Polarization-dependent femtosecond laser filamentation in air

We report on a systematic study of the laser polarization effect on a femtosecond laser filamentation in air.By changing the laser's ellipticity from linear polarization to circular polarization, the onset position of laser filament formation becomes farther from the focusing optics, the filament length is shorter, and less laser energy is deposited. The laser polarization effect on air filaments is supported by a simulation and analysis of the polarization-dependent critical power and ionization rates in air.     

Terahertz radiation source in air based on bifilamentation of femtosecond laser pulses

A new terahertz (THz) source in air based on the bifilamentation of femtosecond laser pulses is reported. This THz radiation is 1 order of magnitude more intense than the transition-Cherenkov THz emission from femtosecond laser filaments reported recently and shows different angular and polarization properties. We attribute it to the emission from a bimodal transmission line created by two plasma filaments.     

飞秒激光诱导钛表面形成高空间频率周期条纹结构

利用波长为800 nm的飞秒激光,在空气和去离子水中诱导钛表面形成不同的周期条纹结构。在空气中,激光能量密度为0.265 J/cm2时,钛表面主要形成周期为500~560 nm低空间频率条纹结构;激光能量密度为0.102 J/cm2时,主要形成的是周期为220~340 nm高空间频率条纹结构。两种条纹均垂直于入射激光偏振方向,且条纹周期随着脉冲重叠数的增大而增大。在水中,除形成垂直激光偏振方向、周期为215~250 nm的高空间频率条纹结构,还形成了平行于激光偏振方向且周期约为入射激光波长八分之一的高空间频率条纹结构。利用表面等离子体理论、二次谐波及Sipe理论对各种周期条纹结构的形成机理进行分析,发现周期条纹结构的形成与钛表面氧化层有密切的关系。     

LIPS技术测量CH_4/空气预混火焰温度

简述了激光诱导偏振光谱技术的测温原理和实验方法,并利用激光诱导偏振光谱技术测量了常压CH4/空气预混火焰的温度分布。该技术灵敏度高,且不受碰撞猝灭的影响,适于诊断各种实际的燃烧过程。通过研究激光能量与信号强度的关系,获得了激光功率密度与信号强度的关系曲线以及实验中的最佳激光功率密度;记录并测量了燃烧场中OH分子A2∑＋-X2Π（0,0）跃迁带中P 1（2）和Q 1（8）两条吸收线的强度,用双线法计算了相应的温度,给出了CH4/空气预混火焰中不同化学配比条件下燃烧场温度的空间分布,实验结果与CARS的测量结果吻合较好。     

对超短脉冲强激光在大气通道中产生的三次谐波偏振特性及白光光谱调制特性的研究

研究了超短脉冲强激光在大气中传输产生电离通道后辐射的三次谐波强度随激光偏振的变化.此外，还研究了产生的白光辐射在短波长上的有规律的谱调制，分析了白光谱调制的产生机理. 关键词： 超短脉冲激光 大气通道 超连续辐射 三次谐波     

1550nm-VCSELs在偏振保持光反馈和正交光注入下的偏振转换特性

基于自旋反转模型,研究了1550 nm垂直腔面发射激光器(1550 nm-VCSELs)在偏振保持光反馈和正交光注入下的偏振转换特性.结果表明:正交光注入下的从激光器会随着注入强度的增加产生偏振转换.在归一化注入电流较小时,改变反馈强度,会使从激光器发生偏振转换的注入强度出现规律不同的变化;改变频率失谐,会使从激光器发生偏振转换的注入强度出现规律相同的变化.     

Hollow-core photonic bandgap fiber polarizer

Broadband, compact in-fiber polarizers were fabricated using a pulsed CO(2) laser to modify the air holes along one side of the hollow-core photonic bandgap fibers. The polarizers have lengths from 3 to 6 mm and exhibit a polarization extinction ratio of better than 20 dB over a wavelength range larger than 100 nm at approximately 1,550 nm.     

