Several closely related third-order nonlinear time-resolved spectroscopic techniques, pump/probe transient absorption, transient grating, and three pulse stimulated photon echo peak shift measurements, are investigated theoretically and experimentally. It is shown in detail, through the consideration of response functions and numerical simulations including both finite pulse durations and detuning from exact resonance, how the solvation dynamics are manifested in these third-order nonlinear time-resolved spectroscopies. It is shown that the three pulse stimulated photon echo peak shift measurement and the transient grating measurement can give accurate dynamical information, whereas transient absorption may not be a reliable technique for a study of solvation dynamics in some cases. The contribution of very slow or static (inhomogeneous) components to the dynamics, however, can only be obtained from the three pulse echo peak shift measurements. Comprehensive experimental measurements are presented to illustrate and corroborate the calculations. We show that it is possible to separate the intramolecular vibrational and solvent contributions to the dephasing (or optical lineshape). Furthermore it is shown that the solvation of polar solutes in polar protic solvents has rather universal characteristics. The initial ultrafast process, usually identified as an inertial response of solvent molecules, occurs on a ∼100 fs time scale, and is essentially identical in methanol, ethanol, and butanol. The amplitude of this ultrafast component does, however, decrease with increasing alcohol size in 1-alkanols. The diffusive (≳0.5 ps) regime of the solvation process shows a strong solvent dependence, and may be described satisfactorily by dielectric relaxation theories.

The underlying dynamics of the B800 absorption band in isolated LH2 of Rb. sphaeroides at room temperature is studied by transient absorption, transient grating, and photon echoes using 30 fs pulses. The energy transfer time from B800 to B850 is determined to be 800 fs, similar to the value reported previously. The three pulse stimulated photon echo identifies several important contributions to the B800 absorption line shape and thereby the dynamics of the system involved: several low frequency intramolecular vibrations, ultrafast bath (solvent and protein) responses, and static inhomogeneity longer than the time scale of B800 to B850 energy transfer make significant contributions. Transient absorption decay is nonexponential as found previously. It is argued that the fast component in the two-exponential analysis of the transient absorption signal originates from vibrational relaxation within the B800 absorption band. Calculations of the nonlinear signals based on the optical transition frequency correlation function, M(t), including all three contributions (intramolecular vibrations, ultrafast bath (solvent and protein) responses, and static inhomogeneity) are presented, and a form for M(t) is obtained by fitting the three pulse photon echo experimental data.

Transport measurements were carried out on 15-35 nm diameter silicon nanowires grown using SiH 4 chemical vapor deposition via Au or Zn particle-nucleated vapor-liquid-solid growth at 440°C. Both Al and Ti/Au contacts to the wires were investigated. The wires, as produced, were essentially intrinsic, although Au nucleated wires exhibited a slightly higher conductance. Thermal treatment of the fabricated devices resulted in better electrical contacts, as well as diffusion of dopant atoms into the nanowires, and increased the nanowire conductance by as much as 10 4 . Three terminal devices indicate that the doping of the wires is p type.

The preparation of 20 ± 5 nm diameter Si nanowires and the electrical characterization of Si nanowire devices are presented. The nanowires were grown at 450−500 °C on solid substrates via the vapor−liquid−solid mechanism using Au or Zn nucleation catalysts and SiH4 as the silicon source. The wires were investigated by high-resolution transmission electron microscopy. Two types of wires were found, as characterized by different growth directions (〈111̄〉 and 〈211〉). Several types of devices, including crossed nanowire devices, four- and six-terminal devices, and three-terminal (gated) devices, were fabricated. For certain devices, various electrode compositions were also studied. The measured resistivity of these nanowires was separated from the contact resistance and could be varied from >105 Ω cm to ∼10-3 Ω cm. The wide variation in resistivity was related to the nature of the electrical contact to the wires (Schottky or Ohmic) and to the doping level of the wires. Doping of the nanowires was performed by the thermal diffusion of metal catalyst into the nanowires at 750−850 °C. Au nucleated nanowires exhibited resistivity values much lower than those of similarly treated Zn nucleated nanowires. This result is attributed to the much larger relative solid solubility of gold in silicon.

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Frequency-resolved, three-pulse photon echo peak shifts of the accessory pigments of the reaction center of Rhodobacter sphaeroides were recorded to study the ultrafast pigment-protein dynamics. For a qualitative understanding of the data, which includes an increase in rephasing capability after 1 ps, it is necessary to take into account the influence of the primary electron donor P on the 800 nm absorption band and direct electron transfer from the excited accessory bacteriochlorohyll B A . The peak shift data and the absorption spectrum are simulated via the time correlation function of the optical transition frequency. We observe subtle differences in the dynamics on the blue and red side of the absorption band, which may be related to differences in the environment between the B A and B B pigments, respectively. The bath correlation time is longer, ∼90 fs, at 810 nm than at 790 nm, where it is ∼60 fs. Since we conclude from our transient grating measurements that the time of energy transfer is 80 fs at 810 nm and 130 fs at 790 nm, this indicates that around 810 nm no significant nuclear relaxation takes place prior to energy transfer. On a longer time scale we observe coherent oscillations of low frequency, ∼3-4 and 36 cm -1 , and a small ∼30 ps decay of rephasing capability.

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