Simplified setup for high-resolution spectroscopy that uses ultrashort pulses.

Using a broadband femtosecond laser and a simple optical setup, we demonstrate narrow-bandwidth-tunable excitation of vibrational modes in CCl4 and CHBr3 liquids. The resolution obtained is 80 times higher than the laser bandwidth. A pair of time-shifted, linearly chirped pulses is created by use of a high-order dispersion-compensated prism-interferometer setup. We use this pulse pair to selectively excite Raman-active transitions. Our setup represents a significant simplification with improved resolution, of previous approaches to the use of ultrashort pulses for chemically selective spectroscopy.

Relationship between dynamical and equilibrium characteristics of glass-forming polymeric liquids.

A connection between measurable equilibrium thermodynamic quantities and a nonequilibrium property of supercooled polymeric liquids, namely, the fragility index, is proposed within the framework of a synthesis of generalized configurational entropy models. The theoretical predictions are compared with experimental data on five glass-forming polymers.

Rapid determination of the three-dimensional orientation of single molecules.

A technique is proposed for determination of the three-dimensional orientation of the transition dipoles of single molecules by use of polarization-sensitive detection of fluorescence through a high-N.A. objective. Molecular orientation can be determined uniquely and rapidly based on the counts from three detectors that are sensitive to different polarizations of light.

Single-molecule detection with a two-photon fluorescence microscope with fast-scanning capabilities and polarization sensitivity.

We describe a laser-scanning two-photon fluorescence microscope that is capable of observing single molecules with excellent temporal resolution and three-dimensional spatial resolution. To demonstrate the capabilities of the instrument we present single-molecule fluorescence data obtained in several different scanning modes. In addition, a polarization-sensitive detection scheme can provide detailed three-dimensional information about the orientations of molecules in any of these scanning modes.

Pulse-length-limited ultrafast pump-probe spectroscopy in a single laser shot.

We demonstrate a scheme for taking single-shot pump-probe data with femtosecond time resolution. Data recorded from a 100-microm-thick dye solution show that pulse-length-limited resolution can be achieved readily with 70-fs laser pulses. This technique opens the door for the study of irreversible ultrafast processes in a wide variety of solids. Also, unlike with conventional techniques, in this method multiple-shot data are not affected by shot-to-shot laser intensity fluctuations.

Extra resonances in time-domain four-wave mixing.

We show that extra resonances, such as those caused in frequency-domain nonlinear wave mixing by pure dephasing or laser fluctuations, can also be induced by operation in the time domain. These pulse-length-induced extra resonances arise in transient-grating experiments when the laser pulses are short enough that a steady state cannot be achieved during the excitation process. We show theoretically that these resonances increase in strength with decreasing excitation pulse length until the pulse length becomes shorter than the dephasing time of the medium and quote an experimental example to support this interpretation.

Picosecond time-resolved four-wave mixing experiments in sodium-seeded flames.

Picosecond four-wave mixing experiments have been used to study collisions in a Na-seeded, premixed, methane-air flame. Population gratings are used to measure Na excited-state quenching collision rates, while polarization gratings are used to measure Na ground-state hyperfine coherence randomizing collision rates and overall Na diffusion rates, even though these processes are slower than the excited-state quenching rate.