Showing papers on "High harmonic generation published in 2006"
TL;DR: In this paper, a number of consequences of relativistic-strength optical fields are surveyed, including wakefield generation, a relativistically version of optical rectification, in which longitudinal field effects could be as large as the transverse ones.
Abstract: The advent of ultraintense laser pulses generated by the technique of chirped pulse amplification (CPA) along with the development of high-fluence laser materials has opened up an entirely new field of optics. The electromagnetic field intensities produced by these techniques, in excess of ${10}^{18}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$, lead to relativistic electron motion in the laser field. The CPA method is reviewed and the future growth of laser technique is discussed, including the prospect of generating the ultimate power of a zettawatt. A number of consequences of relativistic-strength optical fields are surveyed. In contrast to the nonrelativistic regime, these laser fields are capable of moving matter more effectively, including motion in the direction of laser propagation. One of the consequences of this is wakefield generation, a relativistic version of optical rectification, in which longitudinal field effects could be as large as the transverse ones. In addition to this, other effects may occur, including relativistic focusing, relativistic transparency, nonlinear modulation and multiple harmonic generation, and strong coupling to matter and other fields (such as high-frequency radiation). A proper utilization of these phenomena and effects leads to the new technology of relativistic engineering, in which light-matter interactions in the relativistic regime drives the development of laser-driven accelerator science. A number of significant applications are reviewed, including the fast ignition of an inertially confined fusion target by short-pulsed laser energy and potential sources of energetic particles (electrons, protons, other ions, positrons, pions, etc.). The coupling of an intense laser field to matter also has implications for the study of the highest energies in astrophysics, such as ultrahigh-energy cosmic rays, with energies in excess of ${10}^{20}\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The laser fields can be so intense as to make the accelerating field large enough for general relativistic effects (via the equivalence principle) to be examined in the laboratory. It will also enable one to access the nonlinear regime of quantum electrodynamics, where the effects of radiative damping are no longer negligible. Furthermore, when the fields are close to the Schwinger value, the vacuum can behave like a nonlinear medium in much the same way as ordinary dielectric matter expanded to laser radiation in the early days of laser research.
1,459 citations
TL;DR: In this article, a technique that uses high-order harmonic generation in molecules to probe nuclear dynamics and structural rearrangement on a sub-femtosecond time scale was demonstrated.
Abstract: We demonstrate a technique that uses high-order harmonic generation in molecules to probe nuclear dynamics and structural rearrangement on a subfemtosecond time scale. The chirped nature of the electron wavepacket produced by laser ionization in a strong field gives rise to a similar chirp in the photons emitted upon electron-ion recombination. Use of this chirp in the emitted light allows information about nuclear dynamics to be gained with 100-attosecond temporal resolution, from excitation by an 8-femtosecond pulse, in a single laser shot. Measurements on molecular hydrogen and deuterium agreed well with calculations of ultrafast nuclear dynamics in the H2+ molecule, confirming the validity of the method. We then measured harmonic spectra from CH4 and CD4 to demonstrate a few-femtosecond time scale for the onset of proton rearrangement in methane upon ionization.
664 citations
TL;DR: In this article, the first experimental demonstration of high harmonic generation in the relativistic limit was obtained on the Vulcan Petawatt laser, achieving high conversion efficiencies (η>10−6 per harmonic) and bright emission (>1022 photons s−1 mm−2 mrad−2 (0.1% bandwidth)) at wavelengths <4nm.
Abstract: The generation of extremely bright coherent X-ray pulses in the femtosecond and attosecond regime is currently one of the most exciting frontiers of physics–allowing, for the first time, measurements with unprecedented temporal resolution1,2,3,4,5,6. Harmonics from laser–solid target interactions have been identified as a means of achieving fields as high as the Schwinger limit2,7 (E=1.3×1016 V m−1) and as a highly promising route to high-efficiency attosecond (10−18 s) pulses8 owing to their intrinsically phase-locked nature. The key steps to attain these goals are achieving high conversion efficiencies and a slow decay of harmonic efficiency to high orders by driving harmonic production to the relativistic limit1. Here we present the first experimental demonstration of high harmonic generation in the relativistic limit, obtained on the Vulcan Petawatt laser9. High conversion efficiencies (η>10−6 per harmonic) and bright emission (>1022 photons s−1 mm−2 mrad−2 (0.1% bandwidth)) are observed at wavelengths <4 nm (the `water-window' region of particular interest for bio-microscopy).
