Showing papers on "High harmonic generation published in 1994"
TL;DR: A simple, analytic, and fully quantum theory of high-harmonic generation by low-frequency laser fields is presented and the exact quantum-mechanical formula for the harmonic cutoff that differs from the phenomenological law Ip+3.17Up is presented.
Abstract: We present a simple, analytic, and fully quantum theory of high-harmonic generation by low-frequency laser fields. The theory recovers the classical interpretation of Kulander et al. in Proceedings of the SILAP III Works hop, edited by B. Piraux (Plenum, New York, 1993) and Corkum [Phys. Rev. Lett. 71, 1994 (1993)] and clearly explains why the single-atom harmonic-generation spectra fall off at an energy approximately equal to the ionization energy plus about three times the oscillation energy of a free electron in the field. The theory is valid for arbitrary atomic potentials and can be generalized to describe laser fields of arbitrary ellipticity and spectrum. We discuss the role of atomic dipole matrix elements, electron rescattering processes, and of depletion of the ground state. We present the exact quantum-mechanical formula for the harmonic cutoff that differs from the phenomenological law Ip+3.17Up, where Ip is the atomic ionization potential and Up is the ponderomotive energy, due to the account for quantum tunneling and diffusion effects.
3,007 citations
TL;DR: In this paper, the results of an analytical description and of a particle-in-cell simulation of the interaction of an ultrashort, relativistically intense laser pulse, obliquely incident on a nonuniform overdense plasma, are presented and several novel features are identified.
Abstract: The results of an analytical description and of a particle‐in‐cell simulation of the interaction of an ultrashort, relativistically intense laser pulse, obliquely incident on a nonuniform overdense plasma, are presented and several novel features are identified. The absorption and reflection of the ultraintense electromagnetic laser radiation from a sharp‐boundary plasma, high harmonic generation, and the transformation into low‐frequency radiation are discussed. In the case of weak plasma nonuniformity the excitation of nonlinear Langmuir oscillations in the plasma resonance region and the resulting electron acceleration are investigated. The vacuum heating of the electrons and the self‐intersection of the electron trajectories are also studied. In the case of a sharp‐boundary plasma, part of the energy of the laser pulse is found to be converted into a localized, relativistically strong, nonlinear electromagnetic pulse propagating into the plasma. The expansion of the hot electron cloud into the vacuum region and the action of the ponderomotive force of the laser pulse in the localized longitudinal electric field of the Langmuir oscillations lead to ion acceleration. The energy increase of a minority population of multicharged ions is found to be much greater than that of the ambient ions.
403 citations
TL;DR: The strong ellipticity dependence of correlated two-electron multiphoton ionization of neon and of the high-harmonic emission from argon and neon are reported, suggesting a common underlying mechanism and quantitatively consistent with a recently developed Keldysh-like model of high harmonic generation.
Abstract: The strong ellipticity dependence of correlated two-electron multiphoton ionization of neon and of the high-harmonic emission from argon (harmonic N=21) and neon (N=41) are reported. These measurements suggest a common underlying mechanism and are quantitatively consistent with a recently developed Keldysh-like model of high harmonic generation which treats the interaction between a newly freed electron and the ion core.
231 citations
TL;DR: In this paper, a model for high-order harmonic emission by one electron in a laser field bound to a zero-range potential is discussed. The model yields an expression for the emission rates in the form of a one-dimensional integral that has to be calculated numerically.
Abstract: High-order harmonic emission by one electron in a laser field bound to a zero-range potential is extensively discussed. The model yields an expression for the emission rates in the form of a one-dimensional integral that has to be calculated numerically. The solution is based on the quasienergy wave function of the ground state. The approach is very significantly facilitated by suppressing the harmonic components of the wave function at the position of the zero-range potential. This approximation is found to be very accurate except for the third harmonic. In spite of the simplicity of the model, the harmonic spectrum exhibits a very involved structure, occasional harmonics being strongly suppressed, with cusps and spikes for certain evenly spaced intensities. The latter are due to channel closings for the same intensities in above-threshold ionization. The harmonics near and beyond the cutoff of the plateau are amenable to a completely analytical approximation. This approximation shows how the classical model of Krause, Schafer, and Kulander [Phys. Rev. Lett. 68, 3535 (1992)] is embedded in a fully-quantum-mechanical description. Results are also given for the harmonic production rates in an elliptically polarized laser field; they display fair agreement with recent measurements. The model should adequately describe harmonic emission by negative ions with just one bound s state. Moreover, it also gives a fair description of harmonic emission by an atom, particularly if the ground-state energy of the zero-range potential is adjusted not to the binding energy of the atom, but rather to the energy difference between the ground state and the first excited state. The reason why this is appropriate is found in lowest-order perturbation theory, which sheds some light on the physical origin of the plateau.
