Spatiotemporal instability in dispersive nonlinear Kerr medium with a finite response time

We investigated the spatiotemporal modulation instability (MI) in a medium with a non-instantaneous Kerr response in which the nonlinear contribution to the refractive index is governed by a relaxing Debye type equation. The expression for the MI gain spectrum in non-instantaneous Kerr medium is obtained, which clearly reveals the influence of the finite time of the nonlinear response (relaxation effect) on the spatiotemporal MI. It is shown that, due to the relaxation effect, the spatiotemporal MI can appear for the case of anomalous dispersion and defocusing nonlinearity, while it cannot appear in instantaneous Kerr medium for the same case, and a new MI gain spectrum appears, adding to the conventional MI gain spectrum similar to that in an instantaneous Ker medium. In addition, the bandwidth of MI gain spectrum is extended and the maximum MI gain is reduced with increasing of the relaxation time. Interestingly, spatiotemporal MI can appear for any spatial frequencies in non-instantaneous Kerr medium for any combination of dispersion and nonlinearity. We have performed an experiment of MI in carbon disulfide (CS₂), a typical non-instantaneous Kerr medium, which shows quantitative agreement with the theoretical analyses.     

Collision of optical solitons with Kerr law nonlinearity

The intra-channel collision of optical solitons, with Kerr law nonlinearity, is studied by the aid of quasi-particle theory. The perturbation terms considered in this paper are all of Hamiltonian type. It is shown that the soliton–soliton interaction can be suppressed in the presence of these perturbations, namely, the self-steepening, the third-order dispersion, the fourth-order dispersion and the frequency separation between the soliton carrier and the gain-center frequency. The prediction of quasi-particle theory are fully confirmed by direct numerical simulations.     

Influence of the effect of librations on energy exchange between ultrashort light counterpulses in a medium with Kerr nonlinearity

The influence of the effect of molecular librations on the interaction of high-power ultrashort counterpulses in Kerr liquids with anisotropic molecules is investigated. Molecular librations are shown to substantially affect the shape of the output pulses. A narrow peak with an intensity fourfold to fivefold higher than the input-pulse intensity develops on the leading edge. This peak is followed by a series of narrow damped spikes, the duration of which correlates with the libration time. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 3, pp. 398–401, May–June, 2000.     

Controllable Kerr nonlinearity with vanishing absorption in a four-level inverted-Y atomic system

The giant enhancement of Kerr nonlinearity in a four-level inverted-Y atomic system is investigated theoretically. Compared with that generated in a generic three-level system, the Kerr nonlinearity can be enhanced by several orders of magnitude with vanishing linear absorption. The physical mechanism leading to the giant enhancement of Kerr nonlinearity is also discussed.     

The optimum regime of compression of Gaussian pulses in opposing interaction in a medium with cubic nonlinearity

In the present article the effect of compression of Gaussian pulses during their opposing propagation in a medium with inertial cubic nonlinearity is considered. Using methods of computer simulation, conditions are found that determine the optimum regime of compression. It is shown that after single passage through the medium the compression degree is equal to two. B. I. Stepanov Institute of Physics, Academy of Sciences of Belarus, 70, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 6, pp. 840–842, November–December, 1997.     

Nonreciprocal microwave transmission under the joint mechanism of phase modulation and magnon Kerr nonlinearity effect

Nonreciprocal microwave devices, in which the transmission of waves is non-symmetric between two ports, are indispensable for the manipulation of information processing and communication. In this work, we show the nonreciprocal microwave transmission in a cavity magnonic system under the joint mechanism of phase modulation and magnon Kerr nonlinearity effect. In contrast to the schemes based on the standard phase modulation or magnon Kerr nonlinearity, we find that the joint mechanism enables the nonreciprocal transmission even at low power and makes us obtain a high nonreciprocal isolation ratio. Moreover, when two microwave modes are coupled to the magnon mode via a different coupling strength, the presented strong nonreciprocal response occurs, and it makes the nonreciprocal transmission manipulating by the magnetic field within a large adjustable range possible, which overcomes narrow operating bandwidths. This study may provide promising opportunities to realize nonreciprocal structures for wave transmission.     

Enhancement of Kerr nonlinearity with vanishing absorption in a tripod scheme

The giant enhancement of Kerr nonlinearity in a four-level tripod type system is investigated theoretically. By tuning the value of the Rabi frequency of the coherent control field, owing to the double dark resonances, the giant-enhanced Kerr nonlinearity can be achieved within the right transparency window. The influence of Doppler broadening is also discussed.     