431 citations
TL;DR: By introducing the second-harmonic component of the white light in the laser-induced plasma as a local oscillator, coherent detection of broadband THz waves with ambient air is demonstrated for the first time.
Abstract: We report the experimental results and theoretical analysis of broadband detection of terahertz (THz) waves via electric-field-induced second-harmonic generation in laser-induced air plasma with ultrashort laser pulses. By introducing the second-harmonic component of the white light in the laser-induced plasma as a local oscillator, coherent detection of broadband THz waves with ambient air is demonstrated for the first time. Our results show that, depending on the probe intensity, detection of THz waves in air can be categorized as incoherent, hybrid, and coherent detection. Coherent detection is achieved only when the tunnel ionization process dominates in gases.
415 citations
TL;DR: In this article, the polarization of a carrier-envelope phase-stabilized short laser pulse is modulated to fine control the electron-wavepacket dynamics, and the signature of a single return of the electron wavepacket over a large range of energies is observed.
Abstract: Attosecond electron wavepackets are produced when an intense laser field ionizes an atom or a molecule1. When the laser field drives the wavepackets back to the parent ion, they interfere with the bound wavefunction, producing coherent subfemtosecond extreme-ultraviolet light bursts. When only a single return is possible2,3, an isolated attosecond pulse is generated. Here we demonstrate that by modulating the polarization of a carrier-envelope phase-stabilized short laser pulse4, we can finely control the electron-wavepacket dynamics. We use high-order harmonic generation to probe these dynamics. Under optimized conditions, we observe the signature of a single return of the electron wavepacket over a large range of energies. This temporally confines the extreme-ultraviolet emission to an isolated attosecond pulse with a broad and tunable bandwidth. Our approach is very general, and extends the bandwidth of attosecond isolated pulses in such a way that pulses of a few attoseconds seem achievable. Similar temporal resolution could also be achieved by directly using the broadband electron wavepacket. This opens up a new regime for time-resolved tomography of atomic or molecular wavefunctions5,6 and ultrafast dynamics.
410 citations
TL;DR: A review of the recent advances in the field and stresses quantum phenomena that require laser field intensities in excess of the relativistic threshold of ∼ 10 18 W / cm 2 is presented in this article.
394 citations
TL;DR: In this article, the second order harmonic amplitude of a Rayleigh surface wave propagating in metallic specimens is measured using a laser-based ultrasonic technique, and the results show that there is a significant increase in the second-order harmonic amplitude at monotonic tensile loads above the material's yield stress.
Abstract: A reliable laser-based ultrasonic technique is developed to measure the second order harmonic amplitude of a Rayleigh surface wave propagating in metallic specimens. Rayleigh waves are experimentally generated with a wedge transducer and detected with a heterodyne laser interferometer. The capability of this system to measure the nonlinear contribution present in Rayleigh surface waves is demonstrated, and these results are interpreted in terms of a parameter developed for Rayleigh surface waves which corresponds to the nonlinear parameter of a longitudinal wave, β. The proposed measurement technique is used to assess damage in nickel-base high temperature alloy specimens, and the evolution of material nonlinearity under various loading conditions is quantitatively measured in terms of the increasing amplitude of the second order harmonic. These results show that there is a significant increase in the second order harmonic amplitude at monotonic tensile loads above the material’s yield stress, and that du...
232 citations
TL;DR: It is shown by classical and quantum-mechanical model calculations that even a minor addition of phase-locked second-harmonic light to the 800 nm fundamental driver pulse for high- Harmonic generation leads to a major difference in the maximum kinetic energies of the recombining electrons in adjacent half-cycles.
Abstract: We present a method of producing single attosecond pulses by high-harmonic generation with multicycle driver laser pulses. This can be achieved by tailoring the driving pulse so that attosecond pulses are produced only every full cycle of the oscillating laser field rather than every half-cycle. It is shown by classical and quantum-mechanical model calculations that even a minor addition (1%) of phase-locked second-harmonic light to the 800 nm fundamental driver pulse for high-harmonic generation leads to a major (15%) difference in the maximum kinetic energies of the recombining electrons in adjacent half-cycles.
