Femtosecond pump-probe studies of nonresonant media

The ultrafast electronic and nuclear responses of optically transparent media have been studied by a novel two-dimensional femtosecond technique: frequency dispersed optical heterodyne detected (OHD) birefringence and dichroism. The interpulse time delay dependence of observed responses as a function of probe pulse frequency are compared with theory. The non-resonant birefringent and dichroic nuclear responses result from in-phase and out-of-phase third-order polarization contributions on the red and blue sides of the probe pulse carrier frequency, respectively. Combined with probe pulse-shaping techniques, the two-dimensional (frequency and time) spectrogram of pump-probe responses provides a unique way to selectively enhance contributions of different nuclear responses to the observed OHD signal. The dispersed OHD technique allows CS ₂ isotropic and anisotropic polarizability contributions to be separately observed unlike the conventional optical Kerr effect (OKE) measurements. The ultrafast responses of liquid CS₂ are compared with results of an instantaneous normal mode (INM) calculation.; The OHD spectrograms (time and frequency resolved responses) of the dispersed “instantaneous” electronic contribution are studied and modeled for various pulse shapes. The characteristic dependence on detuning frequency is explained in terms of the two-photon off-resonant density matrix time evolution history. Both the experimental and theoretical electronic spectrograms are studied and compared with the well-known FROG (frequency resolved optical grating) traces. The analysis demonstrates that the OHD spectrogram of non-resonant electronic responses potentially offers a more efficient technique for characterizing ultrafast pulses due to the amplitude-level origin of these responses.; The ultrafast OHD birefringence of a non-resonant gas phase system has been examined for the first time. The OHD birefringence of CO₂ as a function of density over the range from high-pressure gas to the liquid has been observed. The investigation bridges the extensive depolarized light scattering studies of rotational dynamics of systems of reduced densities near or below the critical density with the advantages of direct time domain probes. Characteristics of the onset of liquid-like character are observed. Such time domain investigations are capable of providing much more rigorous tests of theoretical models of molecular interactions than possible via frequency domain methods.