10 cm-1 > OMEGA> -10 0 cm-1. This dip conservation has been interpreted as a manifestation of a rather slow phase transition being responsible for the pairing of carriers.