Heating of nonequilibrium electrons by laser radiation in solid transparent dielectrics

A computer simulation of the heating of nonequilibrium electrons by an intense high-frequency electromagnetic field leading to the bulk damage of solid transparent dielectrics under single irradiation has been carried out. The dependences of the avalanche ionization rate on threshold field strength have been derived. Using the Fokker-Planck equation with a flux-doubling boundary condition is shown to lead to noticeable errors even at a ratio of the photon energy to the band gap ∼0.1. The series of dependences of the critical fields on pulse duration have been constructed for various initial lattice temperatures and laser wavelengths, which allow the electron avalanche to be identified as a limiting breakdown mechanism. The ratio of the energy stored in the electron subsystem to the excess (with respect to the equilibrium state) energy of the phonon subsystem by the end of laser pulse action has been calculated both with and without allowance for phonon heating. The influence of phonon heating on the impact avalanche ionization rate is analyzed.     

光伏半导体器件对能量小于禁带宽度光子的响应机理研究

利用光子能量为0.12 eV的10.6 μm连续激光分别辐照了禁带宽度为0.91和0.33 eV的光伏碲镉汞探测器. 实验表明,激光辐照下禁带宽度为0.91 eV的探测器输出正电压,而禁带宽度为0.33 eV的探测器对激光的响应方向却与之相反. 为了研究此现象,利用功率密度一定的10.6 μm激光辐照不同开路电压状态下禁带宽度为0.91 eV的探测器,实验结果证实初始开路电压是产生输出电压反向现象的原因. 对这一机理进一步分析发现,光伏探测器在光子能量小于禁带宽度的激光辐照下,其开路电压是热激发载流子导致的热生电动势和自由载流子吸收导致的晶格热效应共同决定的. 关键词： 能量小于禁带宽度的光子 光伏碲镉汞探测器 热生电动势 晶格热效应     

Nd:YAG laser ablation characteristics of thin CIGS solar cell films

This work reports that the ablation characteristics of thin CuIn_1?x Ga _x Se₂ (CIGS) solar cell film differ significantly with elemental composition and laser pulse energy. From in situ shadowgraphs measured during Nd:YAG laser (1,064 nm) irradiation of CIGS films and crater morphologies, it was found that strong surface evaporation is dominant for low Ga concentration films of which band gap is well below the photon energy. As the band gap of CIGS film becomes close to or over the laser photon energy due to increased Ga content, surface absorption diminishes and at low laser energy, laser heating of the film plays an important role. It is demonstrated that for the CIGS films with Ga/(Ga + In) ratio being approximately over 0.2, the laser irradiation leads to solid phase removal of the film due to thermomechanical fracture at low laser energy but to ablative evaporation at elevated energy.     

Photo and thermally induced properties change in Bi/Ag/Se trilayer thin film

In this article, we have demonstrated the optical and structural properties change in Bi/Ag/Se trilayer thin films by the influence of thermal and photon energy. The trilayer films prepared by thermal evaporation technique were annealed and laser irradiated at room temperature. The X-ray diffraction study revealed the Ag₂Se phase formation and the surface morphology change is being studied by Field emission scanning electron microscopy. The optical properties of the studied films were characterized by using FTIR spectrophotometer in the wavelength range 400–1200?nm. The reduction of optical band gap by both thermal and laser irradiation is being discussed on the basis of chemical disorderness, defect states and density of localized states in the mobility gap. The Raman shift due to annealing and irradiation supports the changes in the film. The large change in optical band gap in thermal annealing is useful for memory device and waveguide fabrication.     

