Showing papers on "Ultrashort pulse published in 2004"
TL;DR: With the current ∼750-nm laser probe and ∼100-eV excitation, the transient recorder is capable of resolving atomic electron dynamics within the Bohr orbit time.
Abstract: In Bohr's model of the hydrogen atom, the electron takes about 150 attoseconds (1 as = 10(-18) s) to orbit around the proton, defining the characteristic timescale for dynamics in the electronic shell of atoms. Recording atomic transients in real time requires excitation and probing on this scale. The recent observation of single sub-femtosecond (1 fs = 10(-15) s) extreme ultraviolet (XUV) light pulses has stimulated the extension of techniques of femtochemistry into the attosecond regime. Here we demonstrate the generation and measurement of single 250-attosecond XUV pulses. We use these pulses to excite atoms, which in turn emit electrons. An intense, waveform-controlled, few cycle laser pulse obtains 'tomographic images' of the time-momentum distribution of the ejected electrons. Tomographic images of primary (photo)electrons yield accurate information of the duration and frequency sweep of the excitation pulse, whereas the same measurements on secondary (Auger) electrons will provide insight into the relaxation dynamics of the electronic shell following excitation. With the current approximately 750-nm laser probe and approximately 100-eV excitation, our transient recorder is capable of resolving atomic electron dynamics within the Bohr orbit time.
1,119 citations
TL;DR: Self-similar propagation of ultrashort, parabolic pulses in a laser resonator is observed theoretically and experimentally, constituting a new type of pulse shaping in mode-locked lasers.
Abstract: Self-similar propagation of ultrashort, parabolic pulses in a laser resonator is observed theoretically and experimentally. This constitutes a new type of pulse shaping in mode-locked lasers: in contrast to the well-known static (solitonlike) and breathing (dispersion-managed soliton) pulse evolutions, asymptotic solutions to the nonlinear wave equation that governs pulse propagation in most of the laser cavity are observed. Stable self-similar pulses exist with energies much greater than can be tolerated in solitonlike pulse shaping, and this has implications for practical lasers.
841 citations
TL;DR: In this article, a carrier-envelope offset (CEO) phase locked few-cycle pulses are generated using self-guiding of intense 43-fs, 0.84 mJ optical pulses during propagation in a transparent noble gas.
Abstract: Intense, well-controlled light pulses with only a few optical cycles start to play a crucial role in many fields of physics, such as attosecond science. We present an extremely simple and robust technique to generate such carrier-envelope offset (CEO) phase locked few-cycle pulses, relying on self-guiding of intense 43-fs, 0.84 mJ optical pulses during propagation in a transparent noble gas. We have demonstrated 5.7-fs, 0.38 mJ pulses with an excellent spatial beam profile and discuss the potential for much shorter pulses. Numerical simulations confirm that filamentation is the mechanism responsible for pulse shortening. The method is widely applicable and much less sensitive to experimental conditions such as beam alignment, input pulse duration or gas pressure as compared to gas-filled hollow fibers.
564 citations
TL;DR: A noninterferometric single beam method to characterize and compensate the spectral phase of ultrashort femtosecond pulses accurately and is ideally suited for the generation of tailored spectral phase functions required for coherent control experiments.
Abstract: We introduce a noninterferometric single beam method to characterize and compensate the spectral phase of ultrashort femtosecond pulses accurately. The method uses a pulse shaper that scans calibrated phase functions to determine the unknown spectral phase of a pulse. The pulse shaper can then be used to synthesize arbitrary phase femtosecond pulses or it can introduce a compensating spectral phase to obtain transform-limited pulses. This method is ideally suited for the generation of tailored spectral phase functions required for coherent control experiments.
425 citations
TL;DR: Ablation of Fe by ultrashort laser pulses with durations 0.1, 1, and 5 ps were investigated experimentally in this paper, showing that the change in the ablation rate is connected to an overheating of the material above the critical point, which results in a steep rise of the pressure developed.
