LiF色心晶体—红宝石激光器被动Q开关

用着色LiF晶体中的色心做红宝石激光器的被动Q开关,得到了能量为50mJ,脉冲宽度为30ns(FWHM)的巨脉冲输出,并和隐花菁(cryptocyanine)等染料调Q作了比较.     

LiF∶F_2晶体中实现三维光数据存储的实验研究

基于LiF∶F2晶体在近红外飞秒脉冲激光作用下产生F2色心向F3 色心的转变,以及两种色心荧光光谱的不同,实现了荧光反射共焦读出和多光子写入的三维光数据存储的原理性实验.钛宝石再生放大器输出的脉冲宽度100fs、中心波长800nm、重复频率1kHz的超短脉冲激光束,用数值孔径0.68的显微物镜聚焦到LiF:F2晶体内部,通过移动晶体实现了三维逐位式数据写入;用405nm的连续蓝光激发存储位,通过探测F3 色心产生的540nm荧光,实现了对信息位进行快速非破坏性的反射共焦读出.与多光子三维存储的透射共焦散射读出和相衬读出相比,格式与现存光盘技术兼容,结构简单;更重要的是,存储信息位是依靠荧光光谱的变化,折射率变化很小,可以有效增加读出层数.     

单次相关仪中非线性晶体KD*P的相位匹配条件分析

本文对应用于单次相关仪中的非线性晶体KD^*P的相位匹配条件进行了理论分析,得到了晶体的最佳相位匹配设计参数。利用此参数加工的非线性晶体已成功地用于测量超短光脉冲实验中,测得CPM Nd:YAG激光器输出的1.06μm光脉冲宽度为14ps.与条纹相机测量的结果二者符合较好。     

光折变光栅在超短脉冲光照射下衍射的研究

利用Kogelnik单色光在体光栅中衍射效率公式推导出超短脉冲光读出稳态光折变体光栅时的衍射光频谱表达式,并在此基础上导出了脉冲光读出时的总衍射光强及总光强衍射效率.研究发现衍射光的频谱分布、总光强及衍射效率与光栅周期Λ、光栅厚度d和读出光脉冲宽度Δτ有密切的关系.通过调整这些参量,光折变光栅可以对入射的超短脉冲光实现不同程度的滤波作用.该结论可用于超短脉冲的整形研究.     

LiF∶F2晶体中实现三维光数据存储的实验研究

基于LiF∶F2晶体在近红外飞秒脉冲激光作用下产生F2色心向F+3色心的转变,以及两种色心荧光光谱的不同,实现了荧光反射共焦读出和多光子写入的三维光数据存储的原理性实验.钛宝石再生放大器输出的脉冲宽度100 fs、中心波长800 nm、重复频率1 kHz的超短脉冲激光束,用数值孔径0.68的显微物镜聚焦到LiF：F2晶体内部,通过移动晶体实现了三维逐位式数据写入；用405 nm的连续蓝光激发存储位,通过探测F+3色心产生的540 nm荧光,实现了对信息位进行快速非破坏性的反射共焦读出.与多光子三维存储的透射共焦散射读出和相衬读出相比,格式与现存光盘技术兼容,结构简单；更重要的是,存储信息位是依靠荧光光谱的变化,折射率变化很小,可以有效增加读出层数.     

超短脉冲在单块晶体中的三次谐波理论模拟

讨论了群速度失配、自相位调制和交叉相位调制、晶体厚度、基波的频率啁啾等因素对超短脉冲单块晶体的三次谐波转换效率、三倍频光脉冲形状以及频谱的影响.结果表明：群速度延迟使三倍频光脉冲展宽；自相位调制和交叉相位调制会降低转换效率,并使三倍频光脉冲形状发生畸变；晶体厚度对三倍频光脉冲频谱展宽有着较大的影响；合适的基频光频率啁啾可以减小三倍频光脉冲的畸变,并有效地提高三倍频转换效率.     

55 fs,510 mW掺铒光纤飞秒激光器

实验搭建了基于分离脉冲放大及光纤非线性压缩的掺铒光纤激光系统.通过三块长度倍增的YVO4晶体进行偏振复用,实现了八脉冲的分离与合成.探究了不同脉冲宽度的注入条件下分离脉冲主放大器的合成效率.将放大后的脉冲耦合入一段单模保偏光纤中进行非线性压缩,通过控制主放大器的抽运光功率和压缩器的光纤长度,对非线性压缩过程进行优化,获得了重复频率为80.4 MHz、平均功率为510 mW、脉冲宽度为55 fs的超短脉冲.最后,采用MgO∶PPLN晶体进行光学倍频处理,在中心波长783.4 nm处获得了平均功率为146 mW、脉冲宽度为75 fs的倍频光,相应的倍频效率为31％.     

