Showing papers on "Femtosecond published in 2002"
TL;DR: This review focuses on the nature of the non-thermal transitions in semiconductors under femtosecond laser excitation.
Abstract: Soon after it was discovered that intense laser pulses of nanosecond duration from a ruby laser could anneal the lattice of silicon, it was established that this so-called pulsed laser annealing is a thermal process. Although the radiation energy is transferred to the electrons, the electrons transfer their energy to the lattice on the timescale of the excitation. The electrons and the lattice remain in equilibrium and the laser simply 'heats' the solid to the melting temperature within the duration of the laser pulse. For ultrashort laser pulses in the femtosecond regime, however, thermal processes (which take several picoseconds) and equilibrium thermodynamics cannot account for the experimental data. On excitation with femtosecond laser pulses, the electrons and the lattice are driven far out of equilibrium and disordering of the lattice can occur because the interatomic forces are modified due to the excitation of a large (10% or more) fraction of the valence electrons to the conduction band. This review focuses on the nature of the non-thermal transitions in semiconductors under femtosecond laser excitation.
837 citations
TL;DR: In this paper, the mechanism of ablation of solids by intense femtosecond laser pulses is described in an explicit analytical form, and the formulas for ablation thresholds and ablation rates for metals and dielectrics, combining the laser and target parameters, are derived and compared to experimental data.
Abstract: The mechanism of ablation of solids by intense femtosecond laser pulses is described in an explicit analytical form. It is shown that at high intensities when the ionization of the target material is complete before the end of the pulse, the ablation mechanism is the same for both metals and dielectrics. The physics of this new ablation regime involves ion acceleration in the electrostatic field caused by charge separation created by energetic electrons escaping from the target. The formulas for ablation thresholds and ablation rates for metals and dielectrics, combining the laser and target parameters, are derived and compared to experimental data. The calculated dependence of the ablation thresholds on the pulse duration is in agreement with the experimental data in a femtosecond range, and it is linked to the dependence for nanosecond pulses.
749 citations
TL;DR: In this paper, the authors studied the generation of supercontinua in air-silica microstructured fibers by both nanosecond and femtosecond pulse excitation.
Abstract: We study the generation of supercontinua in air–silica microstructured fibers by both nanosecond and femtosecond pulse excitation. In the nanosecond experiments, a 300-nm broadband visible continuum was generated in a 1.8-m length of fiber pumped at 532 nm by 0.8-ns pulses from a frequency-doubled passively Q-switched Nd:YAG microchip laser. At this wavelength, the dominant mode excited under the conditions of continuum generation is the LP11 mode, and, with nanosecond pumping, self-phase modulation is negligible and the continuum generation is dominated by the interplay of Raman and parametric effects. The spectral extent of the continuum is well explained by calculations of the parametric gain curves for four-wave mixing about the zero-dispersion wavelength of the LP11 mode. In the femtosecond experiments, an 800-nm broadband visible and near-infrared continuum has been generated in a 1-m length of fiber pumped at 780 nm by 100-fs pulses from a Kerr-lens model-locked Ti:sapphire laser. At this wavelength, excitation and continuum generation occur in the LP01 mode, and the spectral width of the observed continuum is shown to be consistent with the phase-matching bandwidth for parametric processes calculated for this fiber mode. In addition, numerical simulations based on an extended nonlinear Schrodinger equation were used to model supercontinuum generation in the femtosecond regime, with the simulation results reproducing the major features of the experimentally observed spectrum.
398 citations
TL;DR: In this paper, the propagation dynamics of femtosecond electron packets in the drift region of a photoelectron gun are investigated with an N-body numerical simulation and mean-field model.
Abstract: Time-resolved electron diffraction harbors great promise for resolving the fastest chemical processes with atomic level detail. The main obstacles to achieving this real-time view of a chemical reaction are associated with delivering short electron pulses with sufficient electron density to the sample. In this article, the propagation dynamics of femtosecond electron packets in the drift region of a photoelectron gun are investigated with an N-body numerical simulation and mean-field model. It is found that space-charge effects can broaden the electron pulse to many times its original length and generate many eV of kinetic energy bandwidth in only a few nanoseconds. There is excellent agreement between the N-body simulation and the mean-field model for both space-charge induced temporal and kinetic energy distribution broadening. The numerical simulation also shows that the redistribution of electrons inside the packet results in changes to the pulse envelope and the development of a spatially linear axia...
