Showing papers on "High harmonic generation published in 1996"

Short‐pulse laser harmonics from oscillating plasma surfaces driven at relativistic intensity

Abstract: The generation of harmonics by interaction of an ultrashort laser pulse with a step boundary of a plane overdense plasma layer is studied at intensities Iλ2=1017–1019 W cm−2 μm2 for normal and oblique incidence and different polarizations. Fully relativistic one‐dimensional particle‐in‐cell (PIC) simulations are performed with high spectral resolution. Harmonic emission increases with intensity and also when lowering the plasma density. The simulations reveal strong oscillations of the critical surface driven by the normal component of the laser field and by the ponderomotive force. It is shown that the generation of harmonics can be understood as reflection from the oscillating surface, taking full account of retardation. Describing the oscillations by one or more Fourier components with adjustable amplitudes, model spectra are obtained that well reproduce the PIC spectra. The model is based on relativistic cold plasma equations for oblique incidence. General selection rules concerning polarization of odd and even harmonics depending on incident polarization are derived.

Enhanced high-harmonic generation using 25 fs laser pulses.

Abstract: We present experimental and theoretical results on high-harmonic generation in noble gases using an 805 nm, 25 fs, titanium-doped sapphire laser. The harmonic energies observed are unexpectedly high when compared with experimental and theoretical results to date for longer excitation pulses. We observe that the efficiency of harmonic production is highest for shorter pulses. Furthermore, the wavelength of the harmonics can be tuned by adjusting the sign of the chirp of the excitation pulse, demonstrating a tunable, ultrashort-pulse, $l25\mathrm{fs}$ soft-x-ray source.

High-order harmonic generation in atom clusters.

Abstract: We report the generation of short-wavelength, high-order harmonics of intense laser radiation from atom clusters. Clusters containing about 1${0}^{3}$ atoms are produced in a high-pressure gas jet. We show them to be a unique nonlinear medium. Compared with monomer gases they yield a higher appearance intensity for a given harmonic order, stronger nonlinear dependence of harmonic signal on laser intensity, higher-order harmonics, and reduced saturation of the harmonic signal at high laser intensity.

16-fs, 1-microJ ultraviolet pulses generated by third-harmonic conversion in air.

Abstract: We describe a simple method for generating sub-20-fs ultraviolet light pulses with useful average powers, using a kilohertz Ti:sapphire laser system. By focusing a 22-fs, 1-mJ laser pulse in air, we obtain ultraviolet pulses with an energy of 1 microJ and at a wavelength of 266 nm and with an average power of 1 mW. The pulse duration of the ultraviolet pulses was measured to be 16 fs with frequency-resolved optical gating.

Spatial Coherence Measurement of Soft X-Ray Radiation Produced by High Order Harmonic Generation.

Abstract: We have executed a series of Young's two-slit experiments to measure the spatial coherence of soft x rays produced by high order harmonic generation in helium within the 270 to 480 \AA{} wavelength range. We find that the harmonics exhibit good fringe visibility and high spatial coherence, though the coherence is somewhat degraded at high intensity because of the production of free electrons through optical field ionization during the harmonic generation.

Theory of high-order harmonic generation by an elliptically polarized laser field

Abstract: We generalize a recently formulated theory of high-order harmonic generation by low-frequency laser fields [Anne L'Huillier et al., Phys. Rev. A 48, R3433 (1993)] to the case of an elliptically polarized light. Our theoretical description includes both the single-atom response and propagation. Phase matching significantly modifies the results obtained in the single-atom response. The results of our calculations, including propagation for both the intensity and polarization properties of harmonics as a function of laser ellipticity, compare very well with recent experimental observations.

Coulomb corrections and polarization effects in high-intensity high-harmonic emission

Abstract: We describe a quasiclassical approach to correct electron motion following tunneling ionization in intense laser fields, for the presence of the Coulomb potential. The technique is applied to model the dependence of high-harmonic generation on the ellipticity of the incident laser field. We present experimental data on the harmonics with photon energies close to the ionization potential of an atom, identify their source, and explain their unusual dependence on the ellipticity of the incident laser field. Our calculations are in good agreement with experimental data. \textcopyright{} 1996 The American Physical Society.

