Showing papers on "Femtosecond published in 1996"

Femtosecond, picosecond and nanosecond laser ablation of solids

Abstract: Laser ablation of solid targets by 0.2–5000 ps Ti: Sapphire laser pulses is studied. Theoretical models and qualitative explanations of experimental results are presented. Advantages of femtosecond lasers for precise material processing are discussed and demonstrated.

Writing waveguides in glass with a femtosecond laser

Abstract: With the goal of being able to create optical devices for the telecommunications industry, we investigated the effects of 810-nm, femtosecond laser radiation on various glasses. By focusing the laser beam through a microscope objective, we successfully wrote transparent, but visible, round-elliptical damage lines inside high-silica, borate, soda lime silicate, and fluorozirconate (ZBLAN) bulk glasses. Microellipsometer measurements of the damaged region in the pure and Ge-doped silica glasses showed a 0.01–0.035 refractive-index increase, depending on the radiation dose. The formation of several defects, including Si E′ or Ge E′ centers, nonbridging oxygen hole centers, and peroxy radicals, was also detected. These results suggest that multiphoton interactions occur in the glasses and that it may be possible to write three-dimensional optical circuits in bulk glasses with such a focused laser beam technique.

Ultrafast spin dynamics in ferromagnetic nickel.

Abstract: The relaxation processes of electrons and spins systems following the absorption of femtosecond optical pulses in ferromagnetic nickel have been studied using optical and magneto-optical pump-probe techniques. The magnetization of the film drops rapidly during the first picosecond, but different electron and spin dynamics are observed for delays in the range 0--5 ps. The experimental results are adequately described by a model including three interacting reservoirs (electron, spin, and lattice).

Far-infrared dielectric properties of polar liquids probed by femtosecond terahertz pulse spectroscopy

Abstract: We report the frequency-dependent optical constants, n(ν) and α(ν), or, equivalently, the complex permittivity e(ω) = e‘(ω) − ie‘‘(ω), over the frequency range from 2 to 50 cm-1 for water, methanol, ethanol, 1-propanol, and liquid ammonia. These spectra have been measured with femtosecond terahertz pulse transmission spectroscopy. These liquids exhibit multiple-Debye behavior, making their frequency-dependent dielectric constants valuable benchmarks for molecular dynamics simulations and other theoretical treatments of liquids.

Conical emission from self-guided femtosecond pulses in air.

Abstract: Conical emission in the forward direction is observed from intense femtosecond light pulses propagating through air over long distances. The conical emission is attributed to Cerenkov radiation from a dynamic self-guiding structure consisting of a weakly ionized core surrounded by Kerr cladding.

Femtosecond X-ray Pulses at 0.4 Å Generated by 90° Thomson Scattering: A Tool for Probing the Structural Dynamics of Materials

Abstract: Pulses of x-rays 300 femtoseconds in duration at a wavelength of 04 angstroms (30,000 electron volts) have been generated by 90° Thomson scattering between infrared terawatt laser pulses and highly relativistic electrons from an accelerator In the right-angle scattering geometry, the duration of the x-ray burst is determined by the transit time of the laser pulse across the ∼ 90-micrometer waist of the focused electron beam The x-rays are highly directed (∼ 06° divergence) and can be tuned in energy This source of femtosecond x-rays will make it possible to combine x-ray techniques with ultrafast time resolution to investigate structural dynamics in condensed matter

Ultrashort Laser Pulse Phenomena: Fundamentals, Techniques, and Applications on a Femtosecond Time Scale

Abstract: Fundamentals. Femtosecond Optics. Light-Matter Interaction. Coherent Phenomena. Ultrashort Sources. Femtosecond Pulse Amplification. Pulse Shaping. Diagnostic Techniques. Measurement Techniques. Examples of Ultrafast Processes in Matter. Extreme Wavelengths. Phase Shifts Upon Transmission-Reflection. F's Dye Laser Configuration. Four Photon Coherent Interaction. Dyes and Solvents. Reconstruction of the Spectral Phase.