Structural,optical and electrical properties of laser irradiated Pb doped Ga–Se chalcogenide thin films

Using the Transverse Electrical Excitation at Atmospheric Pressure (TEA) nitrogen laser, we had irradiated the amorphous thin films of Ga₁₀Se₈₁Pb₉ chalcogenide glass and the results have been discussed in terms of the structural aspects of Ga₁₀Se₈₁Pb₉ glass. The observed changes are associated with the interaction of the incident photon and the lone-pairs electrons which affects the band gap. The X-ray structural characterization revealed the amorphous nature of as prepared films and polycrystalline nature of the laser irradiated films. The optical band gap of these thin films is measured by using the absorption spectra as a function of photon energy in the wavelength region 400–1200 nm. It is found that the optical band gap decreases while the absorption coefficient increases with increasing the irradiation time. The decrease in the optical band gap has been explained on the basis of change in nature of films, from amorphous to polycrystalline state, with the increase in exposure time. The dc conductivities and activation energies of these thin films are measured in temperature range 303–403 K. It has been found that the activation energy in Ga₁₀Se₈₁Pb₉ chalcogenide thin films decreases whereas the dc conductivity increases at each temperature by increasing the irradiation time.     

On the mechanism of the absorption of femtosecond laser pulses in the melting and ablation of Si and GaAs

The melting and ablation thresholds have been measured for Si and GaAs irradiated by 1240-nm femtosecond pulses of a chromium-forsterite laser, i.e., when the photon energy is lower than the bandgap width. A small difference of these quantities from the respective melting and ablation thresholds measured for the action of the 620-nm second-harmonic pulses with a photon energy higher than the bandgap width cannot be explained using available theoretical models. A new approach to the mechanism of the appearance of the electron-hole plasma and the formation of a thin, strongly absorbing surface layer in semiconductors irradiated by femtosecond laser pulses of the visible and infrared spectral ranges has been proposed on the basis of the avalanche mechanism of the filling of the conduction band.     

脉冲激光辐照硅材料引起表面波纹的特性研究

叙述了激光与材料相互作用过程中引起相干受激光散射的机制,以及形成材料表面波纹的特性。在激光波长1．06μm、能量15mJ、光斑直径2mm、脉冲半峰全宽约10ns和入射方向为布儒斯特角的条件下,进行了脉冲激光辐照硅材料形成表面波纹的实验研究。在脉冲激光辐照硅材料表面功率密度略大于材料损伤阈值的条件下,发现了硅材料表面形成的平行等间距直线条纹结构。用光学显微镜和原子力显微镜分别测量了被辐照硅材料表面的波纹形貌特征。在假设硅材料表面波纹的产生与声波在材料中的传播速度有关的条件下,由声波传播速度和激光辐照硅材料的脉冲宽度较好地解释了材料表面形成条纹的宽度．并认为在形成表面波纹的过程中,热应力起主要作用。     

脉冲激光对类金刚石（DLC）薄膜的热冲击效应研究

强激光辐照红外热像系统时，可造成系统的干扰和破坏，激光的波长不同，对系统的破坏效果也不同.为了保护红外系统窗口以及提高窗口的透过率，红外窗口广泛沉淀类金刚石（DLC）薄膜.当入射的激光波长位于红外系统响应波段外时，激光对系统的破坏首先是激光对DLC薄膜的破坏.以波长为1.06μm的激光为例，研究了脉冲激光对DLC薄膜的损伤机理，建立了DLC薄膜的热冲击效应模型，并通过求解热传导和应力平衡方程，得出了薄膜的温度场和应力场分布.理论分析表明，热应力破坏在脉冲强激光对DLC膜的损伤机理中占主导地位.当 辐照能量密度为E₀=100mJ·cm^-2时，在薄膜表面距光斑中心约 40μm区域内的压应 力明显超出其断裂强度，将造成膜层的剥离、脱落.理论分析与实验结果基本相符，表明建 立热冲击效应模型的正确性. 关键词： 激光辐照 类金刚石（DLC）薄膜 热冲击效应     

Quantum efficiency of technical metal photocathodes under laser irradiation of various wavelengths

Quantum efficiency studies for various laser wavelengths and various technical metal surfaces were carried out in a dedicated unbaked vacuum chamber in the absence of a significant electrical field. Copper, magnesium, aluminum, and aluminum–lithium photocathodes were irradiated by two different high power, high repetition rate, laser systems. We have observed an emission of electrons for photon energies below the work function of the material. This is explained by multiple photon absorption by the photocathode. We have not observed any degradation of the QE for these materials, but an improvement when irradiating them over a long period of time. This is contrary to observations made in RF photoguns.     