Abstract: Ablation of Fe by ultrashort laser pulses with durations 0.1, 1, and 5 ps were investigated experimentally. The laser fluence varied from the ablation threshold up to 100 J cm−2. Above 1 J cm−2, the ablation rate depended on the laser pulse duration, with the shortest pulse producing the highest value. A change in the ablation rate as the laser fluence increased was also observed. These results were analysed using molecular dynamics simulations. We show that the change in the ablation rate is connected to an overheating of the material above the critical point, which results in a steep rise of the pressure developed. Furthermore, due to the electron heat diffusion, the overheated volume increases and involves material located deeper than the skin depth. An increase in the pulse duration results in a decrease in the degree of overheating.
340 citations
TL;DR: In this article, a semiconductor photoconductive switch with femtosecond laser illumination is used as a "pulse generator" to produce jitter-free, sub-100 ps rise time step function-like electrical pulses.
Abstract: We present an experimental approach to study the ultrafast polarization switching dynamics in thin-film ferroelectrics. A semiconductor photoconductive switch with femtosecond laser illumination is used as a “pulse generator” to produce jitter-free, sub-100 ps rise time step-function-like electrical pulses. Quantitative measurements yield a polarization switching time, ts, of ∼220 ps when measured with a 5 V, 68 ps rise time input electrical pulse. Modeling of the switching transients using the Merz–Ishibashi model and Merz–Shur model of switching kinetics yields a quantitative estimate of the characteristic switching time constant, t0, of ∼70–90 ps.
290 citations
TL;DR: In this paper, the interaction of ultrashort laser pulses with materials involves a number of special features that are different from laser-matter interaction for longer pulse durations, and the fundamental physical processes such as energy deposition, melting and ablation are separated in time.
Abstract: The interaction of ultrashort laser pulses with materials involves a number of special features that are different from laser–matter interaction for longer pulse durations. For femtosecond laser excitation the fundamental physical processes such as energy deposition, melting, and ablation are separated in time. By choosing proper time windows, the various processes can be investigated separately. We present selected examples of theoretical studies of free electron excitation in metals, timescales of different melting processes, and peculiarities of near-threshold ablation. Depending on the timescales and intensity ranges, the discussed processes are combined in an overall picture of possible pathways of the material from excitation to ablation.
285 citations
TL;DR: By combining simulation and experiments, the generation mechanism of the visible peak is explored and it is demonstrated that the blue peak is generated only when the input pulse is so strongly compressed that the short-wavelength tail of the spectrum includes the wavelength predicted for the dispersive wave.
Abstract: We study the nonlinear propagation of femtosecond pulses in the anomalous dispersion region of microstructured fibers, where soliton fission mechanisms play an important role. The experiment shows that the output spectrum contains, besides the infrared supercontinuum, a narrow-band 430-nm peak, carrying about one fourth of the input energy. By combining simulation and experiments, we explore the generation mechanism of the visible peak and describe its properties. The simulation demonstrates that the blue peak is generated only when the input pulse is so strongly compressed that the short-wavelength tail of the spectrum includes the wavelength predicted for the dispersive wave. In agreement with simulation, intensity-autocorrelation measurements show that the duration of the blue pulse is in the picosecond time range, and that, by increasing the input intensity, satellite pulses of lower intensity are generated.
285 citations
TL;DR: This work shows the generation of intense isolated pulses from a single harmonic by using a sub-10-femtosecond blue laser pulse, producing a large dipole moment at the relatively low (ninth) harmonic order nonadiabatically.
