超短脉冲激光烧蚀熔融硅的理论模型

在Fokker_P1anck方程的基础上,对超短脉冲激光烧蚀熔融硅的机理进行分析研究,建立了雪崩电离、多光子吸收电离导致的熔融硅烧蚀机理的数学模型.其计算得出的激光能量密度和临界烧蚀阀值与实验结果很好的吻合,定量解释了超短脉冲激光对熔融硅烧蚀损伤微观过程的影响.     

原子分子在强激光场中的电离机制

由于超强超短脉冲激光技术的不断发展以及获取超短超强和高功率激光脉冲的能力上取得了重大进步,使得原子分子在强激光场下的电离和解离机制成为原子分子物理中的研究热点,随着激光场强度的增加,原子分子的电离机制分为多光子电离、阈上电离、场致电离和多电子电离。     

光学薄膜损伤的单光子吸收电离的电子雪崩模型

分析了实际光学薄膜与其块状材料在微观结构及宏观尺寸上的差异,指出了单光子吸收电离与雪崩倍增可分为两个相对独立的先后过程处理,提出了实际光学薄膜激光诱导损伤的单光子吸收电离引发电子雪崩模型,讨论了原初电子数密度与入射激光波长的关系,以及实际光学薄膜激光诱导损伤阈值与其原初电子数密度的关系,提出了确定原初电子数密度的实验方法。     

沉积温度对三倍频ZrO2薄膜能隙的影响

采用电子束蒸发沉积制备了不同基底温度的ZrO2单层薄膜.计算了薄膜在三倍频处的折射率、消光系数,分析了基底温度对薄膜带隙的影响及薄膜性质与损伤阈值的关系,得出了薄膜能隙随温度升高而降低,薄膜在三倍频处的损伤阈值与能隙成正比关系,这与薄膜损伤机理-多光子吸收、雪崩电离机理相符.     

外场中电子脉冲的离散自展宽特性

根据电子脉冲中电子之间的多体相互作用特点,建立了用于分析电子脉冲离散自展宽特性的"电子切片"物理模型,定量分析了外加速场环境中电子脉冲的自展宽动态演化特性。结果表明,电子脉冲动态自展宽过程可分为两步:首先是极短距离内(1μm以内)急剧的雪崩式展宽过程,之后是缓慢的准线性自展宽过程。对雪崩式展宽过程,电子脉冲自身初始参数对其最终自展宽过程的影响并不显著,而提高外加速场电位则可显著抑制该脉冲展宽现象。同时也给出了电子脉冲各初始参数对准线性自展宽过程的耦合影响。该分析方法可用于以电子脉冲为媒介的各类电子枪优化设计分析。     

多层介质膜脉冲压缩光栅激光损伤特性研究进展

本文结合国内外研究情况,概括性介绍了用于啁啾脉冲放大系统中的多层介质膜脉冲压缩光栅的激光损伤特性研究进展,包括多层介质膜的损伤、表面浮雕结构的损伤,以及介质膜光栅损伤的影响因素。在关于介质膜光栅激光损伤的影响因素中又分别介绍了槽型结构、制备工艺、激光参数、脉冲数量、热应力和杂质缺陷对其抗激光损伤阈值的影响。最后,从结构设计、制备工艺以及后期处理等方面,介绍了提高多层介质膜光栅抗激光损伤阈值的常用方法。     

纳秒激光辐照对类金刚石薄膜的损伤及改性

利用Nd:YAG纳秒激光(波长分别为355、532和1 064 nm)辐照由电子束蒸发技术制备的类金刚石(DLC)薄膜,通过光学显微镜、光学轮廓仪和拉曼光谱仪等分析了辐照后的薄膜样品,结果表明:不同波长的单脉冲激光辐照时,DLC膜的激光损伤阈值不同;同一波长的多脉冲激光辐照时,损伤阈值低于单脉冲辐照阈值;脉冲激光辐照对DLC膜具有改性作用,受辐照薄膜区域表层发生了石墨化、剥落和气化效应,致使DLC膜表面出现了隆起和弹坑,隆起高度和弹坑深度与激光能量密度大小和脉冲个数有关.     

