Polarization dependence of optical Kerr effect in metallophthalocyanine-doped inorganic-organic hybrid materials

Ultrafast optical Kerr effect of metallophthalocyanine-doped inorganic-organic materials was investigated using a femtosecond optical Kerr shutter at wavelength of 800 nm. Experimental results showed that the dependence of the Kerr signals on the polarization angle between the pump beam and the probe beam could be controlled by changing the pump-probe intensity ratio. The pump-intensity dependence of the polarization characters of the Kerr signals probably arose from the contribution of light induced transient grating (LITG) to the Kerr signals.     

CS_2飞秒超快非线性特性的实验研究

采用中心波长800 nm、脉宽30 fs的超短激光脉冲,通过飞秒光开关技术对CS_2的飞秒超快非线性特性进行了实验研究.在探测光强与抽运光强比为1∶10时,得到了较理想的光克尔时间分辨曲线.通过实验测定的光克尔信号强度与激发光和探测光偏振方向夹角的依赖关系表明:30 fs的超短激光脉冲激发CS_2的克尔信号主要是源于光诱导双折射效应,而非用200 fs的超短激光脉冲时来自瞬态栅的自衍射效应.     

Defocusing role in femtosecond filamentation: Higher-order Kerr effect or plasma effect?

The femtosecond filamentation in the classical and high-order Kerr(HOK) models is numerically investigated by adopting multi-photon ionization(MPI) cross section with different values. It is found that in the case that the MPI cross section is relatively small, there exists a big difference between the electron density as well as clamped intensity calculated in the classical model and those calculated in the HOK one, while in the case that the MPI cross section is relatively large, the electron density and clamped intensity calculated in the two models are nearly in agreement with each other, and under this circumstance, even if the higher-order nonlinear terms do exist, the free-charge generation and the associated defocusing in a filament are enough to mask their effects. The different behaviors of the maximum intensity and on-axis electron density at the collapse position with the pulse duration provides an approach to determine which effect plays the dominant defocusing role. These results demonstrate that it is ionization that results in the difference between the two models.     

聚合物Poly（2，5－thienyleneVinylene）中超快光学克尔效应的实验研究

实验研究了聚合物Ｐｏｌｙ（２，５－ｔｈｉｅｎｙｌｅｎｅｖｉｎｙｌｅｎｅ）薄膜的超快共振光学克尔效应。获得了３×１０－１１ｅｓｕ的光学克尔极化率。一个双激发子弛豫机制被用来解释克尔信号动力学特性对入射光强度的依赖性。     

Pump power dependence of the spatial gating properties of femtosecond optical Kerr effect measurements

The pump power dependence of the spatial gating properties of femtosecond optical Kerr gate (OKG) was investigated using coaxial two-color optical Kerr measurements in CS₂. As the pump power increased, the spatial pattern of the optical Kerr signals changed from a Gaussian spot to a ring form, and then a spot surrounded by a concentric ring, successively. By comparing the experimental data with the calculation results and measuring the pump power dependence of the OKG signal intensity, we demonstrated that the spatial variation of OKG transmittance could be attributed to the non-uniform spatially distributed phase change of the probe beam, due to the transient birefringence effect induced by pump beam with transverse mode of a Gaussian distribution.     

Arrest of self-focusing collapse in femtosecond air filaments: higher order Kerr or plasma defocusing?

Experimentally measured conical emission rings on the blue side of the filament supercontinuum of a 800 nm 50 fs pulse in air are reproduced in simulations with plasma and the third-order Kerr as the nonlinear terms. This agreement indicates plasma as the dominant mechanism arresting the self-focusing collapse. The higher order Kerr terms with the recently measured coefficients stop the collapse at a lower intensity than the plasma does and lead to the spherical angle-wavelength spectrum without blueshifted rings.     

CS_2分子激发态的紫外多光子电离——双色激光单共振离化的功率密度关系

本文报道气相CS_2紫外双色激光单共振离化的功率密度关系和对高激发态(1~Ⅱg),共振离化特性的研究结果,实验指出通过中间共振态(1~Ⅱ_g)的双光子共振激发三光子离化具有比通过中间共振态((?)A_2)的单光子共振激发三光子离子为高的总离化率,在10~7W/cm~2的功率密度下,观察到饱和效应.实验显示了双色激光单共振离化功率密度关系的研究提供了一种新的光谱学分析方法.这对研究分子高激发态的结构是很重要的.     

