Showing papers on "Standing wave published in 1996"

Bloch oscillations of atoms in an optical potential.

Abstract: Ultracold cesium atoms are prepared in the ground energy band of the potential induced by an optical standing wave. We observe Bloch oscillations of the atoms driven by a constant inertial force. We measure the momentum distribution of Bloch states and effective masses different from the mass of the free atom. {copyright} {ital 1996 The American Physical Society.}

Observation of atomic wannier-stark ladders in an accelerating optical potential

Abstract: We report observation of Wannier-Stark ladders with ultracold sodium atoms in an accelerating one-dimensional standing wave of light. Atoms are trapped in a far-detuned standing wave that is accelerated for a controlled duration. A small oscillatory component is added to the acceleration, and the fraction of trapped atoms is measured as a function of the oscillation frequency. Resonances are observed where the number of trapped atoms drops dramatically. The separation between resonant peaks is found to be proportional to the acceleration. We show that this resonant structure can also be understood as a temporal quantum interference effect. [S0031-9007(96)00369-9]

Oscillations in the Photoionization Cross Section of C 60

Abstract: Recent photoelectron spectroscopy results from gas phase ${\mathrm{C}}_{60}$ exhibit the same partial cross section variation with photon energy as has been observed in its solid phase. We assume that the variations originate from a fullerene specific ability to form a spherical standing wave of the final state electron by intramolecular interference or virtual reflection at the center of the photoionized molecule. The calculated photon energies of the cross section minima based on the boundary conditions of the standing wave agree fairly well with experimental data.

Radiation force on a spherical object in an axisymmetric wave field and its application to the calibration of high‐frequency transducers

Abstract: The general formulation for the radiation force on a spherical object in an axisymmetric acoustic field is provided. The sphere is described in general by three parameters: its density, compressional wave speed, and shear wave speed. Other types of spheres, including the rigid and immovable sphere and the infinitely soft sphere (void), are treated as limiting cases. Specialized formulations of the radiation force function are provided for several types of incident waves of common interest. A low‐frequency expansion for each case is provided for comparison with results from the literature. Among the solutions provided are the scattering function of an elastic sphere in a focused acoustic field and the radiation force on the sphere. The radiation force function is used to calibrate high‐frequency transducers. Experimental data are provided for a focused transducer for frequencies up to 10 MHz, where the size of the elastic sphere is comparable to or larger than the −3‐dB beamwidth of the sound field.

Moderate and steep Faraday waves: instabilities, modulation and temporal asymmetries

Abstract: Mild to steep standing waves of the fundamental mode are generated in a narrow rectangular cylinder undergoing vertical oscillation with forcing frequencies of 3.15 Hz to 3.34 Hz. A precise, non-intrusive optical wave profile measurement system is used along with a wave probe to accurately quantify the spatial and temporal surface elevations. These standing waves are also simulated by a two-dimensional spectral Cauchy integral code. Experiments show that contact-line effects increase the viscous natural frequency and alter the neutral stability curves. Hence, as expected, the addition of the wetting agent Photo Flo significantly changes the stability curve and the hysteresis in the response diagram. Experimentally, we find strong modulations in the wave amplitude for some forcing frequencies higher than 3.30 Hz. Reducing contact-line effects by Photo-Flo addition suppresses these modulations. Perturbation analysis predicts that some of this modulation is caused by noise in the forcing signal through ‘sideband resonance’, i.e. the introduction of small sideband forcing can generate large modulations of the Faraday waves. The analysis is verified by our numerical simulations and physical experiments. Finally, we observe experimentally a new form of steep standing wave with a large symmetric double-peaked crest, while simulation of the same forcing condition results in a sharper crest than seen previously. Both standing wave forms appear at a finite wave steepness far smaller than the maximum steepness for the classical standing wave and a surface tension far smaller than that for a Wilton ripple. In both physical and numerical experiments, a stronger second harmonic (in time) and temporal asymmetry in the wave forms suggest a 1:2 resonance due to a non-conventional quartet interaction. Increasing wave steepness leads to a new form of breaking standing waves in physical experiments.

