Читать книгу Spectroscopy for Materials Characterization - Группа авторов - Страница 62

A typical FLUC experiment involves the excitation of a sample by the pump beam and the collection of the largest possible fraction of the isotropically emitted fluorescence [2–4,41–44]. This is achieved by large solid‐angle collection optics, such as a parabolic mirror with short focal length and large diameter. The fluorescence is then collimated and refocused on a nonlinear crystal such as a BBO or KDP, by a pair of parabolic mirrors arranged in a telescope. In the BBO, the fluorescence spatially overlaps with the second beam, the gate, so that sum frequency generation occurs and a frequency upconverted pulse is generated and sent to detection. Because SFG occurs only within the temporal duration of the gate, SFG involves only a defined femtosecond temporal “slice” of the emission, even if the latter is nanosecond‐lived. By measuring the intensity of the upconverted pulse as a function of excitation–gate delay, the entire emission kinetics can then be reconstructed.