Spectral and Divergence Characteristics of Plateau High-Order Harmonics Generated by Femtosecond Chirped Laser Pulses in a Semi-Infinite Gas Cell (original) (raw)
Related papers
2009
The intensity enhancement of harmonics and change in harmonic profiles have been observed in a few plasma plumes (La, In, Mn) under variable phase modulation of Ti:sapphire laser pulses. It is seen that variation of relative harmonic intensities in the plateau region could be achieved by chirp variation of narrowband (ϳ10 nm bandwidth) radiation. The effect of the self-phase modulation (SPM) of broadband (ϳ20 nm bandwidth) laser radiation on the harmonic emission from nanoparticles, C 60 , and Ag plasma while passing through a glass slab is also studied. The observation of broadband harmonic emission and redshift in harmonic wavelengths for an unchirped laser pulse are explained in terms of SPM. The observation of blueshift of harmonic radiation with both positively and negatively chirped pulses was attributed to the different effects of SPM on the chirped pulses.
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.
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.
Physical Review A, 2005
We present a time-dependent analysis of high-order harmonics generated by a self-guided femtosecond laser pulse propagating through a long gas jet. A three-dimensional model is used to calculate the harmonic fields generated by laser pulses, which only differ by the sign of their initial chirp. The time-frequency distributions of the single-atom dipole and harmonic field reveal the dynamics of harmonic generation in the cutoff. A time-dependent phase-matching calculation was performed, taking into account the self-phase modulation of the laser field. Good phase matching holds for only few optical cycles, being dependent on the electron trajectory. When the cutoff trajectory is phase matched, emitted harmonics are locked in phase and the emission intensity is maximized.
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.
Control of high harmonic generation processes using chirped and self-guided femtosecond laser pulses
2007
High-order harmonic generation in a long gas jet is controlled in the space and time domains using chirped and self-guided femtosecond laser pulses. Since high-order harmonic generation is intrinsically connected to the ionization process of harmonic generation medium, ionization effects on high-order harmonic generation should be properly understood and taken into account. Here, we present a method to control high-order harmonic generation process by controlling the propagation mode of intense femtosecond laser pulses through the ionizing medium. Experimental results and theoretical analysis show that self-guided and chirped laser pulses can optimize high-order harmonics for achieving high brightness, low beam divergence, and narrow spectral bandwidth.
Optics Communications, 2006
The characteristics of harmonic radiation due to electron oscillation driven by an intense femtosecond laser pulse are analyzed considering a single electron model. An interesting modulated structure of the spectrum is observed and analyzed for different polarization. Higher order harmonic radiations are possible for a sufficiently intense driving laser pulse. We have shown that for a realistic pulsed photon beam, the spectrum of the radiation is red shifted as well as broadened because of changes in the longitudinal velocity of the electrons during the laser pulse. These effects are more pronounced at higher laser intensities giving rise to higher order harmonics that eventually leads to a continuous spectrum. Numerical simulations have further shown that by increasing the laser pulse width broadening of the high harmonic radiations can be limited.
Journal of Physics B: Atomic, Molecular and Optical Physics, 2006
Bright harmonic generation in a long Ne gas jet achieved using self-guided and chirped femtosecond laser pulses has been analysed. Three-dimensional modelling was performed to look into the details of laser pulse propagation, harmonic generation and attosecond pulse characteristics. The theoretical analysis and the comparison to the experimental results showed emitted harmonics form a bright attosecond pulse train with good synchronization in time. In the space domain the synchronization is especially good in the self-guided region.
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.
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.