Metal-insulator transitions in an expanding metallic fluid: particle formation kinetics

Core-level photoemission spectroscopy provides a local probe of expansion dynamics and associated transient chemical properties as a highly pressurized, metallic fluid expands into vacuum following impulsive heating of a semiconductor by an intense, ultrashort laser pulse. Transient photoemission peak shifts reveal that metal-insulator transitions occur rapidly following laser heating. These experiments probe constituents species and solidification kinetics occurring in the early moments of material ejection and provide insight into how particles arise in the current laser ablation regime.     

Enhanced ablation of silica by the superposition of femtosecond and nanosecond laser pulses

We investigate the ablation process in SiO₂ by the superposition of 180 fs laser pulse (_center=800 nm) with a 15 ns laser pulse (_center=532 nm). Compared to femtosecond laser pulses alone, we measured an increase of 270±30% in volume of the ejected material with only a total increase of 40% of lasers fluences. This increase of ablation is the result of thermal and incubation effects generated by the femtosecond laser pulse. PACS 78.20.Nv; 61.80.Ba     

Quantum fog and the degradation of information by the gravitational field

In this paper we discuss how information transferred optically through a gravitational field is degraded as the quanta interact with the madium (vacuum state). We quantify information by means of Shannon's entropy, and consider information carriers that are quanta of some field. Next, we obtain the quantum noise (quantum fog) produced by the gravitational field and derive the appropriate channel capacity formula, which quantifies the maximum amount of information that can be transmitted per pulse, in the face of this noise. We show that the channel capacity formula vanishes if the source of information is a space-time singularity because a very intense noise is produced in the vicinity of the singularity. In other words, space-time singularities are hidden behind a very intense quantum fog and cannot be optically observed. A second consequence is that information is degraded as anisotropies (lumpiness) develop in the universe.     

Laser projection-patterned etching of (100) GaAs by gaseous HCl and CH3Cl

Laser projection-patterned etching of GaAs in a HCl and CH₃Cl atmosphere performed using a pulsed KrF-excimer laser (=248 nm, =15 ns) and deep-UV projection optics (resolution 2 m) is reported. The etching process carried out in a vacuum system having a base pressure of 10^–6 mbar is shown to result from a purely thermochemical reaction. Etching takes place in two steps: (i) between the laser pulses, the etchant gas reacts with the GaAs surface-atomic layer to form chlorination products (mainly As and Ga monochlorides), (ii) local laser surface heating results in the desorption of these products allowing further reaction of the gas with the surface. The influence of the etching parameters (laser energy density, gas pressure and pulse repetition rate) on the etch rate and the morphology of the etched features was studied. Etch rates up to 0.15 nm per pulse, corresponding to the removal of 0.5 GaAs molecular layer, are achieved. The spatial resolution of the etching process is shown to be controlled by the heat spread in the semiconductor and by the nonlinear dependence of the etch rate on the surface temperature. As a result, etched features smaller or larger than the projected features of the photomask are achieved depending on the laser energy density. Etched lines having a width of 1.3 m were obtained at low fluences by the projection of 2 m wide lines onto the GaAs surface.     

Laser-induced CVD of rhodium

Stripes of rhodium metal were deposited by focusing an Ar⁺ laser (514.5 nm) onto glass and polyimide substrates in a heated vacuum cell that contained Rh(CO)₂acac vapor. Stripes were characterized by scanning profilometry, electrical resistivity, SEM and Auger measurements. Most stripes were 100–200 m wide and 1–3 m high. Very broad stripes (>500 m) were deposited when the Rh(CO)₂acac vapor pressure was greater than 1 Torr and when the laser power was more than 200 mW. Stripe resistivities were in general around 30 times that of the bulk material. Auger spectra show the presence of carbon in the stripes.     

A high power N2 laser using water filled strip lines

A fast Blümlein circuit with a water strip line having an impedance of Z=0.078 is used to excite a TE high power nitrogen laser at =337 nm. A pseudospark switch works as a high power switch. The laser operates without any preionization and delivers an energy of 15 mJ per pulse (7.5 ns FWHM) at 10 kPa nitrogen pressure. The shot to shot reproducibility is better than 5%.     

