Showing papers on "High harmonic generation published in 2008"
TL;DR: This work demonstrates a method of high-harmonic generation that requires no extra cavities by exploiting the local field enhancement induced by resonant plasmons within a metallic nanostructure consisting of bow-tie-shaped gold elements on a sapphire substrate.
Abstract: High-harmonic generation by focusing a femtosecond laser onto a gas is a well-known method of producing coherent extreme-ultraviolet (EUV) light. This nonlinear conversion process requires high pulse intensities, greater than 10(13) W cm(-2), which are not directly attainable using only the output power of a femtosecond oscillator. Chirped-pulse amplification enables the pulse intensity to exceed this threshold by incorporating several regenerative and/or multi-pass amplifier cavities in tandem. Intracavity pulse amplification (designed not to reduce the pulse repetition rate) also requires a long cavity. Here we demonstrate a method of high-harmonic generation that requires no extra cavities. This is achieved by exploiting the local field enhancement induced by resonant plasmons within a metallic nanostructure consisting of bow-tie-shaped gold elements on a sapphire substrate. In our experiment, the output beam emitted from a modest femtosecond oscillator (100-kW peak power, 1.3-nJ pulse energy and 10-fs pulse duration) is directly focused onto the nanostructure with a pulse intensity of only 10(11) W cm(-2). The enhancement factor exceeds 20 dB, which is sufficient to produce EUV wavelengths down to 47 nm by injection with an argon gas jet. The method could form the basis for constructing laptop-sized EUV light sources for advanced lithography and high-resolution imaging applications.
1,320 citations
TL;DR: A novel optical switch to control the high-order harmonic generation process so that single attosecond pulses can be generated with multiple-cycle pulses and a unique dependence of the harmonic spectra on the carrier-envelope phase of the laser fields is discovered.
Abstract: We demonstrated a novel optical switch to control the high-order harmonic generation process so that single attosecond pulses can be generated with multiple-cycle pulses. The technique combines two powerful optical gating methods: polarization gating and two-color gating. An extreme ultraviolet supercontinuum supporting 130 as was generated with neon gas using 9 fs laser pulses. We discovered a unique dependence of the harmonic spectra on the carrier-envelope phase of the laser fields, which repeats every 2 pi radians.
386 citations
TL;DR: Measurements of the HHG spectrum of N2 molecules aligned perpendicular to the laser polarization showed a maximum at the rotational half-revival, which indicates the influence of electrons occupying the orbital just below the N2 HOMO, referred to as the HomO-1.
Abstract: Molecular electronic states energetically below the highest occupied molecular orbital (HOMO) should contribute to laser-driven high harmonic generation (HHG), but this behavior has not been observed previously. Our measurements of the HHG spectrum of N2 molecules aligned perpendicular to the laser polarization showed a maximum at the rotational half-revival. This feature indicates the influence of electrons occupying the orbital just below the N2 HOMO, referred to as the HOMO-1. Such observations of lower-lying orbitals are essential to understanding subfemtosecond/subangstrom electronic motion in laser-excited molecules.
353 citations
TL;DR: In this paper, the authors discuss the control of high-harmonic spectra by temporal and spatial pulse shaping of the driving laser pulses and its implications on time-resolved XUV spectroscopy and attosecond pulse shaping.
Abstract: High-harmonic generation provides an attractive light source of coherent radiation in the extreme-ultraviolet (XUV) and soft-x-ray regions of the spectrum and allows for the production of single attosecond pulses or pulse trains. This Colloquium covers the control of high-harmonic spectra by temporal and spatial pulse shaping of the driving laser pulses and its implications on time-resolved XUV spectroscopy and attosecond pulse shaping. It summarizes important steps for extending existing pulse shaping techniques and control schemes from the near-infrared or visible part to shorter wavelengths. Using adaptive pulse shaping of the driving laser pulses, several groups have demonstrated control of the high-harmonic spectrum, including the author's work on the complete control over the XUV spectrum of high-order harmonics, generated in a gas-filled hollow fiber. It is possible to achieve both the enhancement and the suppression of single or several selected harmonic orders. These arbitrarily shaped soft-x-ray spectra will allow for important modifications of the resulting harmonic pulses in the temporal domain. This constitutes first steps towards direct attosecond pulse shaping in the soft-x-ray domain. Moreover, high-harmonic generation in a hollow-core fiber can be enhanced by coupling into a single fiber mode using a feedback-controlled adaptive two-dimensional spatial light modulator.
