Processing of transparent materials using visible nanosecond laser pulses

Laser micromachining of transparent materials is an intensively studied research area from the point of view of microoptical element fabrication. One of the most promising indirect processing methods is the laser-induced back-side dry etching (LIBDE). During this method, transparent targets are contacted with solid thin layers, which absorb and transform the pulse energy resulting in etching. The applicability of LIBDE technology for processing of fused silica using a visible nanosecond dye laser (λ=500 nm, FWHM=11 ns) and a 100-nm-thick aluminium absorbing layer was investigated. The applied fluence was varied in the range of 0–3050 mJ/cm²; the illuminated area was 0.1 mm². The threshold fluence of the LIBDE etching of fused silica was found to be approximately 540 mJ/cm². The chemical composition of the surface layers on and around the etched holes was investigated by field-emission scanning electron microscopy and energy-dispersive X-ray spectrometry. It was found that on average 0.4±0.3 at. % aluminium is built into the upper ∼1-μm-thick volume of the illuminated fused silica, while the aluminium content fell below the detection limit in the case of the original surface. Our experiments proved that the LIBDE procedure is suitable for microprocessing of transparent materials using visible nanosecond laser light. PACS 42.62.-b; 61.80.Ba; 81.16.Rf; 81.65.Cf     

On the liquid explosive boiling on a substrate under action of subnanosecond laser pulse

The conditions and capability of acoustic recording of pressure pulses from explosive boiling of a transparent liquid on an absorbing substrate heated by nanosecond and subnanosecond laser pulses are analyzed.     

Explosive boiling in water exposed to q-switched erbium laser pulses

Pressure signals generated in water under the action of erbium laser pulses (100–200 ns, 2.94 μm) are investigated with lithium niobate piezoelectric transducer. For the first time multiply short (subnanosecond) pressure pulses standing against smooth pressure background are observed when the laser intensity exceeds explosive boiling threshold.     

On the theory of explosive boiling of a transparent liquid on a laser-heated target

The possible manifestations of thermodynamical instability (explosive vaporisation) are discussed for different regimes of laser heating of the metal/transparent liquid system. The present calculations show that the explosive vaporisation in the metastable region may occur if the nucleation rate is high enough. This condition is achievable if the surface tension of the superheated liquid tends to zero near the spinodal. It is also shown that the dependence of the phase explosion time on laser intensity markedly changes its behaviour when the water temperature reaches the spinodal.     

Analysis of explosive boiling of a liquid film on an impulsively heated substrate

Explosive boiling of a thin liquid film on a substrate heated by nanosecond laser pulse, which results in film peeling from a substrate, is considered. It was shown that, to explain the experimental data [1] on the maximum film peeling velocity, the features of evaporation kinetics in the appearing cavity and the shaking effect associated with the nonlinear thermal expansion of the film immediately before its detachment from the substrate should be taken into account.     

Self-organization of thin metal films by irradiation with nanosecond laser pulses

Modifications in thin metal films under intensive laser irradiation were studied. Gold, silver, copper, chromium and aluminum films with the thickness of 100 nm were deposited on the glass substrate. Back-side irradiation through the substrate with a burst of nanosecond pulses tightly focused to a line was applied. The film removal threshold with a single pulse F_th was estimated for every material and laser fluence was kept above it in the range of 1.5-3 F_th during experiments. Diverse behavior of the films depending on the metal, the shift between pulses and laser fluence was observed. In chromium, the regular structures were developed in a quite wide range of processing parameters. In gold, three kinds of ripples were observed: transverse (similar to ripples in chromium), longitudinal and a structure of ripples oriented at 60° to each other. The combination of physical properties facilitated the regular assembly of the molten metal in chromium and to some extent in gold.     