175 citations
TL;DR: A previously undescribed spectroscopic probe that makes use of electrons rescattered during the process of high-order harmonic generation using impulsive stimulated Raman scattering with a short laser pulse is reported.
Abstract: We report a previously undescribed spectroscopic probe that makes use of electrons rescattered during the process of high-order harmonic generation. We excite coherent vibrations in SF6 using impulsive stimulated Raman scattering with a short laser pulse. A second, more intense laser pulse generates high-order harmonics of the fundamental laser, at wavelengths of ≈20–50 nm. The high-order harmonic yield is observed to oscillate, at frequencies corresponding to all of the Raman-active modes of SF6, with an asymmetric mode most visible. The data also show evidence of relaxation dynamics after impulsive excitation of the molecule. Theoretical modeling indicates that the high harmonic yield should be modulated by both Raman and infrared-active vibrational modes. Our results indicate that high harmonic generation is a very sensitive probe of vibrational dynamics and may yield more information simultaneously than conventional ultrafast spectroscopic techniques. Because the de Broglie wavelength of the recolliding electron is on the order of interatomic distances, i.e., ≈1.5 A, small changes in the shape of the molecule lead to large changes in the high harmonic yield. This work therefore demonstrates a previously undescribed spectroscopic technique for probing ultrafast internal dynamics in molecules and, in particular, on the chemically important ground-state potential surface.
157 citations
TL;DR: When fully antisymmetrized multielectron wave functions and electronic relaxation in the cation are considered, molecular orbital tomography records the image of the Dyson orbital plus exchange contributions from inner shells, which can be interpreted as a complete Hartree-Fock wave function.
Abstract: It was recently shown that the highest molecular orbital of ${\mathrm{N}}_{2}$ could be reconstructed from a series of high harmonic measurements. Existing theories of high harmonic generation and orbital tomographic imaging are based on the single active electron approximation that ignores essential quantum mechanical properties such as the indistinguishability of identical particles and the Pauli exclusion principle. We show that, when fully antisymmetrized multielectron wave functions and electronic relaxation in the cation are considered, molecular orbital tomography records the image of the Dyson orbital plus exchange contributions from inner shells. The mixing of contributions from more than one molecular orbital gives access to additional wave function information. By utilizing the exchange term, harmonic emission from a closed-shell 4-electron system can be interpreted as a complete Hartree-Fock wave function.
138 citations
TL;DR: Applications of the heterodyne-mixing concept are the extension of the harmonic cutoff to higher photon energies and the temporal gating of attosecond pulse production.
Abstract: The concept of heterodyne mixing of laser fields is theoretically applied to the process of high-harmonic generation to enhance and modulate the kinetic energy of the active electron on subcycle time scales. A very small amount of intensity in the heterodyne field creates a significant modification of the electron kinetic energy, due to its amplification by the strong fundamental field in the kinetic-energy term, in which the heterodyne mixing occurs. Quantum calculations are carried out to verify the predictions of the classical results, demonstrating very good qualitative and quantitative agreement. Applications of the heterodyne-mixing concept are the extension of the harmonic cutoff to higher photon energies and the temporal gating of attosecond pulse production.
TL;DR: In this paper, the authors present ab initio quantum and classical investigations on the production and control of a single attosecond pulse by using few-cycle intense laser pulses as the driving field.
Abstract: We present ab initio quantum and classical investigations on the production and control of a single attosecond pulse by using few-cycle intense laser pulses as the driving field. The high-harmonic-generation power spectrum is calculated by accurately and efficiently solving the time-dependent Schr\"odinger equation using the time-dependent generalized pseudospectral method. The time-frequency characteristics of the attosecond xuv pulse are analyzed in detail by means of the wavelet transform of the time-dependent induced dipole. To better understand the physical processes, we also perform classical trajectory simulation of the strong-field electron dynamics and electron returning energy map. We found that the quantum and classical results provide complementary and consistent information regarding the underlying mechanisms responsible for the production of the coherent attosecond pulse. For few-cycle $(5\phantom{\rule{0.3em}{0ex}}\mathrm{fs})$ driving pulses, it is shown that the emission of the consecutive harmonics in the supercontinuum cutoff regime can be synchronized and locked in phase resulting in the production of a coherent attosecond pulse. Moreover, the time profile of the attosecond pulses can be controlled by tuning the carrier envelope phase.
