Optical heterodyne detected birefringence and dichroism of transparent liquids and gases

Femtosecond nonresonant pump-probe experiments using multiple polarization configurations are implemented to measure the anisotropic and isotropic electronic and nuclear responses in transparent liquids, glasses and gases. The optical heterodyne detected (OHD) birefringences for the liquid and gas phases of CO₂ in the reduced density range ρ/ρ_c = 0.09 to 0.82 are observed. Each birefringent response is separated into electronic, rotational diffusive and non-diffusive nuclear components. The onset of liquid-like character in CO₂ as a function of density is noted by changes in the rotational diffusive and non-diffusive nuclear response functions. Liquid CO₂ and CS₂ OHD components are compared in order to better understand the nature of the interactions that govern the low frequency intermolecular Raman subspaces of these related liquids.; A frequency dispersed OHD birefringence and dichroic technique was developed. A weak dichroic signal in transparent liquids arises from it out-of-phase contributions from coherent Stokes Raman scattering (CSRS) and coherent anti Stokes Raman scattering (CARS) resonances on the red and blue sides of the ferntosecond probe pulse spectrum. By observing the dispersed probe pulse, the various density matrix components contributing to either the dichroic or birefringent signals can be selectively viewed. Spectrally narrow pulses permit the separation of different nuclear contributions to the total OHD responses. The total and dispersed OHD birefringence and dichroism of chloroform illustrate these effects. Polarization selective dispersed OHD dichroisms are used to measure the anisotropic and isotropic intermolecular ultrafast response of CS₂. A τ ∼ 500 fs exponential decay, previously reported as a component of the anisotropic decay of CS₂, dominates the decay of the observed isotropic response.; The dispersed OHD birefringence and dichroism resulting from the electronic response of nonresonant materials is described for Gaussian, sech2 and exponential ultrafast time resolved pulse shapes. Our experimentally measured OHD frequency resolved optical gating (FROG) spectrograms, are compared for linearly chirped, sech2, and exponential pulse shapes. The analysis of these OHD FROG spectrograms should provide a better tool for the characterization of the phase and amplitude structure of these laser pulses.; Lastly, an analysis of femtosecond OHD birefringence and dichroism is developed within the framework of the Z-scan technique previously used to describe the measurement of nonlinear refractive indices. The phase of the effective local oscillator field is shown to depend on the sample position (z) and the spatially detected portion of the signal pulse for pump-probe responses due to focused Gaussian beams. This analysis of apertured OHD spectroscopy is required to exploit a relatively simple experimental technique for quantitative determinations of polarization-specific electronic and nuclear responses.