Nonequilibrium motion of a metal surface exposed to submicrosecond laser pulses

The results of experimental studies and theoretical analysis of the displacement of a metal mirror surface upon heating by submicrosecond laser pulses (t ₀ = 15 × 10⁻⁹ s) are reported. The dynamics of the surface displacement in the irradiation zone was monitored by a Michelson interferometer with photoelectron count of fringes. A substantial delay of the motion of the exposed surface relative to a heating pulse was observed in experiments. It is shown that under thermal perturbations with high temperature gradients corresponding to large optical absorption coefficients, the contribution of nonequilibrium processes to the thermomechanical response of metals becomes decisive even for a submicrosecond duration of heating. The allowance for nonequilibrium processes in theoretical analysis leads to the generalized Duhamel law with thermal memory, which can be used for describing the observed effect correctly. The resultant characteristic time of a transient process (∼(2–6) × 10⁻⁸ s) considerably exceeds the available estimates of the phonon-phonon interaction time and is due to the effect of nonequilibrium processes on the scale of structural elements, which is considerably larger than the atomic scale.     

Electrical breakdown in ionic crystals exposed to nanosecond pulses

Electrical breakdown in alkali halide crystals subjected to ∼10-ns-long electrical pulses is studied. Two, primary and basic, channels of the anodic discharge are noted. In the presence of the primary channel, the other arises at static breakdown voltages. Otherwise, the basic channel forms at voltages exceeding the static breakdown voltage by more than four times. The basic channel is assumed to form via a cascade of Auger transitions. The generation and migration of linear defects seem to play a significant role in the basic channel formation. The enhancement of the pulsed dielectric strength of the crystals is related to conditions of current passage through the metal-insulator interface.     

Surface modification of a WTi thin film on Si substrate by nanosecond laser pulses

Interaction of a nanosecond transversely excited atmospheric (TEA) CO₂ laser, operating at 10.6 μm, with tungsten-titanium thin film (190 nm) deposited on silicon of n-type (1 0 0) orientation, was studied. Multi-pulse irradiation was performed in air atmosphere with laser energy densities in the range 24-49 J/cm². The energy absorbed from the laser beam was mainly converted to thermal energy, which generated a series of effects. The following morphological changes were observed: (i) partial ablation/exfoliation of the WTi thin film, (ii) partial modification of the silicon substrate with formation of polygonal grains, (iii) appearance of hydrodynamic features including nano-globules. Torch-like plumes started appearing in front of the target after several laser pulses.     

纳秒激光烧蚀下液态金属表面微结构的形成(英文)

首次发现了在不同保护气体及多脉冲UV-IR激光的照射下,液态金属的微型突起和微结构的形成。测量表明,针对不同的金属和保护气,这种结构的单脉冲生成速率可达(5～20)μm/pulse,形成了长度为1～2 mm,直径约为焦点两倍的单个微型突起。最后,介绍了控制微结构形状的可能性,并讨论了它们的应用潜力。     

纳秒激光烧蚀下液态金属表面微结构的形成

首次发现了在不同保护气体及多脉冲uV—IR激光的照射下,液态金属的微型突起和微结构的形成。测量表明,针对不同的金属和保护气,这种结构的单脉冲生成速率可达（5—20）μm／pulse,形成了长度为1～2mm,直径约为焦点两倍的单个微型突起。最后,介绍了控制微结构形状的可能性,并讨论了它们的应用潜力。     

Action of femtosecond laser pulses on metal nanoparticles in a liquid

A theoretical model describing the influence that ultrashort laser pulses have on spherical metal nanoparticles placed in a liquid is proposed. The kinetics of excitation and damping of acoustic vibrations in gold nanoparticles is studied, and the propagation processes of acoustic disturbances in their environment are discussed.     

Interaction of nanosecond laser pulses with plastic foams

Some results of an experimental and theoretical study of the interaction of laser pulses with plastic foams are reported. The propagation velocity of a hydrodynamic peturbation which was initiated in foam target under the action of a laser pulse with intensityq≈2·10¹³ W/cm² and the velocity distribution function of plasma ions were measured; the preliminary results of time-integrated spectroscopic measurements of an intense red-shifted signal are also reported. A self-consistent model of the foam target’s laser plasma formed in a hydrodynamic mode was derived. The predictions of this model are consistent with experimental results. A model of microprocesses of laser plasma formation in a structured material was also developed. The results of numerical simulations by 1D and 2D computer codes are also reported. Original article submitted in English May 25, 1997.     