TL;DR: A multiphoton microscopy system using a 12-fs Ti:sapphire laser with adjustable dispersion precompensation with efficiencies of two-photon-excited fluorescence (TPEF) and second harmonic generation (SHG) was developed to examine the impact of pulse duration on nonlinear optical signals.
Abstract: We have developed a multiphoton microscopy MPM system using a 12-fs Ti:sapphire laser with adjust- able dispersion precompensation in order to examine the impact of pulse duration on nonlinear optical signals. The efficiencies of two-photon-excited fluorescence TPEF and second harmonic generation SHG were studied for various pulse durations, measured at the sample, ranging from 400 fs to sub-20 fs. Both TPEF and SHG increased proportionally to the inverse of the pulse duration for the entire tested range. Because of improved signal-to-noise ratio, sub-20-fs pulses were used to enhance MPM imag- ing depth by approximately 160%, compared to 120-fs pulses, in human skin. © 2006 Society of Photo-Optical Instrumenta-
TL;DR: In this paper, an experimental study of high-order harmonic generation in In, InSb, InP, and InGaP plasmas using femtosecond laser radiation with variable chirp is presented.
Abstract: An experimental study of high-order harmonic generation in In, InSb, InP, and InGaP plasmas using femtosecond laser radiation with variable chirp is presented. Intensity enhancement of the 13th and 21st harmonics compared to the neighboring harmonics by a factor of 200 and 10, respectively, is observed. It is shown that the harmonic spectrum from indium-containing plasma plumes can be considerably modified by controlling the chirp of the driving laser pulse.
TL;DR: In this article, the surface magnetic and structural properties of magnetite thin films have been probed by nonlinear second-harmonic generation optical method in high- (centrosymmetric, Oh) and low- (noncentro symmetric, C1) temperature phases.
Abstract: The surface magnetic and structural properties of magnetite thin films have been probed by nonlinear second-harmonic generation optical method in high- (centrosymmetric, Oh) and low- (noncentrosymmetric, C1) temperature phases A model taking into account the cubic magnetocrystalline anisotropy and magnetic symmetry reduction due to a magnetically modified surface layer is supposed to describe the azimuth variations of the nonlinear response The metal-isolator transition (Verwey) manifests itself in an increase of nonlinear response (∼25%) and a decrease in magnetic contrast (∼50%)
TL;DR: In this article, a two-color laser field consisting of a sub-10-fs fundamental and its parallel-polarized second harmonic was applied to high-order harmonic generation in argon, a continuum spectrum centered at 30 nm was successfully obtained with an energy as high as 10 nJ.
Abstract: We have proposed and demonstrated a novel approach for generating high-energy extreme-ultraviolet (XUV) continuum radiation. When a two-color laser field consisting of a sub-10-fs fundamental and its parallel-polarized second harmonic was applied to high-order harmonic generation in argon, a continuum spectrum centered at 30 nm was successfully obtained with an energy as high as 10 nJ. This broadband emission indicates the possibility of generating intense single attosecond pulses in the XUV region.
TL;DR: By irradiating He and Ne atoms with 3mJ, 12fs, near infrared laser pulses from a tabletop laser system, the authors generated spatially and temporally coherent x rays up to a photon energy of 3.5keV.
Abstract: By irradiating He and Ne atoms with 3mJ, 12fs, near infrared laser pulses from a tabletop laser system, the authors generated spatially and temporally coherent x rays up to a photon energy of 3.5keV. With this source it is possible to use high-harmonic radiation for x-ray absorption spectroscopy in the keV range. They were able to clearly resolve the L absorption edges of titanium and copper and the K edges of aluminum and silicon. From the fine structure of the x-ray absorption they estimated the interatomic distances.
TL;DR: In this article, a coupled modes analysis of second-harmonic generation in microdisk resonators is proposed, where whispering gallery modes can be used to obtain a combination of modal and geometrical quasi-phase matching to obtain efficient conversion in isotropic and nonferroelectric materials such as III-V semiconductor compounds.