310 citations
TL;DR: The precept of the design parameter for a single-shot live-cell imaging by contact microscopy is presented and the proposed generation scheme, combining a 1.6 microm laser driver and a neutral Ne gas medium, is efficient and scalable in output yields of the water window x ray.
Abstract: We demonstrate the generation of a coherent water window x ray by extending the plateau region of high-order harmonics under a neutral-medium condition. The maximum harmonic photon energies attained are 300 and 450 eV in Ne and He, respectively. Our proposed generation scheme, combining a 1.6 microm laser driver and a neutral Ne gas medium, is efficient and scalable in output yields of the water window x ray. Thus, the precept of the design parameter for a single-shot live-cell imaging by contact microscopy is presented.
310 citations
TL;DR: In this article, a complete understanding of the way in which attosecond pulses arrive at a target where they can be characterized and used in an experiment is discussed, and a number of results from calculations of the attoscond pulse generation obtained by simultaneous solution of the time-dependent Schrodinger equation and the Maxwell wave equation are discussed.
Abstract: Attosecond pulses are generated by a macroscopic number of ionizing atoms interacting with a focused laser pulse, via the process of high harmonic generation. The physics of their generation consists of an interplay between the microscopic laser–atom interaction and macroscopic effects due to ionization and phase matching in the nonlinear medium. In this review, we focus on a complete understanding of the way in which attosecond pulses arrive at a target where they can be characterized and used in an experiment. We discuss a number of results from calculations of attosecond pulse generation obtained by simultaneous solution of the time-dependent Schrodinger equation and the Maxwell wave equation. These results, which allow for a clean separation of microscopic and macroscopic factors, illustrate how macroscopic effects are used to select attosecond pulses from the radiation that is emitted by atoms interacting with a strong laser field.
304 citations
TL;DR: The results show that high harmonic generation and strong-field ionization in polyatomic molecules undergoing bonding or configurational changes involve the participation of multiple molecular orbitals.
Abstract: The attosecond time-scale electron-recollision process that underlies high harmonic generation has uncovered extremely rapid electronic dynamics in atoms and diatomics. We showed that high harmonic generation can reveal coupled electronic and nuclear dynamics in polyatomic molecules. By exciting large amplitude vibrations in dinitrogen tetraoxide, we showed that tunnel ionization accesses the ground state of the ion at the outer turning point of the vibration but populates the first excited state at the inner turning point. This state-switching mechanism is manifested as bursts of high harmonic light that is emitted mostly at the outer turning point. Theoretical calculations attribute the large modulation to suppressed emission from the first excited state of the ion. More broadly, these results show that high harmonic generation and strong-field ionization in polyatomic molecules undergoing bonding or configurational changes involve the participation of multiple molecular orbitals.
296 citations
TL;DR: It is established that laser-generated high-energy electron momentum spectra and high-order harmonic spectra can be used to extract accurate differential elastic scattering and photo-recombination cross sections of the target ion with free electrons, respectively.
Abstract: By analyzing accurate theoretical results from solving the time-dependent Schrodinger equation of atoms in few-cycle laser pulses, we established the general conclusion that laser-generated high-energy electron momentum spectra and high-order harmonic spectra can be used to extract accurate differential elastic scattering and photo-recombination cross sections of the target ion with free electrons, respectively. Since both electron scattering and photoionization (the inverse of photo-recombination) are the conventional means for interrogating the structure of atoms and molecules, this result implies that existing few-cycle infrared lasers can be implemented for ultrafast imaging of transient molecules with temporal resolution of a few femtoseconds.
