Showing papers on "High harmonic generation published in 1998"
TL;DR: In this article, the temporal coherence of high-order harmonics (up to the 15th order) produced by focusing 100 fs laser pulses into an argon gas jet was studied.
Abstract: We have studied the temporal coherence of high-order harmonics (up to the 15th order) produced by focusing 100 fs laser pulses into an argon gas jet. We measure the visibility of the interference fringes, produced when two spatially separated harmonic sources interfere in the far field, as a function of the time delay between the two sources. In general, we find long coherence times, comparable to the expected pulse durations of the harmonics. For some of the harmonics, the interference pattern exhibits two regions, with significantly different coherence times. These results are interpreted in terms of different electronic trajectories contributing to harmonic generation. [S0031-9007(98)05569-7].
270 citations
TL;DR: In this article, the phase sensitivity of the harmonic spectra to the phase of the laser was investigated in terms of both quantum and classical dynamics, and it was shown that this phase sensitivity may be exploited in order to probe the laser phase for ultrashort pulses.
Abstract: We consider harmonic generation by atoms exposed to an intense laser pulse of a few femtoseconds. Our results, obtained by solving numerically the corresponding three-dimensional time-dependent Schrodinger equation, demonstrate the strong sensitivity of the harmonic spectra to the phase of the laser. These results are explained in terms of both quantum and classical dynamics. We show that this phase sensitivity may be exploited in order to probe the laser phase for ultrashort pulses. Our discussion about this new method of diagnosis takes into account propagation effects.
189 citations
137 citations
TL;DR: In this paper, the authors investigated the generation of high harmonics from the interaction of 150 fsec, 790 nm, and 395 nm laser pulses with solid targets, and demonstrated a strong dependence of the conversion efficiency on the temporal pulse shape and the resulting density scale length of the preformed plasma.
Abstract: We have investigated the generation of high harmonics from the interaction of 150 fsec, 790 nm, and 395 nm laser pulses with solid targets. Experiments are presented that demonstrate a strong dependence of the conversion efficiency on the temporal pulse shape and the resulting density scale length $(L/\ensuremath{\lambda})$ of the preformed plasma. The highest conversion efficiencies are achieved for short density scale lengths $(L/\ensuremath{\lambda}l~0.4),$ which result from high contrast ratio pulse interactions.
135 citations
TL;DR: In this article, a strong dependence of the soft-x-ray spectra generated by high harmonic emission on the chirp of the excitation pulse, when an ultrashort laser drives the process, was reported.
Abstract: We report a strong dependence of the soft-x-ray spectra generated by high harmonic emission on the chirp of the excitation pulse, when an ultrashort laser drives the process. For identical pulse durations, distinct harmonic peaks can be observed for positively chirped excitation pulses, while for negatively chirped pulses, the harmonic peaks become irregular. This behavior is explained by simulations that combine the chirp of the laser with the intrinsic phase shift of the harmonics. This work resolves an outstanding discrepancy between theory and experiment by demonstrating that high-order harmonic generation driven by short-duration pulses can result in distinct harmonic peaks. This work conclusively demonstrates the role of the intrinsic phase in determining harmonic emission spectra, and control this phase during the emission process.
128 citations
TL;DR: In this paper, a self-interaction-free time-dependent density functional theory (TDDFT) was proposed for nonperturbative treatment of multiphoton processes of many-electron atomic systems in intense laser fields.
Abstract: We present a self-interaction-free time-dependent density-functional theory (TDDFT) for nonperturbative treatment of multiphoton processes of many-electron atomic systems in intense laser fields. The theory is based on the extension of the time-dependent Kohn-Sham formalism. The time-dependent exchange-correlation potential with proper short-and long-range behavior is constructed by means of the time-dependent optimized effective potential (TDOEP) method and the incorporation of an explicit self-interaction correction (SIC) term. The resulting TDOEP-SIC equations are structurally similar to the time-dependent Wartree-Fock equations, but include the many-body effects through an orbital-independent single-particle local time-dependent exchange-correlation potential. We also introduce a generalized pseudospectral lime-propagation method, allowing optimal spatial grid discretization, for accurate and efficient numerical solution of the TDOEP-SIC equations. The theory is applied to the study of the role of dynamical electron correlation on the multiple high-order harmonic generation (HHG) processes of He atoms in intense laser fields. We also perform a detailed study of the mechanisms responsible for the production of the higher harmonics in He atoms observed in a recent experiment that cannot be explained by the single-active-electron model. We found that both the dynamical electron correlation and the He+ ion are important to the generation of the observed higher harmonics. The present TDDFT is thus capable of providing a unified and self-consistent dynamical picture of the HHG processes. [S1050-2947(98)04601-0].
