Showing papers on "High harmonic generation published in 1999"
TL;DR: In this article, the authors studied high-order harmonic generation in the general case of absorbing and dispersive atomic gas media and showed that the harmonic conversion efficiency tends to a limit mainly imposed by the harmonic reabsorption in the gas.
Abstract: We study high-order harmonic generation in the general case of absorbing and dispersive atomic gas media. For ultrashort laser pulses, the harmonic conversion efficiency tends to a limit mainly imposed by the harmonic reabsorption in the gas. This limit, independent on the gas density, is the same for both the case of a loosely focused beam or a beam guided in a gas-filled hollow-core fiber. Under optimum conditions, we measured the highest conversion efficiency to date $(4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5})$ for the 15th harmonic generated in xenon using a 40 fs, 1.5 mJ, 800 nm pulse at a 1 kHz repetition rate.
563 citations
TL;DR: In this article, the phase matching of a high-harmonic generation by intense, 20 fs laser pulses in a gas-filled capillary waveguide is investigated, and the role of atomic dispersion is examined by measuring and calculating the harmonic signal for several gases.
Abstract: We investigate the phase matching of a high-harmonic generation by intense, 20 fs laser pulses in a gas-filled capillary waveguide. We identify several regimes in which the harmonic field can build up coherently: a balance of atomic and waveguide dispersions, noncollinear Cerenkov phase matching, and a balance of atomic and plasma dispersions. The role of atomic dispersion is examined by measuring and calculating the harmonic signal for several gases. We also predict and provide preliminary evidence for a regime where phase matching occurs only at specific fractional ionization levels, where the harmonic signal is sensitive to the absolute phase of the carrier wave.
283 citations
TL;DR: In this paper, the authors describe the physical process of cumulative second-harmonic generation of Lamb-mode propagation in a solid plate, and the analytical expression of the cumulative second harmonic of Lambmode propagation has been determined.
Abstract: This article describes the physical process of cumulative second-harmonic generation of Lamb-mode propagation in a solid plate. In general, cumulative second-harmonic generation of a dispersive guided wave propagation does not occur. However, the present results show that the second harmonic of Lamb-mode propagation arising from the nonlinear interaction of partial bulk waves and the restriction of the two boundaries of the solid plate does have a cumulative effect once some conditions are satisfied. Through boundary and initial conditions of excitation, the analytical expression of the cumulative second harmonic of Lamb-mode propagation has been determined. Numerical results reveal that the cumulative second-harmonic fields are symmetrical regardless of whether Lamb-mode propagation is symmetrical or antisymmetrical, and that the cumulative second-harmonic field patterns of Lamb-mode propagation are associated with the position of excitation source.
190 citations
TL;DR: Bellini et al. as mentioned in this paper showed that the harmonic radiation can largely be described as a sum of two fields having a phase proportional to the intensity of the generating field, and discussed how the temporal and spatial coherence properties are influenced by this effect.
Abstract: We present a spatiotemporal analysis of high harmonic generation, showing evidence for the presence of several quantum path contributions to the atomic dipole moment. We show that the harmonic radiation can largely be described as a sum of two fields having a phase proportional to the intensity of the generating field. We compare our results to recent experimental results demonstrating this separation [M. Bellini et al., Phys. Rev. Lett. 81, 297 (1998)]. We show how the temporal and spatial coherence properties are influenced by this effect, and discuss how it could be used to obtain better control of the generated harmonic radiation. [S1050-2947(99)00402-3].
150 citations
TL;DR: In this article, a phase-matched high-harmonic generation of 100 fs Ti:sapphire laser pulses by a self-guided beam in 30 Torr of neon gas was demonstrated.
Abstract: We demonstrate a phase-matched high-harmonic generation of 100 fs Ti:sapphire laser pulses by a self-guided beam in 30 Torr of neon gas. Phase matching of high harmonics is experimentally examined by changing the propagation length up to 7 mm. Phase-matched propagation over 7 mm magnifies the conversion efficiency around the 49th harmonic (in the cutoff region) by 40 times. On the other hand, harmonics around the 25th order (in the plateau region) peak at 4 mm, resulting in an enhancement of 4. This scheme enhances the conversion efficiencies in the cutoff region up to ${10}^{\ensuremath{-}6}$, corresponding to a few nJ per high-harmonic pulse in the 76 eV region.
