Expansion velocities of 0.5 ps KrF excimer laser induced plasma by Doppler-shift analysis of pump and probe measurements

High gradient laser plasma is formed by focused KrF laser pulses (248.3 nm, 450 fs, 10¹³ W/cm²) on liquids (water, styrene) and solids (silicon, aluminum, and polyimide). The hydrodynamic expansion of the plasma was studied by measuring the blue Doppler-shift of reflected probe pulses which was produced by a delayed dye laser (496.6 nm, 450 fs). The Doppler-shift corresponds to the velocity of the reflecting surface of the plasma which is defined by the critical electron density. Expansion is investigated as a function of delay time and laser intensity. The reflecting surface of the plasma accelerates over 1–2 ps after the onset of the ablating laser pulse. With increasing intensity up to 2×10¹³ W/cm² the maximum average velocities are monotonously increasing up to 1–2×10⁵ m/s. PACS 52.38.Kd; 52.50.Jm, 52.70.Kz     

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.     

Generation of monoenergetic ultrashort electron pulses from a fs laser plasma

Ultrashort high-energy electron beams are generated by focusing fs Ti:sapphire laser pulses on a thin metal tape at normal incidence. At laser intensities above 10¹⁶ W/cm² , the fs laser plasma ejects copious amounts of electrons in a direction parallel to the target surface. These electrons are directly detected by means of a backside illuminated X-ray CCD, and their energy spectrum is determined with an electrostatic analyzer. The electrons were observed for two laser polarization directions, parallel and perpendicular to the observation direction. At the maximum applied intensity of 2×10¹⁷ W/cm², the energy distribution peaks at around 35 keV with a hot tail detectable up to about 300 keV. The number of electrons per shot at 35 keV is about 5×10⁸ per sterad per keV. Quasi-monoenergetic electron pulses with a relative energy spread of 1% were produced by using a 50-m slit in the beam path after the analyzer. This approach offers great potential for time-resolved studies of plasma, liquid, and surface structures with atomic-scale spatial resolution. PACS 41.75.Fr; 52.38.Kd; 52.70.Nc     

Femtosecond-pulse laser ablation of human corneas

A femtosecond pulse laser in the visible spectral region shows promise as a potentially new powerful corneal sculpting tool. It combines the clinical and technical advantages of visible wavelengths with the high ablation quality observed with nanosecond-pulse excimer lasers at 193 nm. A femtosecond and a nanosecond dye laser with pulse durations of 300 fs and 7 ns, and centre wavelengths at 615 nm and 600 nm, respectively, both focused to an area of the order of 10^–5 cm², have been applied to human corneal ablation. Nanosecond laser pulses caused substantial tissue disruption within a 30–100 m range from the excision edge at all fluences above the ablation threshold of F _th60 J cm^–2 (I _th9 GW cm^–2). Completely different excisions are produced by the femtosecond-pulse laser: high quality ablations of the Bowman membrane and the stroma tissue characterised by damage zones of less than 0.5 m were observed at all fluences above ablation threshold of F _th1 J cm^–2 or I _th3 TW cm^–2 (3×10¹² W cm^–2). The transparent cornea material can be forced to absorb ultrashort pulses of extremely high intensity. The fs laser generates its own absorption by a multiphoton absorption process.     

Processing of W/Si and Si/W bilayers and multilayers with single and multiple excimer-laser pulses

Interdiffusion phenomena, thermal damage and ablation of W/Si and Si/W bilayers and multilayers under XeCl-excimer laser (=308 nm) irradiation at fluences of 0.15, 0.3 and 0.6 J/cm² were studied. Samples were prepared by UHV e-beam evaporation onto oxidized Si. The thickness of W and Si layers and the total thickness of the structures were 1–20 nm and 40–100 nm, respectively. 1 to 300 laser pulses were directed to the same irradiation site. At 0.6 J/cm² the samples were damaged even by a single laser pulse. At 0.3 J/cm² WSi₂ silicide formation, surface roughening and ablation were observed. The threshold for significant changes depends on the number of pulses: it was between 3–10 pulses and 10–30 pulses for bilayers with W and Si surfaces, respectively, and more than 100 pulses for multilayers with the same total thickness of tungsten. At 0.15 J/cm² the periodicity of the multilayers was preserved. Temperature profiles in layered structures were obtained by numerical simulations. The observed differences of the resistance of various bilayers and multilayers against UV irradiation are discussed.     