Light absorption in laser-heated cavities

The absorption of laser light in 0.25–1 mm diameter gold cavities, irradiated for the purpose of generating high-temperature blackbody radiation with intense laser radiation of either =0.44 m or =1.3 m wavelength, was investigated. For =0.44 m radiation the absorption exceeded 0.9 for all conditions, but dropped to only 0.3 for the smallest cavities irradiated at =1.3 m. Entrance hole and cavity filling with plasma seems important for the understanding of the observations.     

Compact,wide aperture X-ray preionized XeCl laser with high specific optical power

An X-ray preionized, discharge-pumped XeCl laser with a variable beam cross-section of up to 6×6 cm² is described. It uses flat electrodes and the beam width is determined by X-ray collimation. Its operation characteristics concerning reduced electric field strength (E/p) and X-ray dose are discussed in detail. The inductance of the discharge loop is minimized using a water capacitor arrangement. A very high specific optical power (90 MW/l) is achieved in an active volume of 1.2 l. The pulse energy exceeds 5 J in a 45 ns pulse (FWHM). Komatsu Ltd., Manda 1200, Hiratsuka-shi, Kanagawa 254, Japan Institute of Optics and Fine Mechanics, Department of Plasma Physics, Academia Sinica, Shanghai, P.R. China     

Ablative thresholds in laser cleaning of substrates from particulates

Laser cleaning thresholds based on the local ablation of substrate material are studied theoretically. Results are compared with the experimental data on the cleaning of silicon wafers from spherical silica particles using laser wavelengths at 248, 532 and 1064 nm. Calculations take into account local enhancement in the laser-light intensity and are based on analytical solutions for the temperature distribution. Influence of vapor atmosphere on cleaning thresholds is studied experimentally and theoretically. Here cleaning is assisted by explosive vaporization of capillary condensed water. A possibility to increase the window for damage-free cleaning by varying the pulse duration and laser wavelength is also discussed. PACS 42.62.Cf; 81.65.Cf; 68.35.Np     

Electron in an Ultrashort Laser Pulse

After the classical approach to acceleration of a charged particle by -form impulsive force, we consider the corresponding quantum theory based on the Volkov solution of the Dirac equation. We determine the modified Compton formula for frequency of photons generated by the scattering of the -form laser pulse on the electron in a rest frame.     

Pulse duration dependence of laser damage mechanisms

A number of mechanisms for internal laser damage in transparent dielectrics are examined for pulse duration dependence and the results are compared with experimental measurements when possible. The differences in dependence on pulse duration and other variables from one mechanism to another suggest that different mechanisms may initiate damage in different parameter ranges. Experimental identification of these mechanisms will be aided by measuring the pulse duration dependence of the damage threshold.Symbols bulk absorption coefficient - thermal volume expansion coefficient - C heat capacity per unit volume - c velocity of light in vacuum - D thermal diffusivity - E instantaneous electric field strength - E ₀ electric field amplitude - e charge on an electron - e spectral emissivity at the laser wavelength - _c band gap energy - _d energy density (energy per unit cross-sectional area) - _e electron energy - k volume compressibility - k wave vector - L sample length - L _f focusing length - wavelength in vacuum - M multiplication factor to correct particle cross-section for Mie effect - m ^* electron effective mass - N conduction electron number density - N _b value ofN required for breakdown - n number of photons absorbed inn-photon ionisation - n refractive index - n ₀ original refractive index - n ₂ coefficient for quadratic term in expression for refractive index - n change in refractive index - P _d power density (power per unit cross-sectional area) - R inclusion radius - R _s sample radius - r distance from centre of inclusion - r _b beam radius - S breaking strength, damaging stress - T temperature - t time - t time required to accelerate an electron to the energy of the conduction band - t pulse duration - time required for acoustic wave to propagate across the beam - _e conduction electron lifetime - U velocity - angular velocity (frequency times 2)     