146 citations
TL;DR: In this article, the superposition of selected higher order harmonics and continuous background, produced in Ar gas through two time delayed 60 fs pulses at 800 nm, is measured as a function of their delay time.
Abstract: The superposition of selected higher order harmonics and continuous background, produced in Ar gas through two time delayed 60 fs pulses at 800 nm, is measured as a function of their delay time. An isolated less than 100 as sharp feature, indicative for pulse production in the attosecond regime, followed by a subfemtosecond beating, indicative for the production of trains of sub-fs XUV pulses, is clearly observable in the resulting temporal trace. The trace Fourier transforms to a frequency spectrum composed of harmonics from the selected ensemble and a continuous background that correlates with the observed ultrashort feature. Numerical simulation spectra encompass similar temporal localization.
135 citations
TL;DR: In this paper, a theory of ionization in the near-infrared and visible-wavelength regime is developed and a condition for the applicability of the quasistatic approximation in the above barrier ionization regime is obtained.
Abstract: The interaction of noble gases with laser pulses in the near-infrared and visible wavelength regime is dominated by above barrier ionization while tunnel ionization plays a subordinate role We develop a theory of ionization in this parameter range A condition for the applicability of the quasistatic approximation in the above barrier ionization regime is obtained Ionization of arbitrary atoms by a laser pulse may then be calculated from static ionization rates The static ionization rate of He is obtained by solving the full two-electron Schr\"odinger equation Our analysis yields a verification of the single active electron approximation and indicates the possibility to create keV radiation by high harmonic generation in a He gas
123 citations
101 citations
TL;DR: In this paper, the authors present results of a high-harmonic generation experiment in argon, performed with a Ti sapphire laser (787 nm, 140 fs) and a spatially shaped focus.
Abstract: In this paper we present some results of a high-harmonic generation experiment in argon, performed with a Ti sapphire laser (787 nm, 140 fs) and a spatially shaped focus (flat top). The range of intensities is 3-100 TW cm(-2) The intensity dependence of the harmonic signal shows an interesting behaviour of the 13th harmonic, namely an initial suppression followed by a strong enhancement at similar or equal to 85 TW cm(-2), which exceeds the magnitude of the 11th harmonic. This behaviour is a consequence of a resonant atomic process. Numerical calculations were also performed. They partly reproduced the experiment.
96 citations
TL;DR: In this paper, aperiodic optical superlattice (AOS) structures realized by inverting poled ferroelectric domains in sample were proposed to enhance harmonic generation.
Abstract: We find that harmonic generation can be enhanced with aperiodic optical superlattice (AOS) structures realized by inverting poled ferroelectric domains in sample. The optimal design of the AOS can be achieved with use of the simulated annealing method. The constructed AOSs can implement multiple wavelength second-harmonic generation and the coupled third-harmonic generation with an identical effective nonlinear coefficient. The simulations show that the constructed AOSs can enhance harmonic generation compared with the Fibonacci optical superlattice. The physical origin of this enhancement is ascribed to the constructive interference effect.
82 citations
TL;DR: In this paper, high harmonic radiation spectra up to 19th order in alkali metal vapors excited by an intense, picosecond mid-infrared (3 -4 mu m) laser are reported and compared to theory.
Abstract: High harmonic radiation spectra up to 19th order in alkali metal vapors excited by an intense, picosecond mid-infrared (3 -4 mu m) laser are reported and compared to theory. The strong-field dynamics in the alkali metal atoms exhibit significant differences from all previously studied systems due to the strong coupling between their ground and first excited states.
TL;DR: In this paper, the phase-matching conditions for high-order harmonic generation as well as sum-and difference-frequency mixing processes in strong laser fields were studied using a graphical approach described in Balcou et al.
Abstract: We study phase-matching conditions for high-order harmonic generation as well as high-order sum- and difference-frequency mixing processes in strong laser fields, using a graphical approach described in Balcou et al (1997 Phys. Rev. A 55 3204-10). This method is based on the analysis of the different quantum paths that contribute, with different phase properties, to the single-atom response. We propose a simple numerical method to disentangle the quantum paths contributing to the generation process. We present graphical maps of the phase matching around the laser focus, which allow one to predict the geometries that optimize the conversion efficiency of the process considered. The method is applied to the study of sum- and difference-frequency mixing processes. The qualitative predictions of the graphical phase-matching approach are confirmed by numerical propagation calculations.
TL;DR: In this paper, it was shown that an ionized clustered material (cluster plasma) permits propagation below the plasma cut-off of electromagnetic (EM) waves whose phase velocity is close to but below the speed of light.