Pressure independence of intensity clamping during filamentation: theory and experiment

Because of the dynamic equilibrium between Kerr self-focussing and plasma induced defocusing and the inexistence of collisional ionization the critical intensity during femtosecond laser filamentation in air is independent of pressure. An analytical analysis is given which is justified by a direct experimental verification. PACS 42.65.Jx; 52.38.Hb; 52.70.Kz     

Excitation and ionization of sodium clusters by a strong electromagnetic field

This paper reports on a study of the excitation and ionization of small sodium clusters by femtosecond light pulses with a maximum intensity of 5×10¹²–1×10¹⁴ W/cm² and photon energy from 1 to 3 eV made in terms of the density functional theory and jellium model through direct numerical solution of the Kohn-Sham time-dependent equation. The dependence of the degree of ionization on the intensity, duration, and frequency of the light pulses, as well as on the cluster size, is studied. The efficiency of the processes is shown to be determined primarily by the field intensity rather than by the total pulse energy.     

Experimental tests of the new paradigm for laser filamentation in gases

Since their discovery in the mid-1990s, ultrafast laser filaments in gases have been described as products of a dynamic balance between Kerr self-focusing and defocusing by free electric charges that are generated via multiphoton ionization on the beam axis. This established paradigm has been recently challenged by a suggestion that the Kerr effect saturates and even changes sign at high intensity of light and that this sign reversal, not free-charge defocusing, is the dominant mechanism responsible for the extended propagation of laser filaments. We report qualitative tests of the new theory based on electrical and optical measurements of plasma density in femtosecond laser filaments. Our results consistently support the established paradigm.     

电子自旋交换作用对极向磁光Kerr效应的影响——能带跃迁模型

用微观量子理论和能带跃迁模型,研究了极向磁光Kerr效应对电子自旋交换劈裂能依赖性.结果表明,磁光Kerr转角θ_K与自旋劈裂能Δ之间仅当Δ很小时才有线性关系;θK的展开式中只出现Δ奇次项而不出现偶次项.     

Transient stimulated Raman scattering in binary mixed liquids

Stimulated Raman scattering (SRS) spectra in several mixed liquids with a large optical Kerr constant are studied with nanosecond and picosecond light pulses. These spectra show a dependence of the SRS intensity on the relaxation times T₂ of Raman active modes.     

强激光场下原子超快动力学过程中的能量交换

利用三维经典系综模型,研究了整个系综两电子(Ar原子为例)从激光场吸收的能量对激光参数(波长、激光强度和椭偏率)的依赖关系.结果显示,当激光强度固定,波长增加时,整个系综两电子从激光场吸收的能量整体呈上升趋势,但不同强度下趋势略有差异.在较低强度时整个系综两电子从激光场吸收的能量对波长的依赖关系呈现持续平稳增加的趋势,在较高强度时呈现先缓慢减小再快速增大的趋势.对强度的依赖关系在不同波长时呈现两个有趣的交叉点.对椭偏率的依赖关系在较低强度时呈现先逐渐减小再缓慢增大的趋势;在中等强度时呈现一个“阶梯型”即先缓慢增大再逐渐减小最后缓慢增大;在更高强度时呈现先逐渐增大再逐渐减小的趋势.为了解释整个系综两电子从激光场吸收的能量对激光参数的依赖关系,把整个系综的动力学过程分为双电离、单电离、受挫单电离和受挫双电离4种通道.然后分析各个通道的特征及其如何主导整个系综两电子从激光场吸收的能量的变化趋势.分析结果表明,整个系综两电子从激光场吸收的能量对波长、激光强度和椭偏率的依赖均是由于某种通道主导整个系综两电子从激光场吸收的能量的结果.     

Comparison study of atomic and molecular single ionization in the multiphoton ionization regime

In this Letter, we report, for the first time in the multiphoton ionization regime, a comparison study of single-electron ionization of diatomic molecules versus rare gas atoms with virtually the same ionization potentials. In comparing N2+ to Ar+, a higher ion signal is seen in N2+ compared to Ar+ for linear polarization but the difference vanishes in circularly polarized light. In comparing O2+ to Xe+, we observe a suppression in O2+ compared to Xe+ for both linear and circular polarization but this suppression exhibits an intensity dependence; i.e., there is little suppression for O2+ at the lowest intensity range, but the suppression becomes increasingly stronger as the laser intensity increases. The multielectron screening model is used to discuss possible mechanisms of this intensity dependent suppression in O2+ in the multiphoton ionization regime.     

