Nonlinear spectroscopy of YBaCuO and Ni thin films by a biharmonic pumping technique

High temperature superconducting (HTSC) (YBaCuO, critical temperature T c=87 K) and metal (Ni) thin films are investigated by a picosecond spectrometer with two tunable dye lasers. The selfdiffraction process efficiency η versus the biharmonic pumping component detuning Ω has been measured at temperatures Θ=300 and 80 K. For the HTSC films at Θ=80 K a well-defined dip on the disperion curve η(Ω) for − 10 > Ω > −50 cm −1 is found. The upper limit of the region of this dip corresponds to a value of 2Δ of the superconducting energy gap. At 10