Laser-induced phonon spectroscopy. Optical generation of ultrasonic waves and investigation of electronic excited-state interactions in solids

Crossed laser pulse excitation generates high amplitude, counterpropagating, ultrasonic waves (acoustic phonons of selected wave vector) via direct coupling between the optical electromagnetic field and the material acoustic field. The technique allows optical generation of ultrasonic waves, conveniently tunable to at least 20 GHz. The coupling mechanism, which does not involve optical absorption, is discussed in detail in terms of electrostriction. The periodic density changes resulting from the acoustic waves cause spectral shifts whose magnitudes reflect the strengths of excited-state intermolecular interactions and excited-state phonon interactions. The first quantitative measurements of spectral shifts by laser-induced phonon spectroscopy (LIPS) are reported. In pentacene in $p$-terphenyl, spectral shifts on the order of 1 ${\mathrm{cm}}^{\ensuremath{-}1}$ are measured using laser-induced phonons propagating along the $b$ crystallographic axis. Orientation of the phonon wave vector along various crystalline directions allows investigation of the anisotropic excited-state intermolecular interactions.