Theoretical analysis of hot-image effect in a high-power laser system with cascaded nonlinear medium

We present a universally applicable and quick method to forecast the intensity and location of the hot-image effects in a high-power laser system structured by cascaded Kerr medium plates. The analytical expressions for the locations and the peak intensity of the hot-images are deduced by using propagation matrix method. The results are useful for laser designers to estimate and minimize the threat of optical damage. Theoretical analysis demonstrates that a maximum of N hot-images may appear in a laser system structured by N cascaded Kerr medium plates and the distance between two adjacent hot-images is two times the interval between two adjacent Kerr medium plates. The number and locations of the hot-images are related with the number of the Kerr medium plates, the distance from the scatterer to the front of the first Kerr medium plate, and the interval between two adjacent Kerr medium plates. The peak intensity of the hot-images depends on the number of the Kerr medium plates, the B-integral of each Kerr medium plate, the amplitude and phase modulation coefficients of the scatterer and the peak intensity of the input beam. The hot-image effects in a laser system with cascaded Kerr medium plates from two to eight are discussed in detail and numerically analyzed. Numerical simulation results are in agreement with the theoretical results.     

Phase-dependent excitation and ionization in the multiphoton ionization regime

We theoretically study the dependence of atomic excitation and ionization on the carrier envelope phase of few-cycle laser pulses in the multiphoton ionization regime. Our theoretical results for the hydrogen atom based on the solution of the 3D time-dependent Schr?dinger equation show that the strong phase dependence can be seen in not only total ionization, but also bound-state population under the weak laser intensity regime.     

Plasma channel formed by ultraviolet laser pulses at 193 nm in air

The propagation of picosecond deep ultraviolet laser pulse at wavelength of 193 nm in air is numerically investigated. Long plasma channel can be formed due to the competition between Kerr self-focusing and ionization induced defocusing. The plasma channel with electron density of above 10^13/cm^3 can be formed over 70 m by 50-ps, 20-mJ laser pulses. The fluctuation of laser intensity and electron density inside ultraviolet （UV） plasma channel is significantly lower UV laser by air is considered in the simulation and it the limit of the length of plasma channel. than that of infrared pulse. The linear absorption of is shown that the linear absorption is important for the limit of the length of plasma channel.     

Investigation of the Kerr effect and third-order susceptibility constants in a nematic liquid crystal

In this paper, the static Kerr effect was investigated for the W-1680 at temperatures above the nematic-isotropic transition temperature and compared with 9CHBT. We experimentally measured the electro-optical Kerr constant and the pre-transitional behavior. The Kerr law has been confirmed and the variation of the inverse Kerr constant, with a temperature above the smectic-isotropic transition temperatures, were determined. The linear dependence of (T − T*)^− 1 on the Kerr constant is found to be in good agreement with the predications of the Landau-De Gennes model. Additionally, the temperature dependence of third order susceptibility has been studied. This value for these samples will reduce when the temperature increases.     

Measuring the Magnetooptic Kerr effect by diffraction

A method is presented to measure the Magnetooptic Kerr Effect (MOKE) by diffraction. This is accomplished by giving the magnetized surface a magnetization distribution which functions like a linear diffraction grating. It is shown that the first and higher orders of the light diffracted by such a grating are produced by MOKE whereas the zero order approximately represents the reflectivity of the surface (given by the Fresnel formulae assuming zero magnetization). The procedure used here to form a grating is restricted to thin soft magnetic films, and has been demonstrated on evaporated Fe-Ni films. It uses the magnetic field of a pulse-driven meandered stripe-conductor placed in close contact with the film in the presence of a homogeneous external magnetic field. Experimentally determined Kerr intensities were compared with those measured by ellipsometric methods. Good agreement was found in the case of films with uniaxial anisotropy. For films with no anisotropy, the relative dependence of Kerr intensity vs wavelength, polarisation and angle of incidence could be derived.     

(Ga,Mn)As中电流诱导自旋极化的磁光Kerr测量

在GaAs吸收带边附近, 利用磁光Kerr效应测量了(Ga,Mn)As和p-GaAs样品的电流诱导Kerr旋转谱和反射谱, 两者都呈现出Lorentz曲线形状. 电流诱导Kerr旋转角和反射率随着电流的增大而增大, Kerr角与电流的大小成正比关系, 反射率与电流的平方成正比关系. (Ga,Mn)As的Kerr旋转角比p-GaAs的大了一个数量级, 这说明Mn原子的掺杂使得电流诱导的自旋极化增强. 另外, 还测量了温度和入射光偏振方向对电流诱导Kerr旋转谱和反射谱的影响. 发现随着温度的升高, Kerr谱和反射谱均向长波方向移动, 这与GaAs带边随温度的变化是一致的.