324 citations
TL;DR: It is predicted and quantitatively evaluated the unique possibility of concentrating the energy of an ultrafast excitation of a nanosystem in a small part of the whole system by means of coherent control (phase modulation of the exciting ultrashort pulse).
Abstract: We predict and quantitatively evaluate the unique possibility of concentrating the energy of an ultrafast excitation of a nanosystem in a small part of the whole system by means of coherent control (phase modulation of the exciting ultrashort pulse). Such concentration is due to dynamic properties of surface plasmons and leads to local fields enhanced by orders of magnitude. This effect exists for both "engineered" and random nanosystems. We also discuss possible applications.
322 citations
TL;DR: Using time-resolved imaging and scattering techniques, the breakdown dynamics induced in water by femtosecond laser pulses are monitored and a 20 ps delay is observed before the laser-produced plasma expands.
Abstract: Using time-resolved imaging and scattering techniques, we directly and indirectly monitor the breakdown dynamics induced in water by femtosecond laser pulses over eight orders of magnitude in time. We resolve, for the first time, the picosecond plasma dynamics and observe a 20 ps delay before the laser-produced plasma expands. We attribute this delay to the electron-ion energy transfer time.
318 citations
TL;DR: The generation of ultrashort radiation pulses became possible due to specific tailoring of the bunch charge distribution and a complete characterization of the linear and nonlinear modes of the SASE FEL operation was performed.
Abstract: Experimental results are presented from vacuum-ultraviolet free-electron laser (FEL) operating in the self-amplified spontaneous emission (SASE) mode. The generation of ultrashort radiation pulses became possible due to specific tailoring of the bunch charge distribution. A complete characterization of the linear and nonlinear modes of the SASE FEL operation was performed. At saturation the FEL produces ultrashort pulses (30–100 fs FWHM) with a peak radiation power in the GW level and with full transverse coherence. The wavelength was tuned in the range of 95–105 nm.
294 citations
TL;DR: In this paper, a method aimed at quantifying the dimensions of the heat-affected zone (HAZ), produced during nanosecond and femtosecond laser-matter interactions was presented.
Abstract: This letter presents a method aimed at quantifying the dimensions of the heat-affected zone (HAZ), produced during nanosecond and femtosecond laser–matter interactions. According to this method, 0.1 μm thick Al samples were microdrilled and observed by a transmission electronic microscopy technique. The holes were produced at laser fluences above the ablation threshold in both nanosecond and femtosecond regimes (i.e., 5 and 2 J/cm2, respectively). The grain size in the samples was observed near the microholes. The main conclusion is that a 40 μm wide HAZ is induced by the nanosecond pulses, whereas the femtosecond regime does not produce any observable HAZ. It turns out that the width of the femtosecond HAZ is less than 2 μm, which is our observation limit.
264 citations
TL;DR: Time-resolved studies using femtosecond laser pulses, accompanied by model calculations, that illuminate the difference in the dynamics of ultrashort pulsed laser ablation of different materials are reported.
Abstract: We report time-resolved studies using femtosecond laser pulses, accompanied by model calculations, that illuminate the difference in the dynamics of ultrashort pulsed laser ablation of different materials. Dielectrics are strongly charged at the surface on the femtosecond time scale and undergo an impulsive Coulomb explosion. This is not seen from metals and semiconductors where the surface charge is effectively quenched.
246 citations
TL;DR: It is reported on the generation of linearly chirped parabolic pulses with 17-W average power at 75 MHz repetition rate and diffraction-limited beam quality in a large-mode-area ytterbium-doped fiber amplifier.
Abstract: We report on the generation of linearly chirped parabolic pulses with 17-W average power at 75 MHz repetition rate and diffraction-limited beam quality in a large-mode-area ytterbium-doped fiber amplifier. Highly efficient transmission gratings in fused silica are applied to recompress these pulses down to 80-fs with an efficiency of 60%, resulting in a peak power of 1.7 MW. Power scaling limitations given by the amplifier bandwidth are discussed.
244 citations
TL;DR: In this paper, the supercontinuum spectrum of an intense femtosecond Ti:sapphire laser pulse propagating in condensed optical media (water, chloroform and glass), at various input laser energies, below and above the threshold for filamentation of the laser pulse was measured.
TL;DR: It is found that higher-order dispersion primarily determines the shape and width of the generated spectrum and that the fine spectral substructure exhibits extreme sensitivity to the initial pulse energy.