105 citations
96 citations
TL;DR: The computational predictions for the imaging potential of the second harmonic produced by finite amplitude distortion were investigated with a simple experiment and it is suggested that the simple one-pulse scheme may be adequate for second harmonic imaging.
Abstract: The computational predictions for the imaging potential of the second harmonic produced by finite amplitude distortion were investigated with a simple experiment. A focused transducer containing concentric 2.5 MHz and 5.0 MHz elements was used to obtain a sequence of radio-frequency (r-f) backscattered signals using a tissue equivalent phantom. The 2.5 MHz element was used as the transmitter and the 5.0 MHz element was used as the receiver. At 0.68 cm in front of the geometric focal point of the transducer, the phantom contained a 0.6 cm diameter cylindrical volume which contained no scatterers. Each of these r-f signals was then processed to produce the corresponding fundamental (2.5 MHz-centered) and second harmonic (5.0 MHz-centered) envelopes. The contrast resolution obtained for the scatterer-free or cyst region of the envelopes was compared against the computed prediction and good agreement was obtained. The results of this experiment also suggest that the simple one-pulse scheme may be adequate for second harmonic imaging.
79 citations
TL;DR: In this article, a three-dimensional zero-range potential is embedded in an electric field with sinusoidal time dependence and arbitrary polarization and orientation with respect to the axis of the two-center potential.
Abstract: One electron bound by a three-dimensional two-center zero-range potential is embedded in an electric field with sinusoidal time dependence and arbitrary polarization and orientation with respect to the axis of the two-center potential. In the absence of the field, the model supports up to two bound states, which have a large transition dipole moment. Hence, the physical systems best described by the model are molecular ions such as ${\mathrm{H}}_{2}^{+}.$ Rates for high-harmonic emission are calculated analytically up to one final quadrature. In terms of the rescattering picture, harmonic emission can be attributed to two different mechanisms: electrons recombine either at the center they started from or at the other one. The latter case allows for three topologically different classes of orbits, which lead to different spectral ranges of harmonics. Two of them are similar to atomic (one-center) harmonic generation, but have different cutoff laws that are no longer proportional to the ponderomotive potential. In the third the electron moves directly from one center to the other. This leads to strong harmonic emission at comparatively low frequencies similar to emission from a two-level atom with the cutoff proportional to the field amplitude rather than the intensity. The molecular dipole phase in this case is almost independent of the field intensity and, at constant intensity, the phases of neighboring harmonics are locked. Different orientations of the two-center system with respect to the field with various polarization configurations are investigated. Most of the observed features lend themselves to interpretation in terms of the simple man's model.
79 citations
TL;DR: In this article, the main features of high harmonic generation by ultrashort laser pulses can be explained in terms of the intensity dependent phase of the atomic polarization, and focusing conditions and chirped driving fields may be used to control the harmonic spectrum.
Abstract: We demonstrate that the main features of high harmonic generation by ultrashort laser pulses can be explained in terms of the intensity dependent phase of the atomic polarization. Focusing conditions and chirped driving fields may be used to control the harmonic spectrum, while temporal compression provides a unique way to tailor the time profile.
73 citations
TL;DR: In this article, first-order multiple-scale perturbation theory is used to derive a set of coupledmode equations valid for electromagnetic-wave propagation in a weakly periodic, nonlinear medium with periodicity on the order of a wavelength.
Abstract: First-order multiple-scale perturbation theory is used to derive a set of coupled-mode equations valid for electromagnetic-wave propagation in a weakly periodic, nonlinear medium with periodicity on the order of a wavelength. We apply this to a problem where the medium has a ${\ensuremath{\chi}}^{(2)}$ response and find that the second-harmonic signal generated is enhanced when the fundamental is tuned near the band edge. Results are given for a possible experiment with optical fibers.
