基于双光子不对称色锁二阶随机关联的阿秒极化拍研究

基于两双光子过程的相位共轭二阶极化干涉，从理论上研究了四能级系统在阿秒量级的不对称和频极化拍频(FASPB).在抽运光束为窄带情形下，场关联对FASPB信号影响很弱.在宽带情形下，FASPB信号表现出共振-非共振交叉关联，双光子信号展现出混合辐射-物质失谐的THz阻尼振荡，即当激光频率偏离双光子共振跃迁的条件时，信号将呈现阻尼振荡，而且其双光子自相关信号的包络还呈现最大值零延时不对称.还研究了由于两双光子自相关过程零延时的偏差导致的阿秒拍频信号的不对称性，和两个双光子信号的最大值分别向相反的方向偏离零延时点的现象.FASPB作为一种阿秒超快调制过程，从理论上说它可以扩展到任何两偶极禁戒跃迁激发态的能级和系统. 关键词： 阿秒和频 极化拍 四能级 双光子     

阿秒极化拍的V型三能级对称二阶相干理论

基于两单光子过程的相位共轭二阶极化干涉，从理论上研究了V型三能级系统在阿秒量级的对称和频极化拍频（SFPB），考虑了抽运光束为窄带线宽或宽带线宽的情形，发现测得的信号对任意带宽的马可夫随机光场的统计特征非常敏感，其在测量偶极禁界跃迁两激发态之间的能级和时并不受激光线宽的限制，仅取决于光学跃迁均匀增宽，而且可以得到消除多普勒增宽的测量精度.在窄带情形下，场关联对SFPB信号影响很弱.在宽带情形下，SFPB信号表现出共振 非共振交叉关联.SFPB作为一种阿秒超快调制过程，从理论上说它可以扩展到任何和频能级系统. 关键词： 阿秒 和频 极化拍 二阶相干     

阿秒和频相位共轭极化拍频的不对称色锁二阶相干控制

研究了级联三能级系统中孪生色锁随机光场的相位共轭阿秒和频极化拍频(ASPB)不对称新效应。从理论上分析了自关联单光子简并四波混频(DFWM)和双光子非简并四波混频(NDFWM)信号辐射场与物质间不对等效应、混合型太赫兹失谐振荡特性,及双光子非简并共振四波混频信号的延时不对称性;同时也分析了由干涉计两臂不平衡色散效应引起的阿秒和频极化拍频信号的延时不对称性,即阿秒和频极化拍频信号的最大值相对于零延时有一个小偏移,结果表明该不对称性并这不影响阿秒和频极化拍频零差探测两偶极禁戒跃迁能级分裂的总体精度。阿秒和频极化拍频作为一种消除多普勒增宽的超快调制过程,从理论上说它可以扩展到任何两偶极禁戒跃迁能级系统。     

Simultaneously phase-matched parametric processes in a single-pass interaction for efficient visible generation

We demonstrate simultaneously phase-matched cascaded optical parametric amplification (OPA) and sum-frequency generation (SFG) processes in a single-pass interaction in a BBO crystal, where the seed pulse is produced through the self-induced superfluorescence generation. Tunable femtosecond pulses in the blue-green spectral range are produced through the single-pass SFG interaction with a maximum conversion efficiency of about 12%.     

NONLINEAR EFFECTS OF THIRD-AND FIFTH-ORDER ON POLARIZATION BEATS IN A FOUR-LEVEL SYSTEM

We have employed second-order coherence function theory to study the nonlinear effects of third-and fifth-order on polarization beats in a four-level system (TPBFS and FPBFS). It is found that the different temporal behavior of the beat signal in TPBFS and FPBFS depends on the stochastic properties of the lasers and transverse relaxation rate of the transition. We have considered the cases that pump beams have either narrow band or broadband linewidth and found that for both cases a Doppler-free precision can be achieved in the measurement of the energy-level difference between two excited states which are dipolar forbidden from the ground state. We also discussed the spatiall modulation behavior of the beat signal.     