High-power fourth- and fifth-harmonic generation of a Nd:YAG laser by means of a CsLiB 6 O 10

Abstract: High pulse energies of nanosecond-level fourth- and fifth-harmonic (4omega and 5omega) generation of a Nd:YAG laser have been obtained with a CsLiB(6)O(10) (CLBO) nonlinear crystal. 500 mJ of 4omega output with a conversion efficiency of 50% from the second-harmonic input was generated. 5omega output at 213 nm of as high as 230 mJ was obtained by sum-frequency generation of the 266- and 1064-nm beams, corresponding to a 10.4% conversion efficiency of the initial fundamental input energy. The characteristics of the CLBO crystal that permit this effective frequency conversion are discussed.

Elastic Rescattering in the Strong Field Tunneling Limit.

Abstract: High precision measurements of helium photoelectron energy and angular distributions for a broad intensity range reflect the change in the continuum dynamics that occurs as the ionization process evolves into the pure tunneling regime. Elastic rescattering of the laser-driven free electron from its parent ion core leaves a distinct signature on the spectra, providing a direct quantitative test of the various theories of strong field multiphoton ionization. We show that it takes a relatively complete semiclassical rescattering model to accurately reproduce the observed distributions. [S0031-9007(96)01922-9] PACS numbers: 32.80.Rm, 31.90.+ s, 32.80.Fb Strong field photoelectron spectra have attracted considerable attention over the past decade and a half, but a comprehensive understanding of the underlying dynamics which produce these spectra and related phenomena has solidified with the development of high repetitionrate, short pulse lasers which can span the entire intensity range of importance [1]. Keldysh [2] showed that at infrared and visible wavelengths the dynamics of strong field atomic ionization undergoes a change in character as the laser intensity increases. In weak fields electrons are promoted into the continuum by the simultaneous absorption of enough photons to increase their energy above the ionization potential. This is called multiphoton ionization (MPI). However, as the laser intensity increases, a completely separate mode of escape becomes possible. At large distances from the nucleus, the electrostatic attraction of the ion core can be overwhelmed by the laser’s instantaneous electric field, producing a barrier through which a valence electron can tunnel. In this regime a quasistatic tunneling picture becomes appropriate: the laser field varies so slowly compared to the response time of the electron that the ionization rate becomes simply the cycle average of the instantaneous dc tunneling rate. Tunneling becomes important when the ratio of the frequency of the applied field to the tunneling rate becomes smaller than unity. This ratio, known as the Keldysh or adiabaticity parameter, g, is given by p Ipys2Upd, where Ip is the binding energy of the electron and Up › Iy4v 2 is the ponderomotive energy in atomic units of a free electron in the laser field of frequency v and intensity I. The majority of experimental studies on neutral atoms exposed to intense, short‐pulse laser fields have been carried out in the MPI regime ( g$ 1). A few experiments [3 ‐ 6] have extended into the tunneling regime, but these measurements have been limited to observations of total ionization rates or electron energy distributions over a small dynamic range. In this Letter, we report upon the first systematic experimental investigation in the strong field tunneling limit. Our study, by virtue of the enhanced dynamic range accessible with kilohertz laser technology, follows the change in the electron spectrum and angular distributions as the ionization evolves from predominantly MPI to pure tunneling. We find that the electron distributions in the tunneling regime are very different from any previous reports obtained in the MPI or mixed regimes [7 ‐ 9]. We achieve a quantitative description of these spectra using a rescattering picture [9,10] which mimics the time evolution of a tunnel-ionized continuum wave packet in the combined fields of the laser and ion core. Since rescattering events are known to be important in other short-pulse, strong-field emission phenomena (e.g., harmonic generation), this investigation better defines the underlying dynamics of these processes. In the experiments presented here, a 150 fs, 1 kHz repetition rate, titanium sapphire laser operating at 0.78 mm was focused by fy4 optics into an ultrahigh vacuum chamber, producing a maximum intensity of 20 PW/cm 2 . The sample gas was 99.999% helium, which was further scrubbed to ,0.1 ppm for O2 ,H 2 ,H 2O, CO2, and hydrocarbon impurities. A 30 cm long time-of-flight photoelectron (PE) spectrometer provides energy and angular resolution of 0.05 eV and 65 mrad, respectively. Data collection used 1 ns binning of discriminated electron events operating at low event probability (#0.25/ shot), ensuring space charge free conditions. The spectrometer’s energy calibration was obtained by recording the high order (.40 photon absorption), long pulse abovethreshold ionization spectrum of xenon. The intensity was calibrated using measurements of the total ion yield, xenon short-pulse PES resonances, and spot size [11]. The uncertainty in the reported intensities is ,25%. The current PE spectra were recorded between 0.5 and 1.5 times the saturation intensity (Isat › 0.8 PW/cm 2 ) for neutral

Harmonic generation by superintense light scattering from relativistic electrons.

