Pulsed optoacoustic spectroscopy of condensed matter

Abstract

The authors discuss the theory and experiments dealing with the pulsed optoacoustic effect (i.e., generation of a transient acoustic wave by absorption of an optical pulse) in condensed matter. Their primary interest lies in the measurement of small absorption coefficients (≪${10}^{-1\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$). At present an experimental capability of measuring absorption coefficients as small as ${10}^{-6\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ has been demonstrated, and further improvement is foreseen. The pulsed optoacoustic absorption measurement technique has been applied to the following linear spectroscopic studies: (1) precise measurements of the optical absorption spectra of ${\mathrm{H}}_2$O and ${\mathrm{D}}_2$O; (b) accurate determination of absorption strengths and profiles of high harmonics ($n=6, 7, \mathrm{and} 8$) of vibrational modes in transparent organic liquids (e.g., benzene); (c) quantitative absorption spectra of thin (∼ 1-10 μm) liquid films; and (d) quantitative absorption spectra of solids and finely powdered crystals. The usefulness of the pulsed optoacoustic technique to nonlinear spectroscopy has been demonstrated in the following studies: (a) quantitative two-photon absorption spectroscopy of the weak two-photon (${}_{}{}^1{}_{}{}^{}B_{2\mu }^{}{}_{}{}^{}\leftarrow {}_{}{}^1{}_{}{}^{}A_{1g}^{}{}_{}{}^{}$) transition in benzene; and (b) optoacoustic Raman-gain spectra for a variety of liquids where an ability to measure Raman gains as small as ${10}^{-5\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ has been demonstrated. In addition to reviewing the above studies the authors discuss future possible applications and compare the pulsed optoacoustic spectroscopy technique with other optoacoustic absorption measurement techniques.