Laser Injection of Ultrashort Electron Pulses into Wakefield Plasma Waves

Abstract: A novel laser-plasma-based source of relativistic electrons is described. It involves a combination of orthogonally directed laser beams, which are focused in a plasma. One beam excites a wakefield electron plasma wave. Another locally alters the trajectory of some of the electrons in such a way that they can be accelerated and trapped by the wave. With currently available table-top terawatt lasers, a single ultrashort-duration electron bunch can be accelerated to multi-MeV energies in a fraction of a millimeter, with femtosecond synchronization between the light pulse, the electron bunch, and the plasma wave. Both analytical and numerical-simulation results are presented.

Breakdown threshold and plasma formation in femtosecond laser-solid interaction

Abstract: Combining femtosecond pump–probe techniques with optical microscopy, we have studied laser-induced optical breakdown in optically transparent solids with high temporal and spatial resolution. The threshold of plasma formation has been determined from measurements of the changes of the optical reflectivity associated with the developing plasma. It is shown that plasma generation occurs at the surface. We have observed a remarkable resistance to optical breakdown and material damage in the interaction of femtosecond laser pulses with bulk optical materials.

Observation of laser assisted photoelectric effect and femtosecond high order harmonic radiation.

Abstract: We report the first observation of laser-induced free-free transitions in the primary photoelectron spectra of gaseous helium ionized by ultrashort soft x-ray pulses. Measured transition amplitudes are well described by projecting the initial electronic state onto a Volkov wavefunction. Additionally, we report the first direct measurements on the temporal duration of femtosecond high order harmonic radiation. The harmonic pulse duration is observed to exhibit strong dependencies upon both laser intensity and position of the generating medium relative to the laser focus.

Measurement of Temperature-Independent Femtosecond Interfacial Electron Transfer from an Anchored Molecular Electron Donor to a Semiconductor as Acceptor

Abstract: Modified perylene chromophores were adsorbed with virtually constant reaction distance on the surface of a spongelike TiO2 electrode. Interfacial electron transfer was probed with femtosecond resolution in ultrahigh vacuum via transient absorption and fluorescence up-conversion measurements. Identical time constants were measured for the decay of the reactant and rise of the product states. The dominant fast time constant was 190 fs. It remained constant between 300 and 22 K.

Time-resolved observations of shock waves and cavitation bubbles generated by femtosecond laser pulses in corneal tissue and water

Abstract: Background and Objective Photodisruption in ocular media with high power pulsed lasers working at non-absorbing frequencies have become a well established surgical tool since the late seventies Shock waves and cavitation bubbles generated by the optical breakdown may strongly influence the surgical effect of photodisruptive lasers We have investigated the shock wave and cavitation bubble effects of femtosecond laser pulses generated during photodisruption in corneal tissue and water The results are compared to those obtained with longer laser pulses Study Design/Materials and Methods Laser pulses with 150 fs duration at ∼620 nm wavelength have been focused into corneal tissue and water to create optical breakdown Time-resolved flash photography has been used to investigate the dynamics of the generated shock waves and cavitation bubbles Results A rapid decay of the shock waves is observed in both materials with similar temporal characteristics and with a spatial range considerably smaller than that of shock waves induced by picosecond (or nanosecond) optical breakdown Cavitation bubbles are observed to develop more rapidly and to reach smaller maximum diameter than those generated by longer pulses In corneal tissue, single intrastromal cavitation bubbles generated by femtosecond pulses disappear within a few tens of seconds, notably faster than cavitation bubbles generated by picosecond pulses Conclusions The reduced shock wave and cavitation bubble effects of the femtosecond laser result in more localized tissue damage Therefore, a more confined surgical effect should be expected from a femtosecond laser than that from picosecond (or nanosecond) lasers This indicates a potential benefit from the applications of femtosecond laser technology to intraocular microsurgery © 1996 Wiley-Liss, Inc

Femtosecond x-ray pulses of synchrotron radiation.

