Experimental evaluation of the measurement of isotope ratios by time of flight mass spectrometry with laser ablation

Time of flight mass spectrometry (TOF-MS) in combination with laser ablation (LA) applied for low isotope composition measurements is limited by energy and spatial dispersion of ions, spatial charge density, and some other factors. In order to reduce these factors that make the extraction of the plasma ions difficult, we used a second laser to create ions by non-isotope-selective photoionization of neutral particles. The photoionization was carried out in uranium vapor beam by the third harmonic of a pulsed Nd:YAG laser. This work presents the results of tentative experiments to estimate the characteristics of the LA-TOF-MS method with non-selective photoionization.     

On the capabilities of sub-Doppler photoionization spectroscopy in thin gas cells

A high-sensitivity photoionization method of registration of narrow sub-Doppler resonances in the spectral distribution of a flow of metastable atoms (or molecules) excited from the ground quantum term by a monochromatic laser beam propagating at normal incidence through an ultrathin gas cell (with a micrometer-scale or even nanoscale gas layer thickness) is proposed. Based on density matrix equations for atomic particles, various mechanisms of broadening of the considered resonances, such as time-of-flight, field, and Doppler broadening, are analyzed. The requirements for laser beam parameters and gas cell dimensions that allow obtaining the narrowest resonances are established. The proposed method can be used in ultrahigh resolution spectroscopy of atoms and molecules, as well as high-precision optical frequency standards.     

Hollow-tip scanning photoelectron microscopy

A new type of microscopy based on scanning in vacuum by a beam of charged particles transmitted through a hollow probe has been implemented. This approach provides controllable motion of spatially localized ion, electron, molecular (atomic), and soft X-ray beams and investigation of the surface in the shear force mode. In the photoelectron mode, in which electrons are transmitted through a 2-μm quartz capillary, a surface profile of gadolinium irradiated by 400-nm femtosecond laser pulses has been visualized with a subwave spatial resolution. The new method of microscopy opens an opportunity of investigations in the field of nanometer local photodesorption of molecular ions (one of the last ideas of V.S. Letokhov).     

基于声光可调谐滤光器的显微光谱成像技术

为了解决传统声光可调谐滤光器(AOTF)成像模糊的缺点,设计了一种新的AOTF。该器件通过在传统的AOTF的出射孔后面放置一个自行设计的等边色散棱镜来实现对衍射光的色散展宽进行补偿,明显地提高了成像的对比度和空间分辨率。将此器件附加在传统光学显微镜上,获得了一种新型的光谱显微成像仪器。其光谱分辨率在575nm波长处为4.2nm、成像空间分辨率为2μm、图像采集速度为毫秒量级。为基于AOTF的光谱成像技术在生物医学等领域的更广泛的应用奠定了基础。     

Visualization of the spatio-temporal structure of a pulsed photoelectron beam formed by femtosecond laser radiation

A method based on an original electron microscope created for investigating photoelectron beams is presented. It ensures a nanometer spatial resolution and picosecond time resolution. Electrons appearing when a metal needle is irradiated by femtosecond laser pulses are transmitted through a dielectric microcapillary and are subjected to a ponderomotive potential created by femtosecond laser radiation focused near the capillary tip. The position-sensitive detection scheme allows for the detection of the spatial profile of a photo-electron beam with a magnification of K ≅ 4 × 10⁴. The time structure of the photoelectron beam is visualized by scanning the delay time between laser pulses irradiating the needle and a laser pulse focused near the capillary tip.     

铁、铬、钒原子激发态的光电离截面

这篇文章分别在278~288 nm、335~361 nm、300~310 nm激光波长范围内分别报道过渡金属中的铁原子、铬原子和钒原子的共振增强多光子电离(REMPI)的实验结果. 实验中采用激光烧蚀和分子束相结合的技术制备金属原子.光离子产物由飞行时间质谱仪检测.根据共振电离离子信号对探测激光功率的依赖关系,得到原子激发态的光电离截面值.     

Research of spatial resolution in external electro-optic probing

A new method, based on zero point of longitudinal electric field, was used to determine the spatial resolution of external electro-optic (EO) probing equipment. Considering the diffraction of Gauss beam, the result of external EO probing was simulated which was in accordance with the experiment. A spatial resolution <1 μm was demonstrated initially in our equipment using 650 nm laser diode as probe beam and semi-insulating GaP as probe tip.     

