Jeffrey S. Meth

Experimental and theoretical analysis of transient grating generation and detection of acoustic waveguide modes in ultrathin solids

A full theoretical analysis of and experimental evidence for the optical generation and detection of acoustic waveguide modes (Lamb waves) in ultrathin solids using the transient grating (TG) technique is presented. The driving force due to the TG excitation is derived for a free, isotropic plate. In contrast to a bulk isotropic material in which a single wave is excited, the TG excites a number of modes with a variety of frequencies but with the same tangential component of the wavevector. The frequencies beat, resulting in a complex time‐dependent signal. Experimental results are presented for anthracene sublimation flakes. In addition to discussing the general features of Lamb wave generation, we also discuss the effects of mounting the crystal on a substrate, of varying the fringe spacing, of resonant probing, and of polarized detection. The nature of Lamb waves in anisotropic materials is illustrated, and the extent to which the isotropic theory can be applied to anisotropic systems is discussed.

Surface selectivity in four-wave mixing : transient gratings as a theoretical and experimental example

A theoretical treatment of transient grating diffraction is derived for gratings that are spatially nonuniform in the direction perpendicular to the sample surface. This treatment is readily generalized to any four-wave mixing experiment. Both reflection and transmission geometries of diffraction are examined for the standard transient grating case, in which both grating excitation beams are incident upon the same side of the sample For samples in which the grating amplitude perpendicular to the sample surface varies slowly relative to the optical wavelength, the reflection geometry is shown to probe only the surface or the interface, while the transmission geometry probes the bulk of the sample. An experimental example using four transient grating geometries (two reflection, two transmission) is shown to yield significantly different temporal responses, illustrating the nature of the theoretical predictions. The sample is a thin molecular crystal upon a substrate Both electronic excitations (excitons) and wave-guided acoustic modes are generated and probed Distinct signals are obtained from the bulk, the crystal–substrate interface, and the free-crystal face. Model calculations are presented that illuminate the behavior of the experimental example.

An examination of radiative and nonradiative excitation transport in thin anthracene crystals: Transient grating experiments

Abstract Picosecond transient grating (TG) experiments have been performed on ultrathin anthracene sublimation flakes at room temperature. The laser excitation wavelength was tuned to the singlet exciton band and the diffracted signal was detected in the reflection geometry. An extension of the expression [1] for the contribution from optical reabsorption to the TG fringe spacing and time dependences is derived. It includes the effects of directional radiation from molecules and a critical angle for total internal reflection. The extension and the original treatment both show that reabsorption will only influence the grating for very large fringe spacings, and therefore does not inhibit the use of the TG for the study of exciton transport. The fringe spacing dependence for small fringes reveals no measurable exciton diffusion in either the a- or b-crystallographic directions. This sets an upper bound on the diffusion coefficient in anthracene at room temperature at D ⩽ 5 × 10−4 cm2 s−1.

Generation and detection of acoustic waveguide modes in ultrathin crystals using the transient grating technique

The generation of acoustic waveguide modes (Lamb waves) in ultrathin solids using the transient grating (TG) technique is described. The driving force due to the TG excitation is derived for a free, isotropic plate. In contrast to a bulk isotropic material in which a single wave is excited, the TG excites a number of modes with a variety of frequencies but with the same wavevector. The frequencies beat, resulting in a complex time-dependent signal. Experimental results for anthracene sublimation flakes of z 1 pm thicknesses are presented. Lamb waves with frequencies that do not match the bulk frequencies are generated.

Pseudolocal modes of guest molecules in mixed molecular crystals: Photon echo experiments and computer simulations

The temperature‐dependent optical dephasing of anthracene, 9‐methylanthracene, and 2‐methylanthracene monomers in phenanthrene host crystals has been measured using photon echo experiments. Despite large linear electron–acoustic phonon coupling, all three systems dephase because of coupling to pseudolocal modes (local motions of the guest molecule). Computer simulations of the three systems calculate the pseudolocal mode eigenvalues and eigenvectors. In contrast to previous discussions in the literature which describe pseudolocal modes as librations, the predicted eigenvalues are in reasonable agreement with the measured pseudolocal mode energies. The predicted eigenvectors are combinations of translational motion along the long molecular axis and rotational motion about the out‐of‐plane axis of the guest. Differences in site energies for various locations and orientations of the methyl group are calculated.

Polariton effects in transient grating experiments performed on anthracene single crystals

Abstract Excited state dynamics in anthracene crystals at low temperatures are studied with the transient grating technique. In contrast to work reported earlier, the excitation wavelength is in the vicinity of the polariton stop band, and there is a pronounced dependence of the time response on the wavelength. A qualitative discussion of the wavelength dependence of the grating transients is presented.