Recent advances in the spectroscopies of the second order processes both in the optical region and in the soft x-ray region are reviewed from a theoretical standpoint. A peculiar role of the breathing and oscillating spectral hole in the femtosecond pump-probe experiments is pointed out. The resonant x-ray emission spectrum in diamonds at the core-exciton excitation is analyzed by a simple model, and it is shown that a ultrafast off-center displacement occurs in the core-excited state.
Wide Frequency Tuning of Continuous Terahertz Wave Generated by Difference Frequency Mixing under Exciton-Excitation Conditions in a
GaAs
/
AlAs
Multiple Quantum Well