Method of Transient Gratings

Traditional methods of absorption spectroscopy depend on the direct investigation of laser light transmitted through a sample. At optical densities of less than about 10-5 classical absorption spectroscopy fails. A possible way out of this situation is TG-spectroscopy (TG = Transient Grating) : Two coherent pump beams of equal intensity in the region of an absorption band of the sample are brought to overlap inside the sample (setup is here). In the crossing region the optical properties of the sample are being modulated periodically, and probe light can be diffracted at the grating-like structure that is generated. The diffracted probe light can easily be detected "backgroundfree" in the above sense. Investigating sensitive samples or samples with low OD is done better by transient grating spectroscopy than by contentional ESA.