Abstract: Relativistic harmonic generation by the scattering of very-high-intensity laser light from fast free electrons is investigated theoretically. A general solution for the trajectory of an electron, moving initially with an arbitrary velocity in a light pulse of arbitrary intensity and polarization, is presented. This solution generalizes the classical analysis of Eberly [Progress in Optics, edited by E. Wolf (North-Holland, Amsterdam, 1969), Vol. 7] and that of Sarachik and Schappert [Phys. Rev. D 1, 2738 (1970)] for the trajectory of an electron initially at rest. The result is then applied to the case of effective harmonic generation in a monochromatic, circularly polarized field under three different initial conditions for the electron, namely, (a) electron initially at rest, (b) electron initially moving in the direction of light propagation (and opposite to it), and (c) electron initially crossing the radiation beam at right angles. Angular distributions of the harmonics generated by the scattering process are presented in terms of the power scattering cross section in each case. Effects of increasing the light intensity and/or the initial electron speed and/or direction on the angular distributions are discussed. It is found, among other results, that at laser intensities higher than, say ${10}^{18}$ W/${\mathrm{cm}}^{2}$, the low-order harmonics are suppressed while the higher-order harmonics are enhanced. \textcopyright{} 1996 The American Physical Society.

Ultraviolet solid state laser, method of using same and laser surgery apparatus

Abstract: A solid state laser system producing coherent radiations at deep ultraviolet wavelengths includes a solid state laser producing a first beam having a wavelength near 1 micron. The 1 micron beam is passed to both a harmonic generation stage and to a tunable optical parametric oscillator. The harmonic generation stage is configured to produce a fifth harmonic of the 1 micron beam, while the optical parametric oscillator produces a tunable beam in the near infrared spectrum (e.g., approximately 2.075 micron). The fifth harmonic and the near infrared beams are mixed in a sum frequency generator to produce a highly coherent beam in the deep ultraviolet (e.g., between approximately 180 nm to 213 nm).

Frequency-resolved optical-gating measurements of ultrashort pulses using surface third-harmonic generation.

Abstract: We demonstrate what is to our knowledge the first frequency-resolved optical gating (FROG) technique to measure ultrashort pulses from an unamplified Ti:sapphire laser oscillator without direction-of-time ambiguity. This technique utilizes surface third-harmonic generation as the nonlinear-optical effect and, surprisingly, is the most sensitive third-order FROG geometry yet.

High-order optical harmonic generation from solid surfaces

Abstract: During the interaction of an intense ultrashort laser pulse with solid targets, a thin layer of surface plasma is generated in which the density drops to the vacuum level in a distance much shorter than the wavelength. This sharp plasma-vacuum boundary performs an oscillatory motion in response to the electromagnetic forces of the intense laser light. It is shown that the generation of reflected harmonics can be interpreted as a phase modulation experienced by the light upon reflection from the oscillating boundary. The modulation side-bands of the reflected frequency spectrum correspond to odd and even harmonics of the laser frequency. Retardation effects lead to a strong anharmonicity for high velocities of the plasma-vacuum boundary. As a result, harmonic generation is strongly enhanced in the relativistic regime of laser intensities.

Nonadiabatic Effects in High-Harmonic Generation with Ultrashort Pulses.

Abstract: High-harmonic generation using ultrashort laser pulses with pulse durations 25 to 200 fs is s theoretically and experimentally. We observe that the harmonic spectrum of argon taken with laser pulses contains harmonics up to 20 orders higher than for 100 fs laser pulses with the intensity. We show that this increase in harmonics is because the atom survives to highe intensities, due in part to the nonadiabatic response of the atomic dipole to the fast rise time pulse. [S0031-9007(96)01018-6]

Probing the linear and nonlinear excitations of Bose-condensed neutral atoms in a trap.

