Generation of high order optical harmonics from solid surfaces

Received: 26 November 1998     

Effects of an ultrashort prepulse on soft X-ray generation from an aluminium plasma produced by femtosecond Ti:Sapphire laser pulses

The influence of a prepulse on soft X-ray emission in the range of 50–200 from an aluminium plasma produced by 130 fs Ti: Sapphire laser pulses with an intensity of 10¹⁴ W/cm² at normal incidence is studied. An ultrashort prepulse with an intensity of 10¹³ W/cm² significantly enhances soft X-ray emission when there is a long time separation ( > 100 ps) between the prepulse and an intense main pulse. It is also observed for the first time that a prepulse with a short pulse time separation can slightly reduce soft X-ray emission, contrary to the previous work done using 248 nm laser pulses. This can be explained qualitatively in terms of the dependence of absorption on the length scale.     

Influence of strong laser fields on the inverse bremsstrahlung collision frequency

A new model is described for the absorption of laser light by a plasma. Two issues important for high laser intensity are considered: changes in the electron velocity distribution and the absorption non-linearity. The effects of anisotropy and non-Maxwellian distribution function are analysed with a hard sphere model. It is shown that anisotropy alters the electron-ion collision frequency by an amount which depends on the ratio of the temperatures in direction of and perpendicular to the laser field.The effect of high laser intensity is considered assuming Coulomb collisions, and a new expression obtained for the collision frequency as a function of quiver velocity. It will be shown that the result is in agreement with the Spitzer result in the case of small fields and with the result of Catto and Speziale in the case of very strong fields, but shows an unexpected maximum of the collision frequency in an intermediate regime.     

Investigation of the interaction of subpicosecond KrF laser pulses with a preformed carbon plasma

The interaction of a subpicosecond KrF laser pulse with a preformed carbon plasma of various scale lengths is investigated. Two different interaction geometries are chosen. In the first one the propagation vector of the short pulse has a component along the density gradient of the preformed plasma (angle of incidence is 45°). In the second geometry the propagation direction of the short pulse is perpendicular to the density gradient of the preplasma (angle of incidence is 90°). The emitted soft X-ray spectrum in the wavelength interval from 10 to 700 is observed while changing several parameters of the experiment. It is found that the emission in the short wavelength part under 200 results from the radiation of ions created by collisional heating near the critical density region. The long wavelength part above 200 , enhanced up to a maximal factor of 20, is mainly produced by radiating particles field—ionized up to the He-like carbon state in the high-intensity laser field. The short wavelength part is missing in the case of 90° angle of incidence because there is no interaction with the critical layer that results in an insufficient collisional heating.     

Intense femtosecond laser-induced second-harmonic generation in atmospheric-pressure air

Second-harmonic generation (SHG) in atmospheric-pressure air is experimentally studied using single focused linear-polarized Ti:sapphire intense femtosecond laser pulses at 810 nm. The efficiency of SHG is found to reach a maximum at the optical breakdown threshold of ≈2.9×10¹⁴ W/cm². The spectral distribution and polarization property of the second harmonic are investigated. The contribution to SHG from electric-field-induced third-order mixing plays the main role even after the optical breakdown had occurred. Received: 23 May 2000 / Published online: 16 August 2000     

Imaging a tunneling ionization front by using a schlieren method

A schlieren method was used to generate time-resolved images of the tunneling ionization front produced when an ultrashort high-power laser pulse irradiates He gas. By superimposing sequential schlieren images, we obtained information about the laser propagation and found that the ionization front propagated farther with decreasing density of the target gas. Ray-tracing suggested that this density dependence is a result of the spatial distribution of the laser intensity. Received: 20 May 1999 / Revised version: 19 August 1999 / Published online: 27 January 2000     

High-order-harmonic generation: towards laser-induced phase-matching control and relativistic effects

We present a review of some recent results on high-order-harmonic generation, aiming at optimizing the photon flux to allow for future applications in extreme-ultra-violet non-linear optics. We first present new schemes to control phase matching of high harmonics in gases, by using the effect of the spatially varying atomic phase displayed by the high harmonics. An enhancement by a factor of 50 is observed in neon in conditions for which the gradient of the atomic dispersion balances the electronic dispersion. A new scheme to manipulate the laser field was demonstrated, and shown to improve phase matching. We then turn to high-harmonic generation by solid targets, and show that high harmonics generated by an intense 30-fs laser pulse remain collimated even at the threshold of the relativistic regime. Received: 5 December 2001 / Published online: 24 April 2002     

Generation of magnetic fields by the non-stationary ponderomotive force of electromagnetic waves in plasmas with streaming electrons

It is shown that the non-stationary ponderomotive force of large amplitude electromagnetic waves in plasmas with streaming electrons can spontaneously create magnetic fields. The present result may account for the magnetic fields in laser-produces plasmas, in cosmic plasmas, as well as in galactic and inter-galactic spaces.     

