Multiwavelength investigation of laser-damage performance in potassium dihydrogen phosphate after laser annealing

The laser-induced damage performance of potassium dihydrogen phosphate and deuterated potassium dihydrogen phosphate nonlinear optical crystals after pre-exposure to lower-energy laser pulses (laser annealing, also known as laser conditioning) is investigated as a function of wavelength for both the damaging and conditioning pulses. We obtain a quantitative evaluation of the bulk damage performance of these materials by measuring the density of damage events as a function of laser parameters. This new method allows for a detailed assessment of the improvement of material performance from laser conditioning and reveals the key parameters for optimizing performance depending on the operational wavelength.     

Effect of laser annealing on the dynamics of hologram recording in self-developed dichromated gelatin layers

The dynamics of hologram recording in self-developed dichromated gelatin layers are analyzed for the case where the emulsion is synthesized using self-development technology with application of high-power IR radiation with a wavelength of 1.06 μm for the laser preexposure annealing of millimeter-thick colloid layers.     

Excimer laser-assisted annealing of silicate glass with ion-synthesized silver nanoparticles

The effect of KrF excimer laser radiation on a composite layer consisting of sodium-potassium silicate glass with silver nanoparticles is studied as a function of the number of laser nanosecond pulses. The silver nanoparticles are synthesized by ion implantation. The measured optical absorption of the composite layer demonstrates that the silver nanoparticle size decreases monotonically as the number of laser pulses increases. Rutherford backscattering shows that laser annealing is accompanied by silver diffusion into the bulk of the glass and partial metal evaporation from the sample surface. The detected decrease in the silver nanoparticle size is discussed in terms of simultaneous melting of silver nanoparticles and the glass matrix due to the absorption of laser radiation.     

Laser pulse design using optimal control theory-based adaptive simulated annealing technique: vibrational transitions and photo-dissociation

We have designed and optimised a combined laser pulse using optimal control theory-based adaptive simulated annealing technique for selective vibrational excitations and photo-dissociation. Since proper choice of pulses for specific excitation and dissociation phenomena is very difficult, we have designed a linearly combined pulse for such processes and optimised the different parameters involved in those pulses so that we can get an efficient combined pulse. The technique makes us free from choosing any arbitrary type of pulses and makes a ground to check their suitability. We have also emphasised on how we can improve the performance of simulated annealing technique by introducing an adaptive step length of the different variables during the optimisation processes. We have also pointed out on how we can choose the initial temperature for the optimisation process by introducing heating/cooling step to reduce the annealing steps so that the method becomes cost effective.     

Effect of XeCl laser irradiation on the defect structure of Nd:YAG crystals

This paper presents the effect of XeCl laser irradiation on Nd:YAG single crystal samples with various number of pulses at different repetition rates and laser fluences. Effects of the irradiation on the optical and structural properties of the crystal are analyzed by UV–vis-NIR spectroscopy. Annihilation of some point defects of the crystal structure is observed following laser irradiation at a fluence of 100 mJ cm⁻² with 100 and 500 pulses. Increasing the laser fluence and pulse numbers leads to saturation and new defects are found to be formed in the crystal. Additional absorption spectra of the irradiated samples show that oxygen vacancies in the Nd:YAG crystals are removed during the low-dose irradiation. The laser irradiation is compared to the thermal annealing process for Nd:YAG crystal modification. Additional absorption spectrum of an annealed sample reveals that induced negative absorption band at 236 nm is correlated with the annihilation of the oxygen vacancy center. Our results also demonstrate that XeCl laser treatment has several advantages upon annealing at high temperatures in the Nd:YAG crystal quality improvement. Thus, the present work can give a new approach to modify Nd:YAG crystals to be used in a wide variety of solid-state laser engineering.     