177 citations
TL;DR: In this paper, a model of the interaction of an acoustic wave with dislocation dipoles and dipole-array approximations to veins and persistent slip bands (substructures) formed during metal fatigue is presented.
Abstract: A model is presented of the interaction of an acoustic wave with dislocation dipoles and dipole-array approximations to veins and persistent slip bands (substructures) formed during metal fatigue. The model predicts the generation of a substantial acoustic second harmonic that depends on the distance between the glide planes of the dipole pair, on the dipole density, and on the particular arrangement and volume fraction of dipoles in a given substructure of the fatigued solid. Experimental evidence which strongly supports the essential features of the model is presented for fatigued aluminium alloy 2024-T4.
167 citations
TL;DR: Two-color phase control in the total ion yield of ionization is successfully demonstrated in a tunneling regime by using a 100-fs Ti:sapphire laser and its third harmonic.
Abstract: Two-color phase control in the total ion yield of ionization is successfully demonstrated in a tunneling regime by using a 100-fs Ti:sapphire laser and its third harmonic. Adding the third harmonic with an intensity of only 10% enhances the ion yield by a factor of 7. In photoelectron spectra, above threshold ionization peaks due to the third harmonic disappear when two colors are superimposed, resulting in a continuum spectrum. This shows two-color interference clearly. The intensities of high-order harmonics in the plateau region are enhanced by an order of magnitude.
157 citations
TL;DR: In this article, a general phenomenological theory for optical second-and third-harmonic generation obtained in reflection from all cubic centrosymmetric single crystals whose faces have macroscopic C 1υ symmetry, including vicinal faces with such symmetry.
Abstract: We present a general phenomenological theory for optical second- and third-harmonic generation obtained in reflection from all cubic centrosymmetric single crystals whose faces have macroscopic C1υ symmetry, including vicinal faces with such symmetry. This extends earlier research [ Phys. Rev. B35, 1129 ( 1987)] for crystals with low-index faces. As a test of the theory we offer experimental results for anisotropic harmonic generation from vicinal, single crystals of Si, with and without an oxide layer, for which we use 130-fs, λ = 765 nm pulses. Overall there is good agreement between the phenomenological theory and experimental results. It is demonstrated that third-harmonic generation, which is dominated by bulk electric dipole contributions, can be used as a diagnostic of crystal orientation and crystal miscut angle. The intensity and the anisotropy of second-harmonic generation (SHG), which arise from surface electric dipole and bulk electric quadrupole effects, are seen to be sensitive to surface steps, an effect that also varies with surface oxidation. Finally, the use of vicinal surfaces allows us to identify bulk and surface contributions to SHG separately.
106 citations
TL;DR: In this paper, the authors compared the harmonic conversion efficiencies of different gas pairs as a function of three parameters: (1) field-free ionization potential (2) mass and (3) static average polarizability.
Abstract: High harmonic generation has been investigated in the atomic gases Ar and Xe as well as in the diatomic molecular gases H2, D2, N2 and O2 using a linearly polarized picosecond dye laser (594 nm) and a linearly polarized subpicosecond Ti:sapphire/Nd:glass laser (1053 nm). The harmonic conversion efficiencies are compared by choosing different gas pairs as a function of three parameters: (1) field-free ionization potential (2) mass and (3) static average polarizability. It is shown that the ionization potential and mass are not sensitive parameters for harmonic efficiency, but the polarizability is an important factor.
93 citations
TL;DR: In this paper, two waveguides consisting of segments, where each segment has a phase-matching condition different from the others, are proposed to obtain a broad bandwidth and simultaneously, a high SHG efficiency.