Detailed study of the effect of a short prepulse on soft X-ray spectra generated by a high-intensity KrF* laser pulse

The effect of a short prepulse (0.5 ps) on soft X-ray spectra from a plasma generated by a high intensity KrF* laser pulse (main pulse: 0.5 ps, intensity I _main=5.3×10¹⁵ W/cm²) on flat targets of Al and Cu has been studied in detail. The spectra have been measured as a function of the pulse separation t between the two pulses and the prepulse intensity I _pre. It was found that both the overall emission and the line emission increased with t (at constant I _pre) and with I _pre (at constant t). In particular, lines in the shorter wavelength region had higher intensity. The influence of the prepulse on the line emission of specific transitions in the Al spectra was investigated systematicly. An explanation for the observed effects is given.     

High-order harmonic generation in nitrogen molecules with subpicosecond visible dye-laser pulses

An experimental study of high-order harmonic generation in nitrogen molecules (N₂) has been made using intense visible (616 nm) dye-laser pulses, where the harmonic radiation up to the 21st order is observed. The harmonic distribution represents a plateau that is preceded by an intensity minimum at the 7th order. The harmonic generation characteristics were atomic-like. It has been found that there are some similarities in the high-order harmonic generation characteristics for N₂ and Ar, including the highest-order harmonics, harmonic distributions, and the influence of the multiphoton ionization on the high-order harmonic generation. These similarities are reasonably attributed to the energetic correspondence of excited levels and ionization potentials. It is pointed out that the ac Stark shift of excited levels and ionization potentials plays an important role also in the high-order harmonic generation in N₂.     

Porous layer effects on soft X-ray radiation emitted from a plasma generated by 130-fs laser pulses irradiating a porous silicon target

Received: 2 May 1997/Revised version: 17 October 1997     

X-ray spectra from highly ionized dense plasmas produced by ultrashort laser pulses

Hydrogen-like and helium-like X-ray spectra (between 7.1 and 8.2 , i.e., 1500 to 17.50 eV, respectively) from solid aluminium targets irradiated with high intensity (up to 10¹⁷ W/cm²) subpicosecond (0.7 ps) laser pulses have been measured. The spectra show that the resonance lines are very broad and very asymmetric. Evidence for a Doppler-shifted reabsorption of the resonance line emission has been found. The spectra have been simulated by a computer code for the calculation of spectral-line intensities and linewidths. Electron densities exceeding the critical density have been estimated for different laser intensities by comparing the observed and simulated intensity ratio of different dielectronic satellite lines. From the X-ray spectra generated byp- ands-polarized radiation fat different laser intensities, the thresholds for the formation of hydrogen-like and helium-like ions have been determined.     

Mechanisms of absorption in pulsed excimer laser-induced plasma

2 . Received: 20 January 1997/Accepted: 23 June 1997     

Second-harmonic generation at the surface of cubic ionic crystals

The strength of the different source terms in the isotropic Second Harmonic (SH) response of the surface has been measured in different ionic crystals LiF, NaF, GGG(Gadolinium-Gallium-Garnet), and MgF₂, using as excitation beam the fundamental output of a Nd:YAG laser (1.06 m). The azimuthal dependences of the optical second-harmonic generation has also been measured at the LiF(100), LiF(111), GGG(100), and GGG(111) surfaces. No experimental evidence of a bulk contribution to the anisotropic part of the nonlineau polarization has been observed.     

Optical second harmonic generation from Pt nanowires

We have measured optical second harmonic intensity from arrays of Pt nanowires of 20 nm and 9 nm average widths, as a function of the incident and output light polarizations, the azimuthal angle, and the excitation photon energy. The nanowires were fabricated through shadow deposition on self-organized NaCl(1 1 0) faceted templates. The anisotropy of the SH intensity from the Pt nanowires was found to be stronger than that from the Au nanowires reported previously. The effective nonlinear susceptibility element , with the suffix 2 indicating the direction , was observed for Pt nanowires, although it was not observed for Au nanowires. This difference is suggested to be due to the weaker suppression of the incident fundamental fields by the depolarization field in the Pt nanowires and the larger anisotropy in the nonlinearity of Pt nanowires due to the thinner widths.     