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     

Control of cluster multielectron ionization in ultraintense laser fields

We performed classical molecular dynamics simulations to explore the controllability of the inner ionization process in Xe_n clusters (n = 2?2171), driven by ultraintense infrared Gaussian laser fields (peak intensity I _M = 10¹⁵?10¹⁸ W cm^?2, temporal pulse length τ = 10?100 fs, and frequency ν = 0.35 fs^?1). Controllability of ion charge abundances and of their spatial distributions inside the cluster emerges from the different pulse length dependences of classical barrier suppression ionization (BSI) and of electron impact ionizations (EII), as well as from the time scale of the Coulomb explosion (CE). For large clusters (Xe₂₁₇₁), low intensities (10¹⁵ W cm^?2), and long pulses (τ = 100 fs), EII is the dominating ionization channel, which favors the formation of maximum charged ions (Xe¹⁰⁺, Xe¹¹⁺) in the cluster center. In contrast, BSI forms an inverse radial charge ordering with the highest charges in the exterior cluster shells. This suggests that the production of the two inverse radial charge distributions with an equal average ion charge can be forced by the choice of multiple pulses with different intensities and pulse lengths. At high intensities (10¹⁷?10¹⁸ W cm^?2), where EII is insignificant and CE sets in much earlier, the BSI radial charge ordering and the enhancement of the ion charges beyond the single-atom limit by the ignition effect is observed only for short pulses.     

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.     

Nonlinear transformation of a spectrum of femtosecond laser pulses in a gas-filled microcapillary

Features of light pulse propagation and nonlinear optical transformation of the spectrum generated by titanium-sapphire laser pulses (τ_0.5 = 27 fs, λ₀ = 790 nm) have been studied experimentally in a 50-cm cylindrical hollow waveguide (microcapillary with 280-μm diameter core) filled with gaseous molecular nitrogen and helium. Stable guided propagation of light pulses with an intensity of ~1.5⋅10¹⁴ W/cm² in the fundamental EH₁₁ mode of the gas-filled capillary has been demonstrated. Exact focusing of the laser light made it possible to obtain rather high relative (≥95%) and absolute (~60%) energy transmission efficiencies for the pulses at gas pressures equal to or lower than 760 Torr. A method to determine the nonlinear phase shift of the pulses has been proposed. Values of the nonlinear refractive index n₂ ≈ 4.5⋅10^–23 cm²/(W⋅Torr) (N₂) and n₂ ≈ 2.8⋅10^–23 cm²/(W⋅Torr) (He) have been found. A short-wavelength shift in addition to the Kerr nonlinearity has been shown to be contributed by the generated electron plasma at high pulse intensities (≥10¹⁴ W/cm²).     

A comparative study of the ionic keV X-ray line emission from plasma produced by the femtosecond,picosecond and nanosecond duration laser pulses

We report here an experimental study of the ionic keV X-ray line emission from magnesium plasma produced by laser pulses of three widely different pulse durations (FWHM) of 45 fs, 25 ps and 3 ns, at a constant laser fluence of ∼1.5 × 10⁴ J cm^− 2. It is observed that the X-ray yield of the resonance lines from the higher ionization states such as H- and He-like ions decreases on decreasing the laser pulse duration, even though the peak laser intensities of 3.5 × 10¹⁷ W cm^− 2 for the 45 fs pulses and 6.2 × 10¹⁴ W cm^− 2 for the 25 ps pulses are much higher than 5 × 10¹² W cm^− 2 for the 3 ns laser pulse. The results were explained in terms of the ionization equilibrium time for different ionization states in the heated plasma. The study can be useful to make optimum choice of the laser pulse duration to produce short pulse intense X-ray line emission from the plasma and to get the knowledge of the degree of ionization in the plasma.     

