Vibrational transient grating measurements have been performed on the Si-H stretch vibration of amorphous silicon using intense picosecond infrared pulses from a free electron laser. From these data, the vibrational lifetime can be obtained directly, providing a valuable probe of the microscopic structure and dynamics in the vicinity of the Si-H bond. The stretch mode lifetime has been studied as a function of temperature and across the absorption band. Unexpectedly, the Si-H stretch vibration is demonstrated to be highly localized, and the bulk of the vibrational energy is shown to flow directly to bend vibrations, rather than to other stretch states or to host phonons.

Generally, high optical intensity is required for effective multiphoton excitation of quantum systems to highly excited states. In certain situations, however, lower-intensity, chirped pulses can provide more efficient transfer of population to the upper states by the process of adiabatic passage. We have studied the relative importance of these two mechanisms in the anharmonic 5s-5p-5d quantum ladder system of rubidium using frequency chirped laser pulses from an amplified Ti:sapphire laser ͑ϭ780 and ⌬ϭ10 nm͒. We measure simultaneously the three-photon ionization signal due to the Ti:sapphire and the population that remains in the 5d state with a postionizing 532-nm Nd:YAG pulse ͑where YAG denotes yttrium aluminum garnet͒. At low infrared fluences ͑80 J/cm 2 ͒, the transfer to the 5d state is significantly enhanced when the pulse frequency is swept from the red to the blue, such that it follows the frequency spacing of the rubidium ladder. Counterintuitively, population is also transferred efficiently for the blue-to-red chirp at high fluences ͑Ͼ5 mJ/cm 2 ͒. We attribute both of these effects to adiabatic passage from the 5s state to the 5d state. Even at the highest fluences, more efficient transfer occurs for either direction of chirp than occurs at zero chirp, where the intensity is maximal. A comparison to theoretical predictions reveals striking agreement in both absolute magnitude and functional form. These results have important implications for the understanding of population transfer in complex ladder systems, such as molecular anharmonic vibrational ladders.

Anderson localization of light refers to an inhibition of wave transport in scattering media due to the interference of multiple scattered waves. We present wavelength dependent midinfrared optical transport measurements in slabs of randomly packed germanium (Ge) micron-sized particles, using a free electron laser as a tunable source of pulsed radiation. Because of their high refractive index and low absorption, Ge and similar semiconductors are excellent systems to study Anderson localization of light. To characterize the samples fully, we have employed several complementary optical techniques: total diffuse transmission, total diffuse reflection, coherent transmission, and time-resolved speckle interferometry. In this way we obtained the scattering (l(s)) and transport (l) mean free paths, the absorption coefficient (alpha), the diffusion constant (D), and the energy transport velocity (v(e)). These measurements have been made as a function of midinfrared wavelength, so that the scattering cross section and absorption coefficients can be varied in the same samples. We found that the Ge samples are close (kl(s) approximately 3) to the localization transition, but still above it. Our measurements of l(s) and l suggest that l is renormalized due to interference at the proximity of the localization transition. We also found that the diffusion constant is significantly reduced in samples thinner than approximately 7l.

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