Showing papers on "High harmonic generation published in 2000"
TL;DR: By carefully tailoring the shape of intense light pulses, this work can control the interaction of light with an atom during ionization, improving the efficiency of X-ray generation by an order of magnitude.
Abstract: When an intense laser pulse is focused into a gas, the light-atom interaction that occurs as atoms are ionized results in an extremely nonlinear optical process--the generation of high harmonics of the driving laser frequency. Harmonics that extend up to orders of about 300 have been reported, some corresponding to photon energies in excess of 500 eV. Because this technique is simple to implement and generates coherent, laser-like, soft X-ray beams, it is currently being developed for applications in science and technology; these include probing the dynamics in chemical and materials systems and imaging. Here we report that by carefully tailoring the shapes of intense light pulses, we can control the interaction of light with an atom during ionization, improving the efficiency of X-ray generation by an order of magnitude. We demonstrate that it is possible to tune the spectral characteristics of the emitted radiation, and to steer the interaction between different orders of nonlinear processes.
649 citations
TL;DR: The fabrication of what is believed to be the first example of a two-dimensional (2D) nonlinear photonic crystal where the refractive index is constant but where the 2nd order nonlinear susceptibility is spatially periodic is reported.
Abstract: We report on the fabrication of what we believe is the first example of a two-dimensional (2D) nonlinear photonic crystal [Berger, Phys. Rev. Lett. 81, 4136 (1998)], where the refractive index is constant but where the 2nd order nonlinear susceptibility is spatially periodic. Such crystals allow for efficient quasi-phase-matched 2nd harmonic generation using multiple reciprocal lattice vectors. External 2nd harmonic conversion efficiencies >60% were measured with picosecond pulses. The fabrication technique is extremely versatile and should allow for the fabrication of a broad range of 2D crystals including quasicrystals.
450 citations
TL;DR: The approach uses a laser-seeded free-electron laser to produce amplified, longitudinally coherent, Fourier transform-limited output at a harmonic of the seed laser, with the ultimate goal of extending the approach to provide an intense, highly coherent source of hard x-rays.
Abstract: A high-gain harmonic-generation free-electron laser is demonstrated. Our approach uses a laser-seeded free-electron laser to produce amplified, longitudinally coherent, Fourier transform-limited output at a harmonic of the seed laser. A seed carbon dioxide laser at a wavelength of 10.6 micrometers produced saturated, amplified free-electron laser output at the second-harmonic wavelength, 5.3 micrometers. The experiment verifies the theoretical foundation for the technique and prepares the way for the application of this technique in the vacuum ultraviolet region of the spectrum, with the ultimate goal of extending the approach to provide an intense, highly coherent source of hard x-rays.
376 citations
TL;DR: In this article, a detailed analysis of the second-harmonic radiation from biological membranes labeled with a styryl dye is presented, where the authors consider the high-numerical-aperture limit appropriate to high-resolution microscopy in which an excitation beam is tightly focused from the side onto a membrane surface.
Abstract: We present a detailed analysis of the generation of second-harmonic radiation from biological membranes labeled with a styryl dye. In particular, we consider the high-numerical-aperture limit appropriate to high-resolution microscopy in which an excitation beam is tightly focused from the side onto a membrane surface. In this limit the active surface area that contributes to second-harmonic generation (SHG) depends only on the tightness of the beam focus and the SHG radiation is confined by phase matching into two well-defined off-axis lobes. We derive expressions for the SHG radiation power, angular distribution, and polarization dependence in the cases of ideal or nonideal molecular alignment in the membrane and uniaxiality of the molecular hyperpolarizability. We define an SHG cross section similar to that used in two-photon-excited fluorescence (TPEF) to permit direct comparison of the two imaging modalities. Finally, we corroborate our results with experiments based on the excitation of a styryl dye in giant unilamellar vesicles with a mode-locked Ti:sapphire laser.
365 citations
TL;DR: In this article, a single-mode nonlinear resonance acoustic spectroscopy (SIMONRAS) method was proposed to quantify the influence of mesoscopic features and damage in quasi-brittle materials.
