Four-Wave Mixing Studies of Energy Transfer Processes

The main objective of the lecture is to investigate the energy transfer processes among dopant ions in inorganic crystals by both four-wave mixing spectroscopy and fluorescence dynamics techniques. Four-wave mixing has been demonstrated to be an effective technique for characterizing long range energy migration and optical dephazing phenomena. In addition, measurements of the fluorescence decay dynamics of active ions are used to characterize the total energy transfer properties between sensitiser and activator ions.

The set-up used in these experiments will be described in the approach of the production of the laser-induced grating signal in solids. Two nonlinear laser beams are focused inside the sample and the path difference is adjusted in such a way that the two beams interfere creating a sinusoidal intensity pattern. Since the wavelength of the laser beam is resonant with an absorption transition of the active ions, optical absorption creates a similar spatial sinusoidal distribution of the excited ions that acts like a population grating. The laser-induced grating signal carries all of the informations about the physical processes influencing the population grating. Signal strengths and decay times are measured as functions of laser-beam crossing angles and temperature.

We shall show the examples of two different activator ions in Garnets: Cr³⁺ transition metal ion and Nd³⁺ rare earth ion which are characterized by two kinds of spectroscopy. The results will be discussed.