TL;DR: A novel quasi-phase-matched frequency converter is fabricated, using a zigzag optical beam path in a thin, polished parallel plate, which demonstrated angle-tuned output at 4.6 to 5.3mum in GaAs and 1.7 to 2.0mUM in ZnSe crystals when pulsed infrared laser sources were used.
Abstract: We fabricated a novel quasi-phase-matched frequency converter, using a zigzag optical beam path in a thin, polished parallel plate. Second-harmonic generation experiments demonstrated angle-tuned output at 4.6 to 5.3 µm in GaAs and 1.7 to 2.0 µm in ZnSe crystals when pulsed infrared laser sources were used.
TL;DR: Scrinzi and Piraux as mentioned in this paper presented a method of solving the time-dependent Schrodinger equation for atoms with two active electrons in a strong laser field, which they used in a previous paper to calculate ionization, double excitation, and harmonic generation in helium by shea laser pulses.
Abstract: We discuss a method of solving the time-dependent Schrodinger equation for atoms with two active electrons in a strong laser field, which we used in a previous paper [A. Scrinzi and B. Piraux, Phys. Rev. A 56, R13 (1997)] to calculate ionization, double excitation, and harmonic generation in helium by shea laser pulses. The method employs complex scaling and an expansion in an explicitly correlated basis. Convergence of the calculations is documented and error estimates are provided. The results for He at peak intensities up to 10(15) W/cm(2) and wavelength 248 nm are accurate to at least 10%. Similarly accurate calculations are presented for electron detachment and double excitation of the negative hydrogen ion. [S1050-2947(98)00208-X].
TL;DR: In this article, the dynamics of multiphoton ionization of helium were investigated through numerical integration of the two-electron time-dependent Schrodinger equation, and a new single-active electron model was introduced that gives agreement with He ionization rates to within a few per cent on average, and gives good agreement with the He harmonic generation spectra over a laser intensity range of to, and frequencies corresponding to four and five-photon ionization.
Abstract: The dynamics of multiphoton ionization of helium are investigated through numerical integration of the two-electron time-dependent Schrodinger equation. Using this work as a benchmark, a new single-active-electron model is introduced that gives agreement with He ionization rates to within a few per cent on average, and gives good agreement with He harmonic generation spectra over a laser intensity range of to , and frequencies corresponding to four- and five-photon ionization.
TL;DR: In this article, the effects of laser polarization, pulsewidth, hydrogen pressure, and focusing conditions on the efficiencies of stimulated Raman scattering, four-wave Raman mixing, self-phase modulation, selffocusing, and harmonic generation are investigated.
Abstract: More than 40 rotational Raman lines are generated using an 800-fs Ti:sapphire laser. The spectral region extends from the near-infrared to the far-ultraviolet with a considerably flat intensity distribution. The effects of laser polarization, pulsewidth, hydrogen pressure, and focusing conditions on the efficiencies of stimulated Raman scattering, four-wave Raman mixing, self-phase modulation, self-focusing, and harmonic generation are investigated. A white light continuum, generated by self-phase modulation, acts as a seed beam for the generation of high-order rotational lines through four-wave Raman mixing. Strong self-phase modulation, however, suppresses the generation of the Raman emission, due to a line broadening of the pump beam. Thus, optimization of experimental conditions is necessary for the efficient generation of high-order rotational lines.
TL;DR: In this article, visible and near UV blind VUV detectors, made from natural and synthetic diamond, have been developed to characterize coherent VUV pulses (λ=125nm, pulse duration at full width half maximum (FWHM) τ FWHM∼7 ns) generated by resonanceenhanced four-wave sum mixing in mercury vapor.
Abstract: A major difficulty in characterizing vacuum ultraviolet (VUV) radiation produced by harmonic generation or four-wave sum frequency mixing arises in differentiating between the desired VUV signal and the remaining fundamental pump laser beam. To overcome this problem, visible and near UV blind VUV detectors, made from natural and synthetic diamond, have been developed. Such detectors have been used to characterize coherent VUV pulses (λ=125 nm, pulse duration at full width half maximum (FWHM) τFWHM∼7 ns) generated by resonance-enhanced four-wave sum mixing in mercury vapor. They allow full characterization of the intensity profile of the VUV pulses, without any significant parasitic signal from simultaneous stray light irradiation at λ=313 nm. Detectors were fabricated exhibiting response times of less than 70 ps at FWHM, corresponding to the lowest response time obtainable with a 7 GHz bandwidth single-shot oscilloscope.