Abstract: We report on theoretical analysis and experiments for bandwidth broadening in quasi-phase-matched (QPM) second-harmonic generation (SHG) in LiTaO/sub 3/. QPM waveguides consisting of segments, where each segment has a phase-matching condition different from the others, are proposed to obtain a broad bandwidth and simultaneously, a high SHG efficiency. The waveguides were fabricated by adjusting the phase-matching condition either by changing the grating period of the SHG coefficient or by controlling the width of the waveguide. Consequently, the bandwidth of QPM-SHG was broadened to 0.35 nm with an SHG efficiency of 57 percent/W by modulation of the waveguide width and to 1.12 nm with an SHG efficiency of 29 percent/W by modulation of the grating period. The SHG efficiency and phase-matching characteristics in both types of waveguides showed good agreement with theoretical results. >
90 citations
TL;DR: In this article, field induced periodic poling is used to create >10 micron period structures in /spl sim/250 micron thick slabs of LiNbO/sub 3.
Abstract: Field induced periodic poling is used to create >10 micron period structures in /spl sim/250 micron thick slabs of LiNbO/sub 3/. Third order quasi-phase-matched second harmonic blue light generation is demonstrated by free propagation of a 937 nm fundamental Gaussian beam in periodically poled LiNbO/sub 3/, with a conversion efficiency of 0.2%/W cm. >
81 citations
TL;DR: In this paper, soft x-ray photons, generated by the interaction of intense 610 nm sub-picosecond laser pulses with rare gases, are used to carry out tunable angle-resolved photoemission on a number of systems.
Abstract: Soft x‐ray photons, generated by the interaction of intense 610 nm subpicosecond laser pulses with rare gases, are used to carry out tunable angle‐resolved photoemission on a number of systems. When the intensity at the focus of the input radiation exceeds ∼1014 W/cm2 significant quantities of harmonic photons at odd multiples of the input 610 nm (2.04 eV) light are produced. To date, harmonics from the 5th (10.2 eV) to the 25th (51 eV) have been generated with fluxes sufficient for generating photoemission spectra. The experimental apparatus for harmonic generation and several experiments demonstrating the applications of this light are described.
TL;DR: High-order harmonics have been generated by the use of an annular laser beam because the nonlinearity of harmonic production and the shorter wavelengths involved cause the harmonics to emerge strongly peaked on the laser axis.
Abstract: High-order harmonics have been generated by the use of an annular laser beam. The nonlinearity of harmonic production and the shorter wavelengths involved cause the harmonics to emerge strongly peaked on the laser axis. Thus the harmonics emerge from the focus inside the missing portion of the laser beam. This permits the laser to be blocked by an aperture that passes the harmonics.
TL;DR: A laser pulse propagating in a uniform plasma or a preformed plasma density channel is found to undergo a combination of hose and modulation instabilities, provided the pulse centroid has an initial tilt.
Abstract: A laser pulse propagating in a uniform plasma or a preformed plasma density channel is found to undergo a combination of hose and modulation instabilities, provided the pulse centroid has an initial tilt. Coupled equations for the laser centroid and envelope are derived and solved for a finite-length laser pulse. Significant coupling between the centroid and the envelope, harmonic generation in the envelope, and strong modification of the wake field can occur. Methods to reduce the growth rate of the laser hose instability are demonstrated.
TL;DR: In this paper, the decomposition phenomenon of a nonlinear wave train passing over a submerged shelf without breaking has been investigated by a previously developed numerical model, which is triggered by higher harmonic generation and nonlinear resonant interaction over the shelf.
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22 Aug 1994
TL;DR: In this article, the authors proposed a blue microlaser, which is a composite cavity formed form a gain medium crystal and a nonlinear frequency doubling material which together have four spaced parallel dielectrically coated faces and which is positioned in close proximity to a diode laser pump source.
Abstract: A monolithic diode pumped solid-state laser (11) comprising as the laser host neodymium-doped yttrium orthovanadate (Nd:YVO4) (12, 52) or neodymium-doped gadolinium orthovanadate (Nd:GdVO4) (57, 67) operating on the 4F3/2 → 4I9/2 (∩914 nm or ∩912 nm respectively) transition, to which a suitable nonlinear optic material (16), such as potassium niobate (KNbO3) or beta barium borate (BBO), is bonded. The nonlinear crystal gives rise to intracavity frequency doubling to ∩457 or ∩456 nm. The microlaser is a composite cavity formed form a gain medium crystal and a nonlinear frequency doubling material which together have four spaced parallel dielectrically coated faces (14, 17, 18, 15) and which is positioned in close proximity to a diode laser pump source (13) for phase-matched harmonic generation of blue light along an axis of propagation which lies substantially perpendicular to the two faces of the composite cavity. By employing specific doping concentration-length products of lasant material and pumping the gain medium which has a specific crystalline orientation the desired efficient blue microlaser is achieved. Alternative embodiments combine the Nd:YVO4 and Nd:GdVO4 elements to enhance certain output characteristics of the laser.
