Sequence Of Femtosecond Pulses Research Articles

Femtosecond Pulse Shaping, Multiple-Pulse Spectroscopy, and Optical Control

A review is presented of femtosecond pulse-shaping methods and their application to spectroscopy of atoms, molecules, and condensed materials. Pulse shaping can be used to generate femtosecond pulse sequences and other optical waveforms whose time-dependent amplitude, phase, frequency, and polarization profiles are all specified precisely. The light-matter interaction mechanisms through which such waveforms can be used for optical control over molecular and material responses are discussed. Most of the spectroscopic experiments conducted to date that involve shaped femtosecond waveforms are reviewed. These have involved control over coherent electronic responses of atoms, small molecules, and multiple quantum wells and control over coherent molecular and lattice vibrations. A selective review is presented of theoretical predictions and qualitative discussions of optical control possibilities involving complex ultrafast waveforms.

High-order echoes in vibrational spectroscopy of liquids

Multidimensional infrared and Raman spectroscopy of liquids conducted using sequences of femtosecond pulses may be used to directly probe intramolecular and intermolecular nuclear motions. These techniques provide an additional insight into the nature of the nuclear spectral density observed in optical Kerr measurements. Photon echoes predicted in these systems are calculated and analyzed using multilevel coherences.

Dual-wavelength self-mode-locked Ti:sapphire laser

A self-mode-locked Ti:sapphire laser system has been constructed that simultaneously generates sequences of femtosecond pulses at two independent wavelength regions. This dual-wavelength, self-mode-locked operation has been shown to exhibit a high degree of synchronization, and the relative timing jitter has been deduced to be 52 fs.

Shaping of femtosecond pulses using phase-only filters designed by simulated annealing

We describe generation of shaped femtosecond waveforms by using frequency-domain phase-only filters designed by numerical optimization techniques. The task of filter design for pulse shaping in the time domain is directly analogous to the design of phase-only filters for beam shaping and array generation in the spatial domain. We experimentally tested phase filters that were designed to produce ultrafast square and triangle pulses and femtosecond pulse sequences. Our results demonstrate the ability to generate high-quality terahertz-repetition-rate sequences of femtosecond pulses by means of low-loss phase-only filtering. On the other hand, experiments that tested generation of individual shaped pulses, such as square pulses, were less successful than previous experiments that used simultaneous phase and amplitude filtering. Our results indicate the importance of building increased robustness against variations in the input pulse shape into the phase-only filter design.

Fluorescence-detected wave packet interferometry: Time resolved molecular spectroscopy with sequences of femtosecond phase-locked pulses

We introduce a novel spectroscopic technique which utilizes a two-pulse sequence of femtosecond duration phase-locked optical laser pulses to resonantly excite vibronic transitions of a molecule. In contrast with other ultrafast pump–probe methods, in this experiment a definite optical phase angle between the pulses is maintained while varying the interpulse delay with interferometric precision. For the cases of in-phase, in-quadrature, and out-of-phase pulse pairs, respectively, the optical delay is controlled to positions that are integer, integer plus one quarter, and integer plus one half multiples of the wavelength of a selected Fourier component. In analogy with a double slit optical interference experiment, the two the two pulse experiments reported herein involve the preparation and quantum interference of two nuclear wave packet amplitudes state of a molecule. These experiments are designed to be sensitive to the total phase evolution of the wave packet prepared by the initial pulse. The direct determination of wave packet phase evolution is possible because phase locking effectively transforms the interferogram to a frame which is referenced to the optical carrier frequency, thereby eliminating the high (optical) frequency modulations. This has the effect of isolating the rovibrational molecular dynamics. The phase locking scheme is demonstrated for molecular iodine. The excited state population following the passage of both pulses is detected as the resultant two-beam dependent fluorescence emission from the B state. The observed signals have periodically recurring features that result from the vibrational dynamics of the molecule on the electronically excited potential energy surface. In addition, coherent interference effects cause the magnitude and sign of the periodic features to be strongly modulated. The two-pulse phase-locked interferograms are interpreted herein by use of a simple analytic model, by first order perturbation theory and by quantum mechanical wave packet calculations. We find the form of the interferogram to be determined by the ground state level from which the amplitude originates, the deviation from impulsive preparation of the wave packet due to nonzero pulse duration, the frequency and anharmonicity of the target vibrational levels in the B state, and the detuning of the phase-locked frequency from resonance. The dependence of the interferogram on the phase-locked frequency and phase angle is investigated in detail.

Femtosecond multiple-pulse impulsive stimulated Raman scattering spectroscopy

Optical control of elementary molecular motion through impulsive stimulated Raman scattering is enhanced by means of timed sequences of femtosecond pulses that are produced by pulse-shaping techniques. In particular, terahertz-rate trains of femtosecond pulses are used for repetitive impulsive excitation of individual phonon modes in an α-perylene molecular crystal. When the pulse repetition rate is matched to the desired phonon frequency, mode-selective vibrational amplification is achieved. A comparison of data acquired with the transient-grating and the pump–probe experimental geometries reveals the timing dynamics of the induced phonon oscillations with respect to the driving femtosecond pulse sequence.

A quantum mechanical study of predissociation dynamics of NaI excited by a femtosecond laser pulse

This work was stimulated by the experiments of Rosker, Rose, and Zewail (RRZ) who used a pump–probe sequence of femtosecond pulses to study the predissociation of NaI. We calculate quantum mechanically the nuclear wave function created by the pump pulse and its subsequent evolution. To make contact with the experiments we assume that, depending on its wavelength, the probe pulse is absorbed by either the free sodium atoms or by the bound molecule in the neutral state, and that the measured fluorescence induced by the probe pulse is proportional to the populations of these two ‘‘species.’’ The similarity between computed populations and the observed signal confirms this conjecture. We study how various factors, such as the pulse length and shape, the initial vibrational state, the temperature, and the diabatic coupling strength affect the populations (and thus the LIF signal). We also show that the RRZ analysis based on classical trajectories and the Landau–Zener formula agrees semiquantitatively with the exact quantum results.