Comparing study of subpicosecond and nanosecond wet etching of fused silica

The effectiveness of the laser induced backside wet etching (LIBWE) of fused silica produced by subpicosecond (600 fs) and nanosecond (30 ns) KrF excimer laser pulses (248 nm) was studied. Fused silica plates were the transparent targets, and naphthalene-methyl-methacrylate (c = 0.85, 1.71 M) and pyrene-acetone (c = 0.4 M) solutions were used as liquid absorbents. We did not observe etching using 600 fs laser pulses, in contrast with the experiments at 30 ns, where etched holes were found. The threshold fluences of the LIBWE at nanosecond pulses were found to be in the range of 360-450 mJ cm⁻² depending on the liquid absorbers and their concentrations. On the basis of the earlier results the LIBWE procedure can be explain by the thermal heating of the quartz target and the high-pressure bubble formation in the liquid. According to the theories, these bubbles hit and damage the fused silica surface. The pressure on the irradiated quartz can be derived from the snapshots of the originating and expanding bubbles recorded by fast photographic setup. We found that the bubble pressure at 460 mJ cm⁻² fluence value was independent of the pulse duration (600 fs and 30 ns) using pyrene-acetone solution, while using naphthalene-methyl-methacrylate solutions this pressure was 4, 5 times higher at 30 ns pulses than it was at 600 fs pulses. According to the earlier studies, this result refers to that the pressure should be sufficiently high to remove a thin layer from the quartz surface using pyrene-acetone solution. These facts show that the thermal and chemical phenomena in addition to the mechanical effects also play important role in the LIBWE procedure.     

Semi-adiabatic Shinnar–Le Roux pulses and their application to diffusion tensor imaging of humans at 7T

The adiabatic Shinnar–Le Roux (SLR) algorithm for radiofrequency (RF) pulse design enables systematic control of pulse parameters such as bandwidth, RF energy distribution and duration. Some applications, such as diffusion-weighted imaging (DWI) at high magnetic fields, would benefit from RF pulses that can provide greater B₁ insensitivity while adhering to echo time and specific absorption rate (SAR) limits. In this study, the adiabatic SLR algorithm was employed to generate 6-ms and 4-ms 180° semi-adiabatic RF pulses which were used to replace the refocusing pulses in a twice-refocused spin echo (TRSE) diffusion-weighted echo planar imaging (DW-EPI) sequence to create two versions of a twice-refocused adiabatic spin echo (TRASE) sequence. The two versions were designed for different trade-offs between adiabaticity and echo time. Since a pair of identical refocusing pulses is applied, the quadratic phase imposed by the first is unwound by the second, preserving the linear phase created by the excitation pulse. In vivo images of the human brain obtained at 7 Testa (7 T) demonstrate that both versions of the TRASE sequence developed in this study achieve more homogeneous signal in the diffusion-weighted images than the conventional TRSE sequence. Semi-adiabatic SLR pulses offer a more B₁-insensitive solution for diffusion preparation at 7 T, while operating within SAR constraints. This method may be coupled with any EPI readout trajectory and parallel imaging scheme to provide more uniform coverage for diffusion tensor imaging at 7 T and 3 T.     

Structural and electronic properties of graphite layers grown on SiC(0 0 0 1)

Photoelectron spectroscopy, low-energy electron diffraction, and scanning probe microscopy were used to investigate the electronic and structural properties of graphite layers grown by solid state graphitization of SiC(0 0 0 1) surfaces. The process leads to well-ordered graphite layers which are rotated against the substrate lattice by 30°. On on-axis 6H-SiC(0 0 0 1) substrates we observe graphitic layers with up to several 100 nm wide terraces. ARUPS spectra of the graphite layers grown on on-axis 6H-SiC(0 0 0 1) surfaces are indicative of a well-developed band structure. For the graphite/n-type 6H-SiC(0 0 0 1) layer system we observe a Schottky barrier height of ?_B,n = 0.3 ± 0.1 eV. ARUPS spectra of graphite layers grown on 8° off-axis oriented 4H-SiC(0 0 0 1) show unique replicas which are explained by a carpet-like growth mode combined with a step bunching of the substrate.     

KrF laser ablation of a polyethersulfone film: Effect of pulse duration on structure formation

Polyethersulfone (PES) films were processed with KrF laser irradiation of different pulse durations (τ). Scanning electron microscopy (SEM) and Raman spectroscopy were employed for the examination of the morphology and chemical composition of the irradiated surfaces, respectively. During ablation with 500 fs and 5 ps pulses, localized deformations (beads), micro-ripple and conical structures were observed on the surface depending on the irradiation fluence (F) and the number of pulses (N). In addition, the number density of the structures is affected by the irradiation parameters (τ, F, N). Furthermore, at longer pulse durations (τ = 30 ns), conical structures appear at lower laser fluence values, which are converted into columnar structures upon irradiation at higher fluences. The Raman spectra collected from the top of the structures following irradiation at different pulse durations revealed graphitization of the ns laser treated areas, in contrast to those processed with ultra-short laser pulses.     