288 citations
TL;DR: In this paper, the emitted radiation from semiconductor nanostructures due to the excitation with intense ultrashort optical laser pulses is analyzed and the Semiconductor Bloch equations that consistently describe the coupled light-field-induced interband and intraband dynamics are solved numerically.
Abstract: The emitted radiation from semiconductor nanostructures due to the excitation with intense ultrashort optical laser pulses is analyzed. Semiconductor Bloch equations that consistently describe the coupled light-field-induced interband and intraband dynamics are solved numerically. It is demonstrated that the intraband dynamics considerably influences the light emission in the regime of extreme nonlinear optics. In particular, the intraband acceleration significantly modifies the dynamics of the interband polarization which results in a strong enhancement of high-harmonic generation.
281 citations
TL;DR: The new VUV laser-based ARPES system exhibits superior performance, including superhigh energy resolution better than 1 meV, high momentum resolution, superhigh photon flux, and much enhanced bulk sensitivity, which are demonstrated from measurements on a typical Bi2Sr2CaCu2O8 high temperature superconductor.
Abstract: The design and performance of the first vacuum ultraviolet (VUV) laser-based angle-resolved photoemission (ARPES) system are described. The VUV laser with a photon energy of 6.994eV and bandwidth of 0.26meV is achieved from the second harmonic generation using a novel nonlinear optical crystal KBe2BO3F2. The new VUV laser-based ARPES system exhibits superior performance, including superhigh energy resolution better than 1meV, high momentum resolution, superhigh photon flux, and much enhanced bulk sensitivity, which are demonstrated from measurements on a typical Bi2Sr2CaCu2O8 high temperature superconductor. Issues and further development related to the VUV laser-based photoemission technique are discussed.
205 citations
TL;DR: Intensity dependent high harmonic generation was investigated when both short and long trajectories contribute to the emission and directly observed for the first time clear indication of quantum-path interference through harmonic spectrum modulations.
Abstract: We have investigated the intensity dependence of high-order harmonic generation in argon when the two shortest quantum paths contribute to the harmonic emission. For the first time to our knowledge, experimental conditions were found to clearly observe interference between these two quantum paths that are in excellent agreement with theoretical predictions. This result is a first step towards the direct experimental characterization of the full single-atom dipole moment and demonstrates an unprecedented accuracy of quantum path control on an attosecond time scale.
TL;DR: It is demonstrated that phase-matched frequency upconversion of ultrafast laser light can be extended to shorter wavelengths by using longer driving laser wavelengths and that phase matching is also obtained at higher pressures using a longer-wavelength driving laser, mitigating the unfavorable scaling of the single-atom response.
Abstract: We demonstrate that phase-matched frequency upconversion of ultrafast laser light can be extended to shorter wavelengths by using longer driving laser wavelengths. Experimentally, we show that the phase-matching cutoff for harmonic generation in argon increases from 45 to 100 eV when the driving laser wavelength is increased from 0.8 to 1.3 microm. Phase matching is also obtained at higher pressures using a longer-wavelength driving laser, mitigating the unfavorable scaling of the single-atom response. Theoretical calculations suggest that phase-matched high harmonic frequency upconversion driven by mid-infrared pulses could be extended to extremely high photon energies.
TL;DR: In this article, the properties of a tunable nonlinear metamaterial operating at microwave frequencies were investigated, where a varactor diode is introduced into each resonator so that the magnetic resonance can be tuned dynamically by varying the input power.
Abstract: We study the properties of a tunable nonlinear metamaterial operating at microwave frequencies We fabricate the nonlinear metamaterial composed of double split-ring resonators and wires where a varactor diode is introduced into each resonator so that the magnetic resonance can be tuned dynamically by varying the input power We show that at higher powers the transmission of the metamaterial becomes power dependent, and we demonstrate experimentally power-dependent transmission properties and selective generation of higher harmonics
TL;DR: A significant improvement in output power of the plateau harmonics of 4 orders of magnitude over previous results means that high-resolution spectroscopy in the extreme ultraviolet region becomes conceivable.
