Gas medium ionization and harmonic wavelength tunability in high-order harmonic generation with ultrashort laser pulses (original) (raw)
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Applied Physics B-lasers and Optics, 1997
We report the realization of a vacuum-ultraviolet radiation source based on high-order harmonic generation in noble-gas samples, operating at high repetition rate. In particular, we observed up to the 13th harmonic (λ=61 nm) of the fundamental frequency of a short pulse, high repetition rate titanium–sapphire laser after its interaction with a Xe gas jet. The effects of the propagation of the fundamental and harmonic beams through an ionized medium are studied by analysing the spectral profile of the 9th and 7th harmonics. Finally, we report a study of the dependence of the harmonic conversion efficiency on relative position of the focus and the gas target.
Harmonic generation by an intense laser pulse in neutral and ionized gases
IEEE Transactions on Plasma Science, 1993
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.
Measured laser-beam evolution during high-order harmonic generation in a semi-infinite gas cell
Optics …, 2007
We report on direct measurements of self-guiding of 800 nm, 30 fs, 5 mJ laser pulses used to generate high-order harmonics in 80 torr helium. We track the spatial evolution of the laser pulses as they propagate several centimeters near the focus under conditions suitable for harmonic generation. The laser is observed to focus, diverge, and refocus. This behavior is accompanied by a flattop beam profile. Both of these features are absent when the laser is focused in vacuum. We also observed a 4 nm spectral blue shift in the center of the laser beam near the focus in contrast with no spectral shift at wider radii.
Below-threshold-harmonics-generation limitation due to laser-induced ionization in noble gases
Physical Review A, 2022
This work introduces and validates a model that explains an observed upper limitation of the emission of below-threshold harmonics generated by noble gases bound electrons in a restricted region of space. The model uses fundamental parameters from the gases and the laser beam, and correctly predicts an intensity threshold above which the harmonics yield cannot be increased by raising the laser power. This restraint is a consequence from the depletion of electrons due to laser-induced ionization, which exactly compensates the harmonics yield increase with the excitation power.
Nature Physics, 2008
Conventional synchrotron radiation sources enable the structure of matter to be studied at near-atomic spatial resolution and picosecond temporal resolution. Free-electron lasers promise to extend this down to femtosecond timescales. The process by which free-electron lasers amplify synchrotron light-known as self-amplified spontaneous emission 1-3 -is only partially temporally coherent, but this can be improved by seeding it with an external laser 4,5 . Here we explore the use of seed light produced by high-order harmonic generation in a gas 6-9 , covering wavelengths from the ultraviolet to soft X-rays. Using the SPring-8 Compact SASE Source test accelerator 10-12 , we demonstrate an increase of three orders of magnitude in the intensity of the fundamental radiation at 160 nm, halving of the free-electron laser saturation length, and the generation of nonlinear harmonics 13 at 54 nm and 32 nm. The low seed level used in this demonstration suggests that nonlinear harmonic schemes should enable the generation of fully coherent soft X-rays at wavelengths down to the so-called 'water window', vital for the study of biological samples.
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.
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...
Seeding High Gain Harmonic Generation with laser harmonics produced in gases
2004
Free electron Lasers employing High Gain Harmonic generation (HGHG) schemes are very promising coherent light sources in the soft X-ray range. They offer both transverse and longitudinal coherence, while Self Amplified Spontaneous Emission schemes have a limited longitudinal coherence. We propose here to seed a HGHG experimental setup with high harmonics produced by a Ti:Sa femtosecond laser focused on a gas jet in the 100-10 nm spectral region. The implementation of this particular laser harmonics source as a seed for HGHG is investigated. Semi analytical and numerical 1D calculations are given, for the cases of the SCSS, SPARC and ARC-EN-CIEL projects.
Characterization of Below Threshold Harmonics Generated in Argon by Ultrashort Laser Pulses
2018 SBFoton International Optics and Photonics Conference (SBFoton IOPC), 2018
This work reports studies on the generation of below threshold harmonics by ultrashort laser pulses in gas nozzles in vacuum. Odd harmonics of the laser, ranging from the 3rd up to the 9th, were generated in the UV and VUV regions, and the frequency conversion occurred in flowing gas. The harmonics were spectrally characterized, and the orders of the nonlinear processes were determined, showing that, for the higher order harmonics, competing processes decrease the energy coupling into the new frequencies. Also, the conversion efficiencies were estimated, showing that a few percent of the laser pulse energy are converted into the harmonics.
Simulated laser-pulse evolution for high-order harmonic generation in a semi-infinite gas cell
Optics Express, 2008
We numerically simulate the propagation of high-intensity laser pulses in helium to investigate the role of nonlinear effects in gas-cell high-harmonics experiments. An aperture located before the focusing lens is also included in the simulation. Numerical results for the radial fluence profile as a function of axial position, as well as for the spectral shift and ionization levels, agree with experimental observations. The simulations confirm that a significant Kerr effect is not required to generate the observed double focus in the fluence. The beam simulation also permits an investigation of high-harmonic phase matching. Most of the harmonic energy is seen to come from the forward portion of the laser pulse, whereas the latter portion gives rise to the incidental double laser focusing. Good phase matching for the harmonics arises in large measure from a balance between the linear phase delay of the neutral atoms and the Gouy shift, which is elongated and nearly linearized when the aperture is partially closed on the beam.