Abstract: Nonlinear responses to an optical field are universal in nature but have been difficult to observe in the extreme ultraviolet (XUV) and soft X-ray regions owing to a lack of coherent intense light sources. High harmonic generation is a well-known nonlinear optical phenomenon and is now drawing much attention in attosecond pulse generation. For the application of high harmonics to nonlinear optics in the XUV and soft X-ray regime, optical pulses should have both large pulse energy and short pulse duration to achieve a high optical electric field. Here we show the generation of intense isolated pulses from a single harmonic (photon energy 27.9 eV) by using a sub-10-femtosecond blue laser pulse, producing a large dipole moment at the relatively low (ninth) harmonic order nonadiabatically. The XUV pulses with pulse durations of 950 attoseconds and 1.3 femtoseconds were characterized by an autocorrelation technique, based on two-photon above-threshold ionization of helium atoms. Because of the small cross-section for above-threshold ionization, such an autocorrelation measurement of XUV pulses with photon energy larger than the ionization energy of helium has not hitherto been demonstrated. The technique can be extended to the characterization of higher harmonics at shorter wavelengths.
282 citations
TL;DR: In this paper, the key SESAM parameters such as saturation fluence, modulation depth, and nonsaturable losses are measured with a high accuracy to obtain stable pulse generation for a given laser.
Abstract: Semiconductor saturable absorber mirror (SESAM) devices have become a key component of ultrafast passive mode-locked laser sources Here we describe in more detail how the key SESAM parameters such as saturation fluence, modulation depth, and nonsaturable losses are measured with a high accuracy These parameters need to be known and controlled to obtain stable pulse generation for a given laser A high-precision, wide dynamic range setup is required to measure this nonlinear reflectivity of saturable absorbers The challenge to measure a low modulation depth and key measures necessary to obtain an accurate calibration are described in detail The model function for the nonlinear reflectivity is based on a simple two-level travelling wave system We include spatial beam profiles, nonsaturable losses and higher-order absorption, such as two-photon absorption and other induced absorption Guidelines to extract the key parameters from the measured data are given
263 citations
TL;DR: It is demonstrated that the use of time-dependent light polarization opens a new level of control over quantum systems, and a polarization-shaped laser pulse increases the ionization yield beyond that obtained with an optimally shaped linearly polarized laser pulse.
Abstract: We demonstrate that the use of time-dependent light polarization opens a new level of control over quantum systems. With potassium dimer molecules from a supersonic molecular beam, we show that a polarization-shaped laser pulse increases the ionization yield beyond that obtained with an optimally shaped linearly polarized laser pulse. This is due to the different multiphoton ionization pathways in K2 involving dipole transitions which favor different polarization directions of the exciting laser field. This experiment is a qualitative extension of quantum control mechanisms which opens up new directions giving access to the three-dimensional temporal response of molecular systems. DOI: 10.1103/PhysRevLett.92.208301
TL;DR: In this article, a velocity matching technique utilizing tilted pulse fronts is analyzed in comparison with quasi-phase matching in periodically poled LiNbO3 crystals, which provides a ten times higher pulse energy conversion efficiency.
Abstract: Optical rectification of ultrashort near-IR laser pulses with tilted pulse fronts and pulse energies of a few μJ in Mg-doped stoichiometric LiNbO3 cooled to low temperature is a powerful technique for efficient generation of THz pulses. The pulse energy critically depends on the Mg doping (necessary for preventing photorefractive damage) and can be easily increased by a factor of three if the MgO content is reduced. Pulse energies up to 400 pJ at repetition rates of 200 kHz and 3.4% quantum conversion efficiency are achieved at 77 K. At 10 K, changing the tilt angle of the pump pulse front results in continuous tuning of the frequency across the 1.0–4.4 THz range. The temporal pulse shapes measured by electro-optic sampling are in good agreement with the signal calculated by a simple theory. This model predicts tunability on a considerably broader range and narrower spectra even at room temperature if GaSe is used instead of LiNbO3. The advantages of the velocity matching technique utilizing tilted pulse fronts are analyzed in comparison with quasi-phase-matching in periodically poled LiNbO3 crystals. The first method provides a ten times higher pulse energy conversion efficiency.
TL;DR: High nonlinearity ranging from 2000 to 27,000 times that of fused silica, depending on the incident optical intensity is revealed in Ag(x)(As2Se3) chalcogenide glasses.