Monitoring nanoparticle formation during laser ablation of graphite in an atmospheric-pressure ambient

Excimer laser ablation of highly oriented pyrolytic graphite (HOPG) was performed at atmospheric pressure in an N₂ and in an air ambient. During the ablation, nanoparticles condensed from the material ejecta, and their size distribution was monitored in the gas phase by a Differential Mobility Analyzer (DMA) in combination with a Condensation Particle Counter (CPC). Size distributions obtained at different laser repetition rates revealed that the interaction between subsequent laser pulses and formed particles became significant above 15 Hz. This interaction resulted in laser heating, leading to considerable evaporation and a decrease in the size of the particles. X-ray photoelectron spectroscopy revealed that approximately 8% nitrogen was incorporated into the CN_x particles generated in the N₂ ambient, and that the nitrogen was mostly bonded to sp³-hybridized carbon. Monodisperse particles were also deposited and were analyzed by means of Raman spectroscopy to monitor size-induced effects. PACS 81.07.-b; 61.46.+w; 79.70.+q     

Dynamics of the ejected material in ultra-short laser ablation of metals

A molecular dynamics model is applied to study the formation and the early stages of ejection of material in ultra-short laser ablation of metals in vacuum. Simulations of the ablation process for iron at a pulse duration of 0.1 ps and at different laser fluences are performed. Different features of the ejection mechanism are observed below, near, and above the ablation threshold. The last is estimated as approximately 0.1 J/cm². The structure of the ablated material is found to depend on the applied laser fluence. The expanded plume consists mainly of large clusters at fluences near to the threshold. With the increase of the laser fluence the presence of the large clusters decreases. Clear spatial segregation of species with different sizes is observed in the direction normal to the surface several tens of picoseconds after the laser pulse onset. The angular distribution of the ejected material is estimated for different regimes of material removal. Above the ablation threshold the distribution is forward peaking. PACS 79.20.Ds; 52.38.Mf; 02.70.Ns; 81.05.Bx     

Structure formation on titanium during oxidation induced by cumulative pulsed Nd:YAG laser irradiation

We investigated local oxidation and surface structure development of Ti targets under multi-pulse, high-repetition-rate Nd:YAG ( =1.064 m, 300 ns, =30 kHz) laser irradiation in air at atmospheric pressure. The experiments were performed at laser intensity levels below the single-laser-pulse melting threshold of Ti. The morphology of the irradiated areas was studied by scanning electron microscopy and profilometry. The variation of the oxide compositions and the crystalline state with increasing laser pulse number was analysed by X-ray diffractometry. Besides the known phenomena related to laser irradiation and oxidation of metal surfaces (micro-crack or pore formation), we evidenced new morphological features such as droplet-like structures inside the surface micro-cracks and micro-columns, and with increasing laser pulse number the formation of a dome-shaped structure over the whole irradiated zone. The occurrence of melting under multi-pulse irradiation was associated with the rise in the surface temperature enhanced by the oxidation, and the progressive evolution of the surface structures was associated with the consecutive melting–solidification processes as well as with the different stages of oxidation. PACS 61.80.Ba; 68.37.Hk; 81.65.Mq     

Determination of the Coefficient of Laser Radiation Reflection from Dielectric Targets in Vacuum

We present the results of experimental determination of the coefficients of laser radiation reflection (₁ 10.6 m and ₂ 1.06 m) from dielectric targets of complex chemical composition in vacuum with allowance for a regime of developed plasma formation.     