Effects of electron relaxation on multiple harmonic generation from metal surfaces with femtosecond laser pulses

Calculations are presented for the first four (odd and even) harmonics of an 800 nm laser from a gold surface, with pulse widths ranging from 100 down to 14 fs. For peak laser intensities above 1 GW/cm² the harmonics are enhanced because of a partial depletion of the initial electron states. At 10¹¹ W/cm² of peak laser intensity the calculated conversion efficiency for 2nd-harmonic generation is 3 × 10⁻⁹, while for the 5th-harmonic it is 10⁻¹⁰. The generated harmonic pulses are broadened and delayed relative to the laser pulse because of the finite relaxation times of the excited electronic states. The finite electron relaxation times cause also the broadening of the autocorrelations of the laser pulses obtained from surface harmonic generation by two time-delayed identical pulses. Comparison with recent experimental results shows that the response time of an autocorrelator using nonlinear optical processes in a gold surface is shorter than the electron relaxation times. This seems to indicate that for laser pulses shorter than ∼30 fs, the fast nonresonant channel for multiphoton excitation via continuum-continuum transitions in metals becomes important as the resonant channel becomes slow (relative to the laser pulse) and less efficient.     

Ultrashort X-ray pulses

In this paper, we discuss recent advances in ultrashort-pulse X-ray technology. Femtosecond laser-plasma based X-ray sources can now generate sub-pico-second soft X-ray pulses, with photon energies from 10 eV to 10⁶ eV, and with a high conversion efficiency of incident laser light to broadband X-rays. Recent advances in high-speed X-ray detectors and soft X-ray optics make it practical to use such sources as experimental tools for time-resolved X-ray science. Other possible generation techniques, such as laser-electron-beam scattering, promise to further expand the experimental capabilities in this area.     

Atmospheric influences in optical third-harmonic generation experiments

Third-harmonic generation in a thin solid plate surrounded by a gas is investigated. We conclude that atmospheric contributions can be suppressed in third-harmonic generation from thin solid plates. A phase-sensitive measurement of the third-order susceptibility of gases is proposed. We calibrated the third-order susceptibility of air to be X⁽³⁾ _a=(3.4±1.5)×10^–18 esu with a phase uncertainty of ±10°.     

High-order harmonic generation from organic molecules in ultra-short pulses

We have studied high-order harmonic generation (HHG) from organic molecules irradiated with near-infrared high intensity laser pulses of 70 fs and 240 fs duration. The molecular systems studied were the aromatics benzene and naphthalene and the alkanes cyclopropane and cyclohexane (cyclic) and n-hexane (linear). Harmonic intensities were measured both as a function of laser intensity (in the range 5×10¹³-5×10¹⁵ W cm^-2) and as a function of ellipticity of the laser field polarisation. The results were compared with those from the xenon atom. For 70 fs pulses, harmonic generation from the organic systems was similar to that of xenon, revealing an atom-like behaviour for molecules when the laser pulse duration is shorter than the fragmentation timescale of the molecule. We note significant differences between molecules with respect to HHG efficiencies and the suppression of HHG in larger species. We discuss these differences in the context of the molecular properties, electronic structure and behaviour of ionisation and fragmentation that result in enhancement of field ionisation in larger systems. Study of the polarisation ellipticity dependence of HHG shows that the harmonic yield in molecules is less sensitive to the polarisation than for atoms (xenon). This is consistent with the expected behaviour given the larger recollision cross-section presented by the core in the molecular system compared to the atom. Our results suggest that study of HHG from molecules exposed to ultra-short pulses is potentially a powerful tool for understanding the electron dynamics of molecules exposed to an intense field. Received 14 September 2000 and Received in final form 6 December 2000     

Absorption of subpicosecond ultraviolet laser pulses in high-density plasma

The absorption of 250 fs KrF laser pulses incident on solid targets of aluminum, copper and gold has been measured for normal incidence as a function of laser intensity in the range of 10¹²–10¹⁴ W cm^–2 and as a function of polarization and angle of incidence for the intensity range of 10¹⁴–2.5×10¹⁵ W cm^–2. As the intensity increases from 10¹² W cm^–2 the reflectivity at normal incidence changes from the low-intensity mirror reflectivity value to values in the range of 0.5–0.61 at 10¹⁴ W cm^–2. For this intensity maximum absorption of 63–80% has been observed for p-polarized radiation at angles of incidence in the range of 54°–57°, increasing with atomic number. The results are compared with the expected Fresnel reflectivity from a sharp vacuum-plasma interface with the refractive index given by the Drude model and also to numerical calculations of reflectivity for various scale length density profiles. Qualitative agreement is found with the Fresnel/Drude model and quantitative agreement is noticed with the numerical calculations of absorption on a steep density profile with normalized collision frequencies, v/, in the range of 0.13–0.15 at critical density and normalized density gradient scale lengths, L/₀, in the range of 0.018–0.053 for a laser intensity of 10¹⁴ W cm^–2.At 2.5×10¹⁵ W cm^–2 a small amount of preplasma is present and maximum absorption of 64–76% has been observed for p-polarized radiation at angles of incidence in the range of 45°–50°.Dedicated to Prof. Dr. Rudolf Wienecke on the occasion of his 65^th birthdayOn leave from: Department of Electrical Engineering, University of Alberta, Edmonton, T6G 2G7, Canada