Third-harmonic generation and self-channeling in air using high-power femtosecond laser pulses

It is shown, both theoretically and experimentally, that during laser pulse filamentation in air an intense ultrashort third-harmonic pulse is generated forming a two-colored filament. The third-harmonic pulse maintains both its peak intensity and energy over distances much longer than the characteristic coherence length. We argue that this is due to a nonlinear phase-locking mechanism between the two pulses in the filament and is independent of the initial material wave-vector mismatch. A rich spatiotemporal propagation dynamics of the third-harmonic pulse is predicted. Potential applications of this phenomenon to other parametric processes are discussed.     

超常介质中超短电磁脉冲的传输特性研究

将超常介质的色散磁导率合并到非线性极化项中,借鉴常规介质中超短脉冲传输方程的推导方法,得到了非线性超常介质中超短脉冲的传输方程。在德鲁德(Drude)色散模型下,根据脉冲中心频率的不同在传输方程中出现了可正、可负、可为零的自陡峭系数,以及高阶非线性色散项。此外,利用矩方法对传输方程进行分析,得到了超常介质中超短脉冲传输方程的能量守恒定律表达式,揭示了色散磁导率导致的超短脉冲传输的新特性,发现二阶非线性色散使超短脉冲的能量、脉冲频移、脉冲宽度、中心位置和啁啾都随传输距离呈现振荡式变化。     

Carrier-envelope phase dependence of few-cycle ultrashort laser pulse propagation in a polar molecule medium

We theoretically investigate carrier-envelope phase dependence of few-cycle ultrashort laser pulse propagation in a polar molecule medium. Our results show that a soliton pulse can be generated during the two-photon resonant propagation of few-cycle pulse in the polar molecule medium. Moreover, the main features of the soliton pulse, such as pulse duration and intensity, depend crucially on the carrier-envelope phase of the incident pulse, which could be utilized to determine the carrier-envelope phase of a few-cycle ultrashort laser pulse from a mode-locked oscillator.     

Distortion of ultrashort laser pulses in a spatial filter

By means of Collins diffraction integral, the propagation expression of an ultrashort laser pulse passing through a spatial filter is derived. The effects of the magnification of spatial filter on pulse broadening and distortion of pulse-front for the ultrashort optical pulse are analytically deduced and numerically simulated. It is found that pulse broadening and propagation time difference of a laser pulse getting through spatial filter is proportional to the magnification. As a conclusion, in an ultrashort pulse laser system with a large aperture, the effect of pulse broadening and distortion of pulse-front should be considered.     

The role of refocusing in dynamic X-wave formation during femtosecond laser filamentation in water

<正>With the evolution of a laser pulse in water,the formation of a nonlinear X wave during femtosecond filamentation is investigated based on numerical simulations.In particular,we analyze the far-field angularly resolved spectra obtained for different temporal portions of the ultrashort pulse during its propagation. Our result shows that the refocusing of ultrashort pulse leads to the formation of dynamic X wave which essentially manifests itself as conical emission.     

（3＋1）-dimensional nonlinear propagation equation for ultrashort pulsed beam in left-handed material

In this paper a comprehensive framework for treating the nonlinear propagation of ultrashort pulse in metamaterial with dispersive dielectric susceptibility and magnetic permeability is presented. Under the slowly-evolving-wave approximation, a generalized （3＋1）-dimensional wave equation first order in the propagation coordinate and suitable for both right-handed material （I~HM） and left-handed material （LHM） is derived. By the commonly used Drude dispersive model for LHM, a （3＋1）-dimensional nonlinear Schrodinger equation describing ultrashort pulsed beam propagation in LHM is obtained, and its difference from that for conventional RHM is discussed. Particularly, the self-steeping effect of ultrashort pulse is found to be anomalous in LHM.     

Theoretical analysis of noncollinear phase-matched optical parametric generation in BBO crystal

In this article we propose a theoretical treatment of noncollinear phase-matched femtosecond parametric interaction process pumped by ultrashort optical pulses, and investigated the pulse characteristics of OPG in BBO crystal. The results show that the major factors, which affect the optical parametric conversion coefficient and durations of the pulses, are the group velocity mismatch and the phase mismatch among the three ultra-short pulses. In addition, the material dispersion can cause the durations of the pulses to increase when pump pulse is below 20 fs, and its influence becomes more obvious at low pump intensity.     

