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.     

The study of spectral, temporal, and spatial characteristics of harmonics generated at solid surfaces

Spectral, temporal, and spatial characteristics of harmonics generated at solid surfaces interacting with laser radiation (t=27 ps and I≤1.5×10¹⁵ W/cm²) are studied. Spectral broadening and a long-wavelength shift of the second harmonic were observed for laser radiation intensities exceeding 5×10¹⁴ W/cm². Results of the study of the conversion of spectral parameters and polarization features for the generation of second and third harmonics are presented. Conversion efficiencies for the second, third, and fourth harmonic are 2×10^?8, 10^?10, and 5×10^?12, respectively. The results obtained are compared with data of analogous studies utilizing shorter pulses.     

Ellipticity dependence of atomic and molecular high harmonic generation

High harmonic generation is compared in the dependence on the ellipticity of the fundamental laser radiation for an atomic and a molecular system. In particular argon and nitrogen are compared employing molecular beams and intense ( 3×10¹⁴ W/cm²) and ultrashort (80 fs) 800 nm laser pulses. It turns out that for all the harmonics under investigation (H5, H13 and H21) the harmonic yield decreases slower with the ellipticity for the molecule than for the atom. This indicates differences in atomic and molecular high harmonic generation. Received 24 April 2002 Published online 24 September 2002     

Tantalum ion acceleration in laser‐generated plasma and dependence on the pulse duration

The laser irradiation of tantalum targets is presented for different pulsed laser intensities ranging from 10¹⁰ up to about 10¹⁸ W/cm² and pulse durations from 9 ns up to 40 fs. The results show that the produced non‐equilibrium plasma accelerates Ta ions in the backward direction from values of the order of keV up to values of about 5 MeV. In thin foils, the forward plasma, developed behind the target along the direction of incoming laser, at intensities of about 10¹⁶ W/cm² and 300 ps pulse duration, accelerates Ta ions at energies of the order of 4.6 MeV and produces charge states up to about 40+. For fs lasers at intensities of the order of 10¹⁸ W/cm², only proton acceleration occurs up to 2.1 MeV while no Ta ions are accelerated, due to the reduced duration of the electric field and to the too high inertial mass of the Ta ions.     

Efficiency of laser initiation and absorption spectra of PETN

A comparative study of the efficiency of the laser initiation of PETN by the first and second harmonics (1060 and 530 nm) of a neodymium-doped phosphate glass laser was performed. A significant difference in the efficiency of PETN initiation by the different harmonics was revealed: as the initial temperature of the sample increased from 373 to 450 K, the threshold initiation fluence decreased from 3.0 to 0.5 J/cm²; at the same time, the second harmonic failed to initiate PETN even at a fluence of 10 J/cm². The absorption spectrum of PETN was found to have a weak absorption band with a maximum at λ _m = 1020 nm. It was assumed that the high efficiency of initiation by the first harmonic is associated with light absorption (photo-initiation) by this band     

Enhanced photoemission and ionization of He+ by simultaneous irradiation of fundamental laser and high-order harmonic pulses

We simulate the response of He⁺ exposed simultaneously to fundamental and 27th harmonic pulses from an intense Ti:sapphire laser. High-order harmonic emission from He⁺ is enhanced by 17 orders of magnitude compared with the case of the fundamental pulse alone. Moreover, while an individual 10 fs laser with a fundamental wavelength of 800 nm and a peak intensity of 3×10¹⁴ W/cm², or its 27th harmonic pulse with a peak intensity of 10¹³ W/cm², ionizes no more than 5×10^-6 of He⁺, their combined pulses lead to a surprisingly high He²⁺ yield of 17%. The underlying mechanism is either harmonic generation from a coherent superposition of states or two-color frequency mixing, depending on the laser wavelength. PACS 32.80.Rm; 42.50.Hz; 42.65.Ky     

