Acousto-optic modulation in magnetised diffusive semiconductors

The modulation of an intense electromagnetic beam induced by the acousto-optic (AO) effect has been analysed in a transversely magnetised semiconductor-plasma medium. The effect of carrier diffusion on the threshold field and gain profile of the modulated wave has been extremely investigated using coupled mode theory. The origin of the AO interaction is assumed to lie in the induced nonlinear diffusion current density of the medium. By considering the modulation process as a four wave parametric interaction an expression for effective third-order AO susceptibility describing the phenomena has been deduced. The modulation is greatly modified by propagation characteristics such as dispersion and diffraction due to dielectric relaxation of the acoustic mode. The threshold pump field and the steady state growth rates are estimated from the effective third-order polarisation in the plasma medium. Analytical estimation reveals that in the presence of enhanced diffusion due to excess charge carriers the modulated beam can be effectively amplified in a dispersionless acoustic wave regime. The presence of an external dc magnetic field is found to be favourable for the onset of diffusion induced modulational amplification of the modulated wave in heavily doped regime. Received 5 November 2001     

北极冰-水耦合系统中声波传播特性

利用固体和流体介质中波传播理论,导出了冰-水两层复合结构中导波频散方程。进一步,利用二分法对频散方程进行了数值求解,得到了ω-k频散曲线(ω与k分别为圆频率和波数),以及相速度和群速度频散曲线。结果表明:冰-水两层复合结构中导波由具有相同厚度水层和冰层中导波耦合而成,但与水层和冰层中导波频散曲线相比,复合结构中导波频散曲线除第1阶模式外,其余高阶模式均发生了很大变化。从原水层第1阶模式的截止频率开始,复合结构第2阶模式的相速度曲线被压低,各高阶(大于2阶)模式的相速度曲线出现一个跃变点,群速度曲线出现一个极大和一个极小值。水层越厚,复合结构各高阶模式的截止频率越低,相同频带内导波模式越丰富。水层厚度保持不变时,复合结构各阶模式的相速度和群速度曲线均随冰层厚度的增加而向低频方向移动。另外,还进一步分析了冰-水复合结构的导波波结构,发现第1阶导波模式的能量主要集中在冰层内和海表面附近,而2阶以上高阶导波模式的振动位移幅度随深度方向呈现周期性特征,并且模式阶数越高,振动越复杂。      

Widely tunable femtosecond fiber optical parametric oscillator

The phase-matching condition in a fiber is discussed. A balance among the different orders of fiber dispersion can be found to achieve a widely tuning modulation instability gain for pumping around the normal dispersion regime. Three coupled nonlinear wave equations are used to simulate the femtosecond fiber optical parametric oscillator. The numerical results show that, through appropriate choice of dispersion, femtosecond pulses with a 180-nm tunable range can be generated when pump wavelength near a fiber’s zero-dispersion wavelength is tuned only 7 nm. Further tuning is limited by the walk-off between the pump and the signal pulses.     

Combination of two basic types of synchronization in a coupled semiconductor laser system

Combination of two basic types of synchronization, anticipatory synchronization and lagged synchronization, is investigated numerically between two coupled semiconductor lasers. It is found that lagged synchronization produced by a backward coupling with a suitable delay can combine with the originally hidden anticipatory synchronization and produce a type of synchronization overcoming the original lagged synchronization produced by a forward coupling. We study the combination synchronization phenomenon when the delay of the backward coupling is different from that of the original anticipatory synchronization. Our results suggest that the synchronization combination phenomenon might allow an interpretation of an experimental observation by Sivaprakasam et al. [Phys. Rev. Lett. 87, 154101 (2001)] that the anticipating time is irrespective of the external-cavity round trip time, which to date remains to be understood.     

Phase locking and chaotic synchronization in an array of three lasers

A linear array of three lasers that are coupled mutually in space is investigated. It is shown that the phase of the laser fields is locked with intermediate coupling while the laser intensities are totally chaotic and chaotically synchronized. When the intensities of lasers reenter the regime of chaotic synchronization at smaller coupling constant, the laser fields show low degree of phase locking. The phase differences in the fields between three lasers show rich patterns when the coupling is changed. Received 3 August 2001 and Received in final form 27 September 2001     

Optical soliton with group delay in microring coupled-resonator optical waveguides

This paper derives the dispersion relation of microring coupled-resonator optical waveguides (CROWs) without any approximation by using the transfer matrix method. Based on the established dispersion relation of CROWs it obtains the slow group velocity and dispersion coefficient. It finds that the effect of dispersion on optical pulses can be adjusted to balance the effect of nonlinearity by changing coupling coefficient or loss, so optical soliton with group delay can be obtained in microring CROWs. The optical soliton with group delay is of great significance for applications of microring CROWs in delay lines and optical buffers of future all-optical communication systems.     