Abstract: We propose a coupled modes analysis of second-harmonic generation in microdisk resonators. We demonstrate that whispering gallery modes can be used to obtain a combination of modal and geometrical quasi-phase-matching (without domain inversion) to obtain efficient conversion in isotropic and nonferroelectric materials such as III-V semiconductor compounds. Finally we use an analytical model to describe the coupling between a bus waveguide and the nonlinear microdisk to achieve an optimization scheme for practical configuration.
TL;DR: It is found that the integration of phase-plates on the focusing mirrors provides the best performance in terms of input coupling efficiency, cavity loss, and output-coupling of the generated high harmonic light.
Abstract: We have investigated the coupling efficiency and cavity loss associated with a ring cavity that has a hole in one of the focusing mirrors. The aperture provides a means through which intracavity high-harmonic generation can be coupled from the cavity. By studying different cavity geometries and input modes we have found that the integration of phase-plates on the focusing mirrors provides the best performance in terms of input coupling efficiency, cavity loss, and output-coupling of the generated high harmonic light.
TL;DR: The theoretical contact pressure dependence of the nonlinearity parameter was found to be in good qualitative agreement with the experimental results.
TL;DR: By considering the tensor and the presence of the other electric field components the possibility of different polarization states of the generated second harmonic as a result of the nonlinear susceptibility tensor is considered, making it possible to generate radially polarized modes with linearly polarized beams.
Abstract: The effects of the axial field components of a focused beam under high NA on the second harmonic generation (SHG) in collagen was examined using a vectorial approach. We find that with high NA, the cross-component terms that are most likely to have an effect on SHG will be ExEx, ExEy, ExEz and EzEz as a result of tight focusing. By considering the tensor and the presence of the other electric field components the possibility of different polarization states of the generated second harmonic as a result of the nonlinear susceptibility tensor making it possible to generate radially polarized modes with linearly polarized beams.
TL;DR: A new HMI technique is described that images tissue displacement induced by a harmonic radiation force using a single focused-ultrasound element to enable a highly-localized force and stiffness-dependent measurements as well as real-time and low-cost HIFU monitoring.
TL;DR: The intensity enhancement of a single high-order harmonic is demonstrated by using low-ionized tin ions in a laser-ablation plume as the nonlinear medium and attributed to multiphoton resonance with a strong radiative transition of the Sn II ion.
Abstract: We have successfully demonstrated intensity enhancement of a single high-order harmonic at a wavelength of 46.76 nm by using low-ionized tin ions in a laser-ablation plume as the nonlinear medium. The ablation plume was produced by irradiating a solid tin target with a 10 mJ energy picosecond laser pulse. Strong 17th-harmonic generation at a wavelength of 46.76 nm was observed with a conversion efficiency of about 10−4. We attribute the strong enhancement of the single high-order harmonic to multiphoton resonance with a strong radiative transition of the Sn II ion.
TL;DR: The breakdown of the frozen-core single active electron approximation is demonstrated, as it predicts roughly the same radiation amplitude in all noble gases, in contradiction with experiments, where heavier noble gases are known to emit much stronger HHG radiation than lighter ones.
Abstract: High harmonic generation (HHG) in many-electron atoms is studied theoretically. The breakdown of the frozen-core single active electron approximation is demonstrated, as it predicts roughly the same radiation amplitude in all noble gases. This is in contradiction with experiments, where heavier noble gases are known to emit much stronger HHG radiation than lighter ones. This experimental behavior of the noble gases can be qualitatively reproduced when many-electron dynamics, within a simple approximation, is taken into account.
TL;DR: In this paper, second-harmonic light can be generated from a diffraction grating of gold nanoparticles with planar inversion symmetry, where the diffraction pattern of the grating is superimposed on the intrinsic secondharmonic radiation patterns of the nanoparticles.
Abstract: We show that second-harmonic light can be generated from a diffraction grating of gold nanoparticles with planar inversion symmetry. By measuring the angular distribution of second-harmonic light, we observe an effect in which the diffraction pattern of the grating is superimposed on the intrinsic second-harmonic radiation pattern of the nanoparticles. This result suggests that the second-harmonic generation may be used to study coherent nonlinear optical effects in symmetric as well as asymmetric metal nanoparticles.
TL;DR: In this article, the authors showed evidence for quantum interference of electron de Broglie waves in the recombination process of high-order harmonic generation (HHG) from aligned CO2 molecules.