四能级系统中五阶极化拍频的理论研究

研究了四能级系统中基于五阶极化的拍频效应,考虑了抽运光束为窄带线宽或宽带线宽的情形．发现其对能级分裂的测量精度决定于光学跃迁的均匀增宽．也就是说,这种技术在测量与基态是偶极禁戒跃迁的两激发态之间的能级分裂时,可得到消除多普勒增宽的精度． 关键词：     

级联三能级系统中高阶非线性效应的研究

本文采用二阶相干函数理论研究了级联三能级系统中三阶及五阶极化拍频(TPBCTS及FPBCTS)的非线性效应.通过对泵浦光为宽带及窄带情形的分析,我们发现三阶及五阶极化拍频信号的强度在时域内的变化取决于激光的统计特性和跃迁的横向驰豫率,从而在测量原子能级分裂时可以得到消除多普勒极限增宽的精度.另外,我们对信号的空间调制也进行了分析.     

Generation of the cascaded fifth-order nonlinear phase shifts with femtosecond pulse

Due to group velocity mismatch, the generation of the cascaded fifth-order nonlinear phase shifts with femtosecond pulse will inevitably cause the severe pulse distortion. By theoretical study and numerical simulations, we show that undistorted femtosecond pulse which is impressed the cascaded fifth-order nonlinear phase shifts can be obtained at large values of the phase mismatching △κL.     

Generation of large fifth-order nonlinear phase shifts by use of lossless<Emphasis Type="Italic">χ</Emphasis><Superscript>(2)</Superscript> cascaded nonlinear processescascaded nonlinear processes

It is shown that the cascaded fifth-order nonlinear phase shifts will increase with energy loss in the cascaded processes. Essentially different from the multi-photon absorption accompanied with inherent material nonlinearities, the loss of fundamental wave in a cascaded process is controllable and suppressible. By introducing difference frequencies generated from the reaction between the fundamental and its second harmonic after the cascaded processes, the fundamental wave can be free of energy loss, while the large cascaded fifth-order nonlinear phase shift is maintained.     

Generation of large fifth-order nonlinear phase shifts by use of lossless χ~((2)) cascaded nonlinear processes

Theinherentthird-ordernonlinearityofmaterialc(3)isnormallyweak.Toaccumulatealargenonlinearphaseshift,highlaserintensityandlongopticalpathinmaterialareneeded.Whilethevalueofnonlinearitycouldbemadelargerneartheregionofabsorptionresonance,astronglosswillbeaccompaniedthroughtheprocessoftwo-photonabsorption.Inordertorealizestrongthird-ordernonlinearitywithcontrollableenergyloss,thec(2)(w;2w,-w)︰c(2)(2w;w,w)cascadingprocesshasbeenproposed,whichisequivalenttothethird-ordernonlinearityc(3)(w;w,w,-w)w…     

Generation of large fifth-order nonlinear phase shifts by use of lossless χ(2) cascaded nonlinear processes

It is shown that the cascaded fifth-order nonlinear phase shifts will increase with energy loss in the cascaded processes. Essentially different from the multi-photon absorption accompanied with inherent material nonlinearities, the loss of fundamental wave in a cascaded process is controllable and suppressible. By introducing difference frequencies generated from the reaction between the fundamental and its second harmonic after the cascaded processes, the fundamental wave can be free of energy loss, while the large cascaded fifth-order nonlinear phase shift is maintained.     

飞秒激光Z扫描技术研究CS_2的非线性特性

利用波长为800 nm的飞秒激光脉冲,通过Z扫描技术研究了CS2在不同光强下的非线性折射和非线性吸收特性发现,随着光强的增加,闭孔Z扫描透过率曲线发生了较大的轴向移位。分析结果表明:在低光强下,这种轴向移位源于双光子吸收的影响;而在高光强下,是双光子吸收和五阶非线性共同作用的结果。给出了决定五阶非线性存在与否的临界光强,以及非线性参量与轴向位移的关系。     

Attosecond pump probe: exploring ultrafast electron motion inside an atom

The attosecond pump probe, in close analogy to the standard femtosecond probing technique, has been proposed and theoretically demonstrated with its application to explore ultrafast electron motions inside atoms. We have performed realistic modeling for the full dynamics of both the femtosecond pumping and the attosecond probing processes. Our simulations have illustrated that an ultrashort oscillation period of 2.0 fs can be mapped out for a wave packet in low-lying excited states of the helium atom. This opens the prospect of a wealth of similar pump probe experiments to examine ultrafast electronic or atomic motions.     