银超微粒子-氧化铯牛导体薄膜的多光子光电发射

我们推导了金属超微粒子-半导休薄膜的多光子光电发射公式。制备了■Ag超微粒子-Cs_2O半导体薄膜,用调Q的Nd:YAG激光源(λ=1.06urn,脉冲宽度为10ns)测量了这种薄膜的多光子光电发射特性。发现它具有对光电发射贡献较大的光吸收系数几和较高的量干效率η_n。     

光学薄膜的激光诱导损伤与材料带隙的关系

采用多光子吸收电离模型讨论了光学薄膜的激光诱导损伤与其材料带隙的关系,报实际光学薄膜存在大量的非化学计量比化合物缺陷时,损伤阈值的变化,给出了光汪膜的损伤阈民其材料带隙的关系曲线,理论上解释了实际光学薄膜较理想光学薄膜的损伤阈值的主要原因可能是实际光学薄膜中存在大量的非化学计量比化合物缺陷。     

飞秒激光烧蚀血管支架材料的数值模拟

为了研究飞秒脉冲激光烧蚀血管支架材料的特性,利用考虑了电子之间热传导项的双温模型,采用有限差分法,对飞秒激光烧蚀NiTi合金的温度场分布进行数值模拟,计算得到了电子温度和晶格温度随时间和烧蚀深度的变化规律,进一步讨论了不同激光能量密度、不同激光脉宽、不同延迟时间对电子和晶格的温度场影响。发现血管支架材料在飞秒激光的作用下,先是电子吸收能量温度快速升高,再通过电声耦合作用将能量传递给晶格,最后两者的温度达到一个平衡状态;激光能量密度主要影响电子的峰值温度和电子与晶格的平衡温度;脉冲宽度主要影响电子的峰值温度和达到峰值温度所用的时间;电子温度随着延迟时间的增加先升高后降低,晶格温度随着延迟时间的增加不断上升。这些理论分析对实际飞秒激光加工血管支架有重要的指导意义。     

Effect of nanosecond laser pre-irradiation on the femtosecond laser-induced damage of Ta2O5/SiO2 high reflector

The effect of nanosecond laser pre-irradiation on the femtosecond laser-induced damage behaviors of 800 nm 0° AOI Ta(2)O(5)/SiO(2) high reflectors fabricated by e-beam evaporation was explored. Laser pre-irradiation was carried out by Raster-scanning with scanning mode of 1-on-1 and scanning velocities timed such that there was a beam overlap at 70% of the peak fluence, utilizing 5 Hz 1064 nm 12 ns Nd:YAG fundamental lasers. Femtosecond laser damage was investigated by 1 kHz 800 nm 135 fs Ti: sapphire laser system with 1-on-1 mode test. The results indicated that nanosecond laser pre-irradiation did not promote the femtosecond laser-induced damage threshold of reflectors. Instead, the thresholds of all the samples with various fluence steps for pre-irradiation were reduced by about 20%. Furthermore, the damage morphologies were analyzed by optical microscope, SEM and AFM, which displayed deterministic field induced breakdown characteristics. To explain these phenomena, a theoretical model including photoionization, avalanche ionization, and decays of electrons was built to simulate the evolution of electron density in the conduction band. Field ionization mechanism was considered to dominate the femtosecond laser damage process, while the electronic defects induced by nanosecond laser pre-irradiation accelerated the femtosecond laser damage evolution.     

Laser-enhanced ionization of mercury atoms in an inert atmosphere with avalanche amplification of the signal

A new method for laser-enhanced ionization detection of mercury atoms in an inert gas atmosphere is described. The method, which is based on the avalanche amplification of the signal resulting from the ionization from a selected Rydberg level reached by a three-step laser excitation of mercury vapor in a simple quartz cell, can be applied to the determination of this element in various matrices by the use of conventional cold atomization techniques. The overall (collisional + photo) ionization efficiency is investigated at different temperatures, and the avalanche amplification effect is reported for Ar and P-10 gases at atmospheric pressure. It is shown that the amplified signal is related to the number of charges produced in the laser-irradiated volume. Under amplifier noise-limited conditions, a detection limit of ～15 Hg atoms/laser pulse in the interaction region is estimated.     

Microstructure and optical characterization of nanometric silicon films prepared by pulsed laser ablation

The effect of laser energy density, during pulsed laser ablation, on the microstructure and optical properties of silicon films has been investigated using techniques such as atomic force microscopy, scanning electron microscopy, X-ray diffraction, and UV–visible absorption/transmission spectroscopy. The thickness of prepared films increases with increase in laser energy density. The crystallite size and hence the crystallinity of prepared films have been estimated by X-ray diffraction and found to be dependent on laser energy density. The transmittance of films changes with laser energy density. The absorption coefficient of films has been found to be?>10⁴?cm^?1 in wavelength region 450–1100?nm. The band gap of silicon films has been determined as 2.27, 2.11, and 1.90?eV corresponding to laser energy density of 1.5, 2.5, and 3.5?J?cm^?2, respectively.     

