Accumulation and Release of Self-Field Generated Vortices in Percolative YBa2Cu3O7-δ-Films

Pulsed laser heating of current biased percolative YBa₂Cu₃O_7--films can be used to generate and release self-field induced vortices. A laser pulse yields a voltage signal due to penetration of bias current induced magnetic flux in absence of an external magnetic field. Upon retooling, strongly pinned vortices remain in the film. These remanent vortices have been detected after disconnecting the bias source in the currentless film. Applying a subsequent second laser pulse, again yields a signal voltage but of inverse sign due to magnetic flux redistribution.     

Effect of acoustic waves on the repetition frequency of high repetition rate TEA-CO2 lasers and calculating optimization condition

The gas channel of a pulse periodic TEA-CO₂ laser is considered as an acoustic resonator. In this paper, a three-dimensional mathematical modeling has been considered for describe of laser action. By calculating of the equations obtained from this model, the effects of cavity dimensions, Mach number and repetition frequency of laser on the acoustic wave spectrum have been investigated. At last optimum conditions for performance of laser operation has been arrived.     

Output characteristics of a 400 MW Nd:glass laser system

The paper discusses the performance characteristics of a Nd: glass laser system designed for laser produced plasma studies. It consists of aQ-switched oscillator followed by two amplifier stages. The output behaviour of the oscillator, i.e. laser pulse duration, peak power and optimum coupling, has been studied and is in good agreement with theory. Gain characteristics of amplifiers were obtained as a function of various parameters. Energies in excess of 7 J with pulse durations as small as 18 nsec were obtained giving rise to 400 MW peak power.     

Ultraviolet photovoltaic characteristic of MgB2 thin film

Fast photoelectric effects have been observed in MgB₂ thin film fabricated by chemical vapour deposition. The rise time was $\sim $10 ns and the full width at half-maximum was \sim185\,ns for the photovoltaic pulse when the film was irradiated by a 308\,nm laser pulse of 25\,ns in duration. X-ray diffraction and the scanning electron microscope revealed that the film was polycrystalline with preferred c-axis orientation. We propose that nonequilibrium electron--hole pairs are excited in the grains and grain boundary regions for MgB₂ film under ultraviolet laser and then the built-in electric field near the grain boundaries separates carriers, which lead to the appearance of an instant photovoltage.     

T1ρ-weighted MRI using a surface coil to transmit spin-lock pulses

T1rho-weighted MRI is a novel basis for generating tissue contrast. However, it suffers from sensitivity to B1 inhomogeneity. First, excitation with a spatially varying B1 causes flip-angle artifacts and second, spin locking with an inhomogeneous B1 results in non-uniform T1rho contrast. In this study, we overcome the former complication with a specially designed spin-locking pulse sequence and we successfully obtain T1rho-weighted images with a surface coil. In this pulse sequence, the spin-lock pulse was divided into segments of equal duration and alternating phase. This "self-compensating" T1rho-preparatory pulse sequence was analyzed and the effect of an inhomogeneous B1 field was simulated using the Bloch equations. T1rho-weighted MR images of a phantom and a human knee joint in vivo were obtained on a clinical scanner with a surface coil to demonstrate the utility of the pulse sequence. The self-compensating T1rho-prepared pulses sequence resulted in substantially reduced image artifacts compared to the conventional, single-phase spin-lock pulse.     

Enhanced field emission from LaB6 thin films with nanoprotrusions grown by pulsed laser deposition on Zr foil

Lanthanum hexaboride (LaB₆) films have been deposited on a zirconium foil by pulsed laser deposition method. The field emission studies of the LaB₆ deposited film have been performed in the planar diode configuration under ultra high vacuum conditions. The Fowler-Nordheim plots were found to be linear in accordance with the quantum mechanical tunneling phenomenon. A typical field emission current of 7.02 μA was drawn at an applied electric field of 2 V/μm. The field enhancement factor is calculated to be 8913 cm⁻¹, indicating that the field emission is from nanoscale protrusions present on the emitter surface. The atomic force microscope (AFM) investigation of the surface clearly shows the conical shaped nanoprotrusions of few hundred nanometers with asperities of 20-40 nm on its top. The emission current-time plot recorded at the pre-set value of emission current of 5 μA over a period of more than 3 h exhibits an initial increase and subsequent stabilization of the current. The results reveal that the LaB₆/Zr field emitter obtained by the pulsed laser deposition (PLD) is a promising cathode material for practical applications in field emission-based devices.     

