Structure of the focal region at a focusing with a time reversal of waves in an inhomogeneous medium

A focusing array with a time reversal of waves in an inhomogeneous medium is considered. It is shown that, at the focus of such an array, an oscillation trap can be formed. In a homogeneous medium, a wave first travels to the array focus, is focused, and then travels away from the focus, whereas, in an inhomogeneous medium, the wave does not travel at all. In the oscillation trap, an intense oscillation is formed, which arrives from nowhere and escapes to nowhere. The size of the oscillation trap is much smaller than that of the focal spot of the array in free space. The physical nature of this phenomenon and the possible areas of its practical application are discussed.     

Direct measurement of the oscillation frequency in an optical-tweezers trap by parametric excitation

We demonstrate a novel technique for direct measurement of the oscillation frequency in an optical-tweezers trap. The technique uses the phenomenon of parametric resonance in an oscillator when the stiffness of the trapping potential is modulated. The trapped particle is a strongly damped oscillator; hence, the signature of parametric resonance is not an increase in the amplitude but an increase in the size of Brownian fluctuations. The trap frequency is measured with an accuracy of 0.1%, which is better than previous techniques and thus opens up new possibilities in experiments with optical tweezers.     

Observation of the Rabi oscillation of light driven by an atomic spin wave

Coherent conversion between a Raman pump field and its Stokes field is observed in a Raman process with a strong atomic spin wave initially prepared by another Raman process operated in the stimulated emission regime. The oscillatory behavior resembles the Rabi oscillation in atomic population in a two-level atomic system driven by a strong light field. The Rabi-like oscillation frequency is found to be related to the strength of the prebuilt atomic spin wave. High conversion efficiency of 40% from the Raman pump field to the Stokes field is recorded and it is independent of the input Raman pump field. This process can act as a photon frequency multiplexer and may find wide applications in quantum information science.     

Wave motion and dispersion phenomena: veering, locking and strong coupling effects

The dispersion curves describe wave propagation in a structure, each branch representing a wave mode. As frequency varies the wavenumbers change and a number of dispersion phenomena may occur. This paper characterizes, analyzes, and quantifies these phenomena in general terms and illustrates them with examples. Two classes of phenomena occur. Weak coupling phenomena-veering and locking-arise when branches of the dispersion curves interact. These occur in the vicinity of the frequency at which, for undamped waveguides, the dispersion curves in the uncoupled waveguides would cross: if two dispersion curves (representing either propagating or evanescent waves) come close together as frequency increases then the curves either veer apart or lock together, forming a pair of attenuating oscillatory waves, which may later unlock into a pair of either propagating or evanescent waves. Which phenomenon occurs depends on the product of the gradients of the dispersion curves. The wave mode shapes which describe the deformation of the structure under the passage of a wave change rapidly around this critical frequency. These phenomena also occur in damped systems unless the levels of damping of the uncoupled waveguides are sufficiently different. Other phenomena can be attributed to strong coupling effects, where arbitrarily light stiffness or gyroscopic coupling changes the qualitative nature of the dispersion curves.     

Dynamics of dark-bright solitons in cigar-shaped Bose-Einstein condensates

We explore the stability and dynamics of dark-bright (DB) solitons in two-component elongated Bose-Einstein condensates by developing effective one-dimensional vector equations and solving the three-dimensional Gross-Pitaevskii equations. A strong dependence of the oscillation frequency and of the stability of the DB soliton on the atom number of its components is found; importantly, the wave may become dynamically unstable even in the 1D regime. As the atom number in the dark-soliton-supporting component is further increased, spontaneous symmetry breaking leads to oscillatory dynamics in the transverse degrees of freedom. Moreover, the interactions of two DB solitons are investigated with an emphasis on the importance of their relative phases. Experimental results showcasing multiple DB soliton oscillations and a DB-DB collision in a Bose-Einstein condensate consisting of two hyperfine states of ⁸⁷Rb confined in an elongated optical dipole trap are presented.     

Broad bandwidth optical and mechanical rheometry of wormlike micelle solutions

We characterize the linear viscoelastic shear properties of an aqueous wormlike micellar solution using diffusing wave spectroscopy (DWS) based tracer microrheology as well as various mechanical techniques such as rotational rheometry, oscillatory squeeze flow, and torsional resonance oscillation covering the frequency range from 10(-1) to 10(6) rad/s. Since DWS as well as mechanical oscillatory squeeze flow and torsional resonance oscillation cover a sufficiently high frequency range, the persistence length of wormlike micelles could be determined directly from rheological measurements for the first time.     