Abstract: A method capable of producing femtosecond pulses of synchrotron radiation is proposed. It is based on the interaction of femtosecond light pulses with electrons in a storage ring. The application of the method to the generation of ultrashort x-ray pulses at the Advanced Light Source of Lawrence Berkeley National Laboratory has been considered. The same method can also be used for extraction of electrons from a storage ring in ultrashort series of microbunches spaced by the periodicity of light wavelength.

Plasma mediated ablation of biological tissues with nanosecond-to-femtosecond laser pulses: relative role of linear and nonlinear absorption

Abstract: Plasma mediated ablation of collagen gels and porcine cornea was studied at various laser pulse durations in the range of 1 ns-300 fs at 1053-nm wavelength. It was found that pulsed laser ablation of transparent and weakly absorbing gels is always mediated by plasma. On the other hand, ablation of strongly absorbing tissues is mediated by plasma in the ultrashort-pulse range only. Ablation threshold along with plasma optical breakdown threshold decreases with increasing tissue absorbance for subnanosecond pulses. In contrast, the ablation threshold was found to be practically independent of tissue linear absorption for femtosecond laser pulses. The mechanism of optical breakdown at the tissue surface was theoretically investigated. In the nanosecond range of laser pulse duration, optical breakdown proceeds via avalanche ionization initiated by heating of electrons contributed by strongly absorbing impurities at the tissue surface. In the ultrashortpulse range, optical breakdown is initiated by multiphoton ionization of the irradiated medium (six photons in case of tissue irradiated at 1053-nm wavelength), and is less sensitive to linear absorption. High-quality ablation craters with no thermal or mechanical damage to surrounding material were obtained with subpicosecond laser pulses. Experimental results suggest that subpicosecond plasma mediated ablation can be employed as a tool for precise laser microsurgery of various tissues.

THz electromagnetic emission by coherent infrared-active phonons

Abstract: The generation and detection of coherent phonons in condensed media with femtosecond laser pulses allows a detailed study of the interaction dynamics of optically excited carriers with coherent phonons. Several experiments revealed the possibility to excite coherently optical phonons in different semiconductor materials. Depending on the peculiarities of the interaction of the optical excitation with the lattice, different excitation mechanisms for coherent phonons could be identified. 1 In polar crystals with large surface fields ~GaAs, InP!, the generation of coherent LO phonons is based on the ultrafast screening of the surface fields by the photoexcited carriers. 2‐ 4 In high-temperature superconductors coherent phonons are excited by the breaking of Cooper pairs, 1 while in other layered materials ~GaSe! the excitation process is directly related to an impulsive Raman process. 1,5 For narrow-band-gap materials like tellurium, it has been shown that a displacive excitation mechanism closely related to the Frank-Condon principle in molecules accounts for the generation of coherent optical phonons. This mechanism is based on the sudden change of the equilibrium position of the lattice atoms after pulsed optical excitation of a dense electron-hole plasma. 6,7 By this mechanism, only modes of high symmetry can be excited, 6,7 such as the breathing mode of A1 symmetry in Te. We have recently shown that other modes of E symmetry, which are Raman and infrared active, are also coherently excited, but appear in the time-resolved reflectivity changes with an amplitude about two orders of magnitude smaller than the A1 mode. 8 Since the elements of the Raman tensors relevant for the detection of these phonons 8 differ only by a factor of 2 in amplitude, 9 the large differences in the detected phonon amplitudes are due to the strong and selective displacive excitation mechanism for the A1 mode. In contrast, the E modes are most probably excited via a stimulated Raman process. 10

Does Interfacial Charge Transfer Compete with Charge Carrier Recombination? A Femtosecond Diffuse Reflectance Investigation of TiO2 Nanoparticles

Abstract: The charge carrier dynamics of opaque, aqueous suspensions of Degussa P-25 TiO2 are probed with femtosecond time-resolved diffuse reflectance spectroscopy. Comparison of ultrafast pump−probe diffuse reflectance measurements of P-25 suspensions with dry P-25 powder and the transient absorption of transparent, aqueous Q-TiO2 solutions allows the observed kinetics to be assigned to charge carrier recombination. The electron−hole recombination kinetics are consistent with a second-order process as demonstrated by a laser fluence dependence study. Interfacial hole transfer dynamics of the P-25 TiO2/SCN- complex are probed as a function of thiocyanate ion concentration. A dramatic increase in the population of trapped charge carriers is observed within the first few picoseconds, demonstrating that interfacial charge transfer of an electron from the SCN- to a hole on the photoexcited TiO2 effectively competes with electron−hole recombination on an ultrafast time scale. The experimental dependence of the charge c...

