Non-parabolic effect for femtosecond laser-induced ultrafast electro-absorption in solids

A theoretical study for femtosecond laser-induced ultrafast electro-absorption of bulk solids is presented. Our numerical results show that, in the case of low intensity of the pump laser where the interaction between the pump laser and solids is in the multi-photon regime, the energy band of solids can be approximately taken as a parabolic band and electro-absorption spectrums from the parabolic band and real band are nearly the same. While, in the case of high intensity where the interaction is in the tunneling regime, spectrums from the parabolic band and real band are quite different.The physical mechanism for the difference in the tunneling regime is found. We find that the non-parabolic parts of the real energy band and Bragger scattering of electrons near the first Brillouin zone boundaries, which are neglected in previous studies, strongly influence the electro-absorption spectrum in the tunneling regime. These two physical processes cause the difference of spectrums. Our theoretical results are in accordance with the experiment result.     

Competition between ultraviolet and infrared nanosecond laser pulses during the optical breakdown of KH2PO4 crystals

This study addresses the initiation of laser-induced breakdown of dielectric materials in the nanosecond regime under multi-wavelength conditions. In particular, the competition between multi-photon absorption and electronic avalanche as ionization mechanisms in KDP crystal is studied. Since they are both dependent on the laser frequency and intensity of incident radiations, we carried out two experiments: in mono-wavelength configuration at 1,064?nm and in multi-wavelengths configuration applying the simultaneous mixing of 1,064 and 355?nm radiations with various fluence ratios. To interpret experimental data, a model based on heat transfer and which includes ionization processes has been developed for both configurations. The comparison between experiments and modeling results first indicates that avalanche can be responsible for optical breakdown at 1,064?nm. Then, the study underlines the existence of a coupling effect in the multi-wavelength configuration where multi-photon absorption and electronic avalanche both contribute to the breakdown. From a general point of view, the model accounts for the experimental trends and particularly reveals that the electronic recombination timescale may have an important role in the scenario of nanosecond laser-induced breakdown.     

Excitation of He atoms in the strong-field tunneling regime

We report on strong-field excitation of He atoms with two interfering laser pulses of various polarization. The results are explained within the frustrated tunneling ionization model which describes multi-photon excitation in the strong-field tunneling regime.     

Tuning the Mott transition in a Bose-Einstein condensate by multiple photon absorption

We study the time-dependent dynamics of a Bose-Einstein condensate trapped in an optical lattice. Modeling the system as a Bose-Hubbard model, we show how applying a periodic driving field can induce coherent destruction of tunneling. In the low-frequency regime, we obtain the novel result that the destruction of tunneling displays extremely sharp peaks when the driving frequency is resonant with the depth of the trapping potential ("multi-photon resonances"), which allows the quantum phase transition between the Mott insulator and the superfluid state to be controlled with high precision. We further show how the waveform of the field can be chosen to maximize this effect.     

飞秒激光作用下光学玻璃和激光玻璃的光致暗化及其ESR研究

报道了多种商用光学玻璃和激光玻璃在800nm,120fs,1kHz脉冲激光作用下,产生光致暗化的阈值.并对照射前后的玻璃进行ESR分析,结果表明,光致暗化是由于玻璃内生成空穴捕获型色心的结果.认为玻璃在800nm飞秒激光照射后所产生的色心来源于玻璃的多光子吸收 关键词： 阈值 暗化 玻璃 飞秒激光 ESR     

飞秒光镊技术及其发展

激光捕获技术是利用光辐射力来捕捉、移动和操纵微粒的先进技术。飞秒光镊在实现粒子微纳操纵的同时还伴随着非线性现象的发生。阐述了飞秒光镊的模型和原理以及系统的各种结构形式,包括单光束梯度力光阱、贝塞耳光阱、双光束光纤光阱和冲击波光阱几种形式,并分析了每种形式的特点。     

多光子光学双稳性的复振幅解析分析

本文应用线性化稳定性分析方法,对用半经典的麦克斯韦-布赫(Maxwell-Bloch)方程描述的多光子光学双稳性进行了复振幅分析.分析的结果表明:在对极化量进行绝热消除后,多光子光学双稳性特性曲线上合作支和单原子支不存在失稳问题,同时说明由软模不稳定性引起的多光子光学双稳性的混沌态不存在,并指出稳定性的第三个条件是双稳性存在的条件.     