Generation of femtosecond vacuum-ultraviolet pulses

High power femtosecond pulses in the Vacuum Ultra Violet (VUV) have been generated through the nonlinear interaction of femtosecond KrF pulses with xenon and argon gas. Under near resonant two photon excitation of xenon by a femtosecond KrF laser, parametric four wave mixing processes lead to VUV pulses at 147 and 108 nm with pulse energies in the 10 µJ range. Tuning is demonstrated by mixing the KrF pulse with a 500 fs dye laser pulse at 497 nm, resulting in 165 nm emission. In argon, a three photon resonance leads to third harmonic generation at 83 nm and micro joule level pulses near 127 nm generated by a six wave mixing process. Since the spectra of the VUV pulses show an ionization-induced blue shift with increasing KrF laser intensity, the VUV pulses can be shown to have temporal duration less than the pulse width (450 fs) of the KrF laser. Blue shifting of the third harmonic of the KrF laser in argon is dominated by a reduction in the neutral gas density rather than by an increase in the electron density.     

Effect of the surface state on pulsed laser etching of ultrananocrystalline nitrogen-doped diamond films

It is shown that the rate of laser oxidation of ultrananocrystalline films in the nanoablation mode is limited by the presence of physically and chemically adsorbed hydrogen-containing molecules on the surface and can be increased by preliminary thermal annealing. It is proposed to explain the observed saturation of the laser etching rate with the number of pulses by water molecule dissociation on the surface and film saturation with hydride and hydroxyl groups.     

Ultrafast optical power limiting in free-standing Pt–polyvinyl alcohol nanocomposite films synthesized in situ

Free-standing platinum–polyvinyl alcohol nanocomposite films have been prepared by a simple in situ method. By thermal annealing, Pt nanoparticles of different sizes and shapes have been obtained. Their optical nonlinearity is measured using ultrafast (100 fs) laser pulses at 404 nm, in the absorption wing region. A strong optical power limiting is found in the films. The timescale of this limiting action is ultrafast, as it happens within the incident laser pulsewidth. Experimental results and numerical simulation indicate that the sign of the nonlinearity can be controlled by varying the film composition and annealing temperature. Use of ultrashort laser pulses in the free-standing film configuration permits a direct and unambiguous determination of the electronic nonlinearity of the material, since accumulative effects occur at later times lying outside the sharp measurement window.     

Generation of the fourth harmonic of a femtosecond Ti:sapphire laser

A high-repetition-rate femtosecond laser system operating, for the first time to our knowledge, in the spectral region near 200 nm is described. Frequency quadrupling of a mode-locked Ti:sapphire laser results in maximum average powers of 6 mW (165-fs pulses) and 15 mW (340-fs pulses) at 82 MHz. The shortest wavelength achieved is 193.7 nm.     

Near field properties of nanoparticle arrays fabricated by laser annealing of thin Au and Ag films

In this work we show the properties of the electromagnetic field in the vicinity of a monolayer nanoparticle array on SiO₂ substrate. The nanoparticle array is produced by a simple experimental procedure, where thin gold and silver films are deposited on a substrate by pulsed laser deposition technique and they are annealed by nanosecond laser pulses. At certain conditions the laser annealing leads to a homogeneous decomposition of the film into nanoparticles with diameters in the range of few tens of nanometers. Using FDTD simulations the near field distribution in array structures taken from SEM images are obtained. The distribution shows presence on “hot spots” where the near field intensity is enhanced more than two orders of magnitude compared to the incident one. The existence of enhanced field intensity is assumed to be the main reason on enhancement of the Raman scattering signal obtained experimentally using the produced structures as active substrates.     