Abstract: Intracavity high harmonic generation is demonstrated in an external cavity, seeded by a Ti:sapphire mode-locked laser at a repetition rate of 10.8MHz. Harmonics up to 19th order at 43 nm were observed with plateau harmonics at the μW power level.
TL;DR: It is shown that the conical beams are formed as a result of the higher-order nonlinear Bragg diffraction involving two parametric processes in which an ordinary fundamental wave is converted simultaneously into ordinary and extraordinary polarized second harmonics.
Abstract: We report on the observation of second-harmonic conical waves generated in a novel geometry of the transverse excitation of an annular periodically poled nonlinear photonic structure by a fundamental Gaussian beam. We show that the conical beams are formed as a result of the higher-order nonlinear Bragg diffraction involving two parametric processes in which an ordinary fundamental wave is converted simultaneously into ordinary and extraordinary polarized second harmonics.
TL;DR: A novel technique for coupling extreme-ultraviolet (XUV) harmonic radiation out of a femtosecond enhancement cavity using a small-period diffraction grating etched directly into the surface of a dielectric mirror is demonstrated.
Abstract: We demonstrate a novel technique for coupling extreme-ultraviolet (XUV) harmonic radiation out of a femtosecond enhancement cavity. We use a small-period diffraction grating etched directly into the surface of a dielectric mirror. For the fundamental light, this element acts as a high reflector. For harmonic wavelengths, it acts as a diffraction grating, coupling XUV radiation out of the cavity. Using this method, we observed the third through twenty-first odd harmonics with a dramatic increase in usable power over previous results of high-harmonic generation at high repetition rates.
TL;DR: In this article, the phase of high-order harmonic generation from transiently aligned molecules was measured using extreme ultraviolet interferometry and the dispersion relation of the returning electron wave packet as a function of harmonic order was analyzed.
Abstract: We use extreme-ultraviolet interferometry to measure the phase of high-order harmonic generation from transiently aligned ${\mathrm{CO}}_{2}$ molecules. We unambiguously observe a reversal in phase of the high-order harmonic emission for higher harmonic orders with a sufficient degree of alignment. This results from molecular-scale quantum interferences between the molecular electronic wave function and the recolliding electron as it recombines with the molecule, and is consistent with a two-center model. Furthermore, using the combined harmonic intensity and phase information, we extract accurate information on the dispersion relation of the returning electron wave packet as a function of harmonic order. This analysis shows evidence of the effect of the molecular potential on the recolliding electron wave.
Journal Article•
TL;DR: Using extreme-ultraviolet interferometry, pi phase shifts in high harmonics generated from transiently aligned molecules are measured to reflect the quantum interferences in the electron wavepacket due to the two-center molecular structure.
TL;DR: In this article, the authors achieved very efficient high-harmonic generation in a two-color laser field using a long gas jet of He and achieved a conversion efficiency as high as 2×10−4.
Abstract: We have achieved very efficient high-harmonic generation in a two-color laser field using a long gas jet of He. With the optimization of laser parameters and target conditions, strong harmonics were produced at 2(2n+1)th orders in an orthogonally polarized two-color field. The strongest harmonic at the 38th order (21.6nm) reached an energy of 0.6μJ with a 6mm gas jet, giving a conversion efficiency as high as 2×10−4.
TL;DR: In this paper, the authors measured the nonlinear refractive indices and nonlinear absorption coefficients of the suspensions of these nanoparticles using the 792nm femtosecond and picosecond pulses.