纳秒激光诱导空气等离子体射频辐射特性研究

开展了波长为532 nm、脉宽为8 ns的纳秒激光诱导空气等离子体射频电磁辐射特性实验研究,基于锥形天线探测空气等离子体在30-800 MHz频谱范围有较强的射频电磁辐射,是等离子体内电偶极子振荡变速运动造成的.实验结果表明:随激光能量增加,30-200 MHz范围内射频辐射强度逐渐变强,但360-600 MHz频率范围射频辐射强度逐渐变弱.等离子体射频辐射的空间分布依赖于入射激光的偏振方向,当激光偏振方向与天线放置方向一致时,该方向上空气等离子体的射频辐射强度高,谱线较丰富.射频辐射总功率随激光能量先增加后降低,采用等离子体电子密度变化对等离子体频率及等离子体衰减系数影响(制约)关系,对射频辐射总功率随激光能量的变化规律进行了解释.     

Control of supercontinuum generation with polarization of incident laser pulses

Supercontinuum generation is dependent on the polarization state of the incident laser. The polarization of the generated supercontinuum is the same as that of the incident laser. The magnitude of the generated supercontinuum depends on the polarization of the incident laser and increases as the polarization changes from circular to linear, irrespective of the nature of the sample, be it isotropic, anisotropic, or chiral. In all samples, the polarization dependence indicates a preference for the linear component of the incident laser beam. The anisotropic sample shows an additional difference in the generated supercontinuum for the two perpendicular directions of the incident laser polarization. PACS 42.25.Ja; 42.65.Ky     

Fluctuations of the polarization state of laser light

The instantaneous polarization of a laser fluctuates between different elliptical states resulting in a partial polarization. The fluctuations are calculated for a laser with partial linear polarization from the known statistical distributions of the amplitude and phase of laser light. A mean square deviation of the polarization direction of 5 s of arc results for a typical He-Ne-laser.     

Fluctuations of the polarization state of laser light

The instantaneous polarization of a laser fluctuates between different elliptical states resulting in a partial polarization. The fluctuations are calculated for a laser with partial linear polarization from the known statistical distributions of the amplitude and phase of laser light. A mean square deviation of the polarization direction of 5 s of arc results for a typical He-Ne-laser.     

Efficient generation of third harmonic radiation in air filaments: A revisit

In this paper we revisit the third harmonic generation from a femtosecond laser filament in air and its significant enhancement (~ 100 times) with a intercepting pump pulse, which has been reported very recently. The enhanced third harmonic is studied as a function of the pulse polarization, time delay between the pump and signal pulses, laser pulse energy, etc. We provide an explanation for the enhancement of third harmonic based on a quenching of interference effects present near filamentation threshold. Simulations based on a two-color propagation code reproduce well the experimental observations and confirm our interpretation.     

Effects of laser intenstiy on the emission direction of fast electrons in laser-solid interactions

The dependence of emission direction of fast electrons on the laser intensity has been investigated. The experimental results show that, at nonrelativistic laser intensities, the emission of fast electrons is mainly in the polarization plane. With the increase of the laser intensity, fast electrons emit towards the laser propagation direction from laser polarization direction. At relativistic laser intensities, fast electrons move away from the laser polarization plane, closely to the reflection direction of the incident laser beam.     

Effects of laser intensity on the emission direction of fast electrons in laser-solid interactions

The dependence of emission direction of fast electrons on the laser intensity has been investigated. The experimental results show that, at nonrelativistic laser intensities, the emission of fast electrons is mainly in the polarization plane. With the increase of the laser intensity, fast electrons emit towards the laser propagation direction from laser polarization direction. At relativistic laser intensities, fast electrons move away from the laser polarization plane, closely to the reflection direction of the incident laser beam.     

Femtosecond laser writing of nanostructures on bulk Al via its ablation in air and liquids

We report on the formation of self-organized nanostructures (NS) on bulk Al under its ablation in air and liquids with femtoseconds (fs) laser pulses. In case of exposure into liquids, NS are regularly formed on the Al surface with an average period of about 200 nm, independent of the laser polarization. A dispersion of Al nanoparticles (NPs) into the liquid additionally occurs. Irregular nano-bumps are produced when the irradiation is performed in air. NP dispersions as well as NS formed on Al surface show a characteristic absorption peak in the near UV which has been attributed to plasmon oscillation of electrons. The wings of this peak extending to the visible, lead to a distinct yellow coloration of the processed Al surface and the liquid dispersions. Ultrafast laser processing of bulk Al in liquids may be potentially a promising technique for efficient production of nanosized aluminum.     