Method for applying high-intensity ultrasonic waves to a target volume within a human or animal body

Abstract: A method for focussing vibrational energy upon a target volume within a surrounding contiguous medium imparts high intensity energy upon the target volume from low level energy sources. A plurality of standing compression waves are established within the medium along corresponding longitudinal axes between opposing pairs of coordinated transducers. The target volume is located at the common intersection of the axes of the standing compression waves. Opposing pairs of transducers are positioned from each other at a distance equal to an integer multiple of half wavelengths of the corresponding standing wave therebetween. The phase angle of each standing compression wave is regulated so as to cause each wave to be at its maximum intensity (amplitude) within the target volume at the point of common intersection with the other standing waves. The plurality of intersecting standing waves constructively interfere within the target volume, thereby imparting more intense vibrational energy upon the target volume that upon the surrounding medium.

Pattern formation in weakly damped Faraday waves

Abstract: We present a theoretical study of nonlinear pattern formation in parametric surface waves for fluids of low viscosity, and in the limit of large aspect ratio. The analysis is based on a quasi-potential approximation to the equations governing fluid motion, followed by a multiscale asymptotic expansion in the distance away from threshold. Close to onset, the asymptotic expansion yields an amplitude equation which is of gradient form, and allows the explicit calculation of the functional form of the cubic nonlinearities. In particular, we find that three-wave resonant interactions contribute significantly to the nonlinear terms, and therefore are important for pattern selection. Minimization of the associated Lyapunov functional predicts a primary bifurcation to a standing wave pattern of square symmetry for capillary-dominated surface waves, in agreement with experiments. In addition, we find that patterns of hexagonal and quasi-crystalline symmetry can be stabilized in certain mixed capillary-gravity waves, even in this case of single frequency forcing. Quasi-crystalline patterns are predicted in a region of parameters readily accessible experimentally.

Characterizing whispering-gallery modes in microspheres by direct observation of the optical standing-wave pattern in the near field.

Abstract: We demonstrated the use of a near-field probe to map the evanescent field of an optical standing wave in a fused-silica whispering-gallery mode microresonator. The periodicity of the observed standing wave allows us to estimate accurately the radial mode number of the whispering-gallery mode resonance that is being excited. We find that the use of a fiber half-coupler to excite these resonances in fused-silica microspheres results in only the lowest radial mode numbers’ being strongly excited, as predicted.

Multisatellite study of nightside transient toroidal waves

Abstract: Transverse magnetic pulsations polarized in the east-west direction (toroidal waves) are a common feature in the dayside magnetosphere, but they are also observed in the nightside magnetosphere in the form of transient oscillations typically lasting 10–20 min Until now, little attention has been paid to these transient toroidal waves (TTWs) To get insight into the propagation mode and generation mechanism of TTWs, we have examined magnetic field records from the elliptically orbiting AMPTE CCE, geostationary GOES 5 (located at a geographic longitude of ∼75° W), and GOES 6 (∼ 100°W) satellites We find that TTW onset times are often simultaneous among the satellites within an accuracy of 1–2 min, but that the wave periods differ from one satellite to another The latter feature is evidence that TTWs are due to standing Alfven waves on individual magnetic field lines Additional evidence of the standing waves is provided from the phase relationship between oscillations in magnetic field and ion flux anisotropy We show by a numerical calculation of the ionospheric damping rate that toroidal mode standing waves can last for a few wave periods once they are excited Thus, the general lack of continuous toroidal waves on the nightside can be attributed to the absence of sources rather than to strong ionospheric damping Using CCE data and ground magnetic field data, we confirm that TTWs can be observed immediately following ∼30% (∼2%) of substorm onsets when the satellite is on the nightside (dayside) at 4 < L < 7 Substorms and pseudo substorm onsets produce magnetohydrodynamic disturbances in the near-Earth magnetotail, and these disturbances appear to be a major source of TTWs