Enhanced Kerr nonlinearity in a negative refractive atomic medium

The Kerr nonlinearity of a left-handed material is analyzed in a four-level atomic system.It is shown that,due to the effect of quantum interference,a large enhanced Kerr nonlinearity accompanied by vanishing absorption can be realized via choosing appropriate parameters in this negative refraction atomic medium.It not only shows the large nonlinearity but also acts as the phase and amplitude compensating effects.     

Surface waves in a crystal with switching Kerr nonlinearity

In this paper, we explore transverse electric surface waves propagating along the crystal with jump change of Kerr nonlinearity in dependence on field amplitude. The dielectric permittivity in the proposed model of nonlinearity is characterized by abruptly changing unperturbed dielectric constant and Kerr nonlinearity coefficient from one value to another when field amplitude exceeds the threshold value of the switching field. This allows to find exact solutions of model equations in different cases of nonlinearity signs, and to obtain the dependence of wave characteristics, including total power flux, on effective refractive index in explicit form. Such solutions describe two new types of nonlinear surface waves with specific structure depending on electric field amplitude. We derive the conditions of surface domain formation. It is found that the largest percentage of radiation is concentrated within the domain.     

非线性克尔效应对飞秒激光偏振的超快调制

研究了近红外飞秒激光的偏振在太赫兹频率的超快调制.利用抽运-探测光谱技术,通过改变两个脉冲之间的延迟时间可以控制光脉冲的旋转角.在Li：NaTb（WO₄）₂磁光晶体中观察到探测光的偏振随延迟时间变化的高速振荡,振荡信号的中心频率为0.19 THz.这种超快偏振调制现象可以解释为,抽运-探测实验构置中,前向传播的抽运光诱导的光学克尔非线性引起被晶体远端表面所反射的背向传播的探测光脉冲偏振面的额外旋转.通过改变抽运光的圆偏振旋性可以控制探测光调制信号的相位和振幅.实验结果表明,非线性光学克尔效应可以作为一种全新的手段,在磁光晶体中实现近红外飞秒激光以太赫兹频率的超快偏振调控.这将在超快磁光调制器等全光器件中得以应用.实验结果将有助于偏振依赖的超快动力学过程的研究.     

The influence of external magnetic field and femtosecond laser pulses on the anomalous Kerr loops in GdFeCo amorphous film

The anomalous loops obtained by applying the traditional Kerr loop technique, in which the sample was pumped by continuous pulses using the method of successive increment scanning external magnetic field, contained memory and accumulation effects originating, respectively, from external field history and multiple pulse excitations. To eliminate the memory effect and study the influence of external magnetic field on the anomalous loops, an initializing field scanning approach was used to replace the method of successive increment scanning external field. And to confirm the existence of accumulation effect and study the influence of pump pulses on the anomalous loops, a controllable pump-pulse-number magneto-optical Kerr technique was used to replace continuous pulses pump Kerr technique. In both ways, measured with different pump fluence and different pump pulses, the results showed clearly the dependence of the hot coercivity and the degree of magnetization reversal on the external field, pump pulse numbers and pump fluence.     

飞秒脉冲激光1维强度调制的传输特性

实验研究了飞秒激光干涉条纹在CS2介质中传输的增长率与条纹间隔以及增长率与中心峰值强度的关系。结果发现,在非线性介质中传输的1维干涉调制特别是当其调制频率位于中心光强对应的最快增长频率附近时,随着传输距离的增加调制场迅速获得增长,将高斯光束分裂成多根峰值强度很高的带状细丝。相应的数值模拟与实验结果具有较好的一致性。     

各阶克尔非线性折射率对强激光大气传输的影响

用数值方法研究了各阶克尔非线性折射系数对强激光传输的影响,结果显示第7阶克尔非线性系数和第9阶克尔非线性系数协同影响激光传输,当第7阶克尔效应起作用时,忽略第9阶克尔效应的作用会导致光强随传输距离震荡剧烈,传输非常不稳定。要得到稳定传输需要第7阶克尔非线性系数和第9阶克尔非线性系数同时起作用或同时不起作用。在自聚焦阶段第3阶非线性折射率对折射率改变起主要作用,在稳定光丝传输阶段第7阶克尔非线性系数,第9阶克尔非线性系数起主要作用。第5阶克尔系数在整个传输过程中对折射率的改变一直比较小。     