激光在大气中驱动的强太赫兹辐射的理论和实验研究

两束双色激光脉冲能在大气中产生MV/cm的强太赫兹波.本文主要介绍了我们最近的三项理论和实验工作,澄清了双色激光方案的物理机制这个长期存在的问题,并对该方案进行了推广.为了在气体中有效地产生太赫兹波,在广泛研究的双色激光方案中两束激光的频率比ω_2/ω_1总是被取为1:2.首先从理论上预测采用其他频率比时,此方案仍能有效地工作,并通过实验进行证实.实验上观察到在新的频率比ω_2/ω_1=1:4,2:3下,也能有效地产生太赫兹波;观察到通过旋转较长波长的激光脉冲的偏振方向,能够有效地调节太赫兹波的偏振,但是旋转波长较短的激光脉冲的偏振方向,太赫兹波的偏振几乎没有变化,这违背了多波混频理论中极化率张量对称性的要求;采用不同的频率比时,太赫兹能量定标率并没有显示出明显的区别,这与多波混频理论预测的能量定标率不符.这些实验结果与等离子体电流模型及粒子模拟结果符合得很好.因此,该研究不仅对双色激光方案进行了推广,而且证实了其物理机制应该归结为等离子体电流模型.     

Elliptically polarized Terahertz emission through four-wave mixing in a two-color filament in air

We diagnosed the polarization characteristics of Terahertz emission from a two-color femtosecond laser filament when the polarizations of ω and 2ω beams are orthogonal. We discovered that the THz pulse is elliptically polarized. The generation mechanism could be through four-wave optical rectification inside the filament zone where the inversion symmetry of air is broken.     

Silicon suboxide (SiOx): laser processing and applications

Two types of periodic surface structures including micro-ripples and nano-ripples were observed on magnesium after femtosecond laser irradiation, and the color effect was reported by scanning a single laser beam over a large area at near damage threshold fluence. Optical reflectance measurement revealed that the color effect was mainly attributed to the nano-ripples as diffraction gratings, and intensity of the color was affected by morphology of the micro-ripples. AFM and SEM images showed that orientation of the micro-ripples was parallel to laser polarization and the period of the micro-ripples increased with scanning numbers, while the orientation of the nano-ripples was perpendicular to laser polarization and the period of the nano-ripples kept constant as laser sub-wavelength with increasing scanning numbers. These results suggested that formation of the micro-ripples was due to the combined effect of initial surface roughness and near-field interference, while formation of the nano-ripples was caused by the near-field interference between incident light and surface plasmons being excited in the air and metal interface. The potential application of such an effect is further proposed.     

激光原型装置光纤前端系统偏振稳定控制技术

 分析了激光原型装置光纤前端系统的偏振特性,针对激光原型装置光纤前端系统窄光谱、短脉冲的特点以及该原型装置对光纤前端系统输出偏振态的要求,提出了一种稳定光纤前端系统输出偏振态的新方法。采用脉冲方式偏置光纤前端系统的幅度调制器,通过光纤偏振稳定装置在线检测幅度调制器泄漏连续激光的偏振态进行自动调整,实现了光纤前端系统偏振态的自动稳偏,解决了波导幅度调制器长时间工作后消光比下降的问题。光纤前端系统长时间工作输出的稳定度达到了2.9％。     

Influence of surroundings on photorefractive effect in lithium niobate crystals

In the paper results of the investigation of the influence of electric properties of the environment surrounding LiNbO₃ crystals on photorefractive effect induced in these crystals by Gaussian Ar⁺ laser beam with various intensities are presented. We show spatial and temporal dependences of changes of the refractive index obtained experimentally in LiNbO₃: Fe and LiNbO₃: Fe:Mn samples surrounded by media with different electric conductivities and different permittivities (water, air, water solution of CaCl₂). The space and time dependences of the refractive index changes induced by the Ar⁺ laser beam are observed by means of the Mach–Zehnder interferometer using light from HeNe laser. The experimentally obtained results are in a good agreement with those following from numerical calculations using the manifold mirroring method. The agreement between calculated and experimental results indicates that the polarization charge at the photorefractive crystal/surrounding medium boundary significantly influences the photorefractive process in the crystal. The experimentally observed slow spontaneous decrease of the refractive index change in a sample placed into a slightly conducting medium (air) after switching off the beam also indicates that the polarization charge in the sample's surroundings affects the photorefraction.