Threshold behavior of semi-linear photorefractive oscillator

A general analysis of the threshold behavior for the photorefractive semi-linear oscillator is performed within the linear approximation on the basis of the classical wave-coupling model. This analysis shows that the well known particular results on the frequency degenerate oscillation are valid only within a restricted range of the external parameters. The theory specifies the conditions for onset and properties of the frequency non-degenerate oscillations, including the necessary values of the coupling strength, pump intensity ratio, the increments of the instability, and the frequency splits. Important generalizations of the basic model are considered.     

On generation of coherent structures induced by modulational instability in linearly coupled cubic-quintic Ginzburg-Landau equations

We study the properties of the coherent structures induced by the modulational instability (MI) of the two linearly coupled complex Ginzburg-Landau equations with both cubic and quintic terms, which in nonlinear optics can model ring lasers based on dual-core fibers. We obtain new stationary solutions different from the previous result and the analytic gain formula as function of the linear coupling constant and the model parameters. The fact that the system can be modulationally unstable for the vast region of the parameters space is demonstrated. The effects of the linear coupling constant on the evolution of a continuous wave under the MI are numerically investigated in the presence of the linear loss or gain. It is found that doubly asymmetric stable solitary pulses and stable breathers can be formed from the perturbed continuous waves state by the MI. The conditions for generating the periodic stable solitary pulses and fronts by the MI are identified by varying the linear coupling constant.     

Nature of quantum recurrences in coupled higher dimensional systems

We investigate recurrence phenomena in coupled two degrees of freedom systems. It is shown that an initial well localized wave packet displays recurrences even in the presence of coupling in these systems. We discuss the interdependence of time scales namely classical period and quantum revival time and explain the significance of initial conditions.     

The influence of pump beam polarization on the signal beam, in TWM in photorefractive crystals: Transmission geometry

Two-wave mixing (TWM) in transmission geometry of photorefractive (PR) crystals of 23 symmetry such as BSO, BGO is investigated by solving a set of coupled wave equations and taking their optical activity as one of the effective parameter. Coupled wave equations are solved numerically by fourth order Runge-kutta method and results are presented in graphical form. The effects of various parameters such as crystal thickness, optical activity, coupling coefficient, absorption, and input polarization angle of pump beam has been studied on the input beam polarization components. It has been observed that by varying the behaviour of pump beam polarization, signal beam can be modified. This can be used in various applications such as spatial light modulation and optical information processing, etc.     

Magnetic excitations in charge ordered a' - {N_a}{V_2}{O_5}

An investigation of the spin excitation spectrum of charge ordered (CO) NaV₂O₅ is presented. We discuss several different exchange models which may be relevant for this compound, namely in-line and zig-zag chain models with weak as well as strong inter-chain coupling and also a ladder model and a CO/MV (mixed valent) model. We put special emphasis on the importance of large additional exchange across the diagonals of V-ladders and the presence of exchange anisotropies on the excitation spectrum. It is shown that the observed splitting of transverse dispersion branches may both be interpreted as anisotropy effect as well as acoustic-optic mode splitting in the weakly coupled chain models. In addition we calculate the field dependence of excitation modes in these models. Furthermore we show that for strong inter-chain coupling, as suggested by recent LDA + U results, an additional high energy optical excitation appears and the spin gap is determined by anisotropies. The most promising CO/MV model predicts a spin wave dispersion perpendicular to the chains which agrees very well with recent results obtained by inelastic neutron scattering. Received 30 April 1999 and Received in final form 5 October 1999     

Threshold Analysis of a THz-Wave Parametric Oscillator

The parametric gain of a terahertz wave parametric oscillator （TPO） is analyzed. Meanwhile the expression of TPO threshold pump intensity is derived and theoretically analyzed with different factors. The effective interaction length between the pump wave and Stokes wave is calculated, and particular attention is paid to the coupling efficiency of the pump wave and Stokes wave. Such an analysis is useful for the experiments of TPO.     

Spiral wave chimeras in populations of oscillators coupled to a slowly varying diffusive environment

Chimera states are firstly discovered in nonlocally coupled oscillator systems. Such a nonlocal coupling arises typically as oscillators are coupled via an external environment whose characteristic time scale τ is so small (i.e., τ → 0) that it could be eliminated adiabatically. Nevertheless, whether the chimera states still exist in the opposite situation (i.e., τ ≫ 1) is unknown. Here, by coupling large populations of Stuart−Landau oscillators to a diffusive environment, we demonstrate that spiral wave chimeras do exist in this oscillator-environment coupling system even when τ is very large. Various transitions such as from spiral wave chimeras to spiral waves or unstable spiral wave chimeras as functions of the system parameters are explored. A physical picture for explaining the formation of spiral wave chimeras is also provided. The existence of spiral wave chimeras is further confirmed in ensembles of FitzHugh−Nagumo oscillators with the similar oscillator-environment coupling mechanism. Our results provide an affirmative answer to the observation of spiral wave chimeras in populations of oscillators mediated via a slowly changing environment and give important hints to generate chimera patterns in both laboratory and realistic chemical or biological systems.     

