Selective detection of real and imaginary parts of refractive index change in solutions induced by photoexcitation using near-field heterodyne transient grating method

The transient responses in solutions induced by photoexcitation in the nanosecond to millisecond range were observed by the near-field heterodyne transient grating method. In this technique, pump and probe beams are impinged on a transmission grating that is placed in front of a sample. By merely translating the transmission grating along the direction of pump and probe propagation, the heterodyne phase could be continuously changed, and then the real and imaginary parts of the refractive index change could be separated. It was demonstrated that thermal and acoustic gratings were observed only in the real part of the refractive index change.