Abstract: High-order harmonic generation (HHG) from atoms and molecules offers potential application as a coherent ultrashort radiation source in the extreme ultraviolet and soft X-ray regions1,2,3. In the three-step model4,5,6 of HHG, an electron tunnels out from the atom and may recombine with the parent ion (emitting a high-energy photon) after undergoing laser-driven motion in the continuum. Aligned molecules7,8,9,10,11 can be used to study quantum phenomena in HHG associated with molecular symmetries; in particular, simultaneous observations of both ion yields and harmonic signals under the same conditions serve to disentangle the contributions from the ionization and recombination processes. Here we report evidence for quantum interference of electron de Broglie waves12,13,14 in the recombination process of HHG from aligned CO2 molecules. The interference takes place within a single molecule and within one optical cycle. Characteristic modulation patterns of the harmonic signals measured as a function of the pump–probe delay are explained with simple formulae determined by the valence orbital of the molecules. We propose that simultaneous observations of both ion yields and harmonic signals can serve as a new route to probe the instantaneous structure of molecular systems.
TL;DR: In this article, three upconverted emission peaks from ZnO nanowires were observed with femtosecond pulse excitation at the wavelength near 800nm, and the ultraviolet peak at about 385nm and green emission band centered at about 515nm were attributed to the near band edge emission and defect level emission, respectively, while the intermediate peak was assigned to the second harmonic generation.
Abstract: With femtosecond pulse excitation at the wavelength near 800nm, we observed three upconverted emission peaks from ZnO nanowires. The ultraviolet peak at about 385nm and the green emission band centered at about 515nm were attributed to the near band edge emission and defect level emission, respectively, while the intermediate peak was assigned to the second harmonic generation. From the quadratic dependence of the emission intensity on the excitation pulse energy, it is confirmed that the 385nm photoluminescence was mainly generated via two-photon absorption in ZnO nanowires under very intense light-matter interaction.
TL;DR: A Ti:sapphire oscillator with an extended cavity generates pulses with 0.5 microJ energy, sufficient for ionizing helium, a nonlinear process where at least 17 photons are absorbed simultaneously.
Abstract: A Ti:sapphire oscillator with an extended cavity generates pulses with 0.5 μJ energy at a repetition rate of 6 MHz and pulse durations of 50 fs. Tight focusing creates peak intensities exceeding 1014 W/cm2, which is sufficient for ionizing helium, a nonlinear process where at least 17 photons are absorbed simultaneously.
TL;DR: A significant extension of the high-order harmonic cutoff is demonstrated by using a fully-ionized capillary discharge plasma as the generation medium, which allows for significantly higher photon energies to be generated from xenon ions, compared with the 70 eV observed previously.
Abstract: We demonstrate a significant extension of the high-order harmonic cutoff by using a fully-ionized capillary discharge plasma as the generation medium. The preionized plasma dramatically reduces ionization-induced defocusing and energy loss of the driving laser due to ionization. This allows for significantly higher photon energies, up to 150 eV, to be generated from xenon ions, compared with the 70 eV observed previously. We also demonstrate enhancement of the harmonic flux of nearly 2 orders of magnitude at photon energies around 90 eV when the capillary discharge is used to ionize xenon, compared with harmonic generation in a hollow waveguide. The use of a plasma as a medium for high-order harmonic generation shows great promise for extending efficient harmonic generation to much shorter wavelengths using ions.
TL;DR: In this article, the quantum-orbit formalism of high-order atomic processes in a strong laser field is presented starting from the strong-field approximation generalized to include higher-order effects.
Abstract: The quantum-orbit formalism of high-order atomic processes in a strong laser field is presented starting from the strong-field approximation generalized to include higher-order effects. It is shown how to apply the quantum-orbit theory to various processes such as high-order above-threshold ionization and detachment, high-order harmonic generation, laser-assisted X-ray–atom scattering, laser-assisted electron–ion recombination, laser-assisted electron–atom scattering and non-sequential double ionization. Particular attention is devoted to high-order above-threshold ionization by few-cycle laser pulses. The results obtained using the strong-field approximation and the theory of quantum orbits are compared with the ab initio solution of the time-dependent Schrodinger equation. It is shown that Coulomb effects are important for low-energy electron spectra.