Abstract: We prepared Ag(x)(As0.4Se0.6)(100-x) chalcogenide glasses by a melt-quenching method and measured their linear and nonlinear optical properties to evaluate their potential applications to all-optical ultrafast switching devices. Their nonlinear refraction and absorption were measured by the Z-scan method at 1.05 microm. The addition of Ag to As2Se3 glass led to an increase in the nonlinear refractive index without introducing an increase in the nonlinear absorption coefficient. The glass with a Ag content of x = 20 at. % revealed high nonlinearity ranging from 2000 to 27,000 times that of fused silica, depending on the incident optical intensity.
TL;DR: In this article, material, spectroscopic, laser and nonlinear optical properties of wide-band Cr 2+ -doped II-VI materials have been reviewed and a considerable modification of the mode-locking techniques and reconsideration of the existing theories are discussed.
TL;DR: A new method for generating a parabolic pulse by use of a dispersion-decreasing fiber with normal group-velocity dispersion with highly linear chirp allows for efficient and high-quality pulse compression.
Abstract: We propose a new method for generating a parabolic pulse by use of a dispersion-decreasing fiber with normal group-velocity dispersion. When a hyperbolic dispersion-decreasing structure is employed, the pulse evolves into a linearly chirped pulse with an exact parabolic intensity profile without radiating dispersive waves. The highly linear chirp in the parabolic pulse allows for efficient and high-quality pulse compression.
TL;DR: A fast and high-accuracy three-dimensional imaging laser radar that can achieve better than 1-mm range accuracy for half a million pixels in less than 1 s is developed, based on range-gating segmentation.
Abstract: We have developed a fast and high-accuracy three-dimensional (3-D) imaging laser radar that can achieve better than 1-mm range accuracy for half a million pixels in less than 1 s. Our technique is based on range-gating segmentation. We combine the advantages of gated viewing with our new fast technique of 3-D imaging. The system uses a picosecond Q-switched Nd:Yag laser at 532 nm with a 32-kHz pulse repetition frequency (PRF), which triggers an ultrafast camera with a highly sensitive CCD with 582 x 752 pixels. The high range accuracy is achieved with narrow laser pulse widths of approximately 200 ps, a high PRF of 32 kHz, and a high-speed camera with gate times down to 200 ps and delay steps down to 100 ps. The electronics and the software also allow for gated viewing with automatic gain control versus range, whereby foreground backscatter can be suppressed. We describe our technique for the rapid production of high-accuracy 3-D images, derive performance characteristics, and outline future improvements.
TL;DR: This work has found that single 200 attosecond pulses could be produced efficiently in a lambda(3) laser pulse reflection, via deflection and compression from the relativistic plasma mirror created by the pulse itself.
Abstract: Lasers that provide an energy encompassed in a focal volume of a few cubic wavelengths (lambda(3)) can create relativistic intensity with maximal gradients, using minimal energy. With particle-in-cell simulations we found, that single 200 attosecond pulses could be produced efficiently in a lambda(3) laser pulse reflection, via deflection and compression from the relativistic plasma mirror created by the pulse itself. An analytical model of coherent radiation from a charged layer confirms the pulse compression and is in good agreement with simulations. The novel technique is efficient (approximately 10%) and can produce single attosecond pulses from the millijoule to the joule level.
TL;DR: In this article, the phase transformation dynamics induced in Ge2Sb2Te5 films by picosecond laser pulses were studied using real-time reflectivity measurements with subnanosecond resolution.
Abstract: The phase transformation dynamics induced in Ge2Sb2Te5 films by picosecond laser pulses were studied using real-time reflectivity measurements with subnanosecond resolution Evidence was found that the thermal diffusivity of the substrate plays a crucial role in determining the ability of the films to crystallize and amorphize A film/substrate configuration with optimized heat flow conditions for ultrafast phase cycling with picosecond laser pulses was designed and produced In this system, we achieved reversible phase transformations with large optical contrast (>20%) using single laser pulses with a duration of 30 ps within well-defined fluence windows The amorphization (writing) process is completed within less than 1 ns, whereas crystallization (erasing) needs approximately 13 ns to be completed
TL;DR: In this paper, the authors report long-range self-channeling in air of multiterawatt femtosecond laser pulses with large negative initial chirps, and show that the peak intensity in the light channels is at least one order of magnitude lower than required for multiphoton ionization of air molecules.
