Effects of higher-order nonlinear processes on harmonic-generation phase matching (original) (raw)
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When an intense, plane-polarized, laser pulse interacts with a plasma, the relativistic nonlinearities fP induce a third harmonic polarization. A phase-locked growth of a third harmonic wave can tak; place, but the difference between the nonlinear dispersion of the pump and driven waves leads to a rapid li
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Physical Review A, 2011
We present an alternative explanation of the high-order-harmonic-generation experimental results published recently by Seres et al. [Nature Phys. 6, 455 (2010)]. We show that the physical interpretation can be comprehensively done in the frame of classical theory of high-order harmonic generation without referring to a parametric effect in the XUV domain. The experimental conditions explored by Seres et al. indeed correspond to the case of long-pulse, low-infrared-energy laser beams for which tight focusing is necessary to reach the minimum intensity required for high atomic response. The positive atomic dispersion can compensate for the Gouy phase and explains the behavior of the experimental variation of the harmonic signal presented. We will show that our interpretation explains not only the global behavior of the curves but also the second-order variation of the signal as a function of experimental parameters.
Phase-matched third harmonic generation in a plasma
IEEE Transactions on Plasma Science, 1993
Relativistic third harmonic generation in a plasma is investigated. The growth of a third harmonic wave is limited by the difference between the phase velocity of the pump and driven waves. This phase velocity mismatch results in a third harmonic amplitude saturation and oscillation. In order to overcome this saturation, we describe a phase-matching scheme based on a resonant density modulation. The limitations of this scheme are analyzed.
Applied Sciences, 2019
Novel methods of coherent short-wavelength sources generation require thorough analysis for their further amendments and practical implementations. In this work, we report on the quasi-phase matching (QPM) of high-order harmonics generation during the propagation of single- and two-color femtosecond pulses through multi-jet plasmas, which allows the enhancement of groups of harmonics in different ranges of extreme ultraviolet. The role of the number of coherent zones; sizes of plasma jets and the distance between them; plasma formation conditions, and the characteristics of the fundamental radiation on the harmonic efficiency at quasi-phase matching (QPM) conditions are analyzed. We demonstrate the ~40× enhancement factor of the maximally-enhanced harmonic with respect to the one generated at ordinary conditions in the imperforated plasma.
Observation of Phase-Matched Relativistic Harmonic Generation
Physical Review Letters, 2000
Phase-matched relativistic harmonic generation in plasmas is observed for the first time. Thirdharmonic 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. PACS numbers: 52.40.Nk, 42.65.Ky It had been predicted theoretically [1,2] and recently verified experimentally that the free electrons in the focus of a high-intensity laser [3] will oscillate relativistically and thus emit harmonics with unique angular patterns . This process, known as relativistic nonlinear Thomson scattering, scatters light into a broad range of angles (ϳ180 ± ) with each lobe having a .30 ± angular width for the low-order harmonics. In this paper we report the experimental observation of the third harmonic emitted into a narrow, hollow cone pointing in the direction of laser propagation. To explain these results, a new theory is presented that describes how phase matching in a plasma can result provided that the harmonic radiation has a highorder transverse structure. The signal level of the harmonic for a circularly polarized pump pulse was found to be on the same order of magnitude as that for a linearly polarized pump pulse, which is characteristically different from harmonic generation from electrons bound to atoms. The increased efficiency due to phase matching might eventually lead to interesting coherent, ultrashort-duration and short-wavelength light sources, in which there is much current interest .
Optics Express, 2015
We present evidence for self-stabilization of the relative spectral phase of high-order harmonic emission against intensity variations of the driving field. Our results demonstrate that, near the laser focus, phase matching of the harmonic field from a macroscopic target can compensate for the intensity dependence of the intrinsic phase of the harmonics emitted by a single radiator. As a consequence, we show experimentally and theoretically the insensitivity of the harmonic spectra produced at the laser focus against variations of the carrier-envelope phase (CEP) of a sub-two-cycle driving field. In addition, the associated attosecond pulse trains exhibit phase locking against CEP changes of the few-cycle driver. . CEP scans of on-axis HHG spectra for different relative positions of the gas jet and the beam focus. Each panel shows XUV spectrum intensity, versus harmonic order (x-axis) and CEP variation in π units (y-axis). The first column presents the experimental data; the second, simulations of HHG for the macroscopic target (including propagation); in the third column, results for a single-atom are presented. We show five relative positions of the gas jet and the beam focus, from z=-3 mm, where the gas jet is placed 3 mm before the focus, towards, z=1 mm, i.e., gas jet placed 1 mm after the focus position. The HHG spectra are sensitive to CEP variations when the gas jet is placed far from the focus, and becomes more insensitive near the focus.
Higher-order nonlinearities revisited and their effect on harmonic generation
Physical review letters, 2015
We report on harmonic generation experiments and calculations in air to investigate the theoretical prediction of Kolesik et al. [Opt. Lett. 35, 2550 (2010)] for testing the recently proposed higher-order Kerr effect model. Our observations show that although the fifth-order nonlinearity is non-negligible, the overall defocusing effect via the higher-order nonlinearities is sufficiently small that plasma formation should be a main defocusing mechanism in high power filamentation. We also explore cross-phase modulation via the optical Kerr effect, and find that the higher-order nonlinearities can significantly alter the phase matching of harmonic generation.
Controlling phase matching of high-order harmonic generation by manipulating the fundamental field
Physical Review A, 1999
We study experimentally how to control and improve phase matching of high-order harmonic generation. We use a birefringent lens and a birefringent compensator to obtain a fundamental laser pulse ͑150 fs, 800 nm, ϳ4 mJ͒ with two foci separated by 6.2 mm along the propagation axis and with a controllable phase delay between the polarizations along the optic axes of the birefringent optical components. This enables us to enhance the high-order harmonic conversion efficiency for the high-order harmonics in neon to 3ϫ10 Ϫ8 , a factor of 4 higher compared to a single-focus setup in similar conditions. The enhancement is achieved by improving the phase matching and at the same time maintaining a high intensity in a large generating volume.