Square patterns and quasipatterns in weakly damped faraday waves

Abstract: Pattern formation in parametric surface waves is studied in the limit of weak viscous dissipation. A set of quasipotential equations (QPEs) is introduced that admits a closed representation in terms of surface variables alone. A multiscale expansion of the QPEs reveals the importance of triad resonant interactions, and the saturating effect of the driving force leading to a gradient amplitude equation. Minimization of the associated Lyapunov function yields standing wave patterns of square symmetry for capillary waves, and hexagonal patterns and a sequence of quasipatterns for mixed capillary-gravity waves. Numerical integration of the QPEs reveals a quasipattern of eightfold symmetry in the range of parameters predicted by the multiscale expansion. {copyright} {ital 1996 The American Physical Society.}

Image processing in 3D standing-wave fluorescence microscopy

Abstract: Standing-wave fluorescence microscopy, a method which utilizes interference to create a periodic excitation pattern along the optical axis, has been shown to provide improved axial resolution in thin, fluorescently labeled specimens. In each plane of focus, a complete standing wave data set is obtained by acquiring an image at each of three distinct positions of the interference fringes. Thicker specimens require through-focus data consisting of three images per plane. In this report we describe the recovery of information from this data using 3D image processing. The effective optical transfer function (OTF) of the standing wave microscope consists of the conventional OTF and two sidebands which are copies of the conventional OTF shifted axially by the spatial frequency of the interference fringes. The large gaps between the central band and the sidebands lead to significant ringing in the 3D reconstruction if linear deconvolution methods are employed. The use of non-linear, constrained image processing techniques has been shown to allow accurate extrapolation outside the OTF band limit. We demonstrate the extent to which the sidebands enhance recovery of information in the gaps, and provide a comparison between deconvolution using inverse-filtering and maximum-likelihood estimation.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Can a single-pulse standing wave induce chaos in atomic motion?

Abstract: Time-dependent Hamiltonian dynamics exhibit a wide range of novel effects in both classical and quantum domains [1,2]. Possibly the simplest time-dependent potential is the turning on and off of an interaction, though even here our intuition is clear only for the two extreme cases of fast passage and adiabatic interactions. Since the majority of cases fall between these two limits, it is important to develop a clear understanding and simple physical pictures at intermediate time scales. We show that when the interaction is nonlinear, the mere act of turning on and off a potential in this intermediate regime can lead to classical chaos. Further, we provide a clean experimental demonstration of the classical mechanism of resonance overlap [3 ‐ 5] which leads to classically diffusive growth. This general problem is posed in the context of atom optics with ultracold atoms. The nonlinear interaction is a single pulse of a one-dimensional standing wave of light. This type of time-dependent interaction is ubiquitous and occurs, for example, whenever an atomic beam passes through a standing wave of light. The starting point for this discussion is the model of a two level atom (transition frequency v0) interacting with a standing wave of near-resonant light (frequency vL) which is turned on and off with a time-dependent function fstd. For sufficiently large detuning dL › v0 2v L(relative to the natural linewidth), the excited state amplitude can be adiabatically eliminated [6], leading to a Hamiltonian for the ground state H › p 2 y2M 2 s ¯ hVeffy8df std cos2kLx.

Acoustic micromanipulation using a multi-electrode transducer

Abstract: Control of position of particles using acoustic radiation pressure in water was studied to develop noncontact micromanipulation technique. The radiation pressure traps particles suspended in water and forms agglomeration every half wavelength in an ultrasonic standing wave field. This paper describes a method to transport them in the transverse direction to the sound beam axis. The principle is based on the shift of sound field by driving selected elements of a multielectrode transducer. The transducer was a rectangle 20/spl times/40 mm in size with fifteen electrodes of 20/spl times/2 mm. Driving a series of electrodes of the transducer, alumina particles were trapped at the nodes of the standing wave field. When the driving electrodes were switched sequentially, the standing wave field shifted laterally and the trapped particles moved to the corresponding nodal points. The resolution of transportation can be made smaller by reducing the size of electrodes. This suggests a new technique to manipulate micro-objects without contact using an ultrasonic standing wave field generated by a multielectrode transducer.