Abstract: A theoretical investigation of the propagation of femtosecond pulses under conditions similar to those of recent experiments in which a white-light continuum was generated in a microstructured fiber is presented. It is found that higher-order dispersion primarily determines the shape and width of the generated spectrum and that the fine spectral substructure exhibits extreme sensitivity to the initial pulse energy.
TL;DR: It is demonstrated that the ablation process involves three different mechanisms as a function of deposited energy, which can result from mechanical fragmentation, which does not require the system to cross any metastability or instability line.
Abstract: We study the basic mechanisms leading to ablation by femtosecond laser pulses using molecular dynamics and a simple two-dimensional Lennard-Jones model. We demonstrate that the ablation process involves three different mechanisms as a function of deposited energy. In particular, it can result from mechanical fragmentation, which does not require the system to cross any metastability or instability line. The relevance of homogeneous nucleation and vaporization for the description of ablation in this regime is also established.
TL;DR: In this article, the authors proposed the Fast CARS (femtosecond adaptive spectroscopic techniques for coherent anti-Stokes Raman spectroscopy) method.
Abstract: Airborne contaminants, e.g., bacterial spores, are usually analyzed by time-consuming microscopic, chemical, and biological assays. Current research into real-time laser spectroscopic detectors of such contaminants is based on e.g., resonance fluorescence. The present approach derives from recent experiments in which atoms and molecules are prepared by one (or more) coherent laser(s) and probed by another set of lasers. However, generating and using maximally coherent oscillation in macromolecules having an enormous number of degrees of freedom is challenging. In particular, the short dephasing times and rapid internal conversion rates are major obstacles. However, adiabatic fast passage techniques and the ability to generate combs of phase-coherent femtosecond pulses provide tools for the generation and utilization of maximal quantum coherence in large molecules and biopolymers. We call this technique FAST CARS (femtosecond adaptive spectroscopic techniques for coherent anti-Stokes Raman spectroscopy), and the present article proposes and analyses ways in which it could be used to rapidly identify preselected molecules in real time.
TL;DR: Direct vibrational-spectroscopic evidence for the isomerization taking place within 0.5 picosecond is presented, as has been suggested by previous optical femtosecond time-resolved experiments but questioned recently by picose Cond time-Resolved vibrational spectroscopy experiments.
Abstract: The vibrational dynamics of the retinal chromophore all-trans–to–13-cis photoisomerization in bacteriorhodopsin has been studied with mid-infrared absorption spectroscopy at high time resolution (about 200 femtoseconds). After photoexcitation of light-adapted bacteriorhodopsin, the transient infrared absorption was probed in a broad spectral region, including vibrations with dominant C–C, C=C, and C=NH stretching mode amplitude. All photoproduct modes, especially those around 1190 reciprocal-centimeters that are indicative for a 13-cis configuration of the chromophore, rise with a time constant of ∼0.5 picosecond. The results presented give direct vibrational-spectroscopic evidence for the isomerization taking place within 0.5 picosecond, as has been suggested by previous optical femtosecond time-resolved experiments but questioned recently by picosecond time-resolved vibrational spectroscopy experiments.
TL;DR: It is demonstrated that by changing only the writing speed, waveguides with a controllable mode number can be produced.
Abstract: With tightly focused femtosecond laser pulses, waveguides are fabricated in fused silica. The guiding and attenuation properties of these waveguides at wavelengths of 514 nm and 1.5 microm are studied. We demonstrate that by changing only the writing speed, waveguides with a controllable mode number can be produced.
TL;DR: The femtosecond operation of a broadband diode-pumped external-cavity surface-emitting semiconductor laser, passively mode locked with a fast quantum-well Semiconductor Saturable Absorber Mirror grown at 735°C, was reported in this paper.
Abstract: We report on femtosecond operation of a broadband diode-pumped external-cavity surface-emitting semiconductor laser, passively mode locked with a fast quantum–well Semiconductor Saturable Absorber Mirror grown at 735 °C. We obtained 477 fs pulses at 1.21 GHz. The average output power is 100 mW at 1040 nm, the pulse peak power 152 W, with ∼1 W of 830 nm pump. The rf spectrum shows a linewidth <50 kHz at the noise level (−65 dB). We believe that the group-delay dispersion is compensated by the negative self-phase modulation in the absorber structure, leading to soliton-like mode locking. This system requires no additional technological step after the growth of the structures.
TL;DR: In this article, the Teramobile system, a femtosecond multi-terawatt laser and detection system based on a CPA laser system embedded in a standard freight container, is described.