Abstract: The presence of mesoscopic features and damage in quasi-brittle materials causes significant second-order and nonlinear effects on the acoustic wave propagation characteristics. In order to quantify the influence of such micro-inhomogeneities, a new and promising tool for nondestructive material testing has been developed and applied in the field of damage detection. The technique focuses on the acoustic nonlinear (i.e., amplitude-dependent) response of one of the material's resonance modes when driven at relatively small wave amplitudes. The method is termed single-mode nonlinear resonance acoustic spectroscopy (SIMONRAS). The behavior of damaged materials is manifested by amplitude dependent resonance frequency shifts, harmonic generation, and nonlinear attenuation. We illustrate the method by experiments on artificial slate tiles used in roofing construction. The sensitivity of this method to discern material damage is far greater than that of linear acoustic methods.
350 citations
TL;DR: In this article, an efficient method for the generation of circularly polarized high-order harmonics by a bichromatic laser field whose two components with frequencies Ω(n) and n 2 are circularly polarized in the same plane, but rotate in opposite directions was investigated.
Abstract: An efficient method is investigated for the generation of circularly polarized high-order harmonics by a bichromatic laser field whose two components with frequencies $\ensuremath{\omega}$ and $2\ensuremath{\omega}$ are circularly polarized in the same plane, but rotate in opposite directions. The generation of intense harmonics by such a driving-field configuration was already confirmed by a previous experiment. With the help of both a semiclassical three-step model as well as a saddle-point analysis, the mechanism of harmonic generation in this case is elucidated and the plateau structure of the harmonic response and their cutoffs are established. The sensitivity of the harmonic yield and the polarization of the harmonics to imperfect circular polarization of the driving fields are investigated. Optimization of both the cutoff frequency and the harmonic efficiency with respect to the intensity ratio of the two components of the driving field is discussed. The electron trajectories responsible for the emission of particular harmonics are identified. Unlike the case of a linearly polarized driving field, they have a nonzero start velocity. By comparison with the driving-field configuration where the two components rotate in the same direction, the mechanism of the intense harmonic emission is further clarified. Depending on the (unknown) saturation intensity for the bichromatic field with counter-rotating polarizations, this scheme might be of practical interest not only because of the circular polarization of the produced harmonics, but also because of their production efficiency.
271 citations
TL;DR: In this article, a numerical model to calculate the high-order harmonics spectrum of a macroscopic gas target irradiated by a few-optical-cycle laser pulse is presented.
Abstract: A numerical model to calculate the high-order harmonics spectrum of a macroscopic gas target irradiated by a few-optical-cycle laser pulse is presented. The single-atom response, calculated within the nonadiabatic strong-field approximation, is the source term of a three-dimensional propagation code. The simulation results show remarkably good agreement with experiments performed in neon using laser pulses with durations of 30 and 7 fs. Both simulations and experiments show discrete and well-resolved harmonics even for the shortest driving pulses.
168 citations
22 Oct 2000
TL;DR: Harmonic imaging was originally developed for micro-bubble contrast agents in the early 90s under the assumption that tissue is linear and all harmonic echoes are generated by the bubbles.
Abstract: Harmonic imaging was originally developed for microbubble contrast agents in the early 90s under the assumption that tissue is linear and all harmonic echoes are generated by the bubbles. In fact, tissue, like bubbles, is a nonlinear medium. Whereas the harmonic echoes from bubbles have their origins in nonlinear scattering, those from tissue are a result of nonlinear propagation. The clinical benefits of tissue harmonic imaging are reduced reverberation noise and overall clutter level, improved border delineation, increased contrast resolution, and reduced phase aberration artifacts. To a large extend these benefits are explained by the properties of nonlinear propagation of the transmitted ultrasonic pulses in the tissue.
160 citations
TL;DR: In this paper, an ab initio three-dimensional quantum study of high-order harmonic generation (HHG) of atomic H in intense pulsed laser fields is presented, which reveals striking details of the spectral and temporal fine structures of HHG, providing insights regarding HHG mechanisms in different energy regimes.