Abstract: We investigate third- and higher-order nonlinear optical processes in Au, Pt, Pd, and Ru nanoparticles. We measured the nonlinear refractive indices and nonlinear absorption coefficients of the suspensions of these nanoparticles using the 792nm femtosecond and picosecond pulses. The highest values of these parameters were observed in the case of Au nanoparticles (−8×10−14m2W−1 and 1.7×10−10mW−1), when the 210ps pulses were used as a probe radiation. The high-order harmonic generation was studied during the propagation of the 120fs pulses through the plasmas produced on the surfaces containing the nanoparticles. The highest harmonic cutoff (33rd order) was observed in the case of gold nanoparticle-contained plasma. These results were compared with the harmonics generating from the plasma produced on the surfaces of the bulk materials of the same origin. A two- to sixfold enhancement of harmonic yield was observed in the case of nanoparticle-contained plumes with regard to the bulk materials.
TL;DR: It is demonstrated that nonlinear frequency upconversion of few-cycle near-infrared (NIR) laser pulses, by means of harmonic generation in noble gases, is a promising approach for extending cutting-edge, few- cycle ultrafast technology into the deep ultraviolet and beyond, without the need for UV dispersion control.
Abstract: We demonstrate that nonlinear frequency upconversion of few-cycle near-infrared (NIR) laser pulses, by means of harmonic generation in noble gases, is a promising approach for extending cutting-edge, few-cycle ultrafast technology into the deep ultraviolet and beyond, without the need for UV dispersion control. In our experiment, we generate 3.7-fs pulses in the deep UV (approximately 4.6 eV) with adjustable polarization and gigawatt-scale peak power. We demonstrate that the implementation of this concept with a quasi-monocycle driver offers the potential for advancing UV pulse generation towards the 1-fs frontier.
TL;DR: A new dynamic two-center interference effect in high-harmonic generation from H2, in which the attosecond nuclear motion of H2+ initiated at ionization causes interference to be observed at lower harmonic orders than would be the case for static nuclei.
Abstract: We report a new dynamic two-center interference effect in high-harmonic generation from ${\mathrm{H}}_{2}$, in which the attosecond nuclear motion of ${\mathrm{H}}_{2}^{+}$ initiated at ionization causes interference to be observed at lower harmonic orders than would be the case for static nuclei. To enable this measurement we utilize a recently developed technique for probing the attosecond nuclear dynamics of small molecules. The experimental results are reproduced by a theoretical analysis based upon the strong-field approximation which incorporates the temporally dependent two-center interference term.
TL;DR: In this article, the ability of Strontium Barium Niobate crystal as a two-dimensional nonlinear optical multiple-wavelength-and-direction converter is demonstrated, showing how harmonic generation can be obtained in an extremely large spectral range.
Abstract: The ability of Strontium Barium Niobate crystal as a two-dimensional nonlinear optical multiple-wavelength-and-direction converter is demonstrated, showing how harmonic generation can be obtained in an extremely large spectral range. The whole visible spectral region can be continuously generated without any angle or temperature tuning by using different nonlinear schemes involving SHG and THG in different configurations. The dominant frequency conversion mechanisms acting in each case are experimentally determined. The system can act as a nonlinear prism since the SHG conical radiation from 430 to 680 nm can be dispersed into an external angle range of 36°. Conical nonlinear processes are also demonstrated under intracavity conditions from a diode pumped Nd3+:SBN laser crystal. Different harmonic generated rings appear simultaneously inside the cavity, while the crystal is lasing at 1063 nm. The system constitutes the first experimental example of a two-dimensional generalized nonlinear photonic glass on a tunable solid state laser and can be useful for novel optical multifunctional devices such as nonlinear prism self-frequency converted solid state lasers.
TL;DR: In this paper, an infrared (IR) parametric amplifier chain, pumped at 800nm by a terawatt Ti:sapphire laser system, has been reported to achieve a total output energy exceeding 10mJ with 40fs pulse duration.
Abstract: Power scaling of an infrared (IR) parametric amplifier chain, pumped at 800nm by a terawatt Ti:sapphire laser system has been reported. A total output energy exceeding 10mJ with 40fs pulse duration has been achieved in the IR region, which is the highest energy and peak power ever reported for an ultrafast optical parametric amplifier scheme. By applying the developed IR pulses to high-order harmonic generation, we have observed a significant cutoff extension compared to the case of 800nm driving wavelength. This source suits as a driver laser for extending high-order harmonic photon energy into the kiloelectronvolts region.
