不同脉冲宽度355 nm波长激光诱导DKDP晶体损伤特性

研究了用于三倍混频的Ⅱ类DKDP晶体在35 ps,850 ps和7.6 ns三种不同脉宽355 nm波长激光作用下的损伤特性。实验对比分析了损伤阈值、概率和损伤针点形貌、尺寸和密度,并根据损伤阈值及概率得到前驱体阈值及密度。结果表明,前驱体的激光能量吸收量与脉宽线性相关。35 ps激光作用下DKDP有一种前驱体吸收激光能量形成熔融状损伤针点。850 ps激光作用下有两种前驱体吸收激光能量并产生力学破坏形成中心熔融四周断裂的损伤针点。7.6 ns激光作用下只有一种前驱体吸收激光能量,并且形成的损伤针点与850 ps对应的损伤针点有相同特征。     

Color center production by femtosecond-pulse laser irradiation in fluoride crystals

Color centers are lattice vacancy defects trapping electrons or holes. They are easily created in single crystals by irradiation with ionizing radiation. We report the production of color centers in LiF and LiYF₄ single crystals by ultrashort high-intensity laser pulses (60 fs, 12.5 GW). An intensity threshold for color center creation of 1.9 and 2 TW/cm² was determined in YLF and LiF, respectively, which is slightly smaller than the continuum generation threshold. Due to the high energy density of the coherent radiation of the focused laser beam, we were able to identify a large amount of F centers, which gave rise to aggregates such as F₂, F ₂ ⁺ , and F ₃ ⁺ . The proposed mechanism of formation is based on multiphoton excitation, which also produces short-lived F ₂ ⁺ centers. It is also shown that it is possible to write tracks in the LiF crystals with dimensional control.     

Glass modification by continuous-wave laser backside irradiation (CW-LBI)

We report on a permanent change in the physical properties inside glass that is rapidly heated and quenched with a continuous-wave (CW) laser beam. The absorption of the glass was enhanced by laser heating, and the heated spot moved by thermal radiation and conduction. To trigger the heating, an absorbent material was placed on the backside of a glass plate and irradiated through the glass. The laser beam can modify borosilicate glass with a high aspect ratio (∼100:1) at a rate of ∼130 mm/s. The modified zone consists of two concentric cylindrical zones and is crack-free.     

不同温度生长DKDP晶体的性质

用点籽晶快速生长法在不同温度区间生长了四块氘含量60%DKDP晶体,观察了不同温度下晶体生长过程,同时测量了晶体的透过光谱、摇摆曲线和1053nm激光损伤阈值。结果表明,在高温区间晶体生长外形优于低温生长的晶体。不同温区晶体的透过光谱并无明显变化,但是随着生长温度的降低,晶体的结构完整性变差,1053nm激光损伤阈值降低。     

New scheme of a petawatt laser based on nondegenerate parametric amplification of chirped pulses in DKDP crystals

The ultra-broadband phase matching was experimentally observed in a DKDP crystal upon parametric amplification of signal radiation with a wavelength of 911 nm in a pump field with a wavelength of 527 nm. The original scheme was used to excite the first parametric amplification stage by chirped pulses of idler radiation with a wavelength of 1250 nm. The saturated gain of a three-stage parametric amplifier was equal to 10⁸.     

-1不同温度生长DKDP晶体的性质

用点籽晶快速生长法在不同温度区间生长了四块氘含量60% DKDP晶体,观察了不同温度下晶体生长过程,同时测量了晶体的透过光谱、摇摆曲线和1053 nm激光损伤阈值。结果表明,在高温区间晶体生长外形优于低温生长的晶体。不同温区晶体的透过光谱并无明显变化,但是随着生长温度的降低,晶体的结构完整性变差,1053 nm激光损伤阈值降低。     

Swelling and modification of silicone surface by F2 laser irradiation

The surface of silicone rubber swelled and was modified by 157-nm F₂ laser irradiation at a laser fluence less than the ablation threshold. The irradiated surface swelled to a height of approximately 3 m. Fourier-transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy showed that the irradiated surface was modified to SiO₂. 193-nm ArF laser irradiation of the silicone rubber induced the surface to swell, but not to modify to SiO₂. The IR peaks of end groups of silicone were observed in the FT-IR spectra of the surface. From these results, it is concluded that the main chains (Si-O) of silicone were photodissociated and generation of low molecular weight silicones caused the swelling. In addition, it was observed that methane and carbon dioxide were released from silicone rubber when each laser beam irradiated it. These gases were generated by photodissociation of the side chains (Si-CH₃) of silicone. An F₂ laser beam can photodissociate the Si-CH₃ bonds and the Si-O bonds of silicone and O₂ effectively even at a laser fluence less than the ablation threshold, resulting in the modification to SiO₂ and the swelling. PACS 61.80.Ba; 61.82.Pv; 82.50.Hp     

