Nonlinear spectroscopy of narrow-gap Pr–Ba–Cu–O semiconductor films with the aid of biharmonic pumping

The dependences of the efficiency $\eta$ of self-diffraction of picosecond pulses on their frequency detuning $\Omega$ were determined for Pr–Ba–Cu–O semiconductor films. These dependences were similar to those already reported (for Ni and Y–Pr–Ba–Cu–O in the resistive phase) and had a central peak ($\eta\sim10^7$ for $\vert\Omega\vert\leq10$см$^{-1}$) and wide wings ($\eta\sim10^9$ for $\vert\Omega\vert\geq10$см$^{-1}$) with a jagged profile because of phonon resonances. An analysis was made of the causes of this similarity, of the asymmetry of $\eta(\Omega)$ relative to the sign of $\eta$, and of specific temperature effects. Although the position of a combination electron resonance (interband transition edge) of Pr–Ba–Cu–O was within the range of variation of $\eta$, its identification was complicated by the presence of nearby (on the scale of $\Omega$) phonon modes.