Method and apparatus for determining the range, direction and velocity of an object

Abstract: The present invention provides a range finding system which utilizes feedback to induce mode hopping in a standing wave generator and which determines the range, direction and velocity of an object with respect to the range finding system based on the mode hop rate. The present invention provides a novel sampling technique which prevents aliasing by modulating the standing wave generator at a frequency equal to or greater than π times the frequency at which the optical path between the object and the standing wave generator is oscillating or vibrating. In order to account for Doppler effect, the standing wave interference coupling is analyzed over one period of the frequency modulation rate of the standing wave generator. A spectral analysis algorithm is implemented which determines the mode hop rate of the standing wave generator and the range, direction and velocity of the object with respect to the range finding system based on the determined mode hop rate. Implementation of the spectral analysis algorithm improves signal-to-noise ratio by avoiding thresholding to provide a range finding system which is substantially amplitude independent.

Rotating spirals in a Faraday experiment.

Abstract: We report experiments and model simulations of patterns of parametrically excited capillary ripples in a large aspect-ratio cell with thin horizontal layer of viscous fluid subjected to sinusoidal vertical oscillations. We found stable rotating spirals with different topological charges in the parameter range where steady straight rolls were observed previously. Formation of multiarmed spirals via dislocations approaching a target core was observed. The direction of the spiral rotation depends on its chirality and is always consistent with wave propagation towards the core. Wave drift towards the spiral core is associated with the shear flow which is generated near the walls by rapidly decaying viscous surface waves. {copyright} {ital 1996 The American Physical Society.}

Nonlinear Phenomena Induced by Finite-Amplitude Oscillation of Air Column in Closed Duct : Analysis of Acoustic Streaming

Abstract: Acoustic streaming is one of the nonlinear phenomena produced by strong sound waves. This type of streaming is driven by acoustic momentum flux in an attenuating sound field. A strong standing wave in a duct generated by finite-amplitude oscillation of the air column dissipating due to friction at the duct wall produces acoustic streaming. The velocity of streaming is estimated from the steady part of the second-order term of a perturbation expansion of a sinusoidal oscillation. However, finite-amplitude oscillation gives rise to shock-wave propagation in the duct. In order to estimate acoustic streaming produced by finite-amplitude oscillation, it is necessary to analyze the response of the oscillatory boundary layer to shock waves in detail. The present paper deals with numerical analysis of the acoustic streaming described above. The fourth-order spatial difference method is applied to two-dimensional analysis of acoustic streaming in this work. Calculated results show velocity distributions in the oscillatory boundary layer and structures of steady streaming for various amplitudes of oscillation.

Amplitude equations and pattern selection in viscoelastic convection.

Abstract: Pattern selection and stability in viscoelastic convection are studied in the framework of amplitude equations derived in the vicinity of stationary and oscillatory instabilities. The oscillatory instability corresponds to a Hopf bifurcation with broken translational symmetry. When this instability is the first to appear with increasing Rayleigh number, such systems may be described by coupled one-dimensional complex Ginzburg-Landau equations for counterpropagating waves. The coefficients of these equations, as computed from the underlying Navier-Stokes equations, are such that the selected pattern corresponds to standing waves. The phase dynamics of these waves is derived and leads to coupled Kuramoto-Sivashinsky equations. Their stability range is also determined for different typical fluid parameters. \textcopyright{} 1996 The American Physical Society.

Spectroscopy of atoms confined to the single node of a standing wave in a parallel-plate cavity.