Derivation of a wave equation for pulse propagation beyond a slowly varying envelope approximation

An iterative method is used to derive an accurate weve equation that governs ultrashort pulse propagation in a single mode fibre. In the derivation, the slowly varying evelope approximation (SVEA) is not required. Therefore, the wave equation can be used to describe the propagation of an ultrashort pulse of a few optical cycles. It is found that, compared with the wave equation obtained by making an SVEA, an accurate wave equation has the same coefficients of the linear dispersion terms and different coefficients of the high-order time derivatives of the non-linear terms.     

拉曼效应对负折射介质中超短脉冲传输的影响

通过数值法对包含拉曼延迟响应的(3+1)维非线性薛定谔方程进行求解,研究了超短脉冲激光在负折射介质中传输时拉曼效应对自聚焦传输特性的影响,着重分析其不同与常规介质的反常传输现象.结果表明:由于负的折射率影响,拉曼效应将导致超短脉冲在自聚焦过程中频谱发生蓝移现象,这与常规介质对应情形相反;而它对负折射介质中超短脉冲的自聚焦特性的影响与常规介质相同,即拉曼效应将诱导自聚焦效应首先发生在脉冲的前沿.本文研究工作对将来利用负折射介质来操控超短光脉冲串产生、自聚焦等许多实际应用领域研究具有重要的指导意义.     

Experimental research on measuring the fine structure of long pulse in time domain by synchronized ultrashort pulse

We propose a method of measuring the fine structure of a long pulse in time domain by a synchronized ultrashort pulse, in which the basic principle is the cross-correlation interaction between the long pulse and the ultrashort pulse in a nonlinear crystal. After a theoretical analysis on the principle and resolution, as an example, the fine structure of a picosecond Nd:YLF laser pulse is characterized by a synchronized femtosecond pulse experimentally. Further, the effect of group velocity mismatch on measurement results is discussed.     

Ultrashort optical pulses in two-level systems with Coulomb interaction and defects

The dynamics of an ultrashort optical pulse in an ensemble of two-level systems (in particular, in the presence of defects) is considered in the framework of the microscopic pseudospin formalism without limitation on the pulse power in the approximation of unidirectional propagation. The effective equations for the electric field amplitude and polarization of two-level systems are derived and solved numerically. Quasi-soliton modes of propagation of ultrashort optical pulses are revealed and the dependences on the macroscopic Hamiltonian parameters are analyzed. The effect of various types of defects on the propagation of ultrashort pulses is detected.     

Ultrashort optical pulse in an order-disorder ferroelectric in the presence of a lattice of defects

The dynamics of an ultrashort optical pulse in an order-disorder ferroelectric in the presence of a lattice of defects is considered within the microscopic pseudospin formalism in the unidirectional propagation approximation, without limitations on the pulse power. The effective equations for the electric field amplitude and ferroelectric polarization are obtained and numerically solved. Quasi-soliton modes of ultrashort optical pulse propagation in a system with regular defects are revealed and the dependences on the microscopic Hamiltonian parameters are analyzed. The effect of the parameters of defects of different types on the pulse propagation is revealed.     

Kuznetsov–Ma soliton and Akhmediev breather of higher-order nonlinear Schrdinger equation

In terms of Darboux transformation, we have exactly solved the higher-order nonlinear Schrdinger equation that describes the propagation of ultrashort optical pulses in optical fibers. We discuss the modulation instability(MI) process in detail and find that the higher-order term has no effect on the MI condition. Under different conditions, we obtain Kuznetsov–Ma soliton and Akhmediev breather solutions of higher-order nonlinear Schrdinger equation. The former describes the propagation of a bright pulse on a continuous wave background in the presence of higher-order effects and the soliton's peak position is shifted owing to the presence of a nonvanishing background, while the latter implies the modulation instability process that can be used in practice to produce a train of ultrashort optical soliton pulses.     

Group-velocity control by quadratic nonlinear interactions

We give direct experimental evidence that the group velocity of ultrashort pulses can be controlled through chi(2)-cascaded interactions, under the condition of large group-velocity mismatch. The group velocity can be finely tuned by acting on pulse intensity and phase mismatch. Group-delay shifts up to 50 fs are achieved by propagating 40 fs pulses around 1400 nm in a 25 mm long periodically poled stoichiometric lithium tantalate crystal.     