Abstract: We investigate the response of a Bose-Einstein condensate of trapped, neutral atoms to weak and strong sinusoidal perturbation of the trapping potential both by solving the Bogoliubov equations and by direct integration of the time-dependent, driven Ginzburg-Pitaevskii-Gross equation We find that the distortion of the condensate is maximal when the frequency of the perturbation equals one of the mode positions of the condensate's excitation spectrum On resonance, the condensate exhibits a strong nonlinear response that can be used as a clear signature of the mode frequency in an experiment where the trap potential is weakly perturbed For strong driving, we find evidence for an array of nonlinear effects such as harmonic generation and frequency mixing These phenomena are the matter-wave analogs of conventional nonlinear optics and should be straightforward to study in evaporatively cooled samples of alkali-metal atoms \textcopyright{} 1996 The American Physical Society

Influence of atomic density in high-order harmonic generation

Abstract: We have investigated how high-order harmonics generated in rare gases depend on the atomic density. The peak and the profile of the atomic density in the interaction region were measured as a function of the backing pressure and the distance from the nozzle by a differential interferometry technique. The conversion efficiency for the harmonics in the plateau was found to increase approximately quadratically over the entire range of peak pressures investigated (3–80 mbar). The intensity of the harmonics in the cutoff region, in contrast, increased only until an optimum peak pressure was reached, beyond which it decreased. This optimum peak pressure was found to be dependent on both the laser intensity and the process order. To understand this effect, we have performed extensive propagation calculations of both the fundamental and the harmonic fields, using ionization rates and dipole moments from a tunnel ionization model. We obtained good agreement with the experimental results. The observed effect is attributed to ionization-induced defocusing of the fundamental laser beam, which reduces the peak intensity obtained in the medium and shortens the extent of the plateau.

Spectral determination of the amplitude and the phase of intense ultrashort optical pulses

Abstract: We present the results of the spectral analysis of intense ultrashort laser pulses propagating through nonlinear transparent media. We discuss the spectral features characteristic of the self-phase-modulation process, which is the most prominent process during nonguided propagation in the weakly nonlinear regime. We describe an application of automatic pulse phase retrieval based on the Kerr-like nonlinearity.

Optical harmonic generation from animal tissues by the use of picosecond and femtosecond laser pulses.

Abstract: Second- and third-harmonic generations of femtosecond and picosecond laser pulses have been measured from chicken skin, muscle, and fat tissues. The magnitude of the harmonic signals showed a strong structural dependence with the signal from skin interface being the strongest. The polarization dependence of the signal was also measured and found to be consistent with the fact that the tissue samples were highly scattering random media. The second-harmonic- and third-harmonic-generation conversion efficiencies were found to be in the range of ~10−7 to ~10−10.

Measurement of the duration of high harmonic pulses

Abstract: Summary form only given. High harmonics are generated during the ionization of an atomic gas and therefore the probability of ionization, the number of unionized atoms, and the phase matching effects will impose a natural temporal envelope on the harmonic generation. We made some simulations which showed the maximal, minimal, and averaged possible values of the intensity of the harmonic as a function of the angular frequency detuning multiplied by the length of the harmonic pulse. From the envelope of the maximum signal we can extract the high harmonic pulse duration.

Ultraviolet generation with passively Q-switched microchip lasers: errata.

Abstract: First, second, third, fourth, and fifth harmonics of the output of a fiber-pumped passively Q-switched Nd:YAG microchip laser have been obtained at pulse energies of 8.0, 3.5, 0.3, 0.7, and 0.01 μJ, respectively, in an optical head occupying a volume of less than 3 cm3. This compact, economical all-solid-state source provides coherent subnanosecond multikilowatt infrared, visible, and ultraviolet pulses at repetition rates in excess of 10 kHz.

Surface-plasmon-enhanced third-harmonic generation in thin silver films.

Abstract: Optical third-harmonic generation (THG) is dipole allowed. I report a 10(3) signal enhancement of THG that is due to the excitation of a surface plasmon in thin silver films by an attenuated-total-internal-ref lection geometry. Because the THG signal depends on the cube of the incident intensity and the second-harmonic generation depends on the square of the intensity, the THG overtakes the second-harmonic generation at laser intensities beyond 6 x 10(11) W/cm(2).