强激光场中模型氢原子和真实氢原子的高次谐波与电离特性研究

利用数值方法求解含时薛定谔方程，研究了一维、二维模型氢原子和真实的三维氢原子在强激光场中产生的高次谐波和电离特性.结果表明，在多光子电离区域和过垒电离区域，模型氢原子与真实的氢原子产生的高次谐波和电离概率差别很小；在隧道电离区域，它们产生的高次谐波的平台特征和截止位置相似，电离概率随时间变化的趋势相近，但其数值有明显的差异.对产生这种差异的原因进行了分析. 关键词： 强激光场 高次谐波 电离概率     

受驱动光学系统多光子量子统计理论(Ⅰ)——Fokker-Planck方程和良腔情况

我们应用广义Wigner分布的Fokker-Planck方程的系统方法,研究了受驱动光学系统的多光子跃迁过程。得到多光子过程普遍的Fokker-Planck方程、态方程以及良腔情况下透射光谱公式。计算了n光子双稳态的透射光谱并与单光子情况进行比较。还证明了在良腔情况,对于多光子过程,原子-原子关联也可以忽略。 关键词：     

CdS0.1Se0.9纳米晶体共振电光效应

采用电吸收谱(EA)的方法研究了在电场作用下,CdS_0.1Se_0.9纳米晶体光学性质的变化.分析了电场效应的物理机制,电场效应是使纳米晶体的吸收谱展宽和移动.第一激发态对外加电场敏感,而其它激发态不敏感.从电吸收谱上得到电光响应信号幅度与外加电场场强的平方成正比,表明纳米晶体的电光效应是 Kerr效应,具有三阶非线性极化率 χ⁽³⁾.     

全光汤姆孙散射

随着激光和加速器技术的发展,激光场强度和粒子能量也有所提升,在高场强和高电子能量的条件下,电子与光子的汤姆孙散射过程将达到高度非线性状态,在这种状态下会发生多光子效应,即单个电子同时与多个光子相互作用并辐射一个高能光子,此过程通常称为多光子汤姆孙散射.当场强和粒子能量变得更高时,需要引入量子电动力学理论来解决极端光场物理中的动理学过程.近期,全球多台数拍瓦激光装置逐渐投入使用,激光等离子体相互作用中的此类效应会变得极其显著.而全光汤姆孙散射成为目前研究极端光场物理最佳的实验方案,因此,系统地研究全光多光子汤姆孙散射是本领域未来十年极其重要的方向.本文对近年来全光汤姆孙散射实验从单光子、低阶多光子到高阶多光子的研究进展进行了综述,并对其未来的发展方向进行了展望.另外,伴随着散射过程产生的准直高亮X/伽马射线,有望发展成为具有重要应用价值的紧凑型超亮高能光源.     

吸收和色散混合受驱动光学系统多光子FPE和稳态方程

应用广义Wigner分布的Fokker-Planck方程的系统方法,研究了混合吸收和色散特征的受驱动光学系统的多光子跃迁过程。得到多光子、混合型Fokker-Plauck方程和稳态方程。并讨论了稳态方程的性质。 关键词：     

极化子效应对ZnS/CdSe量子点三阶极化率的影响

在有效质量近似下,利用量子力学的密度矩阵理论,采用无限深势阱模型,从理论上计算了考虑极化子效应后在导带子带间跃迁时ZnS/CdSe柱型核壳结构量子点二次电光效应(QEOE)和电吸收过程(EA)的三阶极化率。通过数值计算,分析了电子-LO声子和电子-IO声子相互作用对ZnS/CdSe柱型核壳结构量子点二次电光效应和电吸收过程的三阶极化率的影响。结果表明,极化子效应对二次电光效应的三阶极化率χ⁽³⁾_QEDE和电吸收过程的三阶极化率χ⁽³⁾_EA都有很大影响,并且影响的大小与量子点的尺寸大小有关。     

Laser-induced helical structure in the isotropic phase of nematic liquid crystal

We report on the first observation of laser-induced helical structure in the isotropic phase of a nematic liquid crystal. The helical structure was induced by two nearly counter-propagating waves circularly polarized in the same sense. Bragg reflection in the optical tunneling region was studied. We made an extensive investigation on the analogy between Bragg reflection from a cholesteric liquid crystal slab and four-wave mixing. It provides a deeper insight into the physics of the optical properties of cholesteric liquid crystal.     