Abstract: Based on high-order harmonic generation (HHG) spectra obtained from solving the time-dependent Schr\"odinger equation for atoms, we established quantitatively that the HHG yield can be expressed as the product of a returning electron wave packet and photorecombination cross sections, and the shape of the returning wave packet is shown to be largely independent of the species. By comparing the HHG spectra generated from different targets under identical laser pulses, accurate structural information, including the phase of the recombination amplitude, can be retrieved. This result opens up the possibility of studying the target structure of complex systems, including their time evolution, from the HHG spectra generated by short laser pulses.
TL;DR: An octave-spanning Ti:sapphire oscillator supporting Fourier-limited pulses as short as 3.7 fs is presented, allowing for full control of the electric pulse field on a sub-femtosecond time-scale.
Abstract: We present an octave-spanning Ti:sapphire oscillator supporting Fourier-limited pulses as short as 3.7 fs. This laser system can be directly CEO-phase stabilized delivering an average output power of about 90 mW with a pulse duration of 4.4 fs. The phase-stabilization is realized without additional spectral broadening using an f-2f interferometer approach allowing for full control of the electric pulse field on a sub-femtosecond time-scale.
TL;DR: Efficient cavity-enhanced second and fourth harmonic generation of an air-cooled, continuous-wave (cw), single-frequency 1064 nm fiber-amplifier system is demonstrated.
Abstract: We demonstrate efficient cavity-enhanced second and fourth harmonic generation of an air-cooled, continuous-wave (cw), single-frequency 1064 nm fiber-amplifier system. The second harmonic generator achieves up to 88% total external conversion efficiency, generating more than 20-W power at 532 nm wavelength in a diffraction-limited beam (M(2) 50%.
TL;DR: A numerical study of the phase properties of high-order harmonics of the incident frequency are presented, and it is demonstrated experimentally that they can be coherently controlled through the phase of the driving laser field.
Abstract: As a high-intensity laser-pulse reflects on a plasma mirror, high-order harmonics of the incident frequency can be generated in the reflected beam. We present a numerical study of the phase properties of these individual harmonics, and demonstrate experimentally that they can be coherently controlled through the phase of the driving laser field. The harmonic intrinsic phase, resulting from the generation process, is directly related to the coherent sub-laser-cycle dynamics of plasma electrons, and thus constitutes a new experimental probe of these dynamics.
TL;DR: In this paper, high-order harmonic generation up to the 55th order was achieved using 15 nm and 110 nm silver nanoparticle-containing plumes, when femtosecond radiation propagated through the preformed plasma.
Abstract: High-order harmonic generation up to the 55th order was achieved using 15 nm and 110 nm silver nanoparticle-containing plumes, when femtosecond radiation propagated through the preformed plasma. These results are compared with the high-order harmonics generated from the plasma produced on the surface of bulk silver at different delays between the subnanosecond prepulse and the femtosecond pulse.
TL;DR: In this paper, the spectral phase of the harmonics of the photoelectron spectra at 2.6 and 3.6 µm has been studied and it has been shown that strong field interaction still keeps some secrets after more than a decade.
Abstract: The recent development of intense, ultrashort, table top lasers in the mid-infrared opens new avenues for research in strong field atomic physics. Electrons submitted to such radiation acquire huge quiver energies, even at moderate intensity and interesting properties arise: first, the wavelength offers a convenient experimental knob to tune the ionisation regime by controlling the Keldysh parameter. Second, many processes like above-threshold ionisation or high harmonic generation, whose characteristics depend directly on this energy, can be pushed to unprecedented limits. Third, the wavelength controls the spectral phase of the harmonics and hence the possibility to improve the generation of pulses in the attosecond regime. Recent studies of rare gas and alkali atoms' photoelectron spectra and harmonic generation at 2 and 3.6 μm have begun to confirm the theoretical predictions. However, unexpected features have also been found showing that strong field interaction still keeps some secrets after more th...