Elliptic X-shaped light bullets

Elliptic X-shaped light bullets, which are generated as nondiffraction elliptic Bessel-like beams with central humps of elliptical shape during the propagation of asymmetric input ultrashort pulses in normal dispersive quadratic media, are demonstrated to be quite stable against wave-packet breakup due to asymmetric input or anisotropic diffraction. Walking elliptic X-shaped light bullets can be observed without spatial wave-packet breakup for strongly coupled fundamental-wave and second-harmonic ultrashort pulses with nonvanishing group-velocity mismatch.     

Temporal and spectral properties of ultrashort pulsed Laguerre-Gaussian beams in dispersive media

The temporal and spectral properties of ultrashort pulsed Laguerre-Gaussian (LG) beams propagating in dispersive media are studied both theoretically and numerically. It is shown that the on-axis pulse broadens with increasing propagation distance and the on-axis spectrum is blueshifted. For the off-axis spectrum the blueshifts decrease and then the spectrum becomes redshifted as the radial distance increases. The propagation of ultrashort pulsed LG beams in free space and of ultrashort pulsed Gaussian beams can be regarded as special cases of our results, and some interesting properties are discussed.     

超短激光脉冲在多能级系统中的传输特性

运用数值方法研究了超短激光脉冲在多能级系统中的传输特性.当激光脉冲能量相对较弱时,适当的脉冲面积可以实现自感应透明;激光脉冲能量逐渐增强,脉冲将产生分裂.脉冲分裂的基本面积（即脉冲实现稳定传播的最小面积）与二能级系统不同,而是A₀=2π/κ,与激发态的能级个数和跃迁偶极矩密切相关.由于分裂和吸收,超短激光脉冲传输了一段距离以后,将演化为一个脉冲宽度更窄、峰值强度更高且面积为A₀的超短激光脉冲. 关键词： 超短激光脉冲 多能级系统 特征时间     

高功率光子晶体光纤放大器内超短脉冲的放大传输特性

由速率方程和功率传输方程得到信号脉冲平均功率和增益系数随脉冲传输距离的变化关系.利用Ginzburg-landau方程,在信号脉冲功率的不断增强和增益系数的不断变化的情况下,研究超短脉冲的传输演化特性,发现信号脉冲平均功率、能量和增益系数等参量受到色散和非线性效应的影响相对较小,而信号脉冲的峰值功率、时域和频域特性则易受到色散和非线性效应的影响.探讨了在非线性作用下,脉冲分裂和展宽等所导致的脉宽不稳定性对传输特性的影响,表明研究脉冲传输问题时,引入脉宽不稳定性有利于提高数值模拟准确度.     

Nonlinear regimes of ultrashort pulse propagation in an array of anisotropic tunneling states

Dynamics of an ultrashort electromagnetic pulse in a system with an array of anisotropic tunneling states spanned by the pulse spectrum are analyzed. A system of nonlinear wave equations is derived for the ordinary and extraordinary components of the pulse propagating at an arbitrary angle to the anisotropy axis. Different regimes of ultrashort pulse propagation parallel and perpendicular to the anisotropy axis are examined. Ultrashort-pulse propagation regimes analogous to self-induced transparency and extraordinary transparency are identified. The properties of rational soliton-like pulses having no quasi-monochromatic analogs are analyzed. A longitudinal electric field component is generated in each regime, whereas off-resonance quasi-monochromatic pulses propagating under similar conditions (parallel and perpendicular to the anisotropy axis) have no longitudinal components. Stability of the solutions obtained and the effect of diffraction on ultrashort pulse dynamics are analyzed. The values of pulse parameters for which defocusing dominates over self-focusing are calculated.     

Interaction of an ultrashort optical pulse with the impurity subsystem of a hydrogen-containing ferroelectric

The dynamics of an ultrashort optical pulse in impurity-containing ferroelectrics with hydrogen bonds has been considered in terms of microscopic pseudospin formalism, which uses the approximation of unidirectional propagation without limitations on the pulse power. The effective equations for the electric field amplitude and ferroelectric polarization have been obtained and numerically solved. Quasi-soliton propagation modes of ultrashort optical pulses are revealed, and their dependences on the microscopic Hamiltonian parameters are analyzed.