Coherent Phase Control of Ultrafast Polarization Beats in Reverse V-Type Three-Level System

We investigate the third-order nonlinear absorption and dispersion of femto- and atto-second polarization beats between the one-photon degenerate four-wave mixing process and the two-photon nondegenerate four-wave mixing process in the pure homogeneously-broadened reverse V-three-level using twin noisy fields. The third-order nonlinear response can be controlled and modified by the colour-locked correlation of twin noisy fields. Most importantly, the coherent phase control in optical heterodyne detection for studying the phase dispersion of the third-order susceptibility is demonstrated. The radiation-matter detuning oscillation is also considered in the freauencv domain.     

Six-wave mixing phase-dispersion by optical heterodyne detection in dressed reverse N-type four-level system

We have investigated the dressed effects of non-degenerate four-wave mixing （NDFWM） and demonstrated a phase-sensitive method of studying the fifth-order nonlinear susceptibility due to atomic coherence in RN-type four-level system. In the presence of a strong coupling field, NDFWM spectrum exhibits Autler-Townes splitting, accompanied by either suppression or enhancement of the NDFWM signal, which is directly related to the competition between the absorption and dispersion contributions. The heterodyne-deteeted nonlinear absorption and dispersion of six-wave mixing signal in the RN-type system show that the hybrid radiation-matter detuning damping oscillation is in the THz range and can be controlled and modified through the eolour-locked correlation of twin noisy fields.[第一段]     

倒V形四能级亚飞秒极化拍的三光子吸收和色散

基于孪生色锁噪声场的互相关效应,从理论上研究了倒V形(RV)四能级飞秒量级差频极化拍与极化拍中五阶非线性极化率,并提出了用一种相位敏感的方法来研究多能级原子相干造成的三光子六波混频过程.在抽泵光束为窄带末端情况下,场关联对信号和极化率的影响都很弱,在非窄带情况下,场关联会使极化强度自相干信号和极化拍信号产生关于零延时的不对称性和时域扫描上的辐射与物质失谐阻尼振荡(RDO),而极化率会产生频域扫描上的RDO振荡.参考光信号是一路传播方向和六波混频信号稍有区别的双光于非简并四波混频信号.五阶非线性响应则由享生 关键词： 外差法 吸收 色散 非线性极化     

倒V形四能级亚飞秒极化拍的三光子吸收和色散

基于孪生色锁噪声场的互相关效应，从理论上研究了倒V形(RV)四能级飞秒量级差频极化拍与极化拍中五阶非线性极化率，并提出了用一种相位敏感的方法来研究多能级原子相干造成的三光子六波混频过程.在抽泵光束为窄带末端情况下，场关联对信号和极化率的影响都很弱，在非窄带情况下，场关联会使极化强度自相干信号和极化拍信号产生关于零延时的不对称性和时域扫描上的辐射与物质失谐阻尼振荡(RDO)，而极化率会产生频域扫描上的RDO振荡.参考光信号是一路传播方向和六波混频信号稍有区别的双光于非简并四波混频信号.五阶非线性响应则由享生     

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.     

Fifth-order attosecond polarization beats using twin color-locked noisy lights in cascade three-level system with Doppler-free approach

Fifth-order attosecond sum-frequency polarization beat (FASPB) is studied in a cascade three level system with the phase-conjugation fourth-order coherence function theory. An improved schematic diagram of geometry, which is different from that inducing fifth-order femtosecond different-frequency polarization beat(FFDPB), is used to obtain FASPB. By analyzing the cases that pump beams have either narrow or broad bandwidth, it is found that the temporal behavior of the sum-frequency polarization beat signal depends on the properties of the lasers and transverse relaxation rate of the atomic energy-level system. Finally,the cascaded four-wave mixing (FWM) processes and the difference between attosecond and femtosecond polarization beats have been discussed, it is found that cascaded or sequential lower processes can often obscure the direct fifth-order polarization beat processes.     

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.     

V型三能级系统中五阶极化拍频的理论研究

从理论上研究了由于五阶极化强度间的干涉所导致的Ｖ型三能级系统相闰共轭超快调制光谱学，考虑了抽运光束为窄带线宽或宽带线宽的情形。发现能级分裂的测量精度决定于光学跃迁的均匀增宽，即这种技术在测量能级分裂时可得到消除多普勒增宽的精度。     

较高阶非线性抽运-探测过程中的空间效应

在飞秒抽运一探测光谱技术中,空间分辨的探测光信号反映了在不同空间位置的材料的非线性效应。当抽运光强度增大时,探测光信号中会出现明显的高阶特别是五阶非线性效应。利用劈裂算子方法直接解决了一维非线性传播方程的问题。在数值模拟中,研究了在不同抽运强度和位置下的抽运一探测过程中的五阶非线性效应。在足够高的抽运场下,探测信号出现清晰的振荡,显示了三阶和五阶非线性效应之间的干涉。当空间位置离抽运场中心足够远时,五阶比三阶非线性效应的衰减快得多,对其物理机制和趋势进行了定性的讨论。     

Investigation of symmetries of second-order nonlinear susceptibility tensor of GaSe crystals in THz domain

Following our measurements and analysis made on several GaSe crystals, we demonstrated that terahertz (THz) generation from ultrafast laser pulses can be developed into a sensitive technique for investigating symmetries of second-order nonlinear susceptibility tensor of a nonlinear crystal. Indeed, for GaSe crystals, both Kleinman's symmetry condition and spatial symmetry were violated due to the contribution of ionic displacement to nonlinear polarization and deviation of GaSe lattice from hexagonal symmetry. When the pump photon energy was increased from that below the bandgap of GaSe to that above it, the mechanism for the THz generation was switched from optical rectification to photocurrent surge.     