An electrohydrodynamics model for non-equilibrium electron and phonon transport in metal films after ultra-short pulse laser heating

The electrons and phonons in metal films after ultra-short pulse laser heating are in highly non-equilibrium states not only between the electrons and the phonons but also within the electrons. An electrohydrodynamics model consisting of the balance equations of electron density, energy density of electrons, and energy density of phonons is derived from the coupled non-equilibrium electron and phonon Boltzmann transport equations to study the nonlinear thermal transport by considering the electron density fluctuation and the transient electric current in metal films, after ultra-short pulse laser heating. The temperature evolution is calculated by the coupled electron and phonon Boltzmann transport equations, the electrohydrodynamics model derived in this work, and the two-temperature model. Different laser pulse durations, film thicknesses, and laser fluences are considered. We find that the two-temperature model overestimates the electron temperature at the front surface of the film and underestimates the damage threshold when the nonlinear thermal transport of electrons is important. The electrohydrodynamics model proposed in this work could be a more accurate prediction tool to study the non-equilibrium electron and phonon transport process than the two-temperature model and it is much easier to be solved than the Boltzmann transport equations.     

超短超强激光脉冲辐照超薄碳膜电离状态研究

为了进一步理解极端条件下物质的电离特性, 特别是超短超强激光脉冲辐照超薄靶时等离子体的形成与分布, 本文以超薄碳膜为例, 细致研究了超短超强激光脉冲辐照下原子的离化过程. 分析和比较了强激光场直接作用电离和靶内静电场电离等两种场致电离形式, 在碰撞电离可以忽略的情况下, 发现更多的电离份额是来自靶内静电场的电离方式. 研究了激光脉冲强度对电离的影响, 发现激光脉冲强度越强, 电离速度越快, 产生的高价态离子所占比例也越高.当激光强度为1×10²⁰ W/cm²时, 尽管该强度高于电离生成C⁺⁶所需要的激光强度阈值, 但该激光脉冲并不能将整个靶电离成C⁺⁶离子, 对此本文进行了详细的分析. 在研究激光脉冲宽度的影响时, 发现激光脉宽越小, 电离速度越快, 但越小的激光脉冲电离获得的高价态离子越少.     

Ultrafast solid–liquid–vapor phase change of a gold film induced by pico- to femtosecond lasers

Melting, vaporization and resolidification processes of thin gold film irradiated by a femtosecond pulse laser are studied numerically. The nonequilibrium heat transfer in electrons and lattice is described using a two-temperature model. The solid–liquid interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. An iterative procedure based on energy balance and gas kinetics law to track the location of liquid–vapor interface is utilized to obtain the material removal by vaporization. The effect of surface heat loss by thermal radiation was discussed. The influences of laser fluence and duration on the evaporation process are studied. Results show that higher laser fluence and shorter laser pulse width lead to higher interfacial temperature, deeper melting and ablation depths.     

脉冲激光沉积法制备的ZnO薄膜的低阈值电抽运紫外随机激射

分别采用直流反应溅射法和脉冲激光沉积法在硅衬底上沉积ZnO薄膜, 用X射线衍射、扫描电镜、光致发光谱等手段对两种方法沉积的ZnO薄膜的结晶状态、 表面形貌和光致发光等进行了表征. 进一步对比研究了以上述两种方法制备的ZnO薄膜作为发光层的金属-绝缘体-半导体结构器件的电抽运紫外随机激射. 结果表明, 与以溅射法制备的ZnO薄膜作为发光层的器件相比, 以脉冲激光沉积法制备的ZnO薄膜为发光层的器件具有更低的紫外光随机激射阈值电流和更高的输出光功率. 这是由于脉冲激光沉积法制备的ZnO薄膜中的缺陷更少, 从而显著地减少了紫外光在光散射过程中的光损耗. 关键词： 随机激射 ZnO薄膜 脉冲激光沉积 溅射     