Abstract: We report long-range self-channeling in air of multiterawatt femtosecond laser pulses with large negative initial chirps. The peak intensity in the light channels is at least one order of magnitude lower than required for multiphoton ionization of air molecules. A detailed comparison is made between experiments and realistic 3+1-dimensional numerical simulations. It reveals that the mechanism limiting the growth of intensity by filamentation is connected with broken revolution symmetry in the transverse diffraction plane.
TL;DR: In this article, a comprehensive study of laser micro-machining with nanosecond, picosecond and femtosecond laser pulses is presented with a focus on fundamental aspects of the processes and on their practical consequences.
Abstract: On the fast growing market of precision micro-machining of metals lasers do not only compete with other methods of structuring There is also strong competition among different laser-processing strategies and, especially, among laser sources with different pulse duration A comprehensive study of laser micro-machining with nanosecond, picosecond, and femtosecond laser pulses will be presented with a focus on fundamental aspects of the processes and on their practical consequences An analysis will be given of the potential or the limitations of these laser processes with respect to their industrial application
TL;DR: It is shown that it is possible to organize regular filamentation patterns in air by imposing either strong field gradients or phase distortions in the input-beam profile of an intense femtosecond laser pulse, and for the first time that a control of the transport of high intensities over long distances may be achieved by forcing this well ordered propagation regime.
Abstract: We show that it is possible to organize regular filamentation patterns in air by imposing either strong field gradients or phase distortions in the input-beam profile of an intense femtosecond laser pulse. A comparison between experiments and 3+1 dimensional numerical simulations confirms this concept and shows for the first time that a control of the transport of high intensities over long distances may be achieved by forcing this well ordered propagation regime. In this case, deterministic effects prevail in multiple femtosecond filamentation, and no transition to the optical turbulence regime is obtained [Mlejnek et al., Phys. Rev. Lett. 83, 2938 (1999)].
TL;DR: In this article, the peculiarities of laser microablation and plasma re-heating with ultrashort (50 fs-10 ps) double pulses were studied and three regimes of laser/metal target interaction were observed as a function of double-pulse delay.
TL;DR: The filamentation of femtosecond light pulses in air is numerically and experimentally investigated for beam powers reaching several TW and evolution of the filament patterns can be qualitatively reproduced by an averaged-in-time (2D+1)-dimensional model derived from the propagation equations for ultrashort pulses.
Abstract: The filamentation of femtosecond light pulses in air is numerically and experimentally investigated for beam powers reaching several TW. Beam propagation is shown to be driven by the interplay between intense, robust spikes created by the defects of the input beam and random nucleation of light cells. Evolution of the filament patterns can be qualitatively reproduced by an averaged-in-time $(2\mathrm{D}+1)$-dimensional model derived from the propagation equations for ultrashort pulses.
TL;DR: In this article, the morphology of plasmas induced by single and double laser pulses was studied, with the purpose to improve the understanding of the formation and evolution of the plume in the two cases and the reasons of the increase of sensitivity and detection limits in the double pulse configuration.
TL;DR: In this paper, the parent νOH decay and the appearance of daughter stretching and bending excitations are simultaneously monitored, which allows for characterization of the stretch decay pathways. But they do not investigate the relationship between the parent and daughter excitations, and the shorter lifetime (∼0.5 ps) of the subband compared to the subbands (0.8−0.9 ps) results primarily from enhanced stretch-to-bend anharmonic coupling.