Abstract: We present an ab initio three-dimensional quantum study of high-order harmonic generation ~HHG! of atomic H in intense pulsed laser fields. Accurate time-dependent wave functions are obtained by means of the time-dependent generalized pseudospectral method recently developed and wavelet transform is used to perform time-frequency analysis of the resulting HHG power spectra. The results reveal striking details of the spectral and temporal fine structures of HHG, providing insights regarding HHG mechanisms in different energy regimes and benchmark data for testing the validity of existing HHG models. PACS number~s!: 42.65.Ky, 32.80.Wr, 42.50.Hz Recently a great deal of attention has been devoted to the study of multiple high-order harmonic generation ~HHG! processes in intense short laser pulses @1‐3#. Besides its fundamental interest for strong-field atomic and molecular physics, the HHG provides a potential tunable coherent light source in the extreme ultraviolet ~XUV! region, a so-called ‘‘tabletop synchrotron’’ @2#. Further, the HHG may lead to a promising way for generating subfemtosecond ~attosecond! pulses of radiation of high frequency @4,5#. For optimal control of the HHG processes, it is essential to have a thorough understanding of the spectral and temporal structures of the high-order harmonics and the detailed underlying mechanisms for HHG. The first experimental observation of the temporal coherence of HHG has recently been reported @6#. In this Rapid Communication, we present an ab initio threedimensional ~3D! precision quantum calculation of the time
146 citations
TL;DR: In this article, the phases of the circularly polarized harmonics with alternating helicity generated by a bichromatic laser field whose two components are cyclically polarized in the same plane but rotate in opposite directions are investigated.
Abstract: The phases of the circularly polarized harmonics with alternating helicity generated by a bichromatic laser field whose two components are circularly polarized in the same plane but rotate in opposite directions are investigated. Only one trajectory contributes to harmonic generation in the plateau region. The dependence of the harmonic phase on the laser field intensity is weak (with the slope $\ensuremath{\sim}{0.2U}_{p}/\ensuremath{\omega}).$ Adjacent harmonics having the same helicity are relatively closely phase locked. As a result, a train of three attosecond pulses per optical cycle of the driving field is generated, each having a width of 80 as. Depending upon whether the two helicity components can be separated the polarization of the pulses is close to circular or close to linear with three different orientations per optical cycle.
129 citations
TL;DR: This paper investigating the three-dimensional evolution of harmonic radiation based on the coupled Maxwell-Klimontovich equations that take into account nonlinear harmonic interactions finds that the gain length, transverse profile, and temporal structure of the first few harmonics are eventually governed by those of the fundamental.
Abstract: In a high-gain free-electron laser (FEL) employing a planar undulator, strong bunching at the fundamental wavelength can drive substantial bunching and power levels at the harmonic frequencies. In this paper we investigate the three-dimensional evolution of harmonic radiation based on the coupled Maxwell-Klimontovich equations that take into account nonlinear harmonic interactions. Each harmonic field is a sum of a linear amplification term and a term driven by nonlinear harmonic interactions. After a certain stage of exponential growth, the dominant nonlinear term is determined by interactions of the lower nonlinear harmonics and the fundamental radiation. As a result, the gain length, transverse profile, and temporal structure of the first few harmonics are eventually governed by those of the fundamental. Transversely coherent third-harmonic radiation power is found to approach 1% of the fundamental power level for current high-gain FEL projects.
TL;DR: In this article, the second harmonic generation (SHG) optical functions of cubic and hexagonal BN, AlN, GaN, and InN have been studied by using the first-principles full-potential linearized augmented plane-wave method.
Abstract: Linear and nonlinear [second harmonic generation (SHG)] optical functions of cubic and hexagonal BN, AlN, GaN, and InN have been studied by using the first-principles full-potential linearized augmented plane-wave method. Equilibrium lattice constants are determined from the total-energy minimization method. The calculated spectra of the second-order optical susceptibility show pronounced structures related to the two-photon resonances. In materials with heavy metals there are remarkable contributions from the single-photon transitions. Line shapes of the linear and particularly the nonlinear optical spectra of GaN and InN crystals are very sensitive to the interactions between the conduction bands and metallic d states. Studies of the nonlinear optical susceptibilities in both wurtzite and cubic crystals show high sensitivity of the SHG spectra to the changes of atomic structure.