Comparison of the influence of the fictive and the annealing temperature on the UV-transmission properties of synthetic fused silica

′ and NBOH). Samples with high OH content exhibit gradual recovery from the absorption band within several minutes after exposure to the KrF laser radiation. The formation of the KrF laser-induced 210 nm absorption band depends on the fictive temperature and on the OH content. Low fictive temperature, as a measure for the number of intrinsic defects, retards E^′ generation at the beginning of intense KrF excimer laser irradiation when the majority of defects are generated from precursor defects. However, for longer irradiation periods with pulse numbers of the order of 10⁵ pulses, a high OH content is the beneficial parameter. The accompanying atomic hydrogen is essential for the suppression of the 210 nm absorption band. This happens by transformation of the E^′ centers into Si-H defects. In contrast to a generally held view, annealing (decreasing of the fictive temperature) of fused silica does not always reduce UV induced defect generation. For example, annealing of the samples in an argon atmosphere causes a significantly higher 210 nm absorption increase during KrF excimer laser irradiation (240000 pulses) compared to nonannealed samples. Two spectroscopic methods to determine the OH content of fused silica were applied: Raman and infrared spectroscopy, which in this work lead to differing results. The energetics of the 210 nm absorption band generation and bleaching is summarized by a diagram explaining the interaction of the 248 nm laser radiation with fused silica. Received: 2 June 1997/Accepted: 13 June 1997     

Multifunctional high-performance 10-J level laser system

In this letter,we report a multifunctional high-performance Nd:glass laser system.Laser pulses from an all-fiber front end are first amplified by a regenerative amplifier to 10 mJ level,and then further amplified in a four-pass amplifier.The laser system can provide 10.3-J,3-ns laser pulses at 1053-nm wavelength.The shot-to-shot energy stability is better than 1.5%(RMS).The output laser beam is near diffraction limit.Binary amplitude masks are used to maintain a flat top near-field profile with a 71%fill factor.After the frequency conversion process,7 and 5.5 J pulses at 526.5 and 351 nm are obtained,respectively.     

Stabilization,attenuation and shaping of picosecond laser pulses by two-photon absorption in dyes

The attenuation of ultrashort laser pulses by two-photon absorption is theoretically described when a dye is used as the absorbing medium. It is shown that two-photon absorption also yields an intensity stabilization of low repetition rate laser emissions without a significant loss of power. Consequently this technique is proposed for pulse attenuation and stabilization near the damage threshold of second harmonic generator crystals, and its performance is theoretically compared with other attenuation methods.     

Space-selective co-precipitation of silver and gold nanoparticles in femtosecond laser pulses irradiated Ag, Au co-doped silicate glass

We report on space-selective co-precipitation of silver and gold nanoparticles in Ag⁺, Au³⁺ co-doped silicate glasses by irradiation of femtosecond laser pulses and subsequent annealing at high temperatures. The color of the irradiated area in the glass sample changed from yellow to red with the increase of the annealing temperature. The effects of average laser power and annealing temperature on precipitation of the nanoparticles were investigated. A reasonable mechanism was proposed to explain the observed phenomena.     

In situ grown epitaxial YBa2Cu3O7 − x thin films by pulsed laser deposition under reduced oxygen pressure during cool-down time

We report novel pulsed laser deposition conditions that were used to obtain superconducting epitaxial YBCO thin films, grown in situ using an oxygen pressure lower than the usual one during the cool-down time. We studied the influence of the PLD conditions as substrate temperature, oxygen pressure, laser fluence, and number of laser pulses on the crystallographic and morphological features, and on the superconducting properties of the films. Good superconducting properties were obtained without a high temperature post-deposition annealing process. A maximum critical temperature of 88.6 K was obtained.     

KrF准分子激光退火氢化非晶碳化硅薄膜的晶化研究

介绍了利用KrF准分子脉冲激光对氢化非晶碳化硅(a-SiC∶H)薄膜进行激光退火以实现薄膜的结晶化。利用等离子增强化学气相沉积(PECVD)在单晶Si(100)衬底上制备a-SiC∶H薄膜, 再用不同能量密度的激光对薄膜样品进行退火。分析表明, 选用合适能量密度的激光退火能够实现a-SiC∶H薄膜的结晶化, 且结晶颗粒大小随着入射激光能量密度的增加而增大; 显微图表明当入射能量密度超过200 mJ/cm2时, 薄膜表面出现由热弹性波引起的表面波纹现象, a-SiC∶H薄膜结晶过程为液相结晶; 傅里叶红外谱(FTIR)表明随着入射能量密度增加, 薄膜中氢含量降低, Si-C峰增强并且峰位出现蓝移, 薄膜的结晶度提高。     