Study on random and coherence states using a family of CML models

A family of coupled map lattice (CML) models has been developed to simulate the interaction of convection, diffusion and dispersion in both weakly and strongly coupled cases. With variation of parameters, the models can simulate those systems of convection dominating, diffusion dominating, and dispersion dominating states. Not only coherent and turbulent properties as well as their interrelations, but also the transitional state between any modes with local coupling and global coupling are analysed to demonstrate the essential characteristics of any state. Numerical results show that the models are capable of simulating both layered coupling and stochastic mechanism, and thus can be conveniently used to analyse an initiative and driven coupled system. Results of numerical simulation also lead us to understand whether or not turbulence coherent structure is formed by modulation of wave packet. Finally, the duality of wave and particle characters of turbulence is illustrated in the numerical simulation.     

Nonlinear gain and dispersion in the four-level Λ-type system

The nonlinear gain and dispersion spectra of a four-level Λ-type system, which driven by a weak probe field and coupling field, is investigated. Using the perturbation method, third-order perturbation iterative solutions of the steady density matrix equations are obtained. We find that effect of the SGC on the nonlinear gain and dispersion are negligible. At the resonant coupling, nonlinear dispersion spectra of the two transition paths are symmetrical about a line, but the nonlinear absorption spectra are symmetrical about a point. The degeneration of the two lower states greatly modifies the nonlinear gain and dispersion spectra.     

Soliton coupling in birefringent optical fiber with third-order dispersion

Nonlinear coupling of polarized solitons in birefringent optical fiber in the presence of third-order dispersion is considered in the framework of the coupled nonlinear Schrödinger equations. The influence of third-order dispersion on the interaction between solitons is investigated. For sufficiently strong third-order dispersion the interaction may even become repulsive. The stable conditions for solitons of partial pulses are analyzed and amplitude threshold, which decreases with third-order dispersion coefficient decreasing, for the capture of solitons of partial pulses into a coupled two-component pulse is obtained.     

On the theory of TE-polarized waves guided by a nonlinear three-layer structure

TE-polarized electromagnetic waves guided by a three-layer structure consisting of a film surrounded by semi-infinite media (all media are characterized by a Kerrlike dielectric function) are investigated by expressing the solution of the field equations in terms of Weierstrass' elliptic functions. Evaluation leads to a universal dispersion relation and its solutions and a universal expression for the power flow. Numerical results are presented for the effective wave number as a function of the intensity of the electric field at the lower surface of the nonlinear film, for various profiles of the field intensity, and for the power flow as a function of the effective wave number.     

Statistical properties for a class of nonlinear squeezed states

Theoretical investigations of dynamical behavior in optical parametric oscillators (OPO) have generally assumed that the cavity detunings of the interacting fields are controllable parameters. However, OPOs are known to experience mode hops, where the system jumps to the mode of lowest cavity detuning. We note that this phenomenon significantly limits the range of accessible detunings and thus may prevent instabilities predicted to occur above a minimum detuning from being evidenced experimentally. As a simple example among a number of instability mechanisms possibly affected by this limitation, we discuss the Hopf bifurcation leading to periodic behavior in the monomode mean-field model of a triply resonant OPO and show that it probably can be observed only in very specific setups.     

Nonlinear three-wave interaction in photonic crystals

We present a multi-scale analysis of nonlinear three-wave-interaction processes in photonic crystals. Based on photonic Bloch functions as carrier waves, we derive the effective nonlinear coupled wave equations that govern pulse propagation in these systems and obtain the corresponding effective photonic crystal parameters directly from photonic band-structure computations. As an illustration, we show how hitherto inaccessible radiation-conversion processes such as wave-front reversal of optical pulses can be realized. Furthermore, we describe a novel regime of nonlinear three-wave interaction in photonic crystals associated with the nearly degenerate case and show how these results may be utilized to study experimentally certain problems from plasma physics and hydrodynamics in the context of nonlinear photonic crystals.     

近垂直传播电子回旋寻常波的传播和吸收特性的解析研究

对近垂直入射的电子回旋寻常波的传播和吸收，用相对论等离子体色散关系进行了分区分析，获得了共振层附近内区O－模与X－模的耦合的准确表达式，该结果与非相对论近似下的结论有重大差别，获得了波衰减因子的解析表达式，波的衰减因子由两部分组成：O－模的相对论衰减及与O－模耦的X－模的衰减。