Analysis of the formation and evolution of oriented microstructures on laser ablated silicon

A theoretical approach and qualitative analysis of the changes induced on the surface morphology and the formation of microstructures on silicon targets irradiated by excimer laser are presented. This study is based on theoretical principles of the laser ablation process, in particular, on the analysis of the contribution of the laser energy density, which involves the laser beam parameters and also the physical properties of the target material. For different laser incident angles, the formation of micro-columns oriented towards the laser incident direction is explained. Moreover, numerical simulations and ablation experiments carried out with an excimer laser corroborate the theoretical analysis.     

The interaction and the surface crack of single-crystal silicon induced by a millisecond laser

When the silicon material is irradiated by laser, it absorbs the laser energy leading to the temperature rise and the thermal stress. The damage effect includes melting, vaporation and thermal stress damage. Once the thermal stress exceeds the stress strength the crack will initiate. The silicon surface cracks induced by a millisecond laser are investigated. The experimental results show that three types of cracks are generated including cleavage crack, radial crack and circumferential crack. The cleavage crack is located within the laser spot. The radial crack and circumferential crack are located outside the laser spot. A two-dimensional spatial axisymmetric model of silicon irradiated by a 1064 nm millisecond laser is established. To assess what stresses generate and explain the generation mechanism of the different cracks, the thermal stress fields during laser irradiation and the cooling process are obtained using finite element method. The radial stress and hoop stress within the laser spot are tensile stress after the laser irradiation. The temperature in the center is the highest but the thermal stress in the center is not always highest during the laser irradiation. The cleavage cracks are induced by the tensile stress after the laser irradiation. The radial crack and the circumferential crack are generated during the laser irradiation.     

Pulse-width influence on the laser-induced structuring of CaF2 (111)

We have investigated the morphology of CaF₂ (111) irradiated by 780 nm laser pulses of varying pulse width (200 fs-8 ns) with fluences above the damage threshold. Large differences can be observed which we relate to the mechanisms and dynamics of defect production in this wide band gap material. The best defined and most controllable ablation is obtained for laser pulse widths of a few picoseconds. For nanosecond and femtosecond pulses strong fracturing of the crystal is observed with damage outside the laser irradiated zone. This has a thermal origin for nanosecond pulses but a non-thermal origin for pulse widths below approximately 1 ps.     

Laser-induced thermal stresses on steel surface

In laser heat treatment of steels, a thin surface layer of austenite forms during heating and subsequent phase change process in the cooling period. However, thermal stress develops due to high-temperature gradient attainment in the surface vicinity which in turn results in microcrack development at the surface. The present study is carried out to compute the temperature profiles due to step input pulse laser radiation and determine the resulting thermal stresses. The study is extended to include three-step input pulses having the same energy content. This provides the comparison for the influence of the pulse length on the resulting thermal stresses. To validate the theoretical predictions, an experiment is conducted to irradiate the AISI 4142 steel surface by an Nd–YAG laser. Microphotography and EDS analysis of the heated regions are carried out. It is found that considerable thermal stress is eveloped at the workpiece surface due to attainment of high-temperature gradient in this region. In addition, microcracks are observed at the surface of the irradiated spot.     

Sub-wavelength surface structures on silicon irradiated by femtosecond laser pulses at 1300 and 2100 nm wavelengths

We present periodic ripples and arrays of protrusions formed on the surface of silicon after irradiation by low-fluence linearly polarized femtosecond laser pulses. Laser-induced periodic surface structures (LIPSS) are observed for irradiation at center wavelengths of 800, ∼ 1300, and ∼ 2100 nm, with the structure periods somewhat less than the incident wavelengths in air. Additionally, we observe structures with spatial periods substantially less than the incident laser wavelengths. These sub-wavelength periodic structures form only when the photon energy is less than the silicon bandgap energy. We discuss a number of factors which may contribute to the generation of this surface morphology.     