Defect-density switching wave in crystals under pulsed laser irradiation

A model is proposed for the ignition of a defect-density switching wave in a crystal by powerful laser pulses. It is shown that the switching wave arises as a result of the nonlinear dependence of the activation energy of the defect-formation process on the strain field due to defects. The conditions under which a switching wave arises and the profile, velocity, and propagation direction of the wave are discussed. Zh. Tekh. Fiz. 68, 73–77 (August 1998)     

Microwave dielectric tangent losses in KDP and DKDP crystals

By adding cubic and quartic phonon anharmonic interactions in the pseudospin lattice coupled mode (PLCM) model for KDP-type crystals and using double-time temperature dependent Green's function method, expressions for soft mode frequency, dielectric constant and dielectric tangent loss are obtained. Using model parameters given by Ganguliet al [9] the dielectric losses are calculated for KDP and DKDP crystals. In the microwave frequency range an increase in frequency (1–35 GHz) is followed by an increase in dielectric tangent loss (1–35) at 98 K and (1–15) × 10⁻² at 333 K for KDP and DKDP crystals respectively. The dielectric tangent loss decreases from 0.052 to 0.042 for KDP crystals with increase in temperature from 130 to 170 K and for DKDP crystals it decreases from 0.0166 to 0.0074 with an increase in temperature from 230–343 K in their paraelectric phases at 10 GHz. This shows Curie-Weiss behavior of the dielectric tangent loss     

不同温度生长DKDP晶体的性质

用点籽晶快速生长法在不同温度区间生长了四块氘含量60% DKDP晶体, 观察了不同温度下晶体生长过程, 同时测量了晶体的透过光谱、摇摆曲线和1053 nm激光损伤阈值。结果表明, 在高温区间晶体生长外形优于低温生长的晶体。不同温区晶体的透过光谱并无明显变化, 但是随着生长温度的降低, 晶体的结构完整性变差, 1053 nm激光损伤阈值降低。     

Near-field-optical-microscopy studies of micro-modifications caused by femtosecond laser irradiation in lithium niobate crystals

Near-field-optical-microscopy has been used to study the micro-modifications caused by femtosecond laser pulses focused at the surface and in the volume of lithium niobate crystals. We have found experimental evidence of the existence, close to femtosecond ablation craters, of periodic modifications in the surface reflectivity. In addition, the potential application of near-field-optical microscopy for the spatial location of permanent modifications caused by femtosecond pulses focused inside lithium niobate crystals has been also demonstrated.     

Etch frustration in congruent lithium niobate single crystals induced by femtosecond ultraviolet laser irradiation

Modification of the chemical etching behaviour of undoped congruent z-cut lithium niobate single crystals is achieved by pre-illumination of the –z face of the crystal with sub-ps ultraviolet laser radiation at 248 nm, at energy fluences below the threshold for ablation. A systematic study of the effect of the energy fluence and total exposure on the etch frustration is presented. Received: 23 July 2001 / Accepted: 28 September 2001 / Published online: 20 December 2001     

Effect of high-energy electron irradiation on forsterite laser crystals

The effect of 21-MeV electron irradiation on the optical absorption characteristics of Czochralski-grown forsterite (Mg₂SiO₄) single crystals (both undoped and chromium-doped) has been investigated. The irradiation is found to induce additional optical absorption (AOA) in the crystals in the range of 225–1200 nm due to the formation of color centers based on intrinsic host point defects and the change in the oxidation state of chromium ions. The AOA spectra have been decomposed into elementary bands. The influence of the chromium concentration in crystals, the oxygen content in the growth atmosphere, and additional doping with lithium on the behavior of these bands has been analyzed. A possible structure of the color centers responsible for the AOA is discussed. It is shown that the electron irradiation somewhat decreases the intensity of the characteristic absorption bands of tri- and tetravalent chromium ions and gives rise to a new absorption band in Mg₂SiO₄:Cr and Mg₂SiO₄:Cr,Li crystals heavily doped with chromium.     