Experimental observation of anomalous high harmonics at low intensities

A variety of complex phenomena occurs when an ultra-short laser pulse interacts with atoms in the 10¹³ W/cm² range: non-sequential ionisation, electron recollision, and Freeman resonances. We show that high-harmonic spectra obtained experimentally in this parameter range also display anomalous features, which are difficult to understand in the framework of the three-step, semiclassical model. The results of a systematic study of these high harmonics generated in argon, xenon, and krypton are presented. From the experimental curves, complex high-order harmonic generation phenomena are discussed. PACS 42.65.Ky; 32.80.Rm     

Second-and third-harmonic generation by carbon nanotubes irradiated with femtosecond laser pulses

Femtosecond pulses of a Cr:forsterite laser are used to study second-and third-harmonic generation in a layer of single-wall carbon nanotubes produced by low-velocity spraying. The harmonic amplitude in our experiments scales as (I _p)ⁿ as a function of the pump intensity I _p, with n=2 and 3 for the second and third harmonics, respectively. This scaling law holds up to pump intensities on the order of 10¹²W/cm². The ratio of the maximum signal to the averaged background in the spectra of the second and third harmonics is estimated as 50 and 30, respectively. The second and third harmonics produced by a linearly polarized pump field are also linearly polarized, with their polarization vectors oriented along the polarization direction of the pump field. The capabilities of nonlinear-optical methods for structural and morphological analysis of carbon nanotubes are discussed, as well as ways to create solid-state carbon-nanotube generators of optical harmonic.     

Mechanisms for second harmonic generation in the interaction of a superintense ultrashort laser pulse with cluster plasma

The effects of the interaction of an intense femtosecond laser pulse with large atomic clusters are considered. The pulse intensity is of the order of 10¹⁸ W cm^?2. New effects appear when the magnetic component of the Lorentz force is taken into account. The second harmonic of laser radiation is generated. The second harmonic generation (SHG) efficiency is proportional to the square of the number of atoms in a cluster and the square of the laser radiation intensity. The resonance increase in the SHG efficiency at the Mie frequencies (both at the second harmonic frequency and fundamental frequency) proved to be insignificant because of the fast passage through the resonance during cluster expansion. The mechanisms of the expansion and accumulation of energy by electrons and ions in the cluster are discussed in detail. The energy accumulation by electrons mainly occurs due to stimulated inverse bremsstrahlung upon elastic reflection of the electrons from the cluster surface. The equations describing the cluster expansion take into account both the hydrodynamic pressure of heated electrons and the Coulomb explosion of the ionized cluster caused by outer shell ionization. It is assumed that both inner shell and outer shell ionization is described by the over barrier mechanism. It is shown that atomic clusters are more attractive for the generation of even harmonics than compared to solid and gas targets.     

Efficient phase-matched third harmonic light generation in hexafluoroisopropanol solutions of a pyrimidonecarbocyanine dye

The phase-matched collinear third harmonic generation of picosecond laser pulses in a 0.0825 molar hexafluoroisopropanol solution of a pyrimidonecarbocyanine dye is studied. The fundamental pulses are generated in a passively mode-locked Nd-phosphate glass laser. The saturation of third harmonic generation at high intensities is investigated. The influences of two-photon absorption, excited-state absorption, and amplified spontaneous emission are discussed. For input peak intensities above 10¹¹ W/cm² a third harmonic energy conversion of about 2×10^–4 is achieved.     

Multiple charge states of titanium ions in laser produced plasma

An intense laser radiation (10¹² to 10¹¹ W/cm⁻²) focused on the solid target creates a hot (≥1 keV) and dense plasma having high ionization state. The multiple charged ions with high current densities produced during laser matter interaction have potential application in accelerators as an ion source. This paper presents generation and detection of highly stripped titanium ions (Ti) in laser produced plasma. An Nd:glass laser (KAMETRON) delivering 50 J energy (λ=0.53 μm) in 2.5 ns was focused onto a titanium target to produce plasma. This plasma was allowed to drift across a space of ∼3 m through a diagnostic hole in the focusing mirror before ions are finally detected with the help of electrostatic ion analyzer. Maximum current density was detected for the charge states of +16 and +17 of Ti ions for laser intensity of ∼10¹¹ W/cm⁻².     

