Nonlinear interference between two sets of frequency conversion processes with widely spaced pump wavelengths in an optical fiber

The processes of nonlinear interaction of two high-power pump waves with widely spaced frequencies are studied in an optical fiber. Substantially increased output power of the Stokes components, generated via stimulated Raman scattering or stimulated four-wave mixing by one of the pump waves have been obtained in the field of the other pump wave and its Stokes components. For the first time the possibility to excited new spectral lines in the spectrum of one of the pump waves with frequency shifts characteristic for the other one, is experimentally demonstrated.     

热晕小尺度不稳定性解析理论的推导

本文在傍轴波近似和等压近似下推导出通过均匀介质传播的高能激光束小尺度不稳定性的线性解析理论。对扰动量方程在横向上进行傅里叶变换、在纵向和时间上进行各自的拉普拉斯变换,可以得到扰动量的解析表达式。最后,通过拉普拉斯反演后求得扰动量之傅里叶分量的解析解是用传播核(或格林函数)K_k(z,t)来表示的。     

A generic approach to boundary reflection in periodic media

Waves in periodic media, whose propagation is governed by nearest neighbour interaction, are investigated. The reflection and transmission coefficients are derived for a plane wave incident from medium 1 upon medium 2, without invoking common approximations. The derivation is valid for a class of waves including magneto- and electro-inductive waves, waves on short loaded dipoles, nanoparticles, coupled waveguides and acoustic waves in monatomic media. For this last case hitherto unknown microscopic reflection and transmission coefficients are derived and shown to reduce in the continuous limit to the well-known expressions in terms of acoustic impedances.     

Short pulse laser microforming of thin metal sheets for MEMS manufacturing

Continuous and long-pulse lasers have been used for the forming of metal sheets for macroscopic mechanical applications. However, for the manufacturing of micro-electro-mechanical systems (MEMS), the applicability of such type of lasers is limited by the long-relaxation-time of the thermal fields responsible for the forming phenomena. As a consequence of such slow relaxation, the final sheet deformation state is attained only after a certain time, what makes the generated internal residual stress fields more dependent on ambient conditions and might make difficult the subsequent assembly process for MEMS manufacturing from the point of view of residual stresses due to adjustment.The use of ns laser pulses provides a suitable parameter matching for the laser forming of an important range of sheet components used in MEMS that, preserving the short interaction time scale required for the predominantly mechanic (shock) induction of deformation residual stresses, allows for the successful processing of components in a medium range of miniaturization but particularly important according to its frequent use in such systems.In the present paper, a discussion is presented on the specific features of laser interaction in the timescale and intensity range needed for thin sheet microforming with ns-pulse lasers along with relevant modelling and experimental results and a primary delimitation of the parametric space of the considered class of lasers for the referred processes.     

Dynamics of atomic decay in a special one dimensional photonic crystal

Recently, Liu et al. [Phys. Rev. B 74, 075314 (2006)] pointed out that the atomic instant decay rate in one dimensional photonic crystal (1DPC) showed a series of pulse-like peaks with time. In this paper, we continue their work, and adopt a special 1DPC, in which the refractive indexes of both constitution layers in a period are the same, to perform the analysis in detail. Our results show that the pulse-like peak of instant decay rate originates from the interaction between the atom and the sub-reservoir, the latter of which corresponds to the group of reflected fields having the same optical distance. The atom interacts with such a sub-reservoir mainly after the time needed for propagation. However, near the arrival time of the reflected field, the atomic level is broadened and couples to all frequency components of the sub-reservoir, and the pulse-like peak of instant decay rate appears. Although our conclusion is deduced with the special 1DPC, it is also valid for more general cases and might be useful to measure the quantum Zeno and anti-Zeno effect, since the interval of repeated measurements may be expanded to several optical cycles in 1DPC, which will facilitate the observation of the quantum Zeno or anti-Zeno effect.     

Experimental attempts to measure non-collinear local magnetisation

Representing atomic magnetic moments as simple vectors has limitations when applied to systems in which orbital moments, or significant spin-orbit coupling is present. These phenomena are associated with interactions leading to non-collinearity in the magnetisation distribution. Several experimental techniques are available to probe such non-collinearity. The most direct is to measure both the magnitude and direction of the magnetic interaction vectors, which give its Fourier components. Such measurements are now becoming possible for antiferromagnets with the advent of a new generation of neutron polarimeters, which will allow both greater geometric flexibility and higher precision. However, up to now non-collinear magnetisation distributions have been revealed by more indirect means. Polarised neutron flipping ratio measurements can give only a single component of the magnetic interaction vector directly. However, the special geometric properties of the interaction vector and the symmetry breaking properties of an applied field can be exploited to obtain evidence of non-collinearity in the magnetisation distribution even from such limited data.     