Ultrafast carrier dynamics in semiconductor quantum dots

Abstract: The dynamics of band-edge photoluminescence (PL) in CdS nanocrystals (NC's) dispersed in a glass matrix are studied with the femtosecond up-conversion technique. The time-resolved PL spectra exhibit several discrete features (three of them are in the NC energy band gap) which are not pronounced in a cw PL spectrum. The initial stage of a PL decay is governed by a depopulation of the lowest extended states due to carrier trapping (localization) on the time scale of 1 ps. The low-energy bands originating from the extended-to-localized state transitions exhibit extremely fast buildup dynamics (rise time is 400--700 fs) which is explained by the preexisting occupation of the localized states. \textcopyright{} 1996 The American Physical Society.

Detection of THz pulses by phase retardation in lithium tantalate

Abstract: Freely propagating THz pulses are recorded in a thin lithium tantalate crystal by monitoring the phase retardation of a femtosecond probe pulse. This technique permits the determination of the temporal shape of the THz pulse in the picosecond time domain and offers the exciting prospect of up-converting the spatial THz beam profile into the optical wavelength range in real time, with potential application in diagnostics.

Terahertz radiation from superconducting YBa2Cu3O7−δ thin films excited by femtosecond optical pulses

Abstract: Ultrashort electromagnetic waves (600 fs width) from superconducting YBCO thin films have been observed by irradiating current‐biased samples with femtosecond optical laser pulses (80 fs width). The Fourier component of the pulse extends up to ∼2 THz. The characteristics of the radiation are studied and the radiation mechanism is ascribed to the ultrafast supercurrent modulation by the laser pulses, which induce the nonequilibrium superconductivity.

Femtosecond Real-Time Probing of Reactions. 19. Nonlinear (DFWM) Techniques for Probing Transition States of Uni- and Bimolecular Reactions

Abstract: Degenerate four-wave mixing (DFWM), using ∼60 femtosecond (fs) laser pulses, is introduced to study transition-state dynamics of chemical reactions in the gas phase. The ultrafast techniques are applied to a range of systems, atomic, unimolecular, and bimolecular. It is shown how fs DFWM can be incorporated in different temporal pulse schemes to extract the dynamics. The DFWM beams are configured in a folded boxcar geometry, producing a spatially separated, background-free, femtosecond signal pulse. Aspects of the technique, such as absorption, are investigated. We have taken advantage of the relatively broad spectral width of the fs pulses and extended the techniques to two-color grating experiments in the gas phase. The unimolecular system, NaI, provided a means of testing this new approach. Our experimental observations of the wave packet motion are in excellent agreement with results obtained using laser-induced fluorescence (LIF). A control experiment was also performed on this system, demonstrating ...

Laser wakefield excitation and measurement by femtosecond longitudinal interferometry

Abstract: Summary form only given. Because the electrostatic fields present in plasma waves can exceed those achievable in conventional accelerators and approach atomic scale values, plasma-based accelerators have received considerable attention as compact sources of high-energy electron pulses. Optical probes such as stimulated Raman scattering or terahertz radiation at /spl omega//sub p/ have provided only spatially averaged indications of the wave's existence. However, the ability to measure plasma wave microstructure directly is important for addressing fundamental issues of wakefield generation and propagation. We report femtosecond time-resolved measurements of the longitudinal and radial structure of laser wakefield oscillations using an all optical technique known as photon acceleration.