Search for giant Franz-Keldysh-like effects in GaSe and other layered semiconductors

Recently, an anomalously large redshift of the absorption edge with electric field was claimed for -GaSe_1–x S _x layered crystals. We have studied Bridgman grown -GaSe crystals as well as vapor transport grown -GaS, and 2H-WSe₂. While we have observed a shift in the absorption edge for -GaSe similar to that reported in previous work, our results demonstrate that the redshift arises from Joule heating, and is thus temperature induced, rather than intrinsic. For -GaS, much larger resistivities virtually eliminate Joule heating, and our measurements of the electric field induced absorption edge shift yield an estimated upper limit of approximately 0.04 meV · cm/kV for a field of 2.4×10³ kV/cm, in good agreement with the theoretical value expected for the Franz-Keldysh effect.Also at University of Konstanz     

Mössbauer Study of the Thermal Behaviour of Garnets Used in High-Energy Water Jet Technologies

A high-energy water jet combined with silicate garnets as abrasives has been proven to be a powerful tool for disintegration of hard materials. Thermal heating of the garnets is one way for structure improvement of the abrasive material. Room-temperature Mössbauer spectra of initial powdered almandine samples are characterised by one doublet corresponding to Fe²⁺ in dodecahedral position 24c. Almandine garnet, industrial product Barton HP 80 as reference material in all experiments, has a second doublet corresponding to Fe³⁺ in octahedral position 16a. In room-temperature spectra of heated almandine garnet samples from locality Ktí and Mdnec (heated under temperatures 200–1000°C by 100 degrees for 1 hour in air) a new doublet originating from -Fe₂O₃ nanoparticles appeared. Under a heating temperature of higher than 800°C, the broad sextets of -Fe₂O₃ and -Fe₂O₃ in spectra were discovered. No additional doublets or sextets appeared in room-temperature Mössbauer spectra of reference material of almandine garnet Barton HP 80 heated under each temperature.     

Aluminium electron-phonon relaxation-time measurement from subpicosecond nonlinear single-photon photoelectric emission at 248 nm

The sensitivity of photoelectric emission of polycrystalline aluminium, produced by 248 nm laser pulses with p-polarization and 450 fs duration, under incidence angles = 74–86°, has been measured. A nonlinear increase of photoemission efficiency, as a function of the incident laser peak intensity in the range of 1–50 GW/cm², was displayed, which confirms earlier observations with gold and tungsten. This nonlinearity is consecutive to the non-thermal distribution of electron gas of laser-heated metal on the time scale of the electron-phonon relaxation time. Analysis of experimental data, using the model previously developed by us [1], gives a value of electron-phonon relaxation time 0.55 ± 0.11 ps.     

Superradiating 4P-4S-3P cascade of sodium vapour arising on the leading edge of the exciting laser pulse

Investigations of the superradiating cascade of sodium vapour 4P-4S-3P ₁ = 2.21 µm and ₂=1.14 µm) arising on the leading edge of the exciting laser pulse were carried out. The dependences of the actual delay time _D of the ₁ pulse on the population rise time of the laser-excited 4P state were measured and compared with those calculated following the existing theoretical model. The dependence of the actual delay time _D on the inverse density of excited atoms 1/N* is also presented. Analysis of this dependence revealed the influence of the Doppler dephasing and of the second, ₂, transition on the formation of the ₁ superradiance.     

Width reduction of laser microdrillings by subsequent mechanically induced plastic deformation

This work reports a novel process combination, which is capable of forming large hole matrices (100 pores/mm²) in thin stainless steel foils (10 mfoil<300 m) maintaining high processing rates and pore widths smaller than 5 m. This technique perforates stainless steel foils with high-speed on-the-fly laser perforation (60000 drillings/min) followed by a cold-roll forming of the laser-treated foil. The cold-roll forming leads to a pore-size reduction (in one dimension) perpendicular to the rolling direction. For the laser processing a diode-pumped, q-switched and frequency-tripled Nd:YAG laser (=355 nm, =30 ns), and a flashlamp-pumped Nd:YAG laser (=1064 nm, =57 s) were applied. Using this process combination, minimum pore sizes of 3.5 m have been achieved. At present, the processing efficiency of cold-rolled percussion drillings inserted with nanosecond pulse durations is lower in comparison with single-pulse on-the-fly perforation, but in terms of quality (straight pore channel, low standard deviation of pore widths and pore widths smaller than 5 m) well suited for various fields in filtration (e.g. particle removal) . PACS  81.20.Hy; 42.62.Cf; 81.40.Ef