Abstract: It is shown that unlike a gas plasma or an electron plasma in a metal, an ionized clustered material (“cluster plasma”) permits propagation below the plasma cut-off of electromagnetic (EM) waves whose phase velocity is close to but below the speed of light. Its unique properties allow a variety of applications, including direct acceleration of particles with its EM fields and the phase matching of waves of high harmonic generation (HHG). The particle-in-cell simulation demonstrates the new optical mode, the cluster mode.
TL;DR: A detailed study of the harmonic technique, which exploits the generation of harmonics resulting from excitation of the nonlinearity of the single Langmuir probe characteristic, is presented in this paper.
Abstract: A detailed study of the harmonic technique, which exploits the generation of harmonics resulting from excitation of the nonlinearity of the single Langmuir probe characteristic, is presented. The technique is used to measure electron temperature and its fluctuations in tokamak plasmas and the technical issues relevant to extending the technique to high bandwidth (200 kHz) are discussed. The technique has been implemented in a fast reciprocating probe in the TEXTOR tokamak, gaining the ability to study denser and hotter plasmas than previously possible. A corrected analytical expression is derived for the harmonic currents. Measurement of the probe current by inductive pickup is introduced to improve electrical isolation and bandwidth. The temperature profiles in the boundary plasma of TEXTOR have been measured with high spatial (∼2 mm) and temporal (200 kHz) resolution and compared to those obtained with a double probe. The exact expansion of the probe characteristic in terms of Bessel functions is compared to a computationally efficient power series. Various aspects of the interpretation of the measurement are discussed such as the influence of plasma potential and density fluctuations. The technique is well suited to study fast phenomena such as transient plasma discharges or turbulence and turbulent transport in plasmas.
TL;DR: In this article, a continuous-wave single-pass second-harmonic generation (SHG) in 4?µm-period 0.5mm-thick backswitch-poled lithium niobate was reported.
Abstract: We report continuous-wave single-pass second-harmonic generation (SHG) in 4?µm-period 0.5-mm-thick backswitch-poled lithium niobate. Pump sources at 920–930 nm include both Ti:sapphire and diode-oscillator–amplifier lasers. SHG of a Ti:sapphire laser at 6.1%/W efficiency, producing 61 mW of power at 460 nm, is demonstrated in 50-mm-long periodically poled lithium niobate samples with a nonlinear coefficient deff?9 pm/V, and 60 mW at 465 nm and 2.8%/W efficiency is obtained by SHG of a laser-diode source.
TL;DR: In this article, the temporal coherence properties of high-order harmonic radiation were investigated and it was shown that the coherence time of the inner and outer regions changes as a function of the harmonic order, the laser intensity, and the focusing conditions.
Abstract: Systematic studies of the temporal coherence properties of high-order harmonic radiation are presented. These complement our previous investigations [Bellini et at, Phys. Rev. Lett. 81, 297 (1998)], where we showed the separation of the far-field pattern of high-order harmonics into two distinct spatial regions with different coherence times. Here we show how the coherence time of the inner and outer regions changes as a function of the harmonic order, the laser intensity, and the focusing conditions. Good agreement with the predictions of the semiclassical model of harmonic generation is obtained. [S1050-2947(99)10312-3].
TL;DR: In this article, the phase dependence of high-harmonic generation with few-cycle laser pulses was theoretically investigated by solving the coupled evolution equations for the laser pulse and for the harmonic field in one space dimension.
Abstract: We have theoretically investigated the phase dependence of high-harmonic generation with few-cycle laser pulses. We performed the analysis by solving the coupled evolution equations for the laser pulse and for the harmonic field in one space dimension. Our calculations reveal phase-dependent features in the cutoff region, such as a phase-dependent shift of the cutoff, that are preserved during propagation.
TL;DR: In this article, a phase-matched high-order-harmonic generation of Ti:sapphire laser pulses was demonstrated using a gas-filled hollow fiber, indicating more than a hundred times enhancement around the 25th harmonic.
Abstract: Phase-matched high-order-harmonic generation of Ti:sapphire laser pulses of $\ensuremath{\approx}{10}^{14}{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ has been demonstrated using a gas-filled hollow fiber. The high-order-harmonic spectrum emitted from the fiber showed an intensity distribution significantly different from that in free space, indicating more than a hundred times enhancement around the 25th harmonic. A simple phase-matching calculation considering positive dispersion due to plasma and negative dispersion due to neutrals indicates that macroscopic phase matching achieved in the fiber is responsible for the strong enhancement.