TL;DR: A generalization of the analytical theory of high harmonic generation in the long wavelength limit and in the single active electron approximation is developed taking into account the magnetic dipole and electric quadrupole interaction, which promises the extension ofHigh harmonic generation well into the sub-nm wavelength regime.
Abstract: A generalization of the analytical theory of high harmonic generation in the long wavelength limit and in the single active electron approximation is developed taking into account the magnetic dipole and electric quadrupole interaction. Quantum mechanical and classical theories are found to be in excellent agreement, which allows one to explain the influence of multipole effects in terms of an intuitive picture. For Ti:S lasers ( 0.8 mm) multipole contributions are found to be small below an intensity of about 10(17) W/cm(2), at which harmonic radiation with photon energies of several keV is generated. This promises the extension of high harmonic generation well into the sub-nm wavelength regime.
TL;DR: This power is, to the authors' knowledge, the highest UV power achieved so far in all-solid-state lasers.
Abstract: We have obtained UV power of 20.5 W with a repetition rate of 10 kHz by the use of a high-brightness high-power all-solid-state green laser and a high-quality CsLiB6O10 crystal. This power is, to our knowledge, the highest UV power achieved so far in all-solid-state lasers.
TL;DR: A corrugated hollow-core fiber modulates the intensity of the fundamental pulse along the direction of propagation, resulting in a periodic modulation of the harmonic emission at wavelengths close to the cutoff, increasing the harmonic yield by up to three orders of magnitude.
Abstract: We describe theoretically a new technique for quasi-phase-matched generation of high harmonics and attosecond pulses in a gas medium, in a high ionization limit. A corrugated hollow-core fiber modulates the intensity of the fundamental pulse along the direction of propagation, resulting in a periodic modulation of the harmonic emission at wavelengths close to the cutoff. This leads to an increase of the harmonic yield of up to three orders of magnitude. At the same time the highest harmonics merge in a broad band that corresponds to a single attosecond pulse, using 15-fs driving pulses.
TL;DR: In this article, a nonlinear simulation code to treat multiple frequencies simultaneously is described and used to study nonlinear harmonic generation in free-electron lasers (FELs), where the odd harmonics are favored with higher power levels since a planar wiggler geometry is employed.
Abstract: A three-dimensional nonlinear simulation code to treat multiple frequencies simultaneously is described and used to study nonlinear harmonic generation in free-electron lasers (FELs). Strong nonlinear harmonic gain is found where the gain length varies inversely with the harmonic number. Substantial power levels are found in the harmonics. The odd harmonics are favored with generally higher power levels since a planar wiggler geometry is employed; however, the second harmonic exhibits substantial power as well. The analysis is relevant to the emission expected from self-amplified spontaneous emission (SASE) free-electron laser schemes.
TL;DR: In this paper, a fast drop of the second-harmonic intensity and the Kerr-signal within 300 fs after optical excitation has been observed which is followed by a partial recovery within a few picoseconds.
Abstract: Relaxation processes on the picosecond and subpicosecond time scale after optical excitation have been studied by time-resolved magnetization-induced second-harmonic generation from ferromagnetic nickel. A fast drop of the second-harmonic intensity and the Kerr-signal within 300 fs after optical excitation has been observed which is followed by a partial recovery within a few picoseconds. We show that the fast initial drop cannot be unambiguously attributed to an ultrafast decrease of the magnetization.
TL;DR: In this article, high-order optical harmonics were generated from solid targets using laser pulses of 35 and 120 femtoseconds, and high conversion efficiency was achieved, e.g., ${10}^{\ensuremath{-}6} to the $10\mathrm{th}$ harmonic.
Abstract: This paper discusses the generation of high-order optical harmonics from solid targets using laser pulses of 35 and 120 femtoseconds. Harmonics up to the 35th order were observed. High conversion efficiency has been achieved, e.g., ${10}^{\ensuremath{-}6}$ to the $10\mathrm{th}$ harmonic. It is demonstrated that the harmonic emission is highly directional and that the harmonic efficiency decreases rapidly with increasing plasma scale length.