TL;DR: In this paper, the fabrication and characterization of 5mm-long quasi-phase-matched diffusion-bonded GaAs crystals designed for frequency doubling of a CO(2) laser were reported.
Abstract: We report on the fabrication and characterization of 5-mm-long quasi-phase-matched diffusion-bonded GaAs crystals designed for frequency doubling of a CO(2) laser. Second-harmonic generation of a pulsed laser yielded up to 24% peak power efficiency at 20 MW/cm(2) . Tunability and laser damage threshold are also investigated.
TL;DR: In this paper, the cumulative second-harmonic generation associated with nonlinear shear horizontal (SH) mode propagation in a solid plate has been studied, and the process of cumulative second harmonic generation is clearly shown, with appropriate boundary and initial conditions of excitation.
Abstract: In this article cumulative second-harmonic generation accompanying nonlinear shear horizontal (SH) mode propagation in a solid plate has been studied. Generally, the amplitude of the driven second harmonic arising from the self-interaction of the shear wave is independent of propagation distance. However, for SH mode propagation consisting of two shear waves, the cross interaction between two shear waves may cause the cumulative effect of the driven second harmonic once the phase velocity of SH mode propagation equals the longitudinal velocity of the plate material. In practical issues, one is especially interested in the case in which the second-harmonic amplitude is dependent of propagation distance. With appropriate boundary and initial conditions of excitation, the process of cumulative second-harmonic generation is clearly shown, and the analytical expression of the cumulative second harmonic is determined. Although the plane second harmonic without the cumulative effect cannot be determined in the present analysis, the cumulative second-harmonic generation accompanying nonlinear SH mode propagation is of substantial significance due to its obvious effect.
TL;DR: In this paper, a self-consistent model that solves both the three-dimensional Schrodinger equation and the wave equation ab initio was used to study the propagation of sub-femtosecond pulses produced by high harmonic generation in a gas.
Abstract: Using a self-consistent model that solves both the three-dimensional Schr\"odinger equation and the wave equation ab initio, we study the propagation of subfemtosecond pulses produced by high harmonic generation in a gas. The presence of a plasma shortens the coherence length for the harmonics. However, for the case of very short pump pulses, ionization is suppressed, and the coherence length can approach that of a neutral gas. Thus, the combination of enhanced single-atom harmonic generation for short pulses and improved phase matching allows for efficient amplification of attosecond x-ray pulses, resulting in 100 times higher output intensity for 5 fs compared to 10-fs pump pulses.
05 Oct 1998
TL;DR: In this article, a pseudo-spectral solver is used to achieve very high accuracy over long paths through lossy, nonlinear abdominal wall and liver, quantifying aberration and second harmonic generation by two-dimensional ultrasound beams in realistic tissue models.
Abstract: Second harmonic imaging systems transmit relatively low frequency pulses, e.g., 2.5 MHz, and image the frequency-doubled second harmonic generated by acoustic nonlinearity. Imaging the second rather than the first harmonic eliminates significant wavefront aberration and attenuation on the forward path, narrows the beam, and suppresses sidelobes. This technique is used successfully in commercial medical imaging systems and may become dominant in the near future. However, system optimization requires a better understanding of second harmonic generation by focused ultrasound pulses in tissue. Data and simulations are presented quantifying aberration and second harmonic generation by two-dimensional ultrasound beams in realistic tissue models. A pseudo-spectral solver is used to achieve very high accuracy over long paths through lossy, nonlinear abdominal wall and liver.
05 Oct 1998
TL;DR: The phase inversion technique appears to be a better choice to realize THI in an ultrasound imaging system because it gives better cancellation of the fundamental frequencies while simultaneously improving SNR.