Sub-100-nm laser-induced periodic surface structures upon irradiation of titanium by Ti:sapphire femtosecond laser pulses in air

The formation of laser-induced periodic surface structures (LIPSS) on titanium upon irradiation with linearly polarized femtosecond (fs) laser pulses (τ=30 fs, λ=790 nm) in an air environment is studied experimentally and theoretically. In the experiments, the dependence on the laser fluence and the number of laser pulses per irradiation spot has been analyzed. For a moderate number of laser pulses (N<1000) and at fluences between ～0.09 and ～0.35 J/cm², predominantly low-spatial-frequency-LIPSS with periods between 400 nm and 800 nm are observed perpendicular to the polarization. In a narrow fluence range between 0.05 and 0.09 J/cm², high-spatial-frequency-LIPSS with sub-100-nm spatial periods (～λ/10) can be generated with an orientation parallel to the polarization (N=50). These experimental results are complemented by calculations based on a theoretical LIPSS model and compared to the present literature.     

Ablation induced by single-and multiple-femtosecond laser pulses in lithium niobate

We present results on the surface-damage threshold of lithium niobate after single-and multiple-femtosecond laser pulse irradiation at the pulse duration of 80 fs, with a 800-nm wavelength, and a repetition rate of 1 kHz. The surface-ablation threshold was found to decrease significantly with an increase in the pulse number applied to the surface until reaching an almost constant level due to an incubation effect, which is attributed to the laser-induced defect formation. The threshold of lithium niobate under a single shot is found to be 2.82 J/cm², and the threshold fluence for an infinite number of pulses F _th(∞) converges to a common value of 0.52 ± 0.06 J/cm² for N > 80. The results have the potential for application in laser micromachining and the fabrication of related optical devices and applications in frequency conversion by a femtosecond laser in lithium niobate.     

Femtosecond laser ablation of cemented carbides: properties and tribological applications

Laser ablation with fs laser pulses was performed in air on cobalt cemented tungsten carbide by means of a Ti : sapphire laser (800 nm, 100 fs). Small and moderate fluences (2, 5, 10 J/cm²) and up to 5×10⁴ pulses per irradiated spot were used to drill holes with aspect ratios up to 10. Cross-section cuts from laser-irradiated samples were produced and they were analysed with optical microscopy and SEM. EDX analyses were carried out on selected zones. Quasi-cylindrical holes were found for 2 J/cm², whereas for 5 and 10 J/cm² irregular shapes (lobes, bottoms wider than hole entrances) were found to occur after a given number of incident pulses. Layers with modified structure were evidenced at pore walls. SEM revealed a denser structure, while EDX analyses showed uniform and almost similar contents of W, C, and Co in these layers. As a direct application, patterning of coated WC-Co was carried out with 2 J/cm² and 100 pulses per pore. The resulted surfaces were tribologically tested and these tests revealed an improved friction and wear behaviour. PACS 42.65.Cs; 79.60.Ds     

Comparison of simulated X-ray conversion efficiency of laser produced iron plasma with experimental results

X-ray resonance lines between 11 Å and 17 Å emitted from iron plasmas created by a modest KrF laser have been simulated by modifying the atomic and hydrodynamic code EHYBRID. Free–free and free–bound emission from the Si-, Al-, Mg-, Na-, Ne- and F-like ions is calculated in the simulation. In the original experiments, a KrF laser (249 nm wavelength) with focused irradiances between 1×10¹² W/cm² and 1×10¹⁵ W/cm² was focused on iron targets. The laser pulse duration was varied between 10 ps and 20 ns. We have calculated X-ray conversion efficiencies to be, for example, 0.5% over 2 sr for 2×10¹³ W/cm² and 20 ns pulse duration, in good agreement with experimental measurements. The simulation of X-ray emission is also presented for an experiment where a train of eight 7 ps KrF laser pulses is incident onto an iron target. PACS 52.50.Jm; 52.38.Ph; 52.65.Kj; 52.30.Ex; 32.30.Rj     