TL;DR: It is demonstrated that high-order harmonics generated by short, intense laser pulses in gases provide an interesting radiation source for extreme ultraviolet interferometry, since they are tunable, coherent, of short pulse duration, and simple to manipulate.
Abstract: We demonstrate that high-order harmonics generated by short, intense laser pulses in gases provide an interesting radiation source for extreme ultraviolet interferometry, since they are tunable, coherent, of short pulse duration, and simple to manipulate. Harmonics from the 9th to the 15th are used to measure the thickness of an aluminum layer. The 11th harmonic is used to determine the spatial distribution of the electron density of a plasma produced by a 300-ps laser. Electronic densities higher than 2–1020 electrons/cm3 are measured.
TL;DR: Phase-matched relativistic harmonic generation in plasmas is observed for the first time and the angular pattern is a narrow forward-directed cone, which is consistent with phase matching of a high-order transverse mode in a plasma.
Abstract: Phase-matched relativistic harmonic generation in plasmas is observed for the first time. Third-harmonic light is detected and discriminated spectrally and angularly from the harmonics generated from competing processes. Its angular pattern is a narrow forward-directed cone, which is consistent with phase matching of a high-order transverse mode in a plasma. The signal level is found to be on the same order of magnitude for a circularly polarized pump pulse as for a linearly polarized pump pulse.
TL;DR: In this paper, the authors investigated the second harmonic generation (SHG) in collagen using a broad excitation spectrum covering the 760-1070 nm wavelength range and found that the SHG signal intensity exhibited a quadratic dependence on the excitation radiation (log[ I 532 ]=1.92*log[I 1064 ]).
TL;DR: By spectroscopic analysis of the cantilever oscillation in tapping-mode atomic-force microscopy (TM-AFM), the authors showed that the transition from an oscillatory state dominated by a net attractive force to the state dominated with repulsive interaction is accompanied by the enhanced generation of higher harmonics.
Abstract: By spectroscopic analysis of the cantilever oscillation in tapping-mode atomic-force microscopy (TM–AFM), we demonstrate that the transition from an oscillatory state dominated by a net attractive force to the state dominated by repulsive interaction is accompanied by the enhanced generation of higher harmonics. The higher harmonics are a consequence of the nonlinear interaction and are amplified to significant amplitudes by the eigenmodes of the cantilever. The results show that in a quantitative description of TM–AFM higher eigenmode excitation must be considered to account for internal energy dissipation.
TL;DR: Using an iodine-stabilized He-Ne laser as a transfer oscillator, absolute measurements of the optical frequency from a traditional frequency synthesis chain based on harmonic generation and from the frequency division technique of an ultrawide bandwidth femtosecond frequency comb are compared.
Abstract: Using an iodine-stabilized He-Ne laser as a transfer oscillator, we compare absolute measurements of the optical frequency from a traditional frequency synthesis chain based on harmonic generation and from the frequency division technique of an ultrawide bandwidth femtosecond frequency comb. The agreement between these two measurements, both linked to the Cs standard, is 220+/-770 Hz, yielding a measurement accuracy of 1.6x10(-12). We report 473 612 353 604.8+/-1.2 kHz as a preliminary updated value of the absolute frequency of the " f" component for the He-Ne laser international standard at 633 nm.
TL;DR: In this article, it was shown that nonadiabatic effects dominate phase-matching high-order harmonic generation with few cycle laser pulses, which is a direct consequence of the large ionization rate realized with few-cycle laser pulses.
Abstract: Our calculations show that nonadiabatic effects dominate phase-matched high-order harmonic generation with few cycle laser pulses. The nonadiabatic behavior is a direct consequence of the large ionization rate realized with few-cycle laser pulses, which introduces a nonlinear growth of the harmonic phase with propagation distance. The deviation from linear growth has two striking consequences. On the one hand the nonadiabatic effect drastically reduces the efficiency of conventional phase-matching mechanisms, such as quasi-phase-matching, and on the other hand, it is responsible for a novel phase-matching mechanism, termed nonadiabatic self-phase-matching that is predicted to make high-order harmonic generation in the x-ray regime possible.
TL;DR: Broad-bandwidth light pulses with different frequencies, extending from the IR to the UV, are simultaneously generated when two noncollinear, ultrafast laser pulses from a visible dual-frequency laser propagate through bulk isotropic transparent media such as common glass.