Annihilation of positronium atoms in an oscillatory field

Annihilation parameters (the angular correlation curve and positronium lifetime) are calculated for annihilation of positronium atoms in a three-dimensional oscillatory well. This well can serve as a model of a positron trap in rigid bodies. The model lends itself to exact mathematical analysis, and it admits separation of variables of the center-of-mass motion and the relative motion of the particle. A calculation is given for the wave functions of a positronium, which oscillatory field. The wave function of the relative motion of the particles at small distances is similar to the wave function of free positronium, which gives us a basis to speak of the annihilation of an electron-positron pair as the annihilation of a positronium atom in an oscillatory field. With a decrease in the size of the trap, broadening of the correlation curve occurs, which has Gaussian form for the annihilation of positronium from the minimum condition for center-of-mass motion. The bound state of the electron and positron exists even in traps of the size of the Bohr radius. The model permits evaluation of the size of traps over an experimentally chosen narrow component of the correlation curve.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 82–87, March, 1989.     

Photostimulated attenuation of hypersound in superlattices

Photostimulated attenuation of hypersound in semiconductor superlattices has been investigated theoretically. It is shown that the attenuation coefficient depends on the phonon wave vectorqin an oscillatory manner and that from this oscillation the band width Δ of the superlattice can be found.     

Effect of Attenuator on BWO Start Oscillation Condition in a Helix Millimeter Wave TWT Under Magnetic Focusing

In a practical helix millimeter wave traveling-wave tube (TWT), there are always magnetic focusing system for constraining the electron beam as it passes through the interaction region and attenuator for suppressing the oscillations, including backward-wave oscillation (BWO) and improving the output power. In view of the attenuator and magnetic focusing system, a 2D linear theory is employed to analyze BWO start oscillation condition. Numerical results show that the start oscillation length of the millimeter wave TWT decreases when the start position of the attenuator is close to the input section of the slow wave structure (SWS), and that it increases with the decrease of the attenuation length or the increase of the attenuation quantity. Therefore, in order to predict the BWO accurately, we should take into the attenuator and magnetic focusing system account.     

Effect of attenuation on backward-wave oscillation start oscillation condition

In a practical helix traveling-wave tube (TWT), there is always attenuator/sever for suppressing the oscillations, including backward-wave oscillation (BWO). The factors of the influencing BWO include start position of the attenuator, its length, and attenuation quantity. In the event that the attenuator/sever and nonuniformities in the phase velocity and beam potential were considered, a linear theory is employed to analyze BWO start oscillation condition. Numerical results show that the start oscillation length of the TWT decreases when the start position of the attenuator is close to the input section of the slow wave structure (SWS), that Start oscillation current of the output section of the SWS increases as the attenuation length decreasing, or the attenuation quantity increasing or the nonuniformities becoming strong, and that, however, when the phase velocity or beam potential exceeds a particular value, no oscillation condition could be found.     

Active traveling wave in the cochlea

A sound stimulus entering the inner ear excites a deformation of the basilar membrane which travels along the cochlea towards the apex. It is well established that this wavelike disturbance is amplified by an active system. Recently, it has been proposed that the active system consists of a set of self-tuned critical oscillators which automatically operate at an oscillatory instability. Here, we show how the concepts of a traveling wave and of self-tuned critical oscillators can be combined to describe the nonlinear wave in the cochlea.     

Theoretical comparison of optical traps created by standing wave and single beam

We used generalised Lorenz–Mie scattering theory (GLMT) to compare submicron-sized particle optical trapping in a single focused beam and a standing wave. We focus especially on the study of maximal axial trapping force, minimal laser power necessary for confinement, axial trap position, and axial trap stiffness in dependency on trapped sphere radius, refractive index, and Gaussian beam waist size. In the single beam trap (SBT), the range of refractive indices which enable stable trapping depends strongly on the beam waist size (it grows with decreasing waist). On the contrary to the SBT, there are certain sphere sizes (non-trapping radii) that disable sphere confinement in standing wave trap (SWT) for arbitrary value of refractive index. For other sphere radii we show that the SWT enables confinement of high refractive index particle in wider laser beams and provides axial trap stiffness and maximal axial trapping force at least by two orders and one order bigger than in SBT, respectively.     

A WAVE MODEL FOR A PNEUMATIC TYRE BELT

A one-dimensional wave equation of an infinite flattened tyre belt is generated. The belt vibration is controlled by bending, tension, shear and the sidewall stiffness. The dispersion relations for two waves in the belt are calculated and used to find both the input impedance and attenuation on a tyre belt of infinite extent. Tension and the sidewall controls the deformation and stiffness below 100Hz. Waves propagate around the belt above this frequency. The wave speeds due to bending and shear were predicted and measured. The model presented here should be valid for the prediction of tyre response above about 400 Hz when for a car tyre the modal behaviour is observed to cease. In this high-frequency region, the tyre at the input appears to be of infinite extent.     

