Nonlinear spectroscopy of Y-Ba-Cu-O nonequilibrium states excited by picosecond optical pumping

Abstract

Nonequilibrium states of HTSC Y-Ba-Cu-O films with a thickness of 300 nm at the temperatures Θ=80–85 K below the phase transition point Θ c ≅91 K have been investigated by a biharmonic pumping (BP) technique. It has been found that as a result of optical excitation by picosecond laser pulses with the energy density of 10 −4-10 −3J/cm 2 at the wavelengths of 1.064, 0.532 and 0.620 μm, thermodynamic 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 equilibrium superconductive phase at Θ<Θ c, we revealed the same specific dip in the self-diffraction efficiency as a function of the BP frequency detuning Ω in the spectral range - 10 cm −1 > Ω > − 100 cm −1. This dip conservation has been interpreted as a manifestation of a rather slow phase transition being responsible for the pairing of carriers.