Characterization of Below Threshold Harmonics Generated in Argon by Ultrashort Laser Pulses (original) (raw)
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Abstruct-Reported are the results of a harmonic generation experiment in a simple gas (hydrogen) using 1-ps, 1-pm laser pulses with a range of intensities extending from below to far above the laser ionization saturation threshold. The scaling with intensity above saturation of the third harmonic generated by a single laser-pulse in a filled gas cell is observed to not fit with a simple model that takes into consideration volume ionization effects alone. In another experiment, a pump-probe type, an upper limit on the conversion efficiency of third harmonic generation in a preformed plasma is determined. It is found to be in agreement with the efficiency predicted by a relativistic harmonic generation theory.
Laser and Particle Beams, 2000
We have investigated the effect of free electrons on the spectral properties of high-order harmonics generated in a neon gas jet by a 30 fs Titanium:Sapphire pumping laser with intensities in the range 5–10 × 1014 W/cm2. The main feature of our observations concerns the possibility of continuously tuning the harmonic wavelength in the spectral region 20–7 nm, by taking advantage of the blue shift of harmonic wavelengths induced by the presence of free electrons to cover the entire spectral region between two consecutive harmonics of the unshifted spectrum. Different amounts of blue shift, which can be as large as 0.3–0.4 nm, are imparted to the given harmonic by simply changing the gas-jet-laser-beam-waist relative position. We have also interpreted our experimental results with a simple model for the generation process based on the “barrier suppression” ionization of an atom exposed to an ultraintense laser field.
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Applied Physics B-lasers and Optics, 1997
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Spectral Features and Modeling of High-Order Harmonics Generated by Sub10-fs Pulses
Physical Review Letters, 2000
Harmonic radiation generated in a neon gas jet by sub-10-fs laser pulses was investigated both experimentally and theoretically. The spectral profile of the harmonics with respect to the order, their intensity and relative spectral shifts were measured as a function of the position of the gas jet. The results point out spectral features typical of the quasi-single-cycle excitation regime. A nonadiabatic three-dimensional numerical model was developed, which provides harmonic spectra in remarkable agreement with the experiments. PACS numbers: 42.65.Ky, 42.50.Hz, 42.65.Re High-order harmonics generated in noble gases provide a unique source of high brightness, coherent extreme ultraviolet radiation [1-3], due to a tabletop size apparatus and a very short duration. When an intense short laser pulse interacts with an atomic gas, tunneling ionization occurs followed by acceleration of the ionized electron by the laser field and generation of high-order harmonics during the recombination process. Upon decreasing the excitation pulse duration, the harmonic conversion efficiency is expected to increase [4-6] and higher photon energies (i.e., shorter wavelengths) are also expected because the electrons are released into stronger laser fields. Generation of coherent emission up to the "water window" (4.4-2.3 nm) was indeed demonstrated in helium by using 26-fs [3], and sub-10-fs [7] pulses obtained by the hollow fiber compression technique . The advent of high-peak power fewoptical-cycle (sub-10-fs) laser pulses extends the scenario of the harmonic generation process, pointing out a number of new physical problems to be addressed both from an experimental and a theoretical point of view.
Analysis of several high order harmonic generation in gaseous media
UPB Scientific Bulletin, Series A: Applied Mathematics and Physics
In this article we present a theoretical analysis of high order harmonic generation (HHG) in gas jets with high intensity laser pulses. We estimate several laser and target parameters (i.e. focusing parameters-focus length, beam radius at the waist, length of the nonlinear medium, and concentration of the gas jet) involved in efficient conversion of a Nd + 3 :YAG nanosecond laser radiation to higher frequencies. First, we evaluate the wave vector mismatches, length of the nonlinear medium and focusing parameters for efficient generation of the fifth and seventh order harmonics generated in mixtures of sodium vapors and xenon. The theoretical findings are in good agreement with the experimental results published in the literature. Second, in the case where the gas jet is produced by laser ablation, the condition for obtaining optimum gas density profile for HHG is evaluated by solving numerically the hydrodynamics of the ablation jet produced by a nanosecond laser pulse.
Applied Physics B-lasers and Optics, 2005
Harmonic generation during the self-guided propagation of femtosecond ultraviolet (UV) laser pulses (248-nm, 450-fs) in argon is investigated. The third (82.7-nm) and fifth (49.6-nm) harmonics are generated in the UV filament. The energy-conversion efficiencies for the harmonics are found to be at least two orders of magnitude higher than those reported in the literature for similar gas pressures. The enhancement is attributed to the quasi-phase matching of the harmonics due to the self-guiding of the driving pulse.
Synthesis of ultrashort laser pulses for high-order harmonic generation
Physical Review A, 2018
We present a technique for the synthesis of ultra-short laser pulses with approximately one cycle (FWHM) of temporal duration. These pulses are characterized by a certain degree of chirp. We show that these pulses produce both an enhancement on the high-order harmonic generation (HHG) cutoff and a noticeable increase of the yield, when interact with an atomic system. Additionally, the asymmetric nature of the driven pulses plays an important role in the efficiency and cutoff extension of the high-order harmonics generated. Starting from the HHG spectra, we demonstrate it is possible to retrieve isolated attosecond pulses by spectral filtering. The analysis and interpretation of the different characteristic present in the HHG driven by this kind of pulses was carried out invoking classic arguments. Furthermore, a more complete description and validation of the HHG properties is performed by a quantum analysis, based on the integration of the time-dependent Schrödinger equation in full dimensionality (3D-TDSE).
Atoms
The generation of high-order harmonics in a semi-infinite cell by femtosecond laser pulses is a common practice for reliable coherent and low divergence XUV source beams for applications. Despite the relative simplicity of the experimental method, several phenomena coexist that affect the generated spectral and divergence characteristics of the high harmonic XUV frequency comb. The ionisation degree of the medium and the consequent plasma formation length imposes a spatiotemporal evolution of the fundamental EM field and XUV absorption. Varying the laser pulse chirp and the focusing conditions, as well as the gas density, we measured intense harmonic spectral and divergence variations attributed mainly to self-phase modulations of the laser EM field in the partially ionised medium. Additionally, low-divergence high harmonics are observed for certain laser chirp values attributed to the strong phase matching of only the short electron quantum path. Thus, a tunable, low divergent, and...
Analysis of Optimum Conditions for High Order Harmonic Generation in Gaseous Medias
UPB Scientific Bulletin, Series A: Applied Mathematics and Physics
In this article we present a theoretical analysis of high order harmonic generation (HHG) in gas jets with high intensity laser pulses. We estimate several laser and target parameters (i.e. focusing parameters-focus length, beam radius at the waist, length of the nonlinear medium, and concentration of the gas jet) involved in efficient conversion of a Nd3+ : YAG nanosecond laser radiation to higher frequencies. First, we evaluate the wave vector mismatches, length of the nonlinear medium and focusing parameters for efficient generation of the fifth and seventh order harmonics generated in mixtures of sodium vapors and xenon. The theoretical findings are in good agreement with the experimental results published in the literature. Second, in the case where the gas jet is produced by laser ablation, the condition for obtaining optimum gas density profile for HHG is evaluated by solving numerically the hydrodynamics of the ablation jet produced by a nanosecond laser pulse.