Studies of the fifth-order nonlinear susceptibility of ultraviolet-grade fused silica

Femtosecond transient absorption pump-probe spectroscopy has been used to study the third- and fifth-order nonlinear susceptibility of UV-grade fused silica. Theoretical simulations support the experimental evidence of interference between the third- and fifth-order contributions to the signal. Approximate values of the second-order nonlinear refractive index n(4) and of the three-photon absorption coefficient gamma are obtained.     

Self-action of light beams in nonlinear media: soliton solutions

The behaviour of light beams in nonlinear media (self-focussing of optical beams as well as self-modulation and transmission of optical pulses) is considered in the case when the nonlinear polarization contains susceptibilities of third- and fifth-order. The soliton solutions are obtained for the nonlinear equations deduced.     

All-optical pulse regeneration in an ultrafast nonlinear interferometer with Faraday mirror polarization stabilization

We demonstrate the folded ultrafast nonlinear interferometer (FUNI) as a 3R all-optical regenerator. Faraday rotation provides inherent polarization stabilization, and the optical fiber nonlinear medium provides ultrafast operation and switching window tunability. We demonstrate 3R regeneration of 10-Gbit/s data with 5-pJ pulse switching energy and 4-ps timing-jitter tolerance.     

Linear and Nonlinear Fiber Propagation of Partially Coherent Fields Exhibiting Temporal Correlations

Using an ultrafast photonic first-order differentiator applied on a partially coherent field, the generation of two correlated temporal waveforms is reported and their correlation properties upon linear and nonlinear propagation along the two orthogonal polarization axes of a dispersive optical fiber are studied. Temporal correlations are maintained in linear propagation whereas Kerr nonlinearity generates anticorrelated temporal intensity patterns for both partially and uncorrelated fields. Experiments are in close agreement with the theoretical analysis.     

双光子激发ZnSe自由载流子超快动力学研究

采用Z扫描和抽运-探测实验技术, 在波长为532 nm、脉冲宽度为41 fs的条件下测得ZnSe晶体的双光子吸收系数, 并获得了不同激发光强下的自由载流子吸收截面、电子-空穴带间复合时间和电子-声子耦合时间. 研究发现, 随着激发光强的增大, 自由载流子吸收截面减小, 复合时间变短. 当激发光强增大导致载流子浓度大于10¹⁸ cm^-3时, 抽运-探测信号出现明显改变, 原因归结为强光场激发导致样品在短时间内带隙变窄和电子-空穴等离子体的形成.     

RAMAN-ENHANCED POLARIZATION BEATS WITH PHASE-CONJUGATION GEOMETRY

The Raman-enhanced polarization beats (REPB) with phase-conjugation geometry in carbon disulfide are investigated using chaotic and phase-diffusion models. The overall accuracy of using REPB to measure the resonant frequency of Raman-active vibrational mode is determined by the relaxation rates of the Raman mode and the molecularreorientational grating. Different stochastic models of the laser field only affect the sixth- and fourth-order coherence function. The difference between the REPB and the fourth-order coherence on ultrafast modulation spectroscopy is discussed as well from a physical viewpoint.     

RAMAN-ENHANCED POLARIZATION BEATS WITH PHASE-CONJUGATION GEOMETRY

The Raman-enhanced polarization beats (REPB) with phase-conjugation geometry in carbon disulfide are investigated using chaotic and phase-diffusion models. The overall accuracy of using REPB to measure the resonant frequency of Raman-active vibrational mode is determined by the relaxation rates of the Raman mode and the molecular-reorientational grating. Different stochastic models of the laser field only affect the sixth- and fourth-order coherence function. The difference between the REPB and the fourth-order coherence on ultrafast modulation spectroscopy is discussed as well from a physical viewpoint.     

Ultrafast light-induced magnetization dynamics of ferromagnetic semiconductors

We develop a theory of ultrafast light-induced magnetization dynamics in ferromagnetic semiconductors. We demonstrate magnetization control during femtosecond time scales via the interplay between nonlinear circularly polarized optical excitation, hole-spin damping, polarization dephasing, and Mn-hole-spin interactions. Our results show magnetization relaxation and precession for the duration of the optical pulse governed by the nonlinear optical polarizations and populations.     

Ultrafast non-resonant third-order optical nonlinearity of GeS2–Ga2S3–CdS chalcogenide glass

Ultrafast third-order nonlinear optical responses of 4(1?x)/5GeS₂-(1?x)/5Ga₂S₃-xCdS (x=0.05, 0.10 and 0.15 in mol%) chalcogenide glasses was investigated by using the femtosecond time-resolved optical Kerr effect technique at a wavelength of 800 nm. The largest third-order nonlinear susceptibility of all samples was estimated to be 1.65×10^?13 esu. An ultrafast nonlinear response time of about 250 fs was observed, which was dominantly assigned to the ultrafast distortion of the electron cloud under femtosecond laser excitation. A nonlinear relationship between the third-order nonlinear susceptibility and the introduced amount of CdS was discussed, suggesting a possible disturbance between different polarizable bonds in the glass system.     

Fifth harmonic generation in sodium vapor

We report experimental investigation of direct fifth harmonic of Nd³⁺: glass laser radiation (1060 nm) in metal vapor. Using various mixtures of sodium vapor and xenon in order to achieve a phase-matching, we observed direct as well as cascade nonlinear processes. The first measurement of effective fifth-order nonlinear susceptibility χ⁽⁵⁾_eff in atoms, is performed.