Pulsed laser photodeposition of a-Se nanofilms by ArF laser

Pulsed laser photodeposition from amorphous selenium aqueous colloid solutions using ArF laser radiation at a wavelength of λ = 193 nm has been investigated. Nanometer thick layers were obtained on UV transparent silica substrates in contact with the solution for various photodeposition parameters. Amorphous Se layers, 20 nm thick, were obtained typically by 40 laser pulses of 30 ns duration with a fluence of 50 mJ/cm². Deposition thresholds for depositing 1 nm thick layers were as low as 5 pulses. The deposited nanometer thin surface morphology was analyzed by Evanescent Field Optical Microscopy, Scanning Electron Microscopy and Atomic Force Microscopy. The nanometer thicknesses were evaluated by utilizing the differential evanescent light pattern emanating from the substrates.     

Femtosecond pulsed laser deposition of diamond-like carbon films: The effect of double laser pulses

The bonding structure of carbon films prepared by pulsed laser deposition is determined by the plasma properties especially the change of the kinetic energy. Using double laser pulses the ablation process and the characteristics of the generated plasma can be controlled by the setting of the delay between the pulses. In our experiments, amorphous carbon films have been deposited in vacuum onto Si substrates by double pulses from a Ti:sapphire laser (180 fs, λ = 800 nm, at 1 kHz) and a KrF laser system (500 fs, λ = 248 nm, at 5 Hz). The intensities have been varied in the range of 3.4 × 10¹² to 2 × 10¹³ W/cm². The morphology and the main properties of the thin layers were investigated as a function of the time delay between the two ablating pulses (0-116.8 ps) and as a function of the irradiated area on the target surface. Atomic force microscopy, spectroscopic ellipsometry and Raman-spectroscopy were used to characterize the films. It was demonstrated that the change of the delay and the spot size results in the modification of the thickness distribution of the layers, and the carbon sp²/sp³ bonding ratio.     

Effect of ultrasound treatment on microstructure,colour and carotenoid content in fresh and dried carrot tissue

The aim of the study was to investigate the influence of ultrasound treatment on the carrot tissue microstructure, colour and carotenoids content. To avoid adverse effects of rinsing out substances contained in the raw material, carrot slices were vacuum-packed and treated with ultrasounds in an ultrasonic bath using 21 and 35 kHz frequency for 10, 20 and 30 min. Images of the carrot tissue made by scanning electron microscope were analysed by calculating the cross-section area for each cell. The colour was measured using CIE L^∗a^∗b^∗ method. To determine total carotenoid content spectrophotometric method was used. Obtained results indicate that the structural properties of carrots treated with ultrasound were significantly different from the samples without any treatment and it was clearly noticed during analysing images of scanning electron microscope. There was observed the influence of ultrasound treatment on colour L^∗, a^∗ and b^∗ parameters, especially for carrot treated with ultrasound for 30 min, independent of the applied frequency of the ultrasonic waves. Similarly, sonic treatment resulted in substantial increase of carotenoid in comparison to raw carrot, especially in the case of 35 kHz frequency ultrasounds. Probably, such significant increase is caused by the destruction of the original structure and thus higher extraction ability of these compounds.     

Nonlinear optical properties of an azo-based dye irradiated by picosecond and nanosecond laser pulses

The nonlinear optical properties of an azo-based dye were investigated using Z-scan technique employing 38 ps pulses at 532 and 1064 nm, and 6 ns laser pulses at 532 nm. Large nonlinear absorption and nonlinear refraction were observed at both ps and ns 532 nm in the azoic dye. When excited at ps 1064 nm, this dye displayed a large two-photon absorption cross-section (σ₂=1810 GM). Meanwhile, the optical nonlinearity mechanism was discussed in terms of molecular structure, excitation wavelength, and pulse width.     

Fermi surface and metallic properties of graphite at high pressures

A potential of superconductivity of pure graphite has been theoretically examined. At normal pressure, the carrier concentration is too low to exhibit superconductivity. On applying pressure, the band dispersion along the c-axis is significantly enhanced, resulting in an increase in the carrier concentration; 10²⁰ cm^-3 at p=30 GPa. This is favorable to observe superconductivity. Accurate Fermi surfaces are illustrated: a new Fermi surface appears around K point at p=25 GPa.     

Growth of ultra-thin TiO2 films by spray pyrolysis on different substrates

In the present study TiO₂ films were deposited by spray pyrolysis method onto ITO covered glass and Si (1 0 0) substrates. The spray solution containing titanium(IV) isopropoxide, acetylacetone and ethanol was sprayed at a substrate temperature of 450 °C employing 1-125 spray pulses (1 s spray and 30 s pause). According to AFM, continuous coverage of ITO and Si substrates with TiO₂ layer is formed by 5-10 and below 5 spray pulses, respectively. XPS studies revealed that TiO₂ film growth on Si substrate using up to 4 spray pulses follows 2D or layer-by-layer-growth. Above 4 spray pulses, 3D or island growth becomes dominant irrespective of the substrate. Only 50 spray pulses result in TiO₂ layer with the thickness more than XPS measurement escape depth as any signal from the substrate could not be detected. TiO₂ grain size remains 30 nm on ITO and increases from 10-20 nm to 50-100 nm on Si substrate with the number of spray pulses from 1 to 125.