质子与中子辐照对电荷耦合器件暗信号参数的影响及其效应分析

对科学级电荷耦合器件(charge-coupled device, CCD)进行了质子和中子辐照试验及退火试验, 应用蒙特卡洛方法计算了质子和中子在CCD中的能量沉积, 分析了器件的辐射损伤机理. 仿真计算了N⁺埋层内沉积的位移损伤剂量, 辐照与退火试验过程中主要考察暗信号的变化规律. 研究结果显示, 质子与中子辐照均会引发暗信号退化, 其退化的规律与位移损伤剂量变化一致; 退火后, 质子辐照所致CCD暗信号大幅度恢复, 其体暗信号增加量占总暗信号增加量的比例最多为22%; 中子辐照引发的暗信号增长主要为体暗信号. 质子和中子在N⁺埋层产生相同位移损伤剂量的情况下, 两者导致的体暗信号增长量相同, 质子与中子辐照产生的体缺陷对体暗信号增长的贡献是同质的.     

Non-destructive reading of laser-induced single-shot incubation in dielectric coatings

Two-dimensional photothermal displacement measurements were carried out on TiO₂, ZrO₂, and HfO₂ coatings to uncover single shot incubation produced by 248 nm laser light at fluences below the damage threshold. The incubation behavior of the three coatings differs and correlates with the ratio of band gap to photon energy. The non-destructive nature of the photothermal displacement technique, its high lateral resolution, and its sensitivity for reading single shot imprints by an excimer laser lends this scheme a capacity for use in optical storage.Alexander von Humboldt fellow 1991/92     

飞秒激光辐照下单晶硅薄膜中超快能量输运的数值模拟

利用载流子输运模型对飞秒激光辐照下单晶硅亚微米薄膜中的能量输运过程进行数值模拟。研究了不同辐照能量密度和不同激光波长对载流子密度和温度超快变化过程的影响规律。结果表明,在800nm激光辐照下,不同入射能量密度仅影响载流子密度和温度响应的峰值,但达到峰值的时刻不变。平衡态的恢复过程受入射能量密度影响很小。在不同波长激光辐照下,光子能量越大,载流子密度和温度达到峰值所用时间越短,对应峰值越大,但衰减速度也越快。当入射光子能量大于单晶硅的直接带隙时,快速衰减时间常数可以与载流子能量弛豫时间相当。     

1064 nm和532 nm纳秒激光同时辐照熔石英损伤规律的研究

利用Nd: YAG激光器研究基频(1064 nm)与倍频(532 nm)单独辐照和同时辐照下熔石英的损伤规律,对损伤几率进行了测试,获得损伤几率曲线与典型损伤形貌。研究结果表明：双波长同时辐照下的初始损伤阈值总是小于单波长辐照下的初始损伤阈值;基频光中加入定量倍频光后,熔石英对基频光的吸收效率提高;并且双波长同时辐照下,熔石英损伤密度增大;原因主要是熔石英表面缺陷对不同波长吸收机制的差异。     

Non-linear photo-induced desorption of GaP

Laser-induced desorption of P from GaP at various photon energies near the absorption edge has been measured. The desorption yield is found to start increasing above a certain threshold laser fluence, of which the dependence on the photon energy exhibits a sharp dip near the indirect band gap besides a gradually decreasing component from the indirect band gap to the direct band gap energy. The sharp dip is ascribed to desorption induced by dense excitation of the surface states.     

Structural and optical properties of laser irradiated nano structured cadmium oxide thin film synthesized by sol-gel spin coating method

Cadmium oxide CdO nanostructured thin films are synthesized using sol-gel spin coating method. The prepared samples of CdO thin films are irradiated with 10 mJ laser from pulsed Q-Switched Nd:YAG laser at 1064 and 532 nm wavelength. The samples were exposed to 45 pulses of 7 ns pulse duration. Morphology and structural analysis were carried out with scanning electron microscope (SEM) micrographs and X-ray diffraction (XRD) patterns. Optical investigations were obtained with spectrometer and fluorospectrometer from Shimadzu. SEM micrographs confirm the nanostructure of the CdO film and indicate agglomeration of nanoparticles with laser irradiation. XRD patterns show decrease in the intensity of orientation peaks after laser irradiation. Variation in band gap energy, absorption peaks, and photoluminescence spectra with laser irradiation are observed.