飞秒脉冲激光1维强度调制的传输特性

实验研究了飞秒激光干涉条纹在CS2介质中传输的增长率与条纹间隔以及增长率与中心峰值强度的关系。结果发现,在非线性介质中传输的１维干涉调制特别是当其调制频率位于中心光强对应的最快增长频率附近时,随着传输距离的增加调制场迅速获得增长,将高斯光束分裂成多根峰值强度很高的带状细丝。相应的数值模拟与实验结果具有较好的一致性。     

啁啾脉冲堆积宽带激光的三次谐波产生

 针对啁啾脉冲堆积方法获得的宽带激光,分析了其形成机制,并给出了其时间波形和光谱分布。采用KDP晶体Ⅰ/Ⅱ类角度失谐的三倍频方案以及Ⅰ/Ⅱ/Ⅱ晶体级联角度失谐方案,定量分析了啁啾脉冲堆积方式宽带激光三倍频转换效率随入射光强、带宽、以及晶体厚度等因素的变化,并与时间相位调制的宽带激光三倍频的相应结果进行了比较。研究结果表明,采用晶体级联方式可以大幅度提高宽带三倍频转换效率,并且啁啾脉冲堆积宽带激光的三倍频转换效率的提高比时间相位调制宽带激光更为明显。     

环形光斑激光束在非线性克尔介质中的分束现象

对于非线性克尔介质中的环形斑调制光束,通过数值求解非线性Schrdinger方程,发现当入射光强度超过一定值时,光束在传输过程中发生分束现象.对光分束的阈值、光分束与环半径及半宽度的关系、分束光的特点,以及自聚焦焦点上光强度的横向分布与光分束之间的关系进行了详细的讨论 关键词： 环形光斑 克尔介质 非线性传输 光分束     

Enhancement of Kerr nonlinearity via spontaneously generated coherence in a four-level N-type atomic system

A theoretical investigation is carried out into the effect of spontaneously generated coherence (SGC) on Kerr nonlinearity of a four-level N-type system. It’s found that the Kerr nonlinearity can be obviously enhanced with SGC present. We attribute the enhancement of Kerr nonlinearity mainly to the generation of extra coherences induced by the superposition of the two SGC channels, and when the superposition is controlled by the interference between two SGC channels properly, the maximal Kerr nonlinearity does not only enter the electromagnetically induced transparency window as the spontaneous generated coherences intensify, but also gets enhanced about 10 times with very large SGC coefficients than that with no SGC effect.     

五阶非线性对啁啾超短脉冲间相互作用的影响

基于包含五次复系数的高阶Ginzburg Landau方程为模型,采用分步傅里叶方法数值研究了啁啾类超短脉冲间的相互作用。结果表明:相邻孤子之间的相互作用对五阶非线性效应非常敏感,即使参数改变很小的值,也会改变其传输特性。适当地选择五阶非线性参数值,能够很好地抑制孤子间的相互作用,提高光纤传输的比特率。当相邻孤子的初始间距为6.8,五阶非线性参数值取-0.001时,可以实现2个孤子长距离的保型传输。最后讨论了五阶非线性作用下多孤子之间的相互作用及抑制。     

利用两束同色激光场和半周期脉冲驱动原子产生单个阿秒脉冲

利用分裂算符方法数值求解一维氦原子的含时薛定谔方程,研究了氦原子在两束同色激光场与半周期脉冲(Hcps)形成的组合场驱动下所发射高次谐波的特点.研究结果表明,氦原子在这种组合场驱动下,高次谐波谱的平台区域能得到很大的扩展,其截止位置可延伸到I_P+9.6U_P,通过构造截止位置附近的高次谐波谱能够得到脉宽为63 as的单个阿秒脉冲.经过分析后发现,半周期脉冲的加入不仅使高次谐波谱平台能够得到扩展,同时还抑制了电子长路径对高次谐波的贡献. 关键词： 两束同色激光场 半周期脉冲 高次谐波 阿秒脉冲     

Control of laser pulse parameters in a cavity with a thermal-nonlinearity shutter

The light pulse generation by a laser controlled by shutter from a finite-thickness absorbing layer with thermal nonlinearity has been studied theoretically. The layer placed in the cavity is oriented with respect to the mirror so that conditions for nonlinear total internal reflection of generated radiation are created. In the plane-wave approximation, a self-consistent system of nonlinear truncated equations for the field amplitudes together with constitutive equations has been obtained. Numerical simulation with the use of the obtained system has shown that it is possible to control the form and duration of the generated pulse by varying the absorption coefficient of the nonlinear medium and the angle of incidence on the absorbing layer, as well as the layer thickness.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 6, pp. 771–777, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date.