一种大面积高能激光光束参数的在线测量方法

 提出了一种大面积连续波高能激光光束参数的在线测量方法——环形光刀扫描测量法。该方法采用偏心安装的斜面环形光刀高速扫描反射,光电探测器阵列沿反射光圆周均匀布置探测,使得绝大部分被测激光沿原光路传播,只有少量取样光被反射到探测器阵列上。通过对采集得到的探测器响应信号进行空间映射计算和图像复原,得到激光束的光强分布参数。该方法可用于光束直径数百mm的高能激光光束测量,测量空间分辨率约2 mm,时间分辨率为30~50 ms。     

用脉冲激光全息干涉术测量稠密等离子体电子密度分布

利用脉冲激光作为探测光源,采用全息双曝光法对激光惯性约束核聚变高温高稠度等离子体的诊断,开发了原理上全新的诊断方法.打靶主激光与探测激光实现严格同步Δt≤10^-11～10^-10s,可获得高空间分辨率Δδ≤1μm等离子体二维图象(阴影图象和干涉图象),并保证时间分辨率Δt达到10^-11s左右.记录等离子体折射率空间分布是测定密度剖面变化和计算等离子体流体动力学参量的基础.     

High resolution scanning microanalysis on material surfaces using UV femtosecond laser induced breakdown spectroscopy

Femtosecond lasers together with high resolution optics have given us the ability to achieve submicron ablation spots which can play an important role in specific micromachining applications. Light emitted from the plasma at the sample surface created by a focused femtosecond laser pulse can also be used in laser induced breakdown spectroscopy (LIBS) and allows us to characterize the chemical composition of the target surface with micron-level lateral resolution. The spatial resolution using LIBS has often been defined by measuring the FWHM of the crater size. In this report, we study the application of femtosecond 266 nm laser pulses with very low energies of 10׳s of nanojoules. We have investigated spatial resolution using the detection of thin strips of chromium on silicon substrates and compared the actual width of the chromium versus the experimentally obtained width using LIBS detection. The variation of signal levels for low pulse energies is investigated on chromium surfaces. A spatial resolution of ~1 μm was obtained for detection of chromium from the emission.     

激光等离子体温度测量方法设计

为了获得高时间和空间分辨率的激光等离子体温度测量结果,基于等离子体温度测量的二谱线法,设计了一种激光等离子体温度测量方法。介绍了该方法的测量原理及时间空间分辨率实现方法。利用这种方法获得了空间分辨率微米量级、时间分辨率纳秒量级的激光等离子体温度测量结果,测量得到的激光等离子体温度为10^4K量级。实验结果表明,该方法能够获得具有很高时间空间分辨率的温度测量结果,具有较好的可行性。     

Photoionization mass-spectrometry of benzene and benzaldehyde molecules with an excimer KrF laser

A laser photoionization time-of-flight mass-spectrometer has been developed and used to investigate the photoionization and photofragmentation of benzene and benzaldehyde molecules by an excimer KrF laser radiation at the wavelength of 249 nm in the intensity range from 5·10⁴ to 5·10⁹ W/cm². It has been found that at low laser intensity ions formed by two-step photoionization are most abundant in mass spectra. By increasing laser intensity an extensive fragmentation of molecules, up to C⁺ ions, was observed. The maximum ionization yield of benzaldehyde comes to 0.1% of the number of molecules in the photoionization volume and, according to calculations, to 10% for benzene molecules, when the radiation intensity is 5·10⁹ W/cm².     

Spaser spectroscopy with subwavelength spatial resolution

We propose a method for high-sensitivity subwavelength spectromicroscopy based on the usage of a spaser (plasmonic nanolaser) in the form of a scanning probe microscope tip. The high spatial resolution is defined by plasmon localization at the tip, as is the case for apertureless scanning near-field optical microscopy. In contrast to the latter method, we suggest using radiationless plasmon pumping with quantum dots instead of irradiation with an external laser beam. Due to absorption at the transition frequencies of neighboring nano-objects (molecules or clusters), dips appear in the plasmon generation spectrum. The highest sensitivity is achieved near the generation threshold.     

大芯径光纤整形飞秒激光脉冲空间分布的研究

通过将1 kHz重复频率的飞秒放大激光脉冲耦合到大芯径(100μm)阶跃光纤,在27 mm长的光纤中产生了环形空间光强分布,并在3 160 mm的长光纤中观察到平台型的空间光强分布,通过自聚焦效应对该现象进行了解释.结果表明,通过选择合适的光纤,可以实现对放大飞秒激光脉冲的有效空间整形,从而达到改善光束质量的效果.     