Abstract: We describe the Teramobile system, a new mobile femtosecond multi-terawatt laser and detection system based on a state-of-the art CPA laser system embedded in a standard freight container, as well as a mobile detection unit allowing a characterization of the nonlinear propagation of high power laser pulses over long horizontal distances. The unique mobility feature of the whole system opens the way to previously unreachable applications for high-power laser pulses in the field of atmospheric research (lidar, laser-triggered lightning), which are also briefly reviewed.
TL;DR: In this article, three different artifacts related to two-photon absorption (TPA), stimulated Raman amplification (SRA), and cross-phase modulation (XPM) are discussed.
Abstract: The paper discusses three different artifacts related to two-photon absorption (TPA), stimulated Raman amplification (SRA) and cross-phase modulation (XPM), all intrinsic to transient absorption measurements with femtosecond time resolution. Certain properties of these signals are analysed and shown to superimpose onto measured transient absorption spectra. Ways of reducing the influence of the artifacts discussed are suggested. A simple correcting procedure based on the linear intensity dependence of the artifacts discussed is proposed.
TL;DR: In this paper, transient absorption spectroscopy has been used to probe the electron injection dynamics of transition metal polypyridyl complexes adsorbed onto nanocrystalline TiO2 photoelectrodes.
Abstract: Transient absorption spectroscopy has been used to probe the electron injection dynamics of transition metal polypyridyl complexes adsorbed onto nanocrystalline TiO2 photoelectrodes. Experiments were performed on photoelectrodes coated with Ru(H2L‘)2(CN)2, Os(H2L‘)2(CN)2, Ru(H2L‘)2(NCS)2, or Os(H2L‘)2(NCS)2, where H2L‘ is 4,4‘-dicarboxylic acid-2,2‘-bipyridine, to study how the excited-state energetics and the nature of the metal center affect the injection kinetics. All of these complexes exhibited electron injection dynamics on both the femtosecond and picosecond time scales. The femtosecond components were instrument-limited (<200 fs), whereas the picosecond components ranged from 3.3 ± 0.3 ps to 14 ± 4 ps (electron injection rate constants k2‘ = (7.1−30) × 1010 s-1). The picosecond decay component became more rapid as the formal excited-state reduction potential of the complex became more negative. Variable excitation wavelength studies suggest that femtosecond injection is characteristic of the nonth...
TL;DR: In this article, a significant improvement in the quality of ultrafast laser microstructuring of dielectrics is demonstrated by using temporally shaped pulse trains with subpicosecond separation.
Abstract: A significant improvement in the quality of ultrafast laser microstructuring of dielectrics is demonstrated by using temporally shaped pulse trains with subpicosecond separation. The sequential energy delivery induces a material softening during the initial steps of excitation changing the energy coupling for the subsequent steps. This leads to lower stress, cleaner structures, and provides a material-dependent optimization process.
TL;DR: A new spatial beam-shaping approach for fabrication of waveguides with a circular transverse profile by femtosecond laser pulses, using an astigmatic beam and controlling both beam waist and focal position in the tangential and sagittal planes is reported.
Abstract: We report on a new spatial beam-shaping approach for fabrication of waveguides with a circular transverse profile by femtosecond laser pulses, using an astigmatic beam and controlling both beam waist and focal position in the tangential and sagittal planes. We apply this technique to write single-mode active waveguides at 1.5microm in Er:Yb-doped glass substrates. The experimental results are well described by a simple nonlinear absorption model.
TL;DR: What is believed to be the first observation of coherent subterahertz (sub-THz) emission from a 1-m string in the atmosphere is reported and evidence of constructive interference between two separate strings is shown.
Abstract: We report on what is believed to be the first observation of coherent subterahertz (sub-THz) emission from a 1-m string in the atmosphere. The sub-THz pulse emitted by the filamentary structure from an intense IR femtosecond laser pulse is detected perpendicularly to the laser propagation axis by use of two heterodyne detectors at 94±1 and 118±1 GHz. We describe the characteristics of this emission and show evidence of constructive interference between two separate strings.
TL;DR: In this paper, the design and characterization of a femtosecond high-intensity laser system emitting a near-diffraction-limited beam was presented, which was dimensioned in order to reach intensities in excess of 1020 W/cm2 at a high repetition rate for ultrahigh-field physics experiments.
