Time-resolved nonlinear infrared spectroscopy of samarium ions in SmFeO3

We demonstrate that resonant 32.3 THz pumping of f-f transitions in the samarium ions in SmFeO${}_{3}$ leads to a nonlinear regime of radiation-matter interaction. The nonlinearity arises from the photoinduced population of the excited state, the dynamics of which was studied in a pump-probe experiment. The measurements have been performed in the spectral range near the ${}^{6}{H}_{\frac{5}{2}}{\phantom{\rule{0.16em}{0ex}}}^{6}{H}_{\frac{7}{2}}$ electronic transition. The observations show dynamics with a lifetime of 4 ps. The sign of the dynamics surprisingly differs in different spectral ranges: This phenomenon is interpreted as an excitation-induced shift of the spectral line. The results are described by the nonlinear optical polarization using the optical Bloch equations for an ensemble of two-level systems.