Optical Heterodyne Detected Accumulated Acoustic Grating Responses in Near Supercritical Fluids Jian Peng and Lawrence D. Ziegler* Department of Chemistry and The Photonics Center, Boston UniVersity, Boston, Massachusetts 02215 ReceiVed: September 6, 2007; In Final Form: October 11, 2007 A novel third-order polarization effect due to an accumulated optical heterodyne detected (OHD) transient acoustic grating response in near critical fluids was observed and experimentally characterized. Femtosecond pump-probe responses in near critical CO 2 and CHF 3 illustrate this phenomenon. This large optically generated acoustic response due to electrostrictive coupling appears only when pump and probe pulses are temporally overlapped and is π out-of-phase with the normal optical Kerr effect (OKE) birefringent signal. The local oscillator, the laser intensity, and the modeled experimental repetition rate dependence identify the accumulated heterodyne origin of these responses. The observed OHD accumulated acoustic birefringent signal is inversely dependent on sound velocity to the fifth power. A corresponding sound velocity dependent dichroic (in- phase) response was also observed for these electronically nonresonant samples. The accumulated effect described here may have applications for the design of efficient modulators and as a simple and sensitive experimental technique for the measurement of near critical fluid thermodynamic and acoustic parameters. Introduction The ability of optical pulses to write and read transient acoustic gratings has been exploited over the past 30 years to learn about sound propagation and attenuation, material struc- tural characterization, weak absorptions, and thermal diffusion in solids, liquids, and gases. 1-7 In one well-studied experimental arrangement for the observation of optically excited acoustic responses, two optical pulses from the same laser source at a carrier wavelength λ exc intersect at an angle θ with respect to each other and temporally and spatially overlap in a sample. The simultaneous spatial overlap of the electric fields of the two laser beams creates an intensity interference pattern in the material, producing a fringe spacing, Λ, given by This periodic spatial intensity pattern results in counter propagating acoustic standing waves due to density changes arising from rapid thermal expansion or electrostriction forces depending on whether the sample is absorbing or transparent at λ exc , respectively. For the typical experimental parameters employed in this study (λ exc ) ∼800 nm and θ ) 8°), Λ corresponds to an angular acoustic frequency (ω) of ∼200 MHz. A time delayed probe pulse incident on the grating, at some different angle and often at some new color, is reflected into a detector at an angle that satisfies the Bragg reflection condition. When the delay between the probe and the pair of excitation or pump pulses is scanned, the time dependence of the generally underdamped acoustic response, or more properly the square of this quantity, is observed. 2-4 This transient grating interaction can be described in a third-order nonlinear polarization, P (3) (t), framework, and the resulting signal in this typical experimental configuration is thus a homodyne response, |P (3) (t)|. 2 When a sufficiently intense additional local oscillator field (|E LO | . |P (3) |) is temporally and spatially overlapped with the P (3) (t) derived signal field, the optically excited acoustic response can also be heterodyne detected. 8 Longitudinal acoustic waves can also be optically generated by strain waves resulting from rapid heating in thin films. 9 Here, we report a new manifestation of optically generated acoustic responses that are only clearly evident in a special class of fluids: near supercritical fluids (SCFs). The purpose of this paper is to provide a detailed experimental characterization of this novel nonlinear effect in near SCFs that identifies the origin of these signals, contrasts their observation in normal liquids, and indicates potential applications for this phenomenon. The samples of study here are near critical CO 2 and CHF 3 fluids. Both CO 2 and CHF 3 have experimentally convenient critical state points (see Table 1): P C ) 72.8 atm and T C ) 31.1 °C for CO 2 and P C ) 47.7 atm and T C ) 25.6 °C for CHF 3 . During recent studies of the decays of the H 2 rotational Raman resonances in near supercritical (SC) CO 2 , which act as a probe of the anisotropic fluctuation dynamics in this solvent, 10 we observed an anomalous feature in the ultrafast optical heterodyne detected (OHD) optical Kerr effect (OKE) responses of H 2 / CO 2 near critical solutions. In this standard OHD-OKE two- beam experimental configuration, 11 a large autocorrelation- shaped feature centered at the t ) 0 pump-probe temporal overlap region appeared in these OHD-OKE responses of SC H 2 /CO 2 mixture fluids. This feature was strongly density dependent, increasing in strength as the critical point was approached and was π out-of-phase with the normal nonresonant electronic and Raman nuclear responses of the H 2 /CO 2 mixture, which remained virtually unchanged over the same temperature region. This effect is illustrated in Figure 1 for a 25% (mol fraction) mixture of H 2 in SC CO 2 obtained with the 76 MHz 22 fs output of a Ti:sapphire oscillator in the standard two- beam OHD configuration. 11 The critical temperature for this H 2 solution (CO 2 density ) 0.8 F C ) is reduced by ∼10 to ∼21 °C from the neat CO 2 T C . Such density dependent features in the region of the nonresonant electronic response were not * Corresponding author. E-mail: lziegler@bu.edu. Λ ) λ exc 2 sin(θ/2) (1) 13457 J. Phys. Chem. A 2007, 111, 13457-13465 10.1021/jp0771771 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/01/2007