Generation of lower and higher harmonics in different plasma media with small <Emphasis Type="Italic">Z</Emphasis>

The generation of lower (third) and higher harmonics of femtosecond laser radiation in plasmas produced by laser ablation of different targets with a small atomic number Z (B, Be, Li) has been investigated. The high (10⁻³) efficiency of third-harmonic generation was observed in plasma produced on the boron surface. Efficient third-harmonic generation was also observed in beryllium plasma using femtosecond pulses of Ti:sapphire laser radiation (λ = 790 nm) and its second harmonic (395 nm). We could tune the higher harmonics generation spectrum by tuning the crystal converter when using 395-nm radiation to be converted. It is shown that, in plasmas formed on targets with small Z, the conversion efficiency and limiting generated harmonic order depend on the delay between the ablation pulse and the pulse to be converted.     

Controlling the divergence of high harmonics from solid targets: a route toward coherent harmonic focusing

Harmonic generation from relativistically oscillating plasma surfaces formed during the interaction of high contrast lasers with solid-density targets has been shown to be an efficient source of extreme ultraviolet (XUV) and X-ray radiation. Recent work has demonstrated that the exceptional coherence properties of the driving laser can be mirrored in the emitted radiation, permitting diffraction limited performance and attosecond phase locking of the harmonic radiation. These unique properties may allow the coherent harmonic focusing (CHF) of high harmonics generated from solid density targets to intensities on the order of the Schwinger limit of 10²⁹ W cm^-2 with laser systems available in the near future [Phys. Rev. Lett. 93, 115002 (2004)] and thus pave the way for unique experiments exploring the nonlinear properties of vacuum on ultra-fast timescales. In this paper we investigate experimentally as well as numerically the prospect of focusing high harmonics under realistic experimental conditions and demonstrate, using particle in cell (PIC) simulations, that precise control of the wavefronts and thus the focusability of the generated harmonics is possible with pre-shaped targets.     

Generation of strong coherent extreme ultraviolet radiation from the laser plasma produced on the surface of solid targets

We demonstrate the generation of high harmonics (up to the 65th order, λ=12.24 nm) of a Ti:sapphire laser radiation after the propagation of femtosecond laser pulses through the low-excited plasma produced by a picosecond prepulse radiation on the surface of different targets. High-order harmonics generated from the surface plasma of most targets showed a plateau pattern. It is assumed that the harmonic generation in these conditions occurs due to the interaction of the femtosecond pulses with the ions. The conversion efficiencies at the plateau region were varied between 1×10^-7 to 8×10^-6, depending on the target. The main contribution to the limitation of harmonic generation efficiency and cutoff energy was attributed to the self-defocusing of main pulse. A considerable restriction of the 27th harmonic generation was observed at different focusing conditions in the case of chromium plasma. Our observation of the resonance-induced enhancement of a single harmonic (λ=61.2 nm) at a plateau region with the efficiency of 8×10^-5 in the case of In plasma can offer some expectation that analogous processes can be realized in other plasma samples in the shorter wavelength range where the highest harmonics were achieved. PACS 42.65.Ky; 52.35.Mw; 52.38.-r     

Lyman alpha (1216 Å) generation in krypton

We measured the nonlinear susceptibilities of Kr up to fifth order via nonresonant third-harmonic generation (THG) of a frequency-doubled dye laser at 3648 Å. In agreement with theory the optimum intensity conversion efficiency saturates at 10^-5 in pure Kr. The phase matching of the third harmonic and fundamental wave breaks up owing to intensity-dependent changes of refractive indices. Phase matching in high-density Kr/Ar mixtures raised the 3648 Å to 1216 Å third harmonic conversion obtained in pure Kr by more than two orders of magnitude, as predicted by theory but hitherto never experimentally confirmed. With laser intensities of 5 × 10⁹ W/cm² at 3648 Å we measured conversion efficiencies of 1.4 × 10^-4 limited by gas breakdown. Taking into account the Lyman alpha absorption of Kr₂ and KrAr molecules in the high-density regime, a conversion efficiency of 10^-3 is expected from extrapolating the experimental results to Kr/Ar mixtures of up to 3 × 10³ torr. It is limited by self-focusing, assuming the minimum threshold intensity of gas breakdown is 10¹¹ W/cm², as predicted by theory.     

