Elastic waves trapped above a cylindrical cavity

The existence of trapped elastic waves above a circular cylindrical cavity in a half-space is demonstrated. These modes propagate parallel to the cylinder, and their amplitude decays exponentially as the observer moves away from it. Dispersion relations connecting the frequency with the wavenumber along the cylinder are obtained using an analytical technique based on multipole expansions, and are solved numerically. Critical frequencies at which modes cut on and off are determined, and a range of contour plots illustrating the displacement fields is presented

Extremely Low Loss Phonon-Trapping Cryogenic Acoustic Cavities for Future Physical Experiments

Low loss Bulk Acoustic Wave devices are considered from the point of view of the solid state approach as phonon-confining cavities. We demonstrate effective design of such acoustic cavities with phonon-trapping techniques exhibiting extremely high quality factors for trapped longitudinally-polarized phonons of various wavelengths. Quality factors of observed modes exceed 1 billion, with a maximum $Q$-factor of 8 billion and $Q\times f$ product of $1.6\cdot10^{18}$ at liquid helium temperatures. Such high sensitivities allow analysis of intrinsic material losses in resonant phonon systems. Various mechanisms of phonon losses are discussed and estimated

Heating in Nanophotonic Traps for Cold Atoms

Laser-cooled atoms that are trapped and optically interfaced with light in nanophotonic waveguides are a powerful platform for fundamental research in quantum optics as well as for applications in quantum communication and quantum information processing. Ever since the first realization of such a hybrid quantum nanophotonic, heating rates of the atomic motion observed in various experimental settings have typically been exceeding those in comparable free-space optical microtraps by about three orders of magnitude. This excessive heating is a roadblock for the implementation of certain protocols and devices. Its origin has so far remained elusive and, at the typical atom-surface separations of less than an optical wavelength encountered in nanophotonic traps, numerous effects may potentially contribute to atom heating. Here, we theoretically describe the effect of mechanical vibrations of waveguides on guided light fields and provide a general theory of particle-phonon interaction in nanophotonic traps. We test our theory by applying it to the case of laser-cooled cesium atoms in nanofiber-based two-color optical traps. We find excellent quantitative agreement between the predicted heating rates and experimentally measured values. Our theory predicts that, in this setting, the dominant heating process stems from the optomechanical coupling of the optically trapped atoms to the continuum of thermally occupied flexural mechanical modes of the waveguide structure. Beyond unraveling the long-standing riddle of excessive heating in nanofiber-based atom traps, we also study the dependence of the heating rates on the relevant system parameters. Our findings allow us to propose several strategies for minimizing the heating. Finally, our findings are also highly relevant for optomechanics experiments with dielectric nanoparticles that are optically trapped close to nanophotonic waveguides.Comment: Published version. 35 pages (including appendices), 7 figures, 18 tables, and 3 pages of supplemental materia

Elastodynamic cloaking and field enhancement for soft spheres

In this paper, we bring to the awareness of the scientific community and civil engineers, an important fact: the possible lack of wave protection of transformational elastic cloaks. To do so, we propose spherical cloaks described by a non-singular asymmetric elasticity tensor depending upon a small parameter $\eta,$ that defines the softness of a region one would like to conceal from elastodynamic waves. By varying $\eta$, we generate a class of soft spheres dressed by elastodynamic cloaks, which are shown to considerably reduce the soft spheres' scattering. Importantly, such cloaks also provide some wave protection except for a countable set of frequencies, for which some large elastic field enhancement (resonance peaks) can be observed within the cloaked soft spheres, hence entailing a possible lack of wave protection. We further present an investigation of trapped modes in elasticity via which we supply a good approximation of such Mie-type resonances by some transcendental equation. Next, after a detailed presentation of spherical elastodynamic PML of Cosserat type, we introduce a novel generation of cloaks, the mixed cloaks, as a solution to the lack of wave protection in elastodynamic cloaking. Indeed, mixed cloaks achieve both invisibility cloaking and protection throughout a large range of frequencies. We think, mixed cloaks will soon be generalized to other areas of physics and engineering and will in particular foster studies in experiments.Comment: V2: major changes. More details on the study of trapped modes in elasticity. Mixed cloaks introduced. Latex files, 27 pages, 14 figures. The last version will appear at Journal of Physics D: Applied Physics. Pacs:41.20.Jb,42.25.Bs,42.70.Qs,43.20.Bi,43.25.Gf. arXiv admin note: text overlap with arXiv:1403.184

Feasibility of Fatigue Crack Detection in Fluid-Filled Cylindrical Holes Using Circumferential Creeping Waves

Recently, the development of a novel ultrasonic inspection technique that detects radial fatigue cracks on the far side of so-called “weep” holes in thin airframe stiffeners was reported [1]. These cracks tend to be located on the upper part of the weep hole (at 12 o’clock position) therefore are not readily detectable by conventional ultrasonic inspection techniques from the lower skin of the wing. The new technique utilizes circumferential creeping waves propagating around the inner surface of the hole to perform the inspection. However, the wet wing has to be emptied and dried out before inspection because even a small amount of fluid fuel trapped in these rather small (approximately 6–7 mm in diameter) holes would strongly affect the propagation of circumferential creeping waves. We have searched the literature for published results on circumferential creeping wave propagation around fluid-filled cylindrical cavities in elastic media. Surprisingly, although the analytical solution of this canonical problem can be readily constructed from existing building blocks, very little was found in terms of numerical results that could be used to gain better understanding of the phenomenon. This motivated us to attack the problem by numerically solving the dispersion equation and constructing the corresponding dispersion and attenuation curves for a specific case of interest, namely, for that of a water-filled cylindrical hole in aluminum

Inconsistencies in the Notions of Acoustic Stress and Streaming

Inviscid hydrodynamics mediates forces through pressure and other, typically irrotational, external forces. Acoustically induced forces must be consistent with arising from such a pressure field. The use of "acoustic stress" is shown to have inconsistencies with such an analysis and generally arise from mathematical expediency but poor overall conceptualization of such systems. This contention is further supported by the poor agreement of experiment in many such approaches. The notion of momentum as being an intrinsic property of sound waves is similarly found to be paradoxical. Through an analysis that includes viscosity and attenuation, we conclude that all acoustic streaming must arise from vorticity introduced by viscous forces at the driver or other solid boundaries and that calculations with acoustic stress should be replaced with ones using a nonlinear correction to the overall pressure field