The effect of grana and inter-grana components of chloroplasts on green light transmission: A preliminary study

In this paper we simulate for the first time the propagation of green light through grana and inter-grana components of chloroplasts. Those components are nano-metric structures and thus regular estimation of the propagation of light cannot be exact without taking into account diffraction effects. The numerical investigation presented in this paper solves Maxwell wave equation using the finite-difference time-domain (FDTD) numerical approach to analyze the propagation of light through a chloroplast model.     

A continuous-mode model of the mode locking in a pump-modulated laser

The active mode-locking process of the multimode laser with an external pump modulation is theoretically investigated in the frequency domain within the framework of the continuous-mode approximation. Intermode interaction and mode-coupling effects, including both AM and FM modulations, are naturally considered in a hierarchical equation of the mode components derived from the multimode Maxwell-Bloch equations. It is reduced to a continuous-mode equation that can be solved analytically in a stationary case, and used to discuss the spectral line shape and the phase dynamics of mode-components as a function of modulation amplitude and detuning of the modulation frequency. We predict a novel oscillation existing below the threshold of the ordinary complete mode-locking: The intensity of the total electric field yields a stable pulse train but its phase varies irregularly in time. This semi-locked state is characterized by a nonlinear chirping, an asymmetric spectrum, and drifting phases of the field mode-components.     

Full-vectorial analysis of optical vortex propagation in anisotropic cubic-quintic non-linear optical medium

This paper presents for the first time a full-vectorial analysis of optical vortex propagation in anisotropic cubic-quintic (CQ) non-linear medium. The purpose is to investigate the energy transfer mechanism and stability between orthogonal field components and how they are affected by the presence of material anisotropy in CQ materials. The numerical simulations were carried out via a three-dimensional finite difference based beam propagation method (3D-FD-BPM) which is capable of handling variations in any of the permittivity tensor components. Therefore, in this work we allowed all tensor components to vary independently and simulate vortex propagation for distances several times longer than the diffraction length. We expect that this work can provide new and important information regarding the behavior of these objects that may become valuable for the design of new photonic devices.     

Optical and non-linear optical properties of vanadium oxide phthalocyanine films

The optical and non-linear optical properties of peripheral-substituted vanadium oxide phthalocyanine (VOPc) film and substituted VOPc/polymer composite film were investigated using stationary and transient spectroscopy techniques. The absorption Q band of the VOPc/polymer composite film shows a red shift relative to that of the peripheral-substituted VOPc film, revealing the monomeric characteristics of VOPc molecules. Effective quenching of PL emission was observed for the VOPc/polymer composite film and could be assigned to the efficient VOPc–polymer interaction. From pump-probe and optical Kerr effect (OKE) measurements, two decay components were obtained by fitting the transients for both VOPc films. The fast component, in a femtosecond time domain, originates from the electron–phonon interaction, and the difference in their slow decay is an indication of an efficient ISC process in the VOPc/polymer composite film. The third-order non-linear optical susceptibilities of these films were determined to be in the order of 10^-11 esu. Received: 25 October 2001 / Revised version: 8 January 2002 / Published online: 7 February 2002     

Photon and axion splitting in an inhomogeneous magnetic field

The axion–photon system in an external magnetic field, when the direction of propagation of axions and photons is orthogonal to the direction of the external magnetic field, displays a continuous axion–photon duality symmetry in the limit the axion mass is neglected. The conservation law that follow in this effective (2+1)$(2+1)$-dimensional theory from this symmetry is obtained. The magnetic field interaction is seen to be equivalent to first order to the interaction of a complex charged field with an external electric potential, where this fictitious “electric potential” is proportional to the external magnetic field. This allows one to solve for the scattering amplitudes using already known scalar QED results. From the scalar QED analog the axion and the photon are symmetric and antisymmetric combinations of particle and antiparticle. If one considers therefore scattering experiments in which the two spatial dimensions of the effective theory are involved nontrivially, one observes that both particle and antiparticle components of photons and axions are preferentially scattered in different directions, thus producing the splitting or decomposition of the photon and axion into their particle and antiparticle components in an inhomogeneous magnetic field. This observable in principle effect is of first order in the axion–photon coupling, unlike the “light shining through a wall phenomena”, which is second order.     