TL;DR: In this paper, the effect of free electrons on the spectral properties of high-order harmonics generated by 30-fs Ti:sapphire laser pulses in a neon gas jet was investigated.
Abstract: We have investigated the effect of free electrons on the spectral properties of high-order harmonics generated by 30-fs Ti:sapphire laser pulses in a neon gas jet. Our observations clearly indicate the possibility of continuously tuning the harmonic wavelength in the region below 20 nm, by taking advantage of the blue shift of the harmonic wavelength induced by the presence of free electrons in the gas. We have experimentally demonstrated that this allows one to cover the entire spectral region between two consecutive harmonics of the unshifted spectrum. Different amounts of blue shift are imparted by simply changing the gas jet position relative to the laser beam waist, namely, by varying the effective laser intensity experienced by the gas jet when it is moved across the focal region. We have also interpreted the experimental results in terms of a simple model of the generation process based on the tunnel ionization of an atom exposed to an ultraintense laser field.
TL;DR: The azimuthal dependence of third-order and cascaded second-order nonlinear coupling was used to measure the relative contributions of each to direct third-harmonic generation in beta-barium borate.
Abstract: The azimuthal dependence of third-order and cascaded second-order nonlinear coupling was used to measure the relative contributions of each to direct third-harmonic generation in ?-barium borate. This permitted the measurement of the values of tensor elements ?103, ?113, and ?163 relative to the known ?ij2. Finally, conversion efficiencies to 3? of up to 6% were achieved with a femtosecond chirped-pulse amplification laser with 200 GW/cm2 in collimated beams.
TL;DR: In this paper, the nonlinear interactions of the instability modes in a hypersonic laminar boundary layer undergoing natural transition are examined using bispectral analysis, and it is shown that in the initial stages of transition the dominant nonlinear interaction is forcing by the fundamental to generate a harmonic.
Abstract: The nonlinear interactions of the instability modes in a hypersonic laminar boundary layer undergoing natural transition are examined using bispectral analysis. The data are from an experiment of a boundary layer flow on a cooled-wall cone in a low-level free-stream disturbance hypersonic wind tunnel, and thus, the bispectral measurements are a good representation of the natural transition processes. The bispectral analysis shows that in the initial stages of transition the dominant nonlinear interaction is forcing by the fundamental to generate a harmonic. Subsequently, mutual forcing of the fundamental and harmonic yield a second harmonic. Difference interactions within the band of unstable disturbances centered on the fundamental and harmonic also generate a low frequency nonlinear interaction. At high amplitudes of the fundamental and harmonic a nonlinear interaction characterized by a low frequency modulation of the fundamental and harmonic then follows. This nonlinear interaction is then the most do...
TL;DR: The three-dimensional coordination polymer [Cd 2 (4,4′-bipy) 2 (H 2 O) 3 (SO 4 ) 2 ]·3 H 2 O (1 ) was synthesized by the hydrothermal reaction of 4, 4′-Bipyridine with CdSO 4 ·8/3H 2O in 1:1 mole ratio in a sealed tube at 120°C as discussed by the authors.
TL;DR: In this paper, the authors investigate the nature of low-power nonlinear effects in high-temperature superconducting microwave devices by measuring third harmonic generation at 76 K in coplanar waveguide transmission lines of different geometries fabricated from YBa2Cu3O7−δ thin films.
Abstract: We investigate the nature of low-power nonlinear effects in high-temperature superconducting microwave devices by measuring third harmonic generation at 76 K in coplanar waveguide transmission lines of different geometries fabricated from YBa2Cu3O7−δ thin films. The measured power in the third-harmonic signal changes systematically with film thickness, center conductor linewidth, and line length. We analyze these results using a simple model for a transmission line with a nonlinear inductance arising from a current-dependent superconducting penetration depth. This analysis describes quantitatively the observed differences in harmonic generation for transmission lines of different dimensions, and yields a single geometry-independent parameter (the nonlinear scaling current density J0) to quantify the observed nonlinear behavior. For the thin film samples studied here J0=3.0×107 A/cm2 at 76 K for all geometries investigated. These results provide the means to establish a lower limit for the expected nonline...
TL;DR: In this article, a method for producing an exciting light pulse with a time-dependent degree of ellipticity from a one-frequency linearly polarized laser pulse by means of linear optics is suggested.