反射式液晶空间光调制器电控光束偏转

为获得较大的光束电控偏转范围,使用空间分辨力高达8 m的反射式液晶空间光调制器实现了对入射632 nm激光的电控偏转。利用双光束共焦干涉方法测量了液晶空间光调制器的电控相位延迟特性,最大相移量可达3。根据二元光学理论和器件的电控相位延迟特性,设计了周期台阶相位模式和相应的加载灰度图,以最大衍射效率实现对入射光束的闪耀电控偏转。结果表明:相位模式台阶数为8时,可以实现10 mrad的光束偏转,闪耀级次衍射效率可达46%。利用二元光学的衍射模型对影响衍射效率的关键因素进行了分析,认为器件较低的填充因子和周期台阶模式相位重置点诱导的指向矢回程区是限制光束衍射效率的主要因素。     

Femtosecond laser processing of indium-tin-oxide thin films

Micro- and nano-scale crystalline indium-tin-oxide (c-ITO) patterns fabricated from amorphous ITO (a-ITO) thin films on a glass substrate using a (low NA 0.26) femtosecond laser pulse that is not tightly focused are demonstrated. Different types of c-ITO patterns are obtained by controlling the laser pulse energies and pulse repetition rate of a femtosecond laser beam at a wavelength of 1064 nm: periodic micro c-ITO dots with diameters of ~1.4 μm, two parallel c-ITO patterns with/without periodic-like glass nanostructures at a laser scanning path and nano-scale c-ITO line patterns with a line width ~900 nm, i.e. ~1/8 of the focused beam׳s diameter (7 μm at 1/e²).     

纳秒强光场下碘甲烷的激光电离中高价离子产生研究

利用25纳秒脉冲Nd-YAG 532 nm的激光,在1011 W cm-2的光场强度下,用飞行时间质谱对不同载气条件下碘甲烷的激光电离过程进行了研究.当利用氩作为载气时,除观察到H+,C+,CH+,CH+3,I+,CH3I+等离子外,还观察到很强的C2+,I2+和I3+离子信号.这些高价离子的最可几平动能分别为55.5 eV,9.5 eV和27 eV.质谱峰形的分裂现象以及不同载气的实验结果表明这些高价离子可能来源于碘甲烷团簇的库仑爆炸过程.     

Saturated Förster resonance energy transfer microscopy with a stimulated emission depletion beam: a pathway toward single-molecule resolution in far-field bioimaging

We theoretically demonstrate that the spatial resolution of stimulated emission depletion (STED) microscopy can be substantially enhanced without increasing the intensity of the STED beam. In our scheme, tiny nanobeads codoped with donor and acceptor molecules are used as fluorescent probes, in which F?rster resonance energy transfer (FRET) can occur with an ~100% efficiency between the donors and acceptors. Enhancement of the depletion of acceptors in the nanobeads with the doughnut-shaped depletion beam can lead to an increase of FRET efficiency in the outer area of the excitation spot, which itself is used for deexciting donor molecules and, consequently, enhancing the optical resolution.     

正交狭缝扫描式激光光束质量测量方法研究

传统激光光束质量测量方法在CCD相机靶面前激光光路上加装可调衰减模块,对激光光束进行衰减。但该方法受限于CCD相机像元尺寸限制,难以进行高精度测量。为此该文提出了一种滚轮狭缝式激光光束质量评价方法,在测量时采用狭缝滚轮上的扫描狭缝直接扫描被测激光光束,使用InGaAs探测器配合聚焦透镜进行测量。经过与电机同轴的高精度增量式编码器确保采集位置与采集数据同步,扫描频率根据被测激光脉冲频率和光斑的直径范围可进行调整,该方法对光斑空间采样分辨率优于1 μm。实验结果表明,采用该方法测得的激光M2因子数值与被测激光器提供数值一致,测量不确定度小于10%。     

Photothermal deflection microscopy for imaging sub-micronic defects in optical materials

Photothermal deflection is widely used to study defects in optical coatings and role of these defects in laser damage. Because defects responsible for laser damage are assumed to be nanometer-sized and lowly absorbing, both high spatial resolution and high sensitivity are required to detect them. In this work we theoretically and experimentally explore the capability of collinear photothermal deflection to give micronic resolution by reduction of the pump beam diameter. Thanks to a model describing temperature distribution and photothermal deflection, we have studied the effects of pump beam focusing on photothermal deflection. Then, we have developed a high resolution, high sensitivity microscope based on the photothermal deflection of a transmitted probe beam. The setup is characterized and the theoretical predictions are checked. We present a test of lateral spatial resolution obtained on specially prepared absorbing resolution targets and show that a lateral spatial resolution of 1 μm is reached on non-isolated defects. In case of single defects, we expect that 10 nm sized defects could be detected.