Diode-pumped doubly Q-switched mode-locked YVO4/Nd:YVO4 laser with AO and GaAs saturable absorber

By using both acousto-optic (AO) modulator and GaAs saturable absorber, a diode-pumped doubly Q-switched and mode-locked (QML) YVO₄/Nd:YVO₄ laser is presented. The average output power and the pulse width of the Q-switched envelope have been measured. The Q-switch pulse energy of the doubly QML laser are higher than that only with GaAs. The stability of the QML laser with the dual-loss-modulation is significantly improved if compared to that only with GaAs.The experimental results show that the doubly QML YVO₄/Nd:YVO₄ laser has nearly 80% modulation depth and deeper than that of the singly passively QML pulse. The doubly Q-switched mode-locked pulse inside the Q-switched envelope has a repetition rate of 111 MHz and its pulse width is estimated to be about 700 ps. By using a hyperbolic secant square function and considering the Gaussian distribution of the intracavity photon density, the coupled equations for diode-pumped dual-loss-modulated QML laser is given and the numerical solutions of the equations are in good agreement with the experimental results.     

Ultrafast temperature relaxation evolution in Au film under femtosecond laser pulses irradiation

Ultrafast temperature relaxation processes in Au film including two temperature relaxation and thermal diffusion relaxation with femtosecond laser pulse excitation were investigated numerically by Finite Element Method (FEM). With the temperature dependent thermal parameters, the full 2D temperature field evolution in picosecond and nanosecond domains were obtained. It is proposed that the heat transfer depth can be alternatively localized or enhanced by the distinct temperature relaxation mechanisms. Moreover, the effect of laser parameters and Au film thickness and surface reflectivity on the two temperature relaxation time were analysed.     

Control of the shape and duration of a laser pulse in the case of nonlinear total internal reflection

A nonlinear device based on nonlinear total internal reflection is suggested which provides a more than ten-fold shortening of the laser pulse and smooth variation of the pulse duration due to double reflection from the interface between transparent and absorbing media. Results are presented for the shape and duration of the pulse components that have been reflected and transmitted by the interface. B. I. Stepanov Institute of Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., 220072, Minsk, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 3, pp. 325–328, May–June, 1997.     

Electrical properties and memory effect in the field effect transistor based on organic ferroelectric insulator and pentacene

We have fabricated a field effect transistor (FET) based on an organic ferroelectric insulator and molecular conductor, and investigated the electrical properties and memory effects on the PEN-FET. We have observed a drastic change in the drain current at around the coercive electric fieldE _c of the organic ferroelectric insulator in not only a FET (PEN-FET) based on a pentacene (PEN) film but also a FET (IPEN-FET) based on an iodine doped PEN film. The magnitude of the change of the drain current for the IPEN-FET is 200 times larger than that for the PEN-FET. It is expected from these results that the PEN-FET (especially the IPEN-FET) is an improvement in such devices, since it operates at a low gate electric field accompanied by the appearance of the spontaneous polarization in the organic ferroelectric insulator. In addition, we have found that the drain current for the PEN-FET does not return to the initial drain current ofE _G =0 V/cm for more than one week, even if the gate electric field is changed to 0 V/cm from 500 V/cm(>E _c ). From these results, it is suggested that the PEN-FET becomes a memory device.     

Laser-induced voltage effects in Ca3Co4O9 thin films on tilted LaAlO3（001） substrates grown by chemical solution deposition

Laser-induced voltage effects in c-axis oriented Ca3Co4O9 thin films have been studied with samples fabricated on 10 tilted LaAlO3(001) substrates by a simple chemical solution deposition method. An open-circuit voltage with a rise time of about 10 ns and full width at half maximum of about 28 ns is detected when the film surface is irradiated by a 308-nm laser pulse with a duration of 25 ns. Besides, open-circuit voltage signals are also observed when the film surface is irradiated separately by the laser pulses of 532 nm and 1064 nm. The results indicate that Ca3Co4O9 thin films have a great potential application in the wide range photodetctor from the ultraviolet to near infrared regions.     