大口径DKDP晶体受激拉曼散射增益系数的测量（英）

大口径KDP/DKDP晶体在强紫外光辐照下产生横向受激拉曼散射效应（TSRS）, 受激放大的拉曼散射光将导致激光能量损失甚至激光损伤, 测量DKDP晶体TSRS增益系数对设置激光装置的运行区间以确保晶体的安全使用非常重要。采用高精度光谱仪探测大口径DKDP晶体（氘含量65%）在351 nm激光辐照下的横向拉曼散射信号, 得到了拉曼散射光的增长曲线, 拟合得到的拉曼增益系数为0.109 cm/GW。同时, 实验结果表明晶体体损伤不影响TSRS增长行为, 表明晶体体损伤对拉曼增益系数测量结果的影响可以忽略。     

大口径DKDP晶体受激拉曼散射增益系数的测量（英）

大口径KDP/DKDP晶体在强紫外光辐照下产生横向受激拉曼散射效应（TSRS）, 受激放大的拉曼散射光将导致激光能量损失甚至激光损伤, 测量DKDP晶体TSRS增益系数对设置激光装置的运行区间以确保晶体的安全使用非常重要。采用高精度光谱仪探测大口径DKDP晶体（氘含量65%）在351 nm激光辐照下的横向拉曼散射信号, 得到了拉曼散射光的增长曲线, 拟合得到的拉曼增益系数为0.109 cm/GW。同时, 实验结果表明晶体体损伤不影响TSRS增长行为, 表明晶体体损伤对拉曼增益系数测量结果的影响可以忽略。     

V centers produced in KI crystals under KrCl excimer laser irradiation

Abstract

Irradiation of KI near 150 K with KrCl excimer laser irradiation (hv=5. 58 eV) produces V centers causing V₂ and V₃ bands. The two bands exhibit 100-type dichroism. In KI containing V centers, the 111 cm^?1 Raman signal attributed to I_3- molecular ions is observed. Under KrCl excimer laser irradiation at low temperatures, resonance Raman scattering effects have been also studied for KI, NaI and LiI.     

不同方法DKDP晶体生长和损伤阈值

生长方法是影响DKDP晶体生长和光损伤阈值的一个重要因素。分别采用传统降温法和点籽晶快速生长法,利用同种原料从氘化程度为85%的溶液生长了DKDP晶体,并选取部分Ⅱ类3倍频晶片进行3倍频光损伤阈值和透过性能测试。实验表明,不同生长方法对DKDP晶体的损伤阈值以及DKDP晶体的生长速度的影响效果正好相反,即由于生长溶液过饱和度的差别,点籽晶快速生长法晶体生长速度为传统方法的10倍,但晶体损伤阈值下降了50%,且紫外波段透过性能下降明显。     

Softening of phonon modes in C60 crystals induced by laser irradiation: Thermal effects

Reversible softening of the intramolecular A _g(2) pentagonal pinch (PP) mode of a C₆₀ single crystal in the face centered cubic phase has been studied as a function of laser power density by means of Raman scattering. The average temperature rise in the laser excitation spot has been determined using the Stokes to anti-Stokes integrated peak intensity ratio for the H _g(1) phonon mode. Softening of the PP-mode was found to be due to heating of the sample resulting from laser irradiation, in good quantitative agreement with experimental results obtained for uniformly heated samples. These findings are in excellent agreement with results obtained by numerical calculations of the local temperature distribution and average temperature in the laser spot based on calculated integrated intensities of the Stokes and anti-Stokes bands of the PP-mode. These calculations were based on experimental data for the temperature dependence of phonon frequency and width, absorbance, and thermal conductivity in solid C₆₀. Zh. éksp. Teor. Fiz. 114, 1785–1794 (November 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor     

大口径DKDP元件的辐照损伤分布特性

大口径高能激光装置是各强国积极研究的重点项目。对装置内大口径光学元件损伤特性进行有效评估具有非常重要的意义,在此研究大尺寸光学元件表面损伤。通过分段拍摄、图像拼合、损伤点记录、统计与归纳等工作发现,不同尺寸损伤点的分布特性差异较大。结合统计学方法与类似实验对比、理论计算等方式对损伤点分布与样品辐照环境特性变化的关系进行分析。结果显示,损伤点的位置分布与辐照光束的能量密度关联紧密;系统光束(351 nm)在低于6 J/cm²时能量分布均匀,高于6.7 J/cm²时呈现较为明显的高斯分布状态。可以为大口径高能辐照环境的元件损伤特性评估提供有价值的参考,对大口径紫外激光器的日常运行与维护具有极其重要的工程意义。     

不同方法DKDP晶体生长和损伤阈值

生长方法是影响DKDP晶体生长和光损伤阈值的一个重要因素。分别采用传统降温法和点籽晶快速生长法,利用同种原料从氘化程度为85%的溶液生长了DKDP晶体,并选取部分Ⅱ类3倍频晶片进行3倍频光损伤阈值和透过性能测试。实验表明,不同生长方法对DKDP晶体的损伤阈值以及DKDP晶体的生长速度的影响效果正好相反,即由于生长溶液过饱和度的差别,点籽晶快速生长法晶体生长速度为传统方法的10倍,但晶体损伤阈值下降了50%,且紫外波段透过性能下降明显。