超短脉冲照射下氟化锂的烧蚀机理及其超快动力学研究

研究了超短脉冲激光照射下LiF晶体的破坏机理及其超快动力学过程,利用扫描电镜和原子力显微镜等测试手段,观测了飞秒激光照射下LiF晶体的烧蚀形貌。利用烧蚀面积与激光脉冲能量的对数关系确定了LiF晶体的破坏阈值,并利用非线性玻璃棒展宽脉宽,得到了800nm激光作用下LiF破坏阈值对激光脉宽（50～1000fs）的依赖关系;利用抽运一探针超快探测平台,探测了LiF烧蚀过程中反射率的变化。采用雪崩击穿模型,并根据晶体材料反射率与材料的介电常量的依赖关系,通过数值计算,模拟了材料烧蚀阈值与脉宽的依赖关系及材料激发过程中反射率的变化关系。结果表明,理论结果与实验结果符合较好。讨论了飞秒激光照射下LiF晶体中导带电子数密度的变化规律,并解释了相应的实验结果。     

共线型CS_2微微秒光学快门

用钕玻璃超短脉冲作为开关脉冲,以CS_2作为光克尔介质,在共线型光路中实现了透过特性曲线宽度为15ps的超高时间分辨光学快门,并测量了超短脉冲的宽度。     

超短脉冲光通过光折变体光栅衍射及透射光强的瞬时变化特性研究

采用Kogelnik的耦合波理论,研究了超短脉冲光通过光折变体光栅的衍射及透射光强在时域的瞬时变化特性.发现衍射和透射脉冲波形与读出脉冲宽度、光栅周期、光栅厚度及光折变材料的折射率调制大小有密切关系.通过调整这些参量的数值,可以获得需要的脉冲形状.同时还发现衍射和透射脉冲在时间轴上有一定的偏移,偏移量的大小与参量选择有关.     

Picosecond light pulses influenced by the passage through a LiF color center crystal

LiF crystals containing different types of color centers were used for pulse shaping of picosecond (ps) light pulses of a Nd-glass laser. Besides the saturable absorption of the F₂-centers a two-photon one-photon step absorption was found. Inserting the crystal in the laser resonator reduces the duration and steeps the envelope of the ps-pulse train. No remarkable pulse shortening of single ps-light pulse could be achieved by passing these crystals.     

Modulation of transient stress distributions for controlling femtosecond laser-induced cracks inside a single crystal

After the photoexcitation by a femtosecond laser pulse inside a LiF single crystal, four cracks appear in the <110> directions of the crystal from the photoexcited region. In our previous study, we found that a femtosecond laser-induced stress wave is responsible for generation and elongation of cracks inside a LiF single crystal. This finding suggests that we can control laser-induced cracks by modulating laser-induced stress waves. In this study, we applied parallel fs laser irradiation with a spatial light modulator to generate multiple stress waves at the same time, and found the modulation of crack formation; one crack became thinner and shorter than any other cracks. By a pump-probe imaging of dynamics of crack generation, we showed that the constructive interference of stress waves at a crack tip could compress the crack, which results in a thinner and shorter crack.     

Ab initio calculations of electronic and optical properties in O-doped LiF crystal

We present a first-principle study of electronic and optical properties in pure LiF and O-doped LiF crystals. The pure LiF crystal exhibits a wide band gap while the O-doped LiF crystal shows the less band gap due to the contribution of O 2p. Some optical constants, such as dielectric functions, reflectivity and the refractive index, have been performed. The calculated reflectivity and refractive index from the pure LiF crystal agree with the experimental and recently calculated results in the low-energy range. Meanwhile, the optical properties have also been predicted from the O-doped LiF crystal. The absorption band in 200 nm has been observed, which is relatively close to the experimental result.      