Abstract: We have performed spectroscopy on sodium atoms that are optically channeled in the single node of a laser standing wave set up across a parallel-plate cavity Using this technique we have extended our previous measurement of the Lennard-Jones van der Waals energy-level shift [Sandoghdar et al, Phys Rev Lett 68, 3432 (1992)] down to a cavity width of \ensuremath{\sim}500 nm We discuss the applications of this technique to the precise measurement of atom-surface distances \textcopyright{} 1996 The American Physical Society

Formation of Electric Field Spikes in Electron-Beam-Plasma Interaction.

Abstract: High-frequency waves, which are driven by a strong electron beam and propagate along a density gradient, can form spatially concentrated {open_quote}{open_quote}HF spikes{close_quote}{close_quote} which extend typically one wavelength (1 cm) in the direction along the beam. Experiments and computer simulations show that the spike is a standing wave with nodes at boundaries to regions with propagating waves. The spikes only form in a plasma density gradient, and attempts to produce them in homogeneous plasma have failed. They form without trapping of the waves in density cavities and remain stable after the formation, i.e., there is no tendency towards collapse of the structure. {copyright} {ital 1996 The American Physical Society.}

A Gibbsian model for finite scanned arrays

Abstract: A finite-by-infinite array of thin half-wave dipoles with H-plane scan is used to show the existence of a Gibbs' phenomenon-type standing wave in scan impedance (normalized by the infinite array value) over the elements of the array The period of this wave is 05/spl lambda/ at broadside for /spl lambda//2 array spacing and increases as the scan angle increases by a grating lobe-type expression A simple empirical model based on Gibbs oscillations is fitted to the scan-impedance wave; the model predicts the 1/(1-sin/spl theta//sub 0/) period variation, and should be useful for systems trades and for preliminary design purposes

Hydrodynamic Modes of the “Beating Mercury Heart” in Varying Geometries

Abstract: A mercury drop covered by an aqueous solution which is brought into contact with a metal tip at a fixed potential can be excited to sustained electromechanical oscillations (“beating mercury heart”). The driving force for these pulsations is the electrocapillary effect. We investigated these excitations for the traditional watch glass geometry and for linear and ring-shaped geometries with different lengths and diameters, respectively, varying the potential of the metal tip. We find standing waves in the linear geometry with the number of nodes depending on the potential. In the ring geometry we observe a fast pulsation mode and a slow mode with 2-fold symmetry axis. In addition, we find solitary waves circulating on the ring.

Penetration to the Earth’s surface of standing Alfvén waves excited by external currents in the ionosphere

Abstract: The problem of boundary conditions for monochromatic Alfven waves, excited in the magnetosphere by external currents in the ionospheric E-layer, is solved analytically. Waves with large azimuthal wave numbers m≫1 are considered. In our calculations, we used a model for the horizontally homogeneous ionosphere with an arbitrary inclination of geomagnetic field lines and a realistic height disribution of Alfven velocity and conductivity tensor components. A relationship between such Alfven waves on the upper ionospheric boundary with electromagnetic oscillations on the ground was detected, and the spatial structure of these oscillations determined.

Resonance gas oscillations in closed tubes

Abstract: The problem of gas motion in a tube closed at one end and driven at the other by an oscillating piston is studied theoretically. When the piston vibrates with a finite amplitude at the first acoustic resonance frequency, periodic shock waves are generated, travelling back and forth in the tube. A perturbation method, based on a small Mach number, M and a global mass conservation condition, is employed to formulate a solution of the problem in the form of two standing waves separated by a jump (shock front). By expanding the equations of motion in a series of a small parameter e = M 1/2 , all hydrodynamic properties are predicted with an accuracy to second-order terms, i.e. to e 2 . It is found that the first-order solution coincides with the previous theories of Betchov (1958) and Chester (1964), while additional terms predict a non-homogeneous time-averaged pressure along the tube. This prediction compares favourably with experimental results from the literature. The importance of the phenomenon is discussed in relation to different transport processes in resonance tubes.

