Spectral, temporal and temperature features of the nonlinear response of high-temperature superconductors in transient nonlinear spectroscopy

It is shown that basic properties of the nonlinear response of high-temperature superconductors (HTSCs) observed in femtosecond and picosecond pump–probe experiments at high and low pump levels in various variants of the pump–probe spectroscopy, including one- and two-photon excited-state probing, can be interpreted by using two assumptions. The spectral and temperature properties of the HTSC response at low pump levels can be explained taking into account the contributions from interband electronic transitions to the dielectric constant. At the same time, drastic variations in the HTSC response kinetics (temporal features) observed at high pump levels (for a typical pump pulse energy of ~10^-7 J in a focal spot of diameter 150 μm) can be explained by assuming the existence of a 'frozen' (metastable) energy gap in the electronic spectrum of a HTSC. In this case, all the conditions required for the interpretation of a drastic decrease in the relaxation rate of a nonlinear response (degeneracy) are realised due to the specific distribution of the electronic state density immediately after the formation of the energy gap in the electronic spectrum of the HTSC.