High-order harmonic generation in molecular gases

Abstract: We present an experimental study of harmonic generation in molecular gases, using the fundamental (800 nm) and the second harmonic (400 nm) of a 150 fs titanium - sapphire laser at an intensity of . We compare the conversion efficiency and the maximum energy obtained in different species: rare gases (Ar, Xe), diatomic molecules (, , , CO) and polyatomic molecules (, O, , , ). The harmonic spectra from molecular gases are very similar to those obtained in the atomic gases, with a plateau and a cutoff whose location is strongly correlated to the value of the ionization potential. The conversion efficiency is not higher than that of the rare gases.

High-gain harmonic generation of soft X-rays with the `fresh bunch` technique

Abstract: We report numerical simulations (using the TDA code) and analytic verification of the generation of 64 {Angstrom} high power soft X- rays from an exponential regime single pass seeded FEL. The seed is generated in the FEL using the High Gain Harmonic Generation (HGHG) technique combined with the `Fresh bunch` technique. A seed pulse at 2944 {Angstrom} is generated by conventional laser techniques. The seed pulse produces an intense energy modulation of the rear part of a I GeV, 1245 {Angstrom} electron beam in a `modulator` wiggler. In the `radiator` wiggler, (resonant to 64 {Angstrom}), the energy modulation creates beam density modulation followed by radiation of the 46{sup th} harmonic of the seed. We use a magnetic delay to position the 64 A{Angstrom} radiation at the undisturbed front of the bunch to serve as a seed for a single pass, exponential growth FEL. After a 9 m long exponential section followed by a 7 m long tapered section the radiation power reaches 3.3 GW.

Highly efficient green-light generation by quasi-phase-matched frequency doubling of picosecond pulses from an amplified mode-locked Nd:YLF laser

Abstract: We report on first-order quasi-phase-matched frequency doubling of picosecond pulses to the green, using a sample of periodically poled lithium niobate. In cw mode-locked operation. 330 mW of average green power was generated with an average conversion efficiency of ~52%. With a quasi-cw mode-locked pulse train an average conversion efficiency of ~65% was achieved, and 1.3 W of green power (average power within the 10-ps pulse envelope) was produced. At these powers there was no significant photorefractive damage, as confirmed by measured M 2 beam-quality factors of ~1.1 for both output fundamental and second-harmonic beams.

Effects of cubic nonlinearity on frequency doubling of high-power laser pulses

Abstract: The effects of the phase mismatch due to cubic nonlinearity in the equations for second-harmonic generation are investigated. We show that the phase mismatch induced by the nonlinear refractive index of a doubling crystal can dramatically reduce the conversion efficiency of high-peak-power laser pulses. Simple, analytic expressions are derived for the conversion efficiency of cw radiation and for the estimation of the dispersion in the nonlinear refractive index of the doubling crystal, which is quite important in the determination of the magnitude of the nonlinear effects on maximum conversion. The consequences that these nonlinearities have on the frequency doubling of ultrashort (≤100 fs) pulses, including the additional effects of group-velocity walk-off between the pulses, are then numerically calculated.

Harmonic-generation control

Abstract: In this paper we examine some effects of quantum interference on high harmonic generation. We demonstrate in particular that preparing the initial state in a coherent superposition of bound states leads to a harmonic spectrum with distinct plateaus with different conversion efficiencies. We show how this scheme may provide a way of controlling the coherent output that is produced in an experiment. \textcopyright{} 1996 The American Physical Society.

High-order harmonic generation in a bichromatic elliptically polarized laser field

Abstract: The quantum theory of high-order harmonic generation by a low-frequency laser field is generalized to the case of a bichromatic elliptically polarized laser field. The quasiclassical cutoff law is analyzed. Numerical results for a linearly polarized bichromatic laser field are presented and analyzed for different laser field frequencies, intensities, and relative phases. Harmonic intensity is many orders of magnitude higher in the bichromatic case than in the monochromatic one, but the cutoff is shifted toward the lower harmonics. The plateau height can be controlled by changing the relative phase of the fields. A qualitative agreement with the recent experiments is shown.