Observation of Sub-femtosecond Polarization Beats Using Twin Noisy Driving Fields

We study the phase-conjugate polarization interference in a V-type three-level system and obtain an analytic closed form for the second-order stochastic correlation of sum-frequency polarization beat. In our Na vapour atomic system pumped by laser pulse of nano-second timescale duration, we experimentally demonstrate an ultrafast modulation of the four-wave mixing signal intensity with a sub-femtosecond timescale period, corre- sponding to the sum-frequency of the resonant transitions from 3S1/2 to 3Pl/2 and 3P3/2.     

Diffractive optics implementation of six-wave mixing

Diffractive optics are applied to six-wave mixing processes to provide a single optic approach to attaining the required, relatively complex, phase-matching geometry to discriminate against lower-order nonlinear responses. The diffractive optics were designed specifically for broad-bandwidth operation and passive phase locking of the appropriate pulse pairs for use in femtosecond two-dimensional Raman studies of the dynamic structure of liquids. The fifth-order signal was studied in liquid CS>(2); two different colors were used for the excitation and the probe to reduce background scatter, as were two different phase-matching geometries with different degrees of suppression of cascaded third-order processes.     

INTERFERENCE EFFECTS IN DIFFERENT FIFTH-ORDER POLARIZATIONS WITH BROADBAND NOISY LIGHT

Based on the second-order coherence function, we have studied a phase-conjugate ultrafast modulation spectroscopy due to the interference between the fifth- and fifth-order optical polarizations in cascade three-level Doppler-broadened system. It is found that the temporal behaviour of the beat signal depends on the stochastic properties of the lasers and the transverse relaxation rate of the transition. The beat signal depends on the second-order coherence function, which is determined by the laser line shape. Since different stochastic models of the laser field only affect higher than second-order coherence functions, they have little influence on the general temporal modulation behaviour of the beat signal. The cases that pump beams have either narrow band or broadband linewidth are considered and it has been found that for both cases the overall accuracy for the energy-level difference measurement is determined by the homogeneous linewidths of the optical transitions. Finally, the spatial modulation behaviour of the beat signal has also been discussed.     

Multistep cascading in periodically poled directional couplers

We suggest a novel scheme for multistep cascaded parametric processes based on a periodically poled nonlinear directional coupler and derive the conditions for simultaneous phase matching of the second-harmonic generation and sum-frequency mixing processes. We predict an efficient wavelength conversion by the cascaded third-harmonic generation from the third optical communication window (1.53-1.56 microm) to visible in a LiNbO3 poled waveguide coupler.     

利用三色组合脉冲激光获得孤立阿秒脉冲发射

孤立阿秒脉冲因可以跟踪和控制原子及分子内电子的运动过程而备受关注.本文从理论上开展了氦原子在3束飞秒脉冲激光组合场辐照下产生的高次谐波和阿秒脉冲辐射的研究.组合激光场由16 fs/1600 nm,15 fs/1100 nm和5.3 fs/800 nm的钛宝石脉冲构成.与前两束脉冲合成的双色场产生谐波谱相比,附加钛宝石脉冲的三色场产生的高次谐波发射谱呈现出高转换效率及宽带超连续特性,超连续谱范围覆盖从230—690次谐波,傅里叶变换后实现了128 as高强度孤立短脉冲的产生.该结果归因于合成的三色场呈现出高功率及少周期的中红外飞秒脉冲激光特性,可以有效控制原子电离以及复合发生在中红外飞秒脉冲的一个有效光学周期内.     

高次谐波产生阿秒极紫外和X光脉冲研究新进展

 近年来在可见光谱范围内已经把激光脉冲压缩到接近一个光学周期(2～3 fs)的物理极限,几fs的时间分辨精度可以描述分子化学反应过程,但是要探测远小于可见光周期的电子跃迁过程则需要阿秒(as)量级的光脉冲。利用脉冲间具有相同载波包络相位的阿秒脉冲序列能把可见光波段的光学频率梳向极紫外波段扩展;利用电子和离子碰撞复合过程短于一个光周期这个时间窗,通过测量激光场椭圆极化率对电子轨迹的微扰实现了as精度的分辨率;通过测量碰撞复合过程中的高能电子的辐射谱可以重构阿秒X光脉冲以及探测强场下束缚态和连续态电子动力学。