Nonparaxial propagation of Hermite-Laguerre-Gaussian beams in uniaxial crystal orthogonal to the optical axis

Analytical expressions for the three components of the nonparaxial propagation of a Hermite-Laguerre-Gaussian （HLG） beam in uniaxial crystal orthogonal to the optical axis are derived. The intensity distribution of an HLG beam and its three components propagating in a uniaxial crystal orthogonal to the optical axis are demonstrated by numerical examples. Although the y and z components of an HLG beam in the incident plane are both equal to zero, they emerge upon propagation inside the uniaxial crystal. Moreover, the beam profile of the x component is relatively stable and the beam profiles of the y and z components have the same evolution law. If the ratio of the extraordinary refractive index to the ordinary refractive index is larger than unity, the beam profile of the HLG beam is elongated in the x direction and generally rotates clockwise. Otherwise, the beam profile of the HLG beam is elongated in the y direction and generally rotates anticlockwise. This research is beneficial to the optical trapping and nonlinear optics involved in the rotation of a beam profile.     

Molecular dynamics investigation of soliton propagation in a two-dimensional Yukawa liquid

We investigate via molecular dynamics simulations the propagation of solitons in a two-dimensional many-body system characterized by Yukawa interaction potential. The solitons are created in an equilibrated system by the application of electric field pulses. Such pulses generate pairs of solitons, which are characterized by a positive and negative density peak, respectively, and which propagate into opposite directions. At small perturbation, the features propagate with the longitudinal sound speed, from which an increasing deviation is found at higher density perturbations. An external magnetic field is found to block the propagation of the solitons, which can, however, be released upon the termination of the magnetic field and can propagate further into directions that depend on the time of trapping and the magnetic field strength.     

Mode-I crack in a two-dimensional fibre-reinforced generalized thermoelastic problem

A general model of the equations of the Lord-Şulman theory including one relaxation time and the Green-Lindsay theory with two relaxation times, as well as the classical dynamical coupled theory, are applied to the study of the influence of reinforcement on the total deformation for an infinite space weakened by a finite linear opening mode-I crack. We study the influence of reinforcement on the total deformation of rotating thermoelastic half-space and their interaction with each other. The material is homogeneous isotropic elastic half space. The crack is subjected to prescribed temperature and stress distributions. The normal mode analysis is used to obtain the exact expressions for displacement components, force stresses, and temperature. The variations of the considered variables with the horizontal distance are illustrated graphically. Comparisons are made with the results obtained in the three theories with and without rotation. A comparison is also made between the two theories for different depths.     

Production of three-dimensional entanglement for two atoms with a single resonant interaction

A scheme is proposed for generating three-dimensional maximally entangled states for two atoms. In the scheme the atoms are trapped in a two-mode cavity. The scheme only requires a single resonant interaction of the atoms with the cavity modes. Therefore, the scheme is very simple and required interaction time is very short, which is important in view of decoherence.     

Finite-time information consensus for multi-agent systems with fixed and switching topologies

In this paper, we study the consensus problems for a group of interacting agents. First, we analytically establish the explicit expression of the consensus state for the entire group. Second, we prove that the agents of the group under a particular type of nonlinear interaction can reach the consensus state in finite time in the scenarios with fixed and switching undirected topologies. The results are also extended to the case where the topology of the group is directed and satisfies a detailed balance condition on coupling weights. Third, some numerical examples are provided to analyze the influencing factors of the convergence time, that is, the parameter of the particular interaction function and the algebraic connectivity of graphs. Finally, an application of the theoretical results in sensor networks is given, namely, computing the maximum-likelihood estimate of unknown parameters.     

Selective preparation of ground state wave-packets: a theoretical analysis of femtosecond pump-dump-probe experiments on the potassium dimer

We present simulations on pump-dump-probe experiments performed on the potassium dimer. The interaction of two time-delayed laser pulses prepares vibrational wave packets in the electronic ground state. The quantum calculations reveal to what extent it is possible to prepare a ground state superposition of states with high versus low vibrational quantum numbers by changing the pump-dump delay time. It is shown that transient signals may exhibit interference effects which are due to characteristics of ground state wave-packets composed of two components showing different vibrational dynamics. In this way the signals are able to yield information about vibrational overtone motion. Received 27 September 2000 and Received in final form 21 November 2000     

Preparation of Cluster States of Atomic Qubits in Cavity QED

We propose an optical scheme to generate cluster states of atomic qubits, with each trapped in separate optical cavity, via atom-cavity-laser interaction. The quantum information of each qubit is encoded on the degenerate ground states of the atom, hence the entanglement between them is relatively stable against spontaneous emission. A single-photon source and two classical fields are employed in the present scheme. By controlling the sequence and time of atom-cavity-laser interaction, we show that the atomic cluster states can be produced deterministically.