Observation of two-wave structure in strongly nonlinear dissipative granular chains

In a strongly nonlinear viscous granular chain impacted by a single grain we observe a wave disturbance that consists of two parts exhibiting two time scales of dissipation. Above a critical viscosity there is no separation of the two pulses, and the dissipation and nonlinearity dominate the shocklike attenuating pulse.     

Stationary Kepler orbits of the electron with velocity-dependent perturbations — classical quantization

In this paper the evolution of Kepler orbits generated by velocity-dependent perturbations is discussed. It is found that in the presence of oscillatory perturbations, of oscillation frequency proportional to the kinetic energy of the moving particle, a discrete set of stationary orbits exists. If the coefficient of proportionality in the frequency—kinetic energy relation is the Planck constant, the orbits are the same as those given by Bohr's quantum postulates. The conclusion is drawn that the Schrödinger wave equation describes, in hidden form, velocity-dependent oscillatory perturbations superimposed upon the basic motion of an electron and has nothing in common with the basic trajectory of the motion.     

Oscillatory waves in inhomogeneous neural media

In this Letter we show that an inhomogeneous input can induce wave propagation failure in an excitatory neural network due to the pinning of a stationary front or pulse solution. A subsequent reduction in the strength of the input can lead to a Hopf instability of the stationary solution resulting in breatherlike oscillatory waves.     

一维爆轰波在扰动来流中传播的数值研究

爆轰波在静止气体或定常来流中的传播得到了广泛研究, 然而在扰动来流中的传播研究较少。这方面的研究不仅是爆轰传播机制的重要组成部分, 还可为爆轰发动机的应用提供参考。文章基于两步诱导-放热总包反应模型, 开展了一维爆轰波在正弦密度扰动来流中的传播数值模拟。通过对数值结果分析, 获得了放热反应控制参数与爆轰波内在不稳定性的关系, 并在此基础上研究了扰动波长和幅值对一维爆轰波动力学过程的影响。研究发现, 在波前施加连续扰动会诱导爆轰波表现出更复杂的动力学行为, 且影响过程与爆轰波的内在不稳定性相关。对于稳定爆轰波, 扰动只在特定波长范围内引起前导激波后的压力振荡。对于不稳定爆轰波, 扰动会进一步强化其内在不稳定性。扰动幅值越大, 对爆轰波动力学过程的影响越显著。      

相对论速调管放大器杂模振荡的抑制

 研究了相对论速调管放大器（RKA）输入腔和中间腔之间的高阶杂模振荡问题。通过模式分析得知杂模在谐振腔内为TM11模式,而在漂移管中表现为TE11模式,针对该模式能在漂移管中传输的特性,利用漂移管内壁涂覆吸波材料吸收杂模功率的方法进行抑制。通过3维粒子模拟程序,分析了吸波材料的电导率及涂覆长度对抑制杂模增长率的影响。利用模拟分析得到的结果,对漂移管中涂覆吸波材料的RKA输入腔及中间腔结构进行了3维模拟研究,结果显示:合适的吸波材料的引入能够很好地抑制RKA输入腔和中间腔之间的杂模振荡。     

Suppression of noise in a noisy optical trap

We have experimentally explored and clarified the concept of noise suppression in intrinsically noisy systems by adding noise at the input using a microscopic bead held in a moving and intensity-modulated optical trap. By coupling the stiffness of the optical trap to its position, we have explicitly constructed an experimental model system in which added fluctuations in the trap position result in reduced variance of the observed bead position as compared to a stationary trap. This reduction in variance and the spectral properties of the observed output noise agree with theoretical predictions. Our experiment demonstrates that the essential aspect of noise reduction in such a system is that the added fluctuations drive the system into states with a reduced intensity of intrinsic noise sufficiently often.     

Simple and high efficient optical trapping using a cylindrical lens and a single plane wave of incidence

We suggest a simple and high efficient method for trapping particles in the evanescent field. In this method, a single plane wave is normally incident on the cylindrical surface of a cylindrical lens and then incident on the plane surface of the lens at an angle larger than the critical angle. Multiple reflections of light within the cylindrical lens create two evanescent waves with different directions in the transmitted field. Interference of two evanescent waves comes into being a standing wave which can stably trap particles close to the top of the cylindrical lens. Based on the Rayleigh approximation, we obtain analytical expressions of optical force acting on a Rayleigh particle placed in the vicinity of the lens. We find that the trap stiffness and trap depth is dependent on the radius of the cylindrical lens, wavelength and polarization of light, and incident angle at the lens–liquid interface.