Abstract: The problem of obtaining a single attosecond pulse with high-order harmonics generation is considered. A method for producing an exciting light pulse with a time-dependent degree of ellipticity from a one-frequency linearly polarized laser pulse by means of linear optics is suggested. The whole of the field generated with a limited beam of this light in a gas target is calculated in the far zone. For this calculation we use an original method based on the time approach of a theory of high-order harmonic generation. Contrasted and intense (focused) single attosecond soft-x-ray pulses are calculated by suppression of the low-frequency part of the field to as many as 11 harmonics of exciting frequency. The possibility of suppressing the low-frequency part of the field with pinholes is shown.
TL;DR: This work extends a recent suggestion for the generation of subfemtosecond pulses by molecular modulation to the rotational spectrum of molecular hydrogen and predicts that this spectrum will compress into a train of pulses with 94-fs pulse separation and 0.5 fs.
Abstract: We extend a recent suggestion for the generation of subfemtosecond pulses by molecular modulation [Phys.??Rev.??Lett.??81, 2894 (1998)] to the rotational spectrum of molecular hydrogen H2. When a rotational transition |a??|b? is strongly driven ?ab=0.5 the generation and phase-slip lengths are of the same order and the Raman spectrum has approximately Bessel function sideband amplitudes. Numerical simulation predicts that this spectrum (generated in a 14-cm-long cell at 1-atm pressure of H2) will compress into a train of pulses with 94-fs pulse separation and a pulse length of 0.5??fs.
TL;DR: In this article, the authors present results of ab initio calculations of generalized cross sections for two-, three-and four-photon ionization of helium by VUV and XUV radiation.
Abstract: We present results of ab initio calculations of generalized cross sections for two-, three- and four-photon ionization of helium by VUV and XUV radiation. These cross sections can be useful in the estimation of the intensity required for observation of these processes. This work has been motivated by recent experimental results using high-order harmonic generation with which we find excellent agreement.
TL;DR: In this paper, the authors described experiments with temperature-modulated calorimetry, in which use is made of multiple modulation frequencies for the evaluation of the heat capacity, and showed that the extrapolation of heat capacity from the amplitudes of the first and higher harmonics of the Fourier-transforms of heat-flow rate and heating rate allowed the correct heat capacity to be obtained from a single experiment.
TL;DR: In this paper, a review of recent achievements in the synthesis and crystal growth of materials for efficient generation of short-wavelength light is presented. Materials exhibiting harmonic generation, stimulated Raman scattering or upconversion are discussed.
TL;DR: In this article, a fully quantum treatment explicitly presents the high harmonic generation as a three-step process: (i) above threshold ionization (ATI) is followed by (ii) electron propagation in a laser-dressed continuum.
Abstract: A fully quantum treatment explicitly presents the high harmonic generation as a three-step process: (i) above threshold ionization (ATI) is followed by (ii) electron propagation in a laser-dressed continuum. Subsequently (iii) stimulated (or laser-assisted) recombination brings the electron back into the initial state with emission of a high-energy photon. Contributions of all ATI channels add up coherently. All three stages of the process are described by simple, mostly analytical expressions that allow a detailed physical interpretation. A very good quantitative agreement with the previous calculations on the harmonic generation by the ${\mathrm{H}}^{\ensuremath{-}}$ ion is demonstrated, thus supplementing the conceptual significance of the theory with its practical efficiency. The virtue of the present scheme is further supported by a good accord between the calculations in length and velocity gauges for the high-energy photon.
TL;DR: The results obtained reveal that carefully designed hollow-fiber chirped-mirror compressors are able to deliver high-power sub-10-fs pulses with a smooth Gaussianlike leading edge that has an intensity contrast of approximately 10(-2) .
Abstract: Frequency-resolved optical gating (FROG) based on second-harmonic generation has been demonstrated to be capable of high-fidelity measurement of the electric-field envelope and of the temporal evolution of the instantaneous carrier frequency of 0.1-TW 5-fs pulses without the need for any correction for systematic experimental errors. At a 1-kHz repetition rate, pulse energies of a few microjoules are sufficient for reliable FROG characterization of pulses with durations down to the single-cycle regime. The results obtained reveal that carefully designed hollow-fiber chirped-mirror compressors are able to deliver high-power sub-10-fs pulses with a smooth Gaussianlike leading edge that has an intensity contrast of ?10-2.