Abstract: Ultrafast nonlinear vibrational spectroscopy with mid-IR pumping and incoherent anti-Stokes Raman probing is used to study v = 1 excitations of OH stretching (νOH) of water and of HOD in D2O solvent (HOD/D2O). The parent νOH decay and the appearance of daughter stretching and bending excitations are simultaneously monitored, which allows for characterization of the stretch decay pathways. At all times and with all pump frequencies within the νOH band, the excited-state spectrum can be fit by two overlapping subbands, a broader red-shifted band and a narrower blue-shifted band . We show these subbands are dynamically distinguishable. They decay with different lifetimes and evidence characteristically different decay pathways. Excitations of the subband generate bending vibrations that does not. The shorter lifetime (∼0.5 ps) of the subband compared to the subband (0.8−0.9 ps) results primarily from enhanced stretch-to-bend anharmonic coupling. The subbands represent persistent structures in the excited sta...
TL;DR: Particle-in-cell simulations demonstrate attosecond bunch generation during pulse propagation through a thin channel or in the course of grazing incidence on a plasma layer, showing how p-polarized ultraintense laser pulses interacting with sharp boundaries of overdense plasmas can produce such bunches.
Abstract: Electron bunches of attosecond duration may coherently interact with laser beams. We show how $p$-polarized ultraintense laser pulses interacting with sharp boundaries of overdense plasmas can produce such bunches. Particle-in-cell simulations demonstrate attosecond bunch generation during pulse propagation through a thin channel or in the course of grazing incidence on a plasma layer. In the plasma, due to the self-intersection of electron trajectories, electron concentration is abruptly peaked. A group of counterstream electrons is pushed away from the plasma through nulls in the electromagnetic field, having inherited a peaked electron density distribution and forming relativistic ultrashort bunches in vacuum.
TL;DR: It is proved that the combination of spatial and temporal chirp also produces pulse-front tilt in ultrashort laser pulses, and this is verified experimentally using a GRENOUILLE.
Abstract: Pulse-front tilt in an ultrashort laser pulse is generally considered to be a direct consequence of, and equivalent to, angular dispersion. We show, however, that, while this is true for certain types of pulse fields, simultaneous temporal chirp and spatial chirp also yield pulse-front tilt, even in the absence of angular dispersion. We verify this effect experimentally using GRENOUILLE.
TL;DR: In this paper, the nonlinear refractive index (γ) of CS2 was measured using the Z-scan technique and laser radiation of various (femto-, pico-, and nano-second) pulse durations.
Abstract: The nonlinear refractive index (γ) of CS2 was measured using the Z-scan technique and laser radiation of various (femto-, pico-, and nano-second) pulse durations. We observed the growth of γ with the increase of the pulse duration (from (3±0.6)×10-15 cm2 W-1 at 110 fs to (4±2)×10-14 cm2 W-1 at 75 ns) due to the additional influence of the molecular reorientational Kerr effect in the case of longer (picosecond and nanosecond) pulses. Acoustic wave induced negative nonlinear refraction was observed using wavefront analysis. We analyzed the simultaneous influence of both electronic and molecular processes leading to the positive nonlinear refraction and acoustic processes leading to the negative nonlinear refraction in carbon disulfide. Variations of the refractive index due to the thermal effect at high pulse repetition rates were also investigated.
TL;DR: The demonstrated peak powers, in excess of 5 MW, are to the authors' knowledge the highest ever achieved from a cw-pumped laser and are expected to be scalable to tens of megawatts by an increase in the pump power and (or) a decrease in the repetition rate.
Abstract: We demonstrate a Kerr-lens mode-locked Ti:sapphire oscillator that generates 130-nJ, 26-fs and 220-nJ, 30-fs pulses at a repetition rate of 11 MHz. The generation of stable broadband, high-energy pulses from an extended-cavity oscillator is achieved by the use of chirped multilayer mirrors to produce a small net positive dispersion over a broad spectral range. The resultant chirped picosecond pulses are compressed by a dispersive delay line that is external to the laser cavity. The demonstrated peak powers, in excess of 5 MW, are to our knowledge the highest ever achieved from a cw-pumped laser and are expected to be scalable to tens of megawatts by an increase in the pump power and (or) a decrease in the repetition rate. The demonstrated source permits micromachining of any materials under relaxed focusing conditions.