Passive Q-switches for Nd:hosted Solid State Lasers

The currently available Nd:hosted lasers utilizing passive Q-switches: a plastic dye sheet, a LiF:F₂- color center crystal, a Cr⁴⁺:YAG crystal or a RG1000 colored glass filter have been investigated in detail and the results summarized for comparison purposes for the first time. We first briefly summarize our own and others’ past studies, and report on recent new results concerning the use of Cr⁴⁺:YAG crystals and RG1000 colored glass filters as Nd:hosted laser passive Q-switchings. We then investigate in detail solid state laser system design using the above four passive modulators. Typical interpretations are given based on characteristic features of the materials, such as their saturation intensity and relaxation times. The results are then explained via adequate equations derived with respect to the relevant physical parameters of the materials. The optical dersity selection of plastic dye sheet passive Q-switching for the Nd:YAG laser system design is verified quantitatively, and the slope efficiencies from plastic dye sheet passive Q-switched operation by single pulse and multiple pulse bursts are investigated. The multi-peak Q-switched pulse timing phenomena are presented as well. The outstanding annealability, long lifetime, inexpensiveness and high repetition rate operation of LiF:F₂- crystals are highly recommended.     

Impurity cathodoluminescence of oxygen-containing LiF crystals

Cathodoluminescence of oxygen-containing LiF crystals (LiF-O, LiF-O,OH, LiF-WO₃) is studied by pulsed spectrometry with nanosecond resolution upon excitation of crystals by a single electronbeam pulse at 15 K.     

Deformation-induced excitation of the luminescence centres in coloured alkali halide crystals

The present paper reports the deformation-induced excitation of the luminescence centres in coloured alkali halide crystals. The peaks of the mechanoluminescence (ML) in γ-irradiated KCl, KBr, KI, NaCl and LiF crystals lie at 455, 463, 472, 450 and 485 nm, i.e. at 2.71, 2.67, 2.62, 2.75 and 2.56 eV, respectively. From the similarity between the ML spectra and the thermoluminescence (TL) and afterglow spectra, the ML of KCl, KBr, KI, NaCl and LiF crystals can be assigned to the deformation-induced excitation of the halide ions in V₂-centres or any other hole centres. For the deformation-induced excitation of the halide ions in V₂-centres, or in other centres, the following four models may be considered: (i) free electron generation model, (ii) electron–hole recombination model, (iii) dislocation exciton radiative decay model and (iv) dislocation exciton energy transfer model. The dislocation exciton energy transfer model is found to be suitable for the coloured alkali halide crystals. According to the dislocation exciton energy transfer model, during the deformation of solids the moving dislocations capture electrons from the F-centres and then they capture holes from the hole centres and consequently the formation of dislocation excitons takes place. Subsequently, the energy released during the decay of dislocation excitons excites the halide ions of the V₂-centres or any other hole centres and the light emission occurs during the de-excitation of the excited halide ions, which is the characteristic of halide ions. The mechanism of ML in irradiated alkali halide crystals is different from that of the TL in which the electrons released form F-centres due to the thermal vibrations of lattices reach the conduction band and the energy released during the electron–hole recombination excites the halide ions in V₂-centres or in any other hole centres. It is shown that the phenomenon of ML may give important information about the dislocation bands in coloured alkali halide crystals.     

Multiple filamentation of femtosecond laser pulses

The formation of fluorescent channels with color centers in LiF crystals under the action of the multiple filamentation of femtosecond laser pulses is studied experimentally and theoretically for pulse powers around four orders of magnitude higher than the critical self-focusing value.     

X-ray scattering factors for ionic crystals with complete electron shells

Expressions of the X-ray scattering factors were derived for ionic crystals with complete electron shells by using the wave functions of free ions. The non-orthogonality of the atomic wave functions in a crystalline state was taken into account by means of Löwdin’s orthogonalization method when presenting the electron density of crystals. In the case of a small overlap between the wave functions localized at the lattice sites, it was plausible to describe mathematically a crystal ion containing small overlap charge contribution in addition to the free-ion contribution. As an application, the scattering factors for the LiF crystal were computed. The theoretical results obtained agree well with recent experimental data.     

Laser and self-Raman-laser oscillations of PBMoO4:Nd3+ crystal under laser diode pumping

Different regimes of laser operation and stimulated Raman scattering (SRS) for a laser diode (LD)-pumped PbMoO4:Nd3+ crystal have been investigated. Under 1.5 W LD pumping, the output power of the PbMoO4:Nd3+ laser in the free-running mode at 1056 nm reached 0.65 W with a slope efficiency of up to 53%. Using different LiF: F-2 saturable absorbers, we achieved a maximum pulse energy of 11 microJ at a 1.4 ns pulse duration and a maximum output power of 0.35 W (10 microJ) at a 7 ns pulse width in the passively Q-switched mode. PbMoO4:Nd3+ self-Raman-laser operation at 1163 nm (first Stokes) was obtained for the first time, to our knowledge. The self-Raman-laser output pulse energy was measured to be as high as 6 microJ and the pulse duration was estimated to be shorter than 500 ps.