Abstract: Tissue Harmonic Imaging (THI) is a new ultrasound imaging technique, which uses the harmonic components generated by nonlinear acoustic propagation through human tissues to form an image. Several factors affect THI image quality. First, strong suppression of the fundamental signals is necessary to allow the full dynamic range of the harmonic signals to be seen. This will allow the full benefits of harmonic imaging to be seen. Second, since the harmonic components are much weaker (15-20 dB lower) than the fundamental, they must be enhanced as much as possible relative to noise. This will allow the maximum possible image penetration. This paper will compare the phase inversion technique with another data acquisition and processing scheme from the literature in terms of suppression of fundamental frequencies, and signal-to-noise ratio (SNR) improvement. The comparison will be both theoretical, using a very simple model, and experimental, using data acquired in vitro. The phase inversion technique appears to be a better choice to realize THI in an ultrasound imaging system. This technique gives better cancellation of the fundamental frequencies while simultaneously improving SNR.
TL;DR: In this paper, the authors present a scheme for the production of circularly polarized multiple high-order harmonic generation (HHG), which involves the use of two-color laser fields, consisting of a linear polarized fundamental laser field and a linearly polarized second-harmonic laser field, in crossed-beam configuration.
Abstract: We present a scheme for the production of circularly polarized multiple high-order harmonic generation (HHG). The proposed experimental setup involves the use of two-color laser fields, consisting of a circularly polarized fundamental laser field and a linearly polarized second-harmonic laser field, in crossed-beam configuration. The feasibility of such a scheme is demonstrated by an ab initio quantal study of the HHG power spectrum of He atoms by means of the time-dependent density-functional theory with optimized effective potential and self-interaction correction recently developed. The theoretical study also provides insights regarding the different mechanisms responsible for the production of HHG in different energy regimes as well as the mechanism for the generation of continuous background radiation.
TL;DR: In this article, the effects of the static electric field on high-order harmonic generation (HOHG) spectrum were investigated and it was found that the presence of static electric fields breaks the inversion and reflection symmetry, as a result, the HOHG spectrum exhibits a double plateau structure.
Abstract: Based on the full quantum theory of high-order harmonic generation (HOHG) (Lewenstein et al 1994 Phys. Rev. A 49 2117-32), we study the effects of the static electric field on HOHG. It is found that the presence of the static electric field breaks the inversion and reflection symmetry, as a result, the HOHG spectrum exhibits a double-plateau structure. Furthermore, the cut-off of the HOHG no longer corresponds to the maximum kinetic energy the ionized electron can obtain from the laser field. Instead, it corresponds to the maximum kinetic energy of the electron with one return. Finally, for ultra-high static electric field, we find a monotonic decrease of the harmonic intensity as the harmonic order is increased.
TL;DR: In this paper, the authors demonstrate kHz-repetition-rate guided-wave high-harmonic generation in helium down to the 10-nm range with high field interactions with plane-wave excitation at intensity levels in excess of 1015 W/cm2 under well-controlled conditions.
Abstract: W/cm2. The high resistance of fused silica to damage in the sub-10-fs regime allows stable reproducible operation without degradation of the capillary waveguide. In preliminary experiments, we demonstrate kHz-repetition-rate guided-wave high-harmonic generation in helium down to the 10-nm range. The reported experiments open up the way to realizing high-field interactions with plane-wave excitation at intensity levels in excess of 1015 W/cm2 under well-controlled conditions.
TL;DR: Using a lossless dispersive apparatus, optimized to match a second-harmonic crystal phase-matching angle versus wavelength to second order, this work efficiently doubled tunable fundamental light near 660 nm over a range of 80 nm, using a 4-mm-long type I beta -barium borate crystal without tuning the crystal angle.
Abstract: Using a lossless dispersive apparatus consisting of six prisms, optimized to match a second-harmonic crystal phase-matching angle versus wavelength to second order, we efficiently doubled tunable fundamental light near 660??nm over a range of 80??nm, using a 4-mm-long type I ?-barium borate crystal without tuning the crystal angle. Another set of six prisms after the crystal realigned the propagation directions of the various second-harmonic frequencies to be collinear to within 1/4 spot diameter in position and 200 µrad in angle. The measured conversion efficiency of a 40-mJ, 5-ns fundamental pulse was 10%.