An experimental investigation of screech noise generation

Abstract: The screech noise generation process from supersonic underexpanded jets, issuing from a sonic nozzle at pressure ratios of 2.4 and 3.3 (fully expanded Mach number, Mj = 1.19 and 1.42), was investigated experimentally. The extremely detailed data provide a fresh, new look at the screech generation mechanism. Spark schlieren visualization at different phases of the screech cycle clearly shows the convection of the organized turbulent structures over a train of shock waves. The potential pressure field (hydrodynamic fluctuations) associated with the organized structures is fairly intense and extends outside the shear layer. The time evolution of the near-field pressure fluctuations was obtained from phase-averaged microphone measurements. Phase-matched combined views of schlieren photographs and pressure fluctuations show the sound generation process. The individual compression and rarefaction parts of the sound waves are found to be generated from similar hydrodynamic fluctuations. A partial interference between the upstream-propagating sound waves and the downstream-propagating hydrodynamic waves is found to be present along the jet boundary. The partial interference manifests itself as a standing wave in the root-mean-square pressure fluctuation data. The standing wavelength is found to be close to, but somewhat different from, the shock spacing. An outcome of the interference is a curious 'pause and go' motion of the sound waves along the jet periphery. Interestingly, a length scale identical to the standing wavelength is found to be present inside the jet shear layer. The coherent fluctuations and the convective velocity of the organized vortices are found to be modulated periodically, and the periodicity is found to match with the standing wavelength distance rather than the shock spacing. The reason for the appearance of this additional length scale, different from the shock spacing, could not be explained. Nevertheless, it is demonstrated that an exact screech frequency formula can be derived from the simple standing wave relationship. The exact relationship shows that the correct spacing between the sources, for a point source model similar to that of Powell (1953), should be a standing wavelength (not the shock spacing).

A new class of instabilities of rotating fluids

Abstract: Standing waves in a rotating ideal fluid are considered. It is shown that all of them are unstable with respect to short‐wavelength perturbations. Moreover, it is demonstrated that the growth rate of the corresponding instabilities tends to infinity when either the amplitude of the standing wave increases or its spatial scale decreases without bound. It is suggested that the observed instabilities are akin to the Hadamard instabilities.

Surface acoustic wave device

Abstract: PURPOSE: To surely and easily eliminate the spurious signal without complicating the constitution of a surface acoustic wave device and also to improve the designing freedom of the device together with suppression of occurrence of the multiple reflection and the standing waves CONSTITUTION: A grating reflector 6 is provided on a surface acoustic wave transmission path that reaches an output side IDT 5 from an input side IDT 4 which are formed on a piezoelectric substrate 1 The reflection band width of the reflector 6 is set at a frequency band where a spurious signal is produced Thus the spurious signal is suppressed The reflector 6 has an arc shape against the transmitting direction of the surface acoustic wave, or the shape of the reflector 6 includes a tilted grating reflector, a reflector IDT and a matching circuit which is externally attached to the IDT

On the observability of finite-depth short-crested water waves

Abstract: A numerical study of the superharmonic instabilities of short-crested waves on water of finite depth is performed in order to measure their time scales. It is shown that these superharmonic instabilities can be significant - unlike the deep-water case - in parts of the parameter regime. New resonances associated with the standing wave limit are studied closely. These instabilities ‘contaminate’ most of the parameter space, excluding that near two-dimensional progressive waves ; however, they are significant only near the standing wave limit. The main result is that very narrow bands of both short-crested waves ‘close ’ to two-dimensional standing waves, and of well developed short-crested waves, perturbed by superharmonic instabilities, are unstable for depths shallower than approximately a non-dimensional depth d = 1 ; the study is performed down to depth d = 0.5 beyond which the computations do not converge sufficiently. As a corollary, the present study predicts that these very narrow sub-domains of shortcrested wave fields will not be observable, although most of the short-crested wave fields will be.