Acceleration in femtosecond laser-produced plasmas

Abstract: The influence of the acceleration of a femtosecond laser‐generated plasma on the reflected spectrum of the plasma‐producing pulse is analyzed quantitatively, and compared to experimental results. It is shown that the spectral positions of the reflected laser light are complicated functions of the temporally varying optical properties of the plasma and the hydrodynamic motion. The linewidths, however, depend only on the acceleration of the plasma mirror and the chirp of the laser pulse and can consequently be used to measure the acceleration in a laser‐produced plasma. Plasma accelerations on the order of 1018 m s−2≊1017 g directed both away from the solid target at intensities of I≤1017 W cm−2 and into the target for I≥3×1017 W cm−2 are obtained from an analysis of the experiments. The results demonstrate that during the short subpicosecond laser pulse the plasma motion is actually dominated by acceleration rather than by a constant expansion velocity. The measured accelerations are among the highest acce...

Femtosecond two-dimensional spectroscopy of molecular motion in liquids.

Abstract: Intermolecular motion in CS2 and benzene is investigated by femtosecond nonresonant four- and six-wave mixing. Impulsive stimulated six-wave mixing yields new information on dephasing of coherent nuclear motion, not accessible from four-wave mixing experiments. The results cannot be modeled by two independent harmonic modes accounting for coherent librations and rotational diffusion.

Femtosecond time-resolved photoionization and photoelectron spectroscopy studies of ultrafast internal conversion in 1,3,5-hexatriene

Abstract: Ultrafast photodynamics in a 1,3,5‐hexatriene are studied using femtosecond time‐resolved photoionization and photoelectron spectroscopy The trans and cis isomers have distinctly different dynamics following excitation at the S2 origin near 250 nm An intermediate, presumably the S1 state, is observed for both trans and cis isomers with lifetimes of 270 fs and 730 fs, respectively Time‐delayed photoelectron spectra of cis‐hexatriene determine a 300 fs time scale for vibrational energy redistribution within the intermediate S1 state

Femtosecond self- and cross-phase modulation in semiconductor laser amplifiers

Abstract: We present detailed derivation of our new model for femtosecond pulse amplification in semiconductor laser amplifiers. The various dynamic nonlinear terms of gain compression and associated self-phase modulation are derived semiphenomenologically, and are discussed physically. Included are the effects of carrier depletion, carrier heating and spectral hole-burning, as well as linear and two photon absorption and the instantaneous nonlinear index. Additionally, we account for dynamically changing gain curvature and slope. We apply the theory to strong signal cross-phase-cross-gain modulation experiments with /spl sim/500 fs pulses in a broad area GaAs amplifier and show that the model accurately describes the observed complex phenomena. We also present experimental results on single beam strong signal amplification in two different quantum-well amplifiers using 150-200 fs duration pulses. For such pulse lengths, carrier heating becomes an integrating nonlinearity and its self-phase modulation is similar to that due to carrier depletion. Additionally, since the pulse spectrum is broad, the gain slope and curvature shift and narrow it. The resultant spectral distortions are very different than observed (and modeled) earlier for the /spl sim/500 fs pulses. The model is again able to correctly describe the evolution of these ultrashort pulses, indicating that it remains valid, even though pulse durations approach the intraband relaxation time.

Femtosecond Real-Time Probing of Reactions. 20. Dynamics of Twisting, Alignment, and IVR in the trans-Stilbene Isomerization Reaction

Abstract: The femtosecond real-time dynamics of the isomerization reaction of trans-stilbene under collisionless conditions are studied using (2+1) resonance-enhanced multiphoton ionization (REMPI) and femtosecond depletion spectroscopy (FDS) in a pump−probe scheme. The observed transients reflect the macroscopic sample anisotropy decay (rotational coherence) and intramolecular vibrational energy redistribution (IVR) as well as the ethylenic twisting isomerization reaction. Polarization-resolved measurements are performed to isolate the influence of rotational dynamics, and the measured anisotropy decay is compared with theoretical calculations of rotational coherence at room temperature. The IVR and nonradiative (isomerization) processes of trans-stilbene are studied as a function of S_1 excess vibrational energy up to ∼6500 cm^(-1). These results are compared with previous measurements of trans-stilbene under collisionless conditions and with predictions of RRK and RRKM theories.