Radiation effects and pulsed laser deposition of titanium by Nd:Yag laser

A study of Ti laser irradiation and thin film deposition produced by an Nd:Yag pulsed laser is presented. The laser pulse, 9?ns width, has a power density of the order of 10¹⁰?W/cm². The titanium etching rate is of the order of 1?µg/pulse, it increases with the laser fluence and shows a threshold value at about 30?J/cm² laser fluence. The angular distribution of ejected atoms (neutrals and ions) is peaked along the normal of the target surface. At high fluence, the fractional ionization of the plasma produced by the laser is of the order of 10%. Time-of-flight measurements demonstrate that the titanium ions, at high laser fluence, may reach kinetic energies of about 1?keV. Obtained results can be employed to produce energetic titanium ions, to produce coverage of thin films of titanium and to realize high adherent titanium-substrate interfaces. The obtained results can be employed to produce energetic titanium ions, to produce a coverage of thin titanium films on polymers, and to realize highly adherent titanium–substrate interfaces.     

A nanohole in a thin metal film as an efficient nonlinear optical element

The nonlinear optical properties of single nanoholes and nanoslits fabricated in gold and aluminum nanofilms are studied by third harmonic generation (THG). It is shown that the extremely high third-order optical susceptibility of aluminum and the presence of strong plasmon resonance of a single nanohole in an aluminum film make possible an efficient nanolocalized radiation source at the third harmonic frequency. The THG efficiency for a single nanohole in a thin metal film can be close to unity for an exciting laser radiation intensity on the order of 10¹³ W/cm².     

High brightness harmonic generation at 13 nm using self-guided and chirped femtosecond laser pulses

We have optimized the brightness of high-order harmonics from a long neon gas jet using self-guided and chirped laser pulses. The self-guided and chirped laser pulses effectively reduced the ionization effects in space and time, producing bright high-order harmonics with narrow bandwidth. The brightness of the 61st harmonic was about 10¹⁵ W/cm²/srad with a bandwidth of 0.7 Å. PACS 42.65.Ky; 42.65.Wi; 32.80.-t; 52.38.-r     

Photoionization mass-spectrometry of benzene and benzaldehyde molecules with an excimer KrF laser

A laser photoionization time-of-flight mass-spectrometer has been developed and used to investigate the photoionization and photofragmentation of benzene and benzaldehyde molecules by an excimer KrF laser radiation at the wavelength of 249 nm in the intensity range from 5·10⁴ to 5·10⁹ W/cm². It has been found that at low laser intensity ions formed by two-step photoionization are most abundant in mass spectra. By increasing laser intensity an extensive fragmentation of molecules, up to C⁺ ions, was observed. The maximum ionization yield of benzaldehyde comes to 0.1% of the number of molecules in the photoionization volume and, according to calculations, to 10% for benzene molecules, when the radiation intensity is 5·10⁹ W/cm².     

High harmonic generation in a laser plasma

The results of investigation of high harmonics of radiation of a Ti:sapphire laser propagating through a laser plasma generated on the surface of different targets are presented. For most of the targets, the intensity distribution of the high harmonics generated is found to form a plateaulike pattern similar to that observed in the case of gas jets. The generation of high harmonics (up to the 65th harmonic, λ = 12.24 nm) is caused by the interaction of femtosecond laser radiation with ions. The conversion efficiency in the plateau region varies from 10^?7 to 8 × 10^?5 depending on the target. The main restriction on the conversion efficiency and the peak intensity of the harmonics generated is caused by the self-defocusing of the femtosecond radiation due to free charge carriers formed as a result of tunnel ionization.