多光子非线性Compton散射对等离子体中颗粒成长的影响

应用等离子体颗粒成长模型和多光子非线性Compton模型,研究了多光子非线性Compton散射对等离子体中颗粒成长的影响。提出了将入射光和多光子非线性Compton散射光作为颗粒成长的新机制,对颗粒凝合截面、凝合系数、颗粒收集的电子流和离子流的表达式进行了修正。结果表明,等离子体参数的变化与多光子非线性Compton散射成分、电子散射前的初始速度、入射角、与电子同时作用的光子数等因素有关。散射使生长过程中收集离子机制和凝合机制增强,颗粒的凝合截面增大,颗粒半径较快的非均匀增长。     

半导体三量子点中的多透明窗口及弱光孤子的调控

考虑半导体量子点间隧穿耦合效应,研究非对称半导体三量子点分子中的弱探测光的传播特性。线性情况下,由于点间隧穿耦合和外部控制光的协同调控,探测光的吸收特性将出现共振吸收、隧穿诱导透明单窗口、隧穿诱导透明双窗口及隧穿诱导透明三窗口的转变。此外,从反常色散到正常色散的开关效应可通过改变隧穿强度及光学控制场强度来实现。对于非线性情况,发现孤子的振幅随着点间隧穿耦合系数增大呈先增大再减小随即再次增大并减小的波动变化趋势且出现最大振幅及其对应的点间隧穿耦合强度随着外部控制光场的增大而减小。此外,发现孤子的群速度随着耦合强度的增加呈逐渐减小的趋势。     

Integrated Electro-Absorption Modulation DFB Laser Based Optical True Time-Delay Feeder for X-Band Phased Array Antennas

A 4-bit optical true-time-delay feeder incorporating an integrated electro-absorption modulation distributed feedback (DFB) laser for X-band phased array antennas is demonstrated. The integrated electro-absorption modulation DFB laser is an attractive device that provides a very compact, low-cost solution for optical true-time-delay systems. The variable delay line is constructed by cascaded magneto-optic switches, which have fast switching speed. The integrated electro-absorption modulation DFB laser system shows high signal-to-noise ratio up to 58dB, and the transmission performance of the system is relatively good and accredited.     

Role of dispersion profile in controlling emission of dispersive waves by solitons in supercontinuum generation

We study the supercontinuum process in optical fibers numerically for a variety of dispersion profiles to investigate how a specific dispersion profile controls the emission of dispersive waves. We conclude that the number of zero-dispersion points in the dispersion profile of a fiber is an excellent predictor of the dispersive-wave peaks when it is pumped with femtosecond pulses in the anomalous dispersion regime. Our study reveals that two or more such peaks can form on the same side of the input wavelength in specially designed and practically achievable dispersion profiles. We show that dispersive waves are emitted even in the case of normal dispersion where soliton fission does not occur. We suggest that a phenomenon related to soliton spectral tunneling is responsible for this radiation. Distinct dispersive peaks may also appear when an optical pulse, launched in the normal dispersion region, later begins to propagate in the anomalous dispersion regime because of its spectral broadening. Several dispersion profiles are numerically employed to show how the soliton fission process creates non-solitonic radiation even under normal dispersion pumping. A time-domain picture clearly shows this radiation when the conventional phase matching condition is satisfied. We also propose a realistic photonic crystal fiber with a dispersion profile that supports dispersive-wave generation in the normal-dispersion region. Our study should prove useful for experiments designed to control the generation of blue light by launching femtosecond pulses into optical fibers.     

Theoretical and experimental investigations on the threshold of a laser-induced breakdown of air

The threshold of a laser-induced breakdown of air is determined experimentally and theoretically. We find that the ionization of air has two steps: the first step is a multi-photon ionization process, which provides enough"seed electrons" to initiate the next step, and the second one is predominated by cascade ionization, which continues to produce free electrons geometrically until the critical free-electron density for breakdown is reached. So a two-step model based on the Morgan ionization model is established to describe the breakdown process. It is found that the time node dividing the two steps is about 9.8 ns in atmospheric air, and the threshold derived from the two-step model proposed here is more consistent with the experimental results than traditional ionization model.     

Reduction of multi-photon ionization in dielectrics due to collisions

The collisional effect due to the multi-photon ionization process in dielectric material has been studied. We found that the breakdown threshold of fused silica is the same for both linearly and circularly polarized light at 55 fs and 100 fs, which we believe is an indication of the suppression of multi-photon ionization in solids. By numerically solving the time-dependent Schrödinger equation with scattering, for the first time, we have observed substantial reduction of the multi-photon ionization rate in dielectrics due to collisions.