High-energy extraction to nanosecond laser pulses from a 60-cm-aperture KrF laser amplifier

The intra-cavity amplified spontaneous emission (ASE) in saturated amplification of nanosecond laser pulses in a 60-cm-aperture KrF laser amplifier is calculated. The experimentally observed high intrinsic efficiency and saturated pulse intensities are numerically simulated. By using the ASE code validated experimentally, high-efficiency energy extraction from a still larger-aperture amplifier for inertial-fusion-energy drivers is predicted. Received: 12 March 2001 / Published online: 27 April 2001     

Formation of nanobumps and nanoholes in thin metal films by strongly focused nanosecond laser pulses

Nanobumps and nanoholes have been formed in gold and silver films with various thicknesses on a dielectric substrate by strongly focused single nanosecond pulses of a Nd:YAG laser. An apertureless dielectric fiber probe and an aspherical lens with a numerical aperture of 0.5 were used to focus laser radiation into a diffraction-limited spot on the surface of gold and silver films, respectively. Atomic force and electron microscopy studies have demonstrated that the shape and dimension of nanostructures, as well as the threshold parameters of laser radiation for their formation, are determined by the thickness of a modified film (“size effect”) and by the duration of a laser pulse owing to the lateral heat conduction in films (nonlocal energy deposition effect). Mechanisms of the dynamic formation of such structures in metallic films by nanosecond laser pulses due to phase transformations of their material have been discussed.     

Characteristic investigation of ablative laser propulsion driven by nanosecond laser pulses

The momentum transfer and the specific impulse of the ablative laser propulsion of nanosecond laser irradiation on copper, lead, aluminum and graphite targets are investigated. The effects of the ambient pressure and laser focal spot sizes on the target momentum are measured. The results show that the target momentum strongly relates to the ambient pressure and target property. The highest target momentum about 2.28 g·cm/s is obtained on lead targets under 1 atmospheric pressure. With the increase of the focal spot sizes, the specific impulse decreases. The highest specific impulse in vacuum is about 950 s on copper targets. PACS 52.75.Di; 52.38.Mf; 52.50.Jm     

Temporal compression of nanosecond laser pulses using coherent control

We present a study of the temporal compression of nanosecond laser pulses resulting from the coherent control peculiarities of the propagation dynamics in a regime of electromagnetically induced transparency. We describe the general theoretical framework and discuss the crucial conditions required in order to experimentally realize such a temporal compression scheme. A proof-of-concept experimental realization of a scheme of this type in a sample of hot sodium vapors is currently being implemented: we describe in detail the experimental setup designed for this purpose.     

On the characteristic properties of oxidation of sulfide minerals exposed to nanosecond electromagnetic pulses

High-power nanosecond electromagnetic pulses cause changes in the chemical and phase surface composition of sulfide minerals (pyrrhotite and pentlandite) and their sorption, flotation, and chemical activities. The influence of the conditions and parameters of the electric-pulse effect on the change in the amount of elemental sulfur and iron oxide on the surface of mineral particles, as well as the concentration of iron and sulfoxide ions in the aqueous mineral suspension, has been studied. The parameters of preliminary pulsed treatment of pyrrhotite and pentlandite that lead to improvement of flotation separation of minerals have been determined.     

飞秒激光与透明介质的相互作用

随着飞秒激光技术的不断成熟,飞秒激光器不但在实验室能产生小于10fs的光脉冲,啁啾放大后的飞秒光脉冲的聚焦峰值功率密度可达到10的21次方W/cm2以上,而且飞秒激光系统已实现全固体、小型化结构,其稳定性和可靠性大大提高,因此在科学技术研究中的应用越来越广.文章重点介绍飞秒激光的主要特性和它与透明介质［如熔融石英、光学玻璃、对激光透明的高分子聚合物(PMMA)等］的相互作用过程,分析它们之间的非线性相互作用过程引起的材料特性或结构变化的物理机制和可能的应用,尤其在高密度大容量三维存储和微光子器件制造等方面的应用可能性.