Abstract: Broad-bandwidth light pulses with different frequencies, extending from the IR to the UV, are simultaneously generated when two noncollinear, ultrafast laser pulses from a visible dual-frequency laser propagate through bulk isotropic transparent media such as common glass. This phenomenon, which is believed to have been previously unreported, can be explained by a cascade of highly nondegenerate four-wave-mixing processes and corresponds to a coherent scattering effect with geometrically minimized phase mismatch. Frequency-upconverted beams were observed up to the 11th order.
TL;DR: A new type of photonic structure is suggested to achieve simultaneous generation of several harmonics and both general analytical results and design parameters for 2D periodically poled LiNbO(3) structures are presented.
Abstract: We analyze harmonic generation in a two-dimensional (2D) χ2 photonic crystal and demonstrate the possibility of multiple phase matching and multicolor parametric frequency conversion. We suggest a new type of photonic structure to achieve simultaneous generation of several harmonics; we also present both general analytical results and design parameters for 2D periodically poled LiNbO3 structures.
TL;DR: The theoretical analysis reveals that tunnel ionization significantly modifies the electric field of few-cycle laser pulses within a single oscillation period, making high harmonic generation in the x-ray regime possible for the first time.
Abstract: Our theoretical analysis reveals that tunnel ionization significantly modifies the electric field of few-cycle laser pulses within a single oscillation period. This subcycle self-modulation is predicted to result in phase matching, making high harmonic generation in the x-ray regime possible for the first time. Such a radiation source opens novel possibilities in the investigation of matter with x-ray techniques, such as time resolved x-ray diffraction and absorption.
TL;DR: In this paper, the performance characteristics of a pulse injection locked, passively mode-locked (PML) external-cavity semiconductor laser system for all-optical clock recovery are investigated in detail.
Abstract: The performance characteristics of a pulse injection locked, passively mode-locked (PML) external-cavity semiconductor laser system for all-optical clock recovery are investigated in detail. It is important to characterize the clock recovery dynamics to understand the fundamental capabilities and limitations of the clock recovery system. It is experimentally shown that these devices offer robust clock recovery with low phase and amplitude noise, low injected data power requirements, large frequency locking bandwidth, large phase tracking bandwidth, short lockup time, long dephasing time and immunity to bit-pattern-effects. Harmonic clock generation and subharmonic clock generation are demonstrated for data-rate conversion applications.
TL;DR: In this article, a colloid of ultrasmall nano particles (∼ 1 nm) is reconstituted into microcrystallites films on device-quality Si. The results are analyzed in terms of second-harmonic generation, a process that is not allowed in silicon due to the centrosymmetry.
Abstract: We dispersed crystalline Si into a colloid of ultrasmall nano particles (∼1 nm), and reconstituted it into microcrystallites films on device-quality Si. The film is excited by near-infrared femtosecond two-photon process in the range 765–835 nm, with incident average power in the range 15–70 mW, focused to ∼1 μm. We have observed strong radiation at half the wavelength of the incident beam. The results are analyzed in terms of second-harmonic generation, a process that is not allowed in silicon due to the centrosymmetry. Ionic vibration of or/and excitonic self-trapping on novel radiative Si–Si dimer phase, found only in ultrasmall nanoparticles, are suggested as a basic mechanism for inducing anharmonicity that breaks the centrosymmetry.
TL;DR: In this article, the direction of laser polarization with respect to the molecular axis of a linear molecule can have a significant effect on high-harmonic generation and the efficiency of some high harmonics can be higher by up to three orders of magnitude when the laser is polarized in the perpendicular direction rather than parallel to a molecular axis.
Abstract: We show that the direction of laser polarization with respect to the molecular axis of a linear molecule can have a significant effect on high-harmonic generation. Numerical integration of the two-dimensional Schrodinger equation of the H2+ and the H32+ model molecular ions shows that the efficiency of some high harmonics can be higher by up to three orders of magnitude when the laser is polarized in the perpendicular direction rather than parallel to the molecular axis. We discuss experiments to test this prediction.