TL;DR: In this article, the authors present theory and numerical simulations of strong nonlinear effects in standing shear Alfven waves (SAWs) in the Earth's magnetosphere, which is modeled as a finite size box with straight magnetic lines and (partially) reflecting boundaries.
Abstract: We present theory and numerical simulations of strong nonlinear effects in standing shear Alfven waves (SAWs) in the Earth`s magnetosphere, which is modeled as a finite size box with straight magnetic lines and (partially) reflecting boundaries. In a low beta plasma it is shown that the ponderomotive force can lead to a large-amplitude SAW spatial harmonic generation due to nonlinear coupling between the SAW and a slow magnetosonic wave. The nonlinear coupling leads to secularly growing frequency shifts, and in the case of driven systems, nonlinear dephasing can lead to saturation of the driven wave fields. The results are discussed on the context of their possible relevance to the theory of standing ionospheric cavity wave modes and field line resonances in the high-latitude magnetosphere.
TL;DR: QPM optimization, being possible for both tight and loose focusing of the fundamental beam, may increase the conversion efficiency of high-order harmonic generation by several orders of magnitude as compared to the efficiency attainable now.
Abstract: We theoretically demonstrate the feasibility of optimal quasi-phase-matching (QPM) of high-order harmonic generation in gases and plasma with modulated density. QPM optimization, being possible for both tight and loose focusing of the fundamental beam, may increase the conversion efficiency of high-order harmonic generation by several orders of magnitude as compared to the efficiency attainable now.
TL;DR: In this paper, surface second harmonic generation (SHG) signals obtained with a linearly polarised fundamental indicate a Langmuir-like adsorption isotherm, and the SHG signals from the LL and DD enantiomer show equal but opposite dependence on the handedness of the fundamental radiation.
TL;DR: In this paper, a new mechanism to enhance second harmonic generation was proposed, where a numerical solution of the inhomogeneous wave equation in an interrupted periodic laminar structure indicates that the high energy density of a local mode within the forbidden zone may greatly enhance the nonlinear interaction.
01 Jan 1994
TL;DR: In this article, Agostini et al. studied the effect of high intensity Lasers on the Xray Yield of Sub-picosecond Laser-produced Plasmas.
Abstract: High Intensity Lasers A. Mysyrowicz. Recent Aspects of Multiphoton Ionization of Atoms P. Agostini, E. Mevel. Theory of Laser-Atom Interactions C.J. Joachain. Harmonic Generation at High Intensities K.J. Schafer, et al. Resonant Degenerate Fourwave Mixing D. Kupiszewska. Semiclassical Calculation of Atomic Processess R.M. More. Experiments of Rydberg Wave Packets L.D. Noordam, et al. Laser Manipulation of Neutral Atoms A. Aspect. Second Harmonic Production from Solid Targets D. von der Linde. Raman Study of Laserinduced Structure Modifications of Ionamorphized Silicon L.P. Avakyants, et al. Properties of Condensed Matter under Planetary Interior Conditions Measured by Femto-second Spectroscopy M.C. Downer, et al. Absorption Spectroscopy of Compressed Matter T.A. Hall. Density Functional and Nonequilibrium Methods for Unusual States of Matter Produced Using Shortpulse Lasers C. Dharma-Wardana. Theory of Complex Spectra from Laser Plasmas J. Bauche, C. BaucheArnoult. Influence of the Prepulse on the Xray Yield of Sub-picosecond Laserproduced Plasma J.C. Gauthier, et al. 4 additional articles. Index.
TL;DR: In this article, the 15th, 17th, and 19th harmonics of the dye laser light are separated by 4.07 eV and possess a narrow energy bandwidth which can be used to generate high resolution core level spectra.
Abstract: High resolution atomic core level spectroscopy is carried out on condensed matter systems using tunable harmonics generated by focusing light from an amplified 150 femtosecond dye laser system operating at 610 nm, into a pulsed source of Ar gas. We show core level spectra collected with the 15th (30.54 eV), 17th (34.61 eV), and 19th (38.68 eV) harmonics of the dye laser light. Each harmonic is separated by 4.07 eV and possesses a narrow energy bandwidth which can be used to generate high resolution core level spectra.
