We report what is to our knowledge the first measurement of linear and nonlinear spectroscopic properties for the (7)F(0)-(5)D(0) transition of Eu(3+):Y(2)SiO(5). Two clearly resolved lines at 579.879 and 580.049 nm, stemming from different sites, show dissimilar photoluminescence and hole spectra. In addition, these two sites have different inhomogeneous and homogeneous linewidths, which suggests that the local-field effect is smaller for one site. Specifically, the less affected site exhibits the longest dephasing time (822 micros) of any solid, which corresponds to a homogeneous linewidth of 387 Hz, and this linewidth is found to persist for hours without apparent spectral diffusion.

We propose and demonstrate a novel type of frequency-selective optical memory that writes and reads the data in both the time domain and the frequency domain. Temporal 16-bit data were stored by accumulated-photon-echo bit-by-bit storage at 103 frequency addresses within the inhomogeneous line of the (7)F(0)-(5)D(0) transition of Eu(3+):Y(2)SiO(5), which yields a total memory capacity of 1.6 kbits in a single spot of 240-microm diameter. The keys to the success of this experiment are this material's long dephasing time and lack of spectral diffusion.

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A stimulated photon-echo study has verified the instantaneous shift (< 1 MHz) and subsequent restoration ( -2 ms) of the transition frequency of an impurity ion in a solid when its neighboring ions are exposed to pulsed optical excitation. Experiments were conducted by monitoring the echo intensity for the 7 Fo-5 Z>o transition of Eu ions in one site of YzSiOs, while exciting the other site. A photon-echo theory taking account of the stochastic frequency recovery is developed to explain the observations. PACS numbers: 78.50.Ec, 42.50.Md A ions and B ions can be the same species, as they are in all the previous cases. The magnitude of the EFS should be random, depending on each A ion, which has its own distribution of the excited B ions around it. It is also important to note that this environmental change is instantaneous but temporary. In fact, sooner or later the original environment is restored when the B ions decay back to the ground state, but experimental verification of this restoration process has not been reported. High-resolution nonlinear laser spectroscopy in solids has been a powerful tool for studying small magnetic or electric interactions between optical centers and their circumstances. In 1987 Liu et al [1] and in 1989 Huang et al. [2,3] used photon-echo experiments to show the existence of a small shift (20 to 300 kHz depending on the excitation intensity) in the optical transition frequency of an impurity ion in a solid, induced by the optical excitation of nearby ions. This excitation-induced frequency shift (EFS), also known as instantaneous spectral diffusion, can only be unveiled by nonlinear spectroscopy because otherwise the EFS would be totally buried within the huge inhomogeneous absorption linewidth ( -GHz). Many recent observations of excitation-intensity-dependent optical dephasing times [1-5] can be well interpreted in terms of the EFS. The physical origin of the EFS is very clear. By optically exciting the surrounding ions (B ions) by an intense pulse, which we now call the scrambler, the two-level system of interest (A ion) experiences a sudden change in the local field (electric or magnetic), and this perturbation shifts the energy levels of the A ion. Of course

Reflection-type degenerate pump-probe spectroscopy was performed for low-temperature grown (LT-) GaAs to study the effects of arsenic pressure during crystal growth and annealing on carrier lifetime and to ascertain the annealing dynamics. It was found that a sample grown under a high arsenic pressure has a shorter carrier lifetime for both as-grown and anneal conditions. It was also found that the carrier decay times of samples changed drastically when the annealing temperature was above 550 °C. We determined the annealing dynamics of LT-GaAs based on a model in which AsGa antisite defects trap photoexcited carriers. An Arrhenius plot of the carrier decay rate vs. annealing temperature in the high temperature regime gave an energy EPA that was different from the true activation energy. The annealing time dependences of EPA obtained by the two diffusion models (self diffusion and VGa vacancy assisted diffusion of defects) were compared with EPAs of our data and other works, which proved that the annealing dynamics of AsGa antisite defects was dominated by VGa vacancy assisted diffusion.