Optical properties of CdS sintered film

Chemical method has been used to prepare cadmium sulphide by using cadmium, hydrochloric acid and H₂S. The reflection spectra of covered and uncovered sintered films of CdS have been recorded by ‘Hitachi spectrophotometer’ over the wavelength range 300–700 nm. The energy band gaps of these films have been calculated from reflection spectra. It is found that the energy band gap of both films is same as 2.41 eV. It is indicated that energy band gap of these films does not change. This value of band gap is in good agreement with the value reported by other workers. The measurement of photocurrent has also been carried out using Keithley High Resistance meter/ Electrometer. This film shows the high photosensitivity and high photocurrent decay. Thus so obtained films are suitable for fabrication of photo detectors and solar cells.     

Laser-induced amorphization and crystallization on Se80Te20−xPbx thin films

Thin films of glassy alloys of a-Se₈₀Te_20−xPb_x (x=2, 6 and 10) was crystallized in a specially designed sample holder under a vacuum of 10⁻² Pa. The amorphous and crystallized films were induced by pulse laser (wavelength: 337.1 nm, frequency: 10 Hz, pulse duration: 4 ns and pulse energy: 0.963 mJ). After laser irradiation on amorphous and crystalline films: optical band gaps were measured. Crystallization and amorphization of chalcogenide films is accompanied by the change in the optical band gap. The change in optical energy gap could be determined by identification of the transformed phase. This change in the optical band gap may be due to the increase in the grain size and the reduction in the disorder of the system.     

Comparisons and analyses of the properties of laser-induced damage to SiO2 and ZnS

To compare the optical damage resistance ability of SiO₂ and ZnS, the processes of electrons reproduction of the two materials have been studied. The relationship of multiphoton ionization rate, avalanche ionization rate and the multiphoton parameter <?x?+?1?> with the intensity of the incident laser is calculated. The damage thresholds induced by the laser with different pulse widths are calculated too. In addition, the respectable role of the recombination and diffusion for an electron in the electronic proliferation processes is examined. Calculation results show that SiO₂ has a higher damage threshold while τ?<?1?ns, and ZnS has a higher damage threshold while τ?>?1?ns.     

Optical characterization of organic material bromoindium phthalocyanine thin films grown by electron beam evaporation

This paper presents the optical properties of organic material bromoindium phthalocyanine (BrInPc) thin films grown by electron beam evaporation technique. The paper describes the optical characteristics of BrInPc thin films, which have been determined using spectrophotometric measurements of the absorbance, transmittance and reflectance at normal incident of light in the spectral range 300–1,100 nm. The optical band gap energy and type of the electronic transition have been determined by analysis of spectral behavior of absorption coefficient, which reveals the probability of both direct and indirect transitions. Other optical constants, such as refractive index, extinction coefficient, complex dielectric constant and optical conductivity of thin films have been evaluated. Moreover, the width of band tails of localized states (Urbach energy), steepness parameter and width of the defect states have been determined by studying the absorption coefficient spectra just below the fundamental absorption edge.     

Doping of rf plasma deposited diamond-like carbon films

Rf plasma deposited diamond-like carbon (DLC) films have been doped n-type with the addition of nitrogen as a feed gas to a magnetically confined rf plasma. Controlled amounts of nitrogen are added to the CH₄/He plasma and the films are characterised. The electronic properties together with the microstructure of the deposited films are examined. Activation energy studies show the Fermi level can be moved from 0.5 eV away from the valence band for the undoped DLC films, through a maximum activation energy of 0.9 eV corresponding to the midgap and to 0.45 eV away from the conduction band with maximum N incorporation. The optical band gap first increases, indicative of a reduction in the band-edge tail states, and then tends to a steady value of ˜2 eV. Activation energy studies together with the optical band gap data are used to analyse the density of states for the deposited films. The preferential doping configuration of the atomic nitrogen and the importance of the π-π* states for electronic conduction for DLC:N films is discussed in the light of the findings.     

金刚石薄膜抗激光破坏研究

介绍了金刚石优异的光学和力学特性,对金刚石薄膜在从紫外到红外波段以及不同脉宽激光参数下的激光损伤行为和损伤阈值进行了评述。分析了不同激光工作参数对金刚石薄膜的激光损伤机理,认为石墨化导致晶格失稳是金刚石薄膜激光损伤的主要原因。金刚石薄膜石墨化有两种方式:垂直表面向体层方向石墨化和平行表面按分层的方式逐层石墨化。