TL;DR: In this article, a simple nonperturbative two-level model of an atom driven by a very strong periodic field results in a rich picture of very high harmonic generation and related phenomena.
Abstract: We show that a simple nonperturbative two-level model of an atom driven by a very strong periodic field results in a rich picture of very high harmonic generation and related phenomena. It reproduces the experimentally observed plateau, yields simple analytic formulas for the plateau cutoff frequency, critical driving intensity, and saturation, and predicts intensity-induced multiresonances.
TL;DR: In this paper, the intensity-dependent switching characteristic of the nonlinear coupler is simulated, where a cascaded second-order nonlinearity is used as the switching power.
Abstract: The intensity-dependent switching characteristic of the nonlinear coupler is simulated. Here, for the first time, a cascaded χ(2) : χ(2)second-order nonlinearity in addition to the direct χ(3) third-order nonlinearity is used as the switching nonlinearity. The nonlinear refraction due to the cascade process can produce much larger effects than the electronic third-order nonlinearity which yields a reduced switching power of the nonlinear coupler. The description of the dynamics of the switching process suggests that switching of subpicosecond pulses is realizable. The resonance of the cascaded nonlinearity is primarily the resonance of second harmonic generation. Thus, the strength of the cascaded nonlinearity can simply be optimized corresponding to the required bandwidth by adjusting the phase-mismatch.
TL;DR: In this paper, the angular distribution of high-order harmonic radiation generated in neon and in argon using a 140 fs Cr:LiSAF laser system is reported. And the divergence of the harmonics is found to depend essentially on whether they belong to the plateau or to the cutoff.
Abstract: Reports the angular distributions of high-order harmonic radiation generated in neon and in argon using a 140 fs Cr:LiSAF laser system. The profiles show an evolution from Gaussian to near flat-top. No substructure is observed except at the highest intensity in a completely ionized gas. The divergence of the harmonics is found to depend essentially on whether they belong to the plateau or to the cutoff. It is approximately constant in the plateau, of the order of 15 mrad and decreases from 15 mrad to 6 mrad in the cutoff region.
TL;DR: In this article, femtosecond Ti: sapphire laser pulses were used for harmonic generation in barium metaborate (BBO) by femto-cond Ti-sparve laser pulses.
TL;DR: In this paper, the scanning surface harmonic microscope is used to detect the capacitance/voltage characteristics of semiconductor samples on a nanometer scale. And the authors demonstrate its sensitivity to a wide range of dopant concentrations on Si.
Abstract: The scanning surface harmonic microscope, in which a microwave signal is applied across a tip‐sample tunneling gap and higher harmonics are detected, is sensitive to the capacitance/voltage characteristics of semiconductor samples on a nanometer scale. We demonstrate its sensitivity to a wide range of dopant concentrations on Si, and its applications as a dopant profiler. Depletion regions are delineated with remarkable sensitivity, and variations in dopant concentration over a 35‐nm scale are discussed. Indications of a 5 nm resolution have been obtained.
TL;DR: In this article, the authors compare the spectra calculated using two different models, one in one dimension with an approximate hydrogenic soft core potential and the other in three dimensions with a Coulomb potential.
Abstract: A commonly used numerical technique for calculating the harmonic spectrum emitted by an atom exposed to an intense laser pulse is the direct integration of the Schr\"odinger equation. We compare the spectra calculated using two different models. The first is in one dimension with an approximate hydrogenic soft-core potential and the second is in three dimensions with a Coulomb potential. We use realistic laser pulse conditions (100 fs,800 nm) and intensities at which there is significant ionization, leading to a saturation of the harmonic-generation process. Although the ionization rates in the two models differ, the harmonic spectra and the positions of the cutoff are remarkably similar. Only a relatively small number of angular momentum states is needed in the three-dimensional calculation to give a reliable estimate of the cutoff, even for intensities at which there is strong ionization.
TL;DR: The ac Stark effect can shift initially nonresonant minibands in semiconductor superlattices into multiphoton resonances and result in strongly enhanced generation of a particular desired harmonic of the driving laser frequency, at isolated values of the amplitude.
Abstract: The ac Stark effect can shift initially nonresonant minibands in semiconductor superlattices into multiphoton resonances. This effect can result in strongly enhanced generation of a particular desired harmonic of the driving laser frequency, at isolated values of the amplitude.