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     

Strong optical fields induce ultrafast rearrangement of H-atoms in ethanol molecules

H-atoms in C₂H₅OH are rearranged by strong optical fields generated by intense, 100 fs long infrared laser pulses to form new bonds that lead to the H ₃ ⁺ molecular ion. This observation appears to be against the expectation that exposure of molecules to intensities of the order of 10¹⁵ W cm^?2 inevitably lead to multiple ionization of molecules followed by instantaneous Coulomb explosion into fragments. The polarization dependence of the H ₃ ⁺ signal and of the energy content of H ₃ ⁺ ions lead to believe that H-atom rear-rangement in ethanol occurs within a single 100 fs pulse.     

High-harmonic generation and plasma radiation from water microdroplets

This work studied the emission of XUV radiation from water microdroplets under excitation with either a single or a pair of intense femtosecond laser pulses (Ti:Sa, 80 fs, 10¹⁴ W/cm² , 800 nm, 1 kHz). When varying the delay between the two pulses a transition from pure incoherent plasma emission to coherent high-harmonic generation was observed. Under optimized conditions high-harmonic radiation up to the 27th order was obtained. PACS 33.80.-b; 39.10.+j; 42.65.ky; 52.50.Jm     

Studies on excimer laser doping of GaAs using sulphur adsorbate as dopant source

Excimer laser doping of GaAs using sulphur adsorbate as a dopant source is demonstrated. Box-like n-type layers of depths of about 100 nm with carrier concentration as high as (23)×10¹⁹ cm^–3 are formed. Passivation of GaAs using a (NH₄)₂S_x solution for 40 min followed by sublimation of the excess sulphur atoms in high vacuum result in an effective dopant for controllable n-type doping. The samples are irradiated using a KrF excimer laser in a N₂ gaseous environment. Secondary ion mass spectrometry (SIMS) measurements show that sulphur is successfully incorporated in the GaAs. The sheet resistance is controlled by adjusting the laser energy fluence and number of laser pulses. Rutherford backscattering spectrometry with channeling (RBS/C) alignment measurement indicates that lattice damage is undetectable for N₂ gas pressures of 760 Torr.     

Observation of X-ray spectra from nitrogen clusters irradiated with high-intensity ultrashort laser pulses

X-ray spectra from nitrogen clusters irradiated with ultrashort laser pulses are reported. The line spectra of H-like and He-like nitrogen ions have been observed by irradiation with 100 fs, 800 nm pulses at 7×10¹⁷ W/cm² irradiance. The generation of highly charged ions of N⁶⁺ and N⁷⁺ is explained by the optical field-induced ionization and the subsequent collisional ionizations in the clusters. The He-δline has anomalously high brightness compared to other He-like lines. It is ascribed to charge exchange, which preferentially populates the n=5 level of N⁵⁺. Received: 7 October 1999 / Published online: 23 February 2000     

Flexible gratings fabricated in polymeric plate using femtosecond laser irradiation

Flexible gratings embedded in poly-dimethlysiloxane (PDMS) were fabricated using femtosecond laser pulses. Photo-induced gratings in a flexible PDMS plate were directly written by a high-intensity femtosecond (130 fs) Ti: Sapphire laser (λ_p=800 nm). Refractive index modifications with 4 μm diameters were photo-induced after irradiation of the femtosecond pulses with peak intensities of more than 1×10¹¹ W/cm². The graded refractive index profile was fabricated to be symmetric around the center of the focal point. The diffraction efficiency of the grating samples is measured by an He-Ne laser. The maximum value of refractive index change (Δn) in the laser-modified regions was estimated to be approximately 3.17×10⁻³.