30 Oct 1998
TL;DR: In this paper, a comprehensive error budget that accounts for effects that are known to influence conversion efficiency is developed, including variations in amplitude and phase of the incident laser pulse, temporal bandwidth of the reported laser pulses, crystal surface figure and bulk non-uniformities, angular alignment errors, Fresnel losses, polarization errors and crystal temperature variations.
Abstract: The design of the National Ignition Facility (NIF) incorporates a type I/type II third harmonic generator to convert the 1.053-{micro}m fundamental wavelength of the laser amplifier to a wavelength of 0.351 {micro}m for target irradiation. To understand and control the tolerances in the converter design, we have developed a comprehensive error budget that accounts for effects that are known to influence conversion efficiency, including variations in amplitude and phase of the incident laser pulse, temporal bandwidth of the incident laser pulse, crystal surface figure and bulk non-uniformities, angular alignment errors, Fresnel losses, polarization errors and crystal temperature variations. The error budget provides specifications for the detailed design of the NIF final optics assembly (FOA) and the fabrication of optical components. Validation is accomplished through both modeling and measurement, including full-scale Beamlet tests of a 37-cm aperture frequency converter in a NIF prototype final optics cell. The prototype cell incorporates full-perimeter clamping to support the crystals, and resides in a vacuum environment as per the NIF design.
TL;DR: In this paper, the authors consider high harmonic generation by ultrashort (27-108 fs) laser pulses and calculate the macroscopic response of a collection of atoms to such a short pulse.
Abstract: We consider high harmonic generation by ultrashort (27-108 fs) laser pulses and calculate the macroscopic response of a collection of atoms to such a short pulse. We show how the harmonic spectrum after propagation through the medium is significantly different from the single-atom spectrum. We use single-atom data calculated by integration of the time-dependent Schrodinger equation and propose a method, based on an adiabatic approximation;:to extract the data necessary to perform a propagation calculation.
TL;DR: In this paper, an optical superlattice of LiTaO3 was fabricated, where two building blocks, each containing a pair of antiparallel 180° ferroelectric domains, were arranged as a Fibonacci sequence.
Abstract: We have investigated and fabricated an optical superlattice of LiTaO3, in which two building blocks, each containing a pair of antiparallel 180° ferroelectric domains, were arranged as a Fibonacci sequence. The second- and third-harmonic generation in this structure have been studied and measured. The results verifies that the second-harmonic spectrum and the third harmonic may be generated in a quadric nonlinear medium by a series of quasi-phase-matching processes. The experimental results are in good agreement with theory.
TL;DR: This compact all-solid-state scheme provides maximum output powers that exceed 1 muW and permits continuous adjustment-free scans larger than 2 cm(-1) across the entire tuning range.
Abstract: We demonstrate difference-frequency generation in the 6.8–12.5-μm range by mixing two high-power single-frequency laser diodes in a type II AgGaS2 crystal. This compact all-solid-state scheme provides maximum output powers that exceed 1 μW and permits continuous adjustment-free scans larger than 2 cm-1 across the entire tuning range.
TL;DR: In this paper, an analysis of high-order harmonics generated by atoms and ions in high-intensity laser beams is given, and a detailed discussion of an analytic quantum-mechanical theory of highorder harmonic generation in a one-electron system with the Coulomb, delta-like, and other potentials.
Abstract: An analysis is made of the generation of high-order harmonics by atoms and ions in high-intensity laser beams. A brief description is given of the main experimental relationships governing such generation, of methods for numerical solution of the Schrodinger equation for an atom in a strong field, and of some approximate models which make it possible to understand the mechanism of the effect (in particular, the 'semiclassical' model). A detailed discussion is made of an analytic quantum-mechanical theory of high-order harmonic generation in a one-electron system with the Coulomb, delta-like, and other potentials. Expressions are provided for the complex amplitudes of harmonics generated by monochromatic and bichromatic excitation. The results of simulation of high-order harmonic generation in an extended medium are given. This simulation takes into account the phases of the harmonics and their dependences on the amplitude of the fundamental-frequency field. The phase-matching problem and ways of solving it, the problem of the spectrum and duration of a pulse of a single harmonic and of the feasibility of controlling them, the problem of the total harmonic field, and other topics are considered.