The mechanism of nanobump formation in femtosecond pulse laser nanostructuring of thin metal films

The physical mechanisms responsible for the formation of nanobump structures on a surface of a thin metal film irradiated by a tightly focused femtosecond laser pulse are investigated in a large-scale molecular dynamics simulation. The simulation is performed with a combined atomistic-continuum model adapted for an adequate representation of laser-induced processes at the length-scale of the entire laser spot. The relaxation of the compressive stresses generated by the fast laser heating is identified as the main driving force responsible for the separation of the metal film from the substrate and formation of the nanobump. The kinetics of the transient melting and resolidification, occurring under conditions of the fast cooling due to the two-dimensional electron heat conduction, defines the shape of the nanobump. The predictions of the simulation are related to the surface structures observed in femtosecond laser nanostructuring.     

Laser plasma propulsion generation in nanosecond pulse laser interaction with polyimide film

The plasma propulsion generated in nanosecond pulse laser interaction with polyimide film is investigated. A comparison of coupling coefficient and specific impulse with glass layer and water layer confinement is given. It shows that polyimide has a higher efficiency in water confinement ablation. Through doped carbon black in polyimide film, a higher coupling coefficient is obtained. In ablated surface images, less re-deposited products on polyimide surface have been observed compared with other polymers at the same laser intensity.     

Control of electrical resistance of TiO2 films by short-pulse laser irradiation

Titanium dioxide (TiO₂) films were irradiated with a femtosecond laser beam to alter their electrical resistances. The TiO₂ film was produced by aerosol beam deposition. The wavelength, pulse duration, and repetition rate of the femtosecond laser scanned across the sample surface were 800 nm, 100 fs, and 1 kHz, respectively. By attenuating the laser fluence on the TiO₂ film, a range was found in which the electrical resistance of the TiO₂ film was varied even though the morphology of the film surface was not changed.     

Effect of experimental conditions on surface hardness measurements of calcified tissues via LIBS

This paper reports on the effects of LIBS experimental conditions on the measurement of the surface hardness of calcified tissues. The technique mainly depends on a previously demonstrated correlation between the intensity ratio of ionic to atomic spectral lines and the hardness of the target material. Three types of calcified tissues have been examined, namely enamel of human teeth, shells, and eggshells. Laser-induced breakdown spectra were obtained under two different experimental conditions. In the first nano and picoseconds, laser pulses were used in a single-pulse arrangement, while in the second, single- and double-pulse regimes with nanosecond laser excitation were utilized. The results show that the ionic to atomic spectral line intensity ratios are higher in the case of picosecond laser pulse for both Ca and Mg spectral lines. This effect has been justified in view of the repulsive force of the laser-induced shock waves which depends clearly on the target surface hardness and on the laser irradiance. The electron densities ratio (pico/nano) is shown to be strongly depending on the laser irradiance too. In the case of calcium, single-pulse ratios are higher than the double-pulse ratios, while there is no appreciable difference between both in the case of magnesium. The results obtained herein suggest that double-pulse nanosecond arrangement and the choice of a minor element such as Mg furnishes the best experimental conditions for estimating the surface hardness via LIBS spectra. To validate this method, it has been applied on two previously measured groups of teeth enamel, the first is of ancient Egyptians, and the second from Nubians and Ugandans. The results support the usefulness of this method for similar real-life applications.     

Laser transfer of diamond nanopowder induced by metal film blistering

Blister-based laser induced forward transfer (BB-LIFT) is a promising technique to produce surface microstructures of various advanced materials including inorganic and organic micro/nanopowders, suspensions and biological micro-objects embedded in life sustaining medium. The transferred material is spread over a thin metal film irradiated from the far side by single laser pulses through a transparent support. Interaction of the laser pulse with the metal–support interface under optimized conditions causes formation of a quickly expanding blister. Fast movement of the free metal surface provides efficient material transfer, which has been investigated for the case of diamond nanopowder and diamond-containing suspension. The unique features of the given technique are universality, simplicity and efficient isolation of the transferred material from the ablation products and laser heating.