Single-shot measurement of terahertz electromagnetic pulses by use of electro-optic sampling.

We demonstrate a simple scheme for capturing the temporal waveforms of a freely propagating terahertz electromagnetic transient in a single shot. The method relies on electro-optic sampling in a noncollinear geometry for the terahertz radiation and the visible probe beam, coupled with multichannel detection. The approach provides time resolution that is comparable to that of conventional electro-optic sampling measurements.

Variable electron-phonon coupling in isolated metallic carbon nanotubes observed by Raman scattering.

We report the existence of broad and weakly asymmetric features in the high-energy (G) Raman modes of freely suspended metallic carbon nanotubes of defined chiral index. A significant variation in peak width (from 12 cm(-1) to 110 cm(-1)) is observed as a function of the nanotube's chiral structure. When the nanotubes are electrostatically gated, the peak widths decrease.

Conductivity of solvated electrons in hexane investigated with terahertz time-domain spectroscopy.

We present investigations of the transient photoconductivity and recombination dynamics of quasifree electrons in liquid n-hexane and cyclohexane performed using terahertz time-domain spectroscopy (THz-TDS). Quasifree electrons are generated by two-photon photoionization of the liquid using a femtosecond ultraviolet pulse, and the resulting changes in the complex conductivity are probed by a THz electromagnetic pulse at a variable delay. The detection of time-domain wave forms of the THz electric field permits the direct determination of both the real and the imaginary part of the conductivity of the electrons over a wide frequency range.

Real-space observation of molecular motion induced by femtosecond laser pulses.

Femtosecond laser irradiation is used to excite adsorbed CO molecules on a Cu110 surface; the ensuing motion of individual molecules across the surface is characterized on a site-to-site basis by in situ scanning tunneling microscopy. Adsorbate motion both along and perpendicular to the rows of the Cu110 surface occurs readily, in marked contrast to the behavior seen for equilibrium diffusion processes. The experimental findings for the probability and direction of the molecular motion can be understood as a manifestation of strong coupling between the adsorbates' lateral degrees of freedom and the substrate electronic excitation produced by the femtosecond laser radiation.

Measurement of the frequency-dependent conductivity in sapphire.

Electron transport of photoexcited single-crystal sapphire (alpha-Al2O3) is characterized by terahertz time-domain spectroscopy. The complex conductivity displays a Drude-type frequency dependence, which yields carrier scattering rates and densities. Carrier scattering is dominated by interactions with acoustical and optical phonons at low and high temperatures, respectively, and follows Matthiessen's law over the measured temperature range of 40-350 K.

Charge transport and carrier dynamics in liquids probed by THz time-domain spectroscopy.

We examine the transport properties and the dynamics of free electrons in n-hexane by means of femtosecond spectroscopy using an ultraviolet pump pulse to create the electrons and a THz electromagnetic pulse as a probe. The complex dielectric response of the photogenerated electrons is determined over a broad range of frequencies, from which we infer the electron scattering time and density through the Drude model. The time evolution of the carrier density reveals nongeminate electron-ion recombination within hundreds of picoseconds at high ion concentration.