Stochastic band structure for waves propagating in periodic media or along waveguides

We introduce the stochastic band structure, a method giving the dispersion relation for waves propagating in periodic media or along waveguides, and subject to material loss or radiation damping. Instead of considering an explicit or implicit functional relation between frequency $\omega$ and wavenumber $k$, as is usually done, we consider a mapping of the resolvent set in the dispersion space $(\omega, k)$. Bands appear as as the trace of Lorentzian responses containing local information on propagation loss both in time and space domains. For illustration purposes, the method is applied to a lossy sonic crystal, a radiating surface phononic crystal, and a radiating optical waveguide. The stochastic band structure can be obtained for any system described by a time-harmonic wave equation

Sublabel-Accurate Relaxation of Nonconvex Energies

We propose a novel spatially continuous framework for convex relaxations based on functional lifting. Our method can be interpreted as a sublabel-accurate solution to multilabel problems. We show that previously proposed functional lifting methods optimize an energy which is linear between two labels and hence require (often infinitely) many labels for a faithful approximation. In contrast, the proposed formulation is based on a piecewise convex approximation and therefore needs far fewer labels. In comparison to recent MRF-based approaches, our method is formulated in a spatially continuous setting and shows less grid bias. Moreover, in a local sense, our formulation is the tightest possible convex relaxation. It is easy to implement and allows an efficient primal-dual optimization on GPUs. We show the effectiveness of our approach on several computer vision problems

Electrostriction and guidance of sound by light in optical fibers

We investigate the generation of phonon wavepackets in optical fibers via electrostriction from coherent optical waves. Solving the elastodynamic equation subject to the electrostrictive force, we are able to reproduce experimental spectra found in standard and photonic crystal fibers. We discuss the two important practical cases of forward interaction, dominated by elastic resonances of the fiber, and backward interaction, for which an efficient mechanism of phonon guidance by light is found. The latter result describes the formation of the coherent phonon wavepacket involved in stimulated Brillouin scattering

La terapia psico-espiritual malāmati

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Light modulation in phoxonic nanocavities

We report on the occurrence of strong nonlinear acousto-optic interactions in phoxonic structures, that support, simultaneously, acoustic and optical localized resonant modes, under the influence of acoustic losses. Deploying a detailed theoretical investigation of the acousto-optic coupling in the specific case of a one-dimensional phoxonic cavity, realized by homogeneous SiO2 and Si layers, we demonstrate the possibility for an enhanced modulation of light with sound through multi-phonon exchange mechanisms. A full electrodynamic and elastodynamic multiple scattering approach is employed to describe the optical and acoustic modes, and to account for their mutual interaction and the underlying effects both in time and frequency domains. In particular, we discuss the influence of hypersonic attenuation on the acousto-optic interaction by considering typical acoustic losses in the GHz regime

Generation of phonons from electrostriction in small-core optical waveguides

International audienceWe investigate the generation of acoustic phonons from electrostriction of optical waves in small core waveguides. We specifically consider simple step-index strip waveguides composed of silica or silicon in air, with sub-micron lateral dimensions. Such waveguides support one or a few optical modes, but a rich spectrum of acoustic phonons that becomes densely populated as the phonon frequency increases. We evaluate rigorously the phonon energy density that results from the electrostriction of two frequency detuned guided optical waves, that are either co- or contra-propagating, including phonon loss. Plotting this energy density as a function of frequency detuning reveals the phonon wave packets that are electrostrictively active and gives a quantitative estimation of the energy transfer from optical waves to particular phonons. Furthermore, in the backward interaction geometry, the dispersion relation of such phonons can be accessed directly by varying the optical wavelength

Annular interdigital transducer focuses piezoelectric surface waves to a single point

We propose and demonstrate experimentally the concept of the annular interdigital transducer that focuses acoustic waves on the surface of a piezoelectric material to a single, diffraction-limited, spot. The shape of the transducing fingers follows the wave surface. Experiments conducted on lithium niobate substrates evidence that the generated surface waves converge to the center of the transducer, producing a spot that shows a large concentration of acoustic energy. This concept is of practical significance to design new intense microacoustic sources, for instance for enhanced acouto-optical interactions