Superconducting V3Si produced by pulsed laser annealing

Pulsed laser annealing has been utilized to fabricate superconducting V₃Si from multilayer V-Si thin film samples. It is demonstrated that a single laser pulse can induce mixing to form V₃Si in the A15 phase. The effects of multiple laser pulses and post thermal annealing on the superconducting transition temperature and width are presented, and a model is proposed to explain the effects of rapid heating and cooling on the V₃Si structure.     

Effect of scanning resolution and fluence fluctuation on femtosecond laser ablation of thin films

Femtosecond-pulse laser pulses have been shown to hold great potential for high-precision micromachining. Much research has been done to characterize the laser parameters for predicting the feature size, and the most important of these is the number of pulses incident at each point of ablation. Theoretical modeling, so far, has been restricted to a single point where the number of pulses incident at each point of ablation depends on the pulse-repetition rate and the dwell time of the laser beam at that machining point. However, to make the theoretical model useful, a laser scanning system with the ability to fabricate complex microfeatures is considered in this work. In this case, the scanning resolution determines the number of pulses incident at each scanning point. This has been taken into account while presenting a theoretical and experimental analysis of the effect of the scanning resolution on the threshold fluence of the material. Results of ablation on a gold thin film were subjected to theoretical analysis to predict the feature size. In addition, the effect of the energy fluctuation of the laser on the feature size has been evaluated to predict the minimum achievable feature size for a gold thin film. PACS 42.65; 61.80     

Removal of graffiti from the mortar by using Q-switched Nd:YAG laser

This paper presents part of the larger study on microstructural features of mortars and it's effects on laser cleaning process. It focuses on the influence of surface roughness, porosity and moisture content of mortars on the removal of graffiti by Nd:YAG laser. The properties of this laser are as follows: wavelength (λ) 1.06 μm, energy: 500 mJ per pulse, pulse duration: 10 ns. The investigation shows that the variation of laser fluence with the number of pulses required for the laser cleaning can be divided into two zones, namely effective zone and ineffective zone. There is a linear relationship observed between number of pulses required for laser cleaning and the laser fluence in the effective zone, while the number of pulses required for the laser cleaning is almost constant even though the laser fluence increases in the ineffective zone. Moreover, surface roughness, porosity and moisture content of mortar samples have influence on the laser cleaning process. The effect of these parameters become however negligible at the high level of laser fluence. The number of pulses required for the laser cleaning is low for smooth surface or less porous mortar. Furthermore, the wetness of the samples facilitates the cleaning process.     

Combined continuum-atomistic modeling of ultrashort-pulsed laser irradiation of silicon

Ultrafast thermomechanical responses of silicon thin films due to ultrashort-pulsed laser irradiation were investigated using an atomic-level hybrid method coupling the molecular dynamics and the ultrafast two-step energy transport model. The dynamic reflectivity and absorption were considered, and the effects of laser fluence and pulse duration on the thermomechanical response were studied. It was found that both the carrier temperature and number density rapidly increase to their maximum while the lattice temperature rises at a much slower rate. The ultrafast laser heating could induce a strong stress wave in the film, with the maximum compressive and tensile stress occurring near the front and back surfaces, respectively. For laser pulses of the same duration, the higher the laser fluence is, the higher the carrier temperature and density and lattice temperature are induced. For the same laser fluence, a longer pulse generally produces lower carrier density and temperatures and weaker stress shock strength. However, for the fluence of 0.2 J/cm², the lowest lattice temperature was simulated for a 100-fs pulse compared to the 1-ps and 5-ps pulses, due to the increase of reflectivity by high carrier density. It is also shown that the optical properties as functions of lattice temperature usually employed are not suited for modeling ultrafast laser interactions with silicon materials.