Abstract: We present the design and characterization of a femtosecond high-intensity laser system emitting a near-diffraction-limited beam. This system was dimensioned in order to reach intensities in excess of 1020 W/cm2 at a high repetition rate for ultrahigh-field physics experiments. We describe the improvements that were added to a conventional chirp pulse amplification configuration in order to decrease the deleterious effects of gain narrowing, gain shifting, thermal focusing in the amplifier stages, and spatial degradation due to multipass amplification processes.
TL;DR: By tailoring the phase of a 100 femtosecond probe pulse, a narrow-band coherent anti-Stokes Raman spectroscopy (CARS) resonant signal is obtained with a width of less than 15 cm(-1), which is an order of magnitude narrower than the CARS signal from a transform limited pulse.
Abstract: By tailoring the phase of a 100 femtosecond probe pulse we are able to obtain a narrow-band coherent anti-Stokes Raman spectroscopy (CARS) resonant signal with a width of less than 15 cm(-1), which is an order of magnitude narrower than the CARS signal from a transform limited pulse. Thus, by measuring the spectrum of the CARS signal we are able to obtain a high-resolution energy level diagram of the probed sample in spite of the broad femtosecond pulse spectrum.
TL;DR: M maskless, single-step fabrication of strongly birefringent Fresnel zone plates by focusing of femtosecond laser pulses deep within silica substrates is demonstrated.
Abstract: We demonstrate maskless, single-step fabrication of strongly birefringent Fresnel zone plates by focusing of femtosecond laser pulses deep within silica substrates. The process allows us to produce alternate zone rings directly by inducing a local refractive-index modification of the order of n~10-2 . The embedded zone plates shown in this Letter exhibit efficiencies that vary by as much as a factor of ~6 for orthogonal polarizations. Focal lengths of primary and secondary foci are shown to compare well with theory.
TL;DR: Using the direct output of an extended cavity femtosecond laser, without the need for a laser amplifier, single mode waveguides can be rapidly fabricated with well controlled parameters.
Abstract: Coupled mode devices are fabricated in transparent glasses by nonlinear materials processing with femtosecond laser pulses Using the direct output of an extended cavity femtosecond laser, without the need for a laser amplifier, single mode waveguides can be rapidly fabricated with well controlled parameters A variety of photonic waveguide devices are demonstrated Directional couplers with various interaction lengths and coupling coefficients are fabricated and their coupling properties are characterized Measurements demonstrate coupled mode behavior consistent with theory An unbalanced Mach-Zehnder interferometer is also fabricated and demonstrated as a spectral filter
TL;DR: The nonlinear response of single GaAs quantum dots is studied in femtosecond near-field pump-probe experiments, providing the first evidence for a perturbed free induction decay of the excitonic polarization.
Abstract: The nonlinear response of single GaAs quantum dots is studied in femtosecond near-field pump-probe experiments At negative time delays, transient reflectivity spectra show pronounced oscillatory structure around the quantum dot exciton line, providing the first evidence for a perturbed free induction decay of the excitonic polarization Phase-disturbing Coulomb interactions between the excitonic polarization and continuum excitations dominate the optical nonlinearity on ultrafast time scales A theoretical analysis based on the semiconductor Bloch equations accounts for this behavior
TL;DR: In this article, a model of the energy transport is used to predict the carrier temperature and lattice temperature as well as the electron and vapor flux emitted from the surface of the material.
Abstract: Mechanisms of ultrafast (femtosecond) laser-induced ablation on crystalline silicon are investigated by time-resolved pump-and-probe microscopy in normal imaging and shadowgraph arrangements. A one-dimensional model of the energy transport is utilized to predict the carrier temperature and lattice temperature as well as the electron and vapor flux emitted from the surface. The temporal delay between the pump and probe pulses is set by a precision translation stage up to about 500 ps and then extended to the nanosecond regime by an optical fiber assembly. The ejection of material is observed at several picoseconds to tens of nanoseconds after the main (pump) pulse by high-resolution, ultrafast shadowgraphs.
TL;DR: In this article, a femtosecond, time-resolved two photon photoemission has been used to map the dynamics of photo-excited electrons at a structured metal/semiconductor surface.
Abstract: Femtosecond, time-resolved two photon photoemission has been used to map the dynamics of photo-excited electrons at a structured metal/semiconductor surface. A photoemission microscope was employed as a spatially resolving electron detector. This novel setup has the potential to visualize variations of hot electron lifetimes in the femtosecond regime on heterogeneous sample surfaces and nanostructures.