הפקולטה בתקשורת

Coordinate-transformation-inspired optical devices have been mostly examined in the continuous-wave regime: the performance of an invisibility cloak, which has been demonstrated for monochromatic excitation, is likely to deteriorate for short pulses.

Here, pulse dynamics of flexural waves propagating in transformed plates is investigated. A practical realization of a waveshifter and a rotator for flexural waves based on the coordinate transformation method is proposed. Time-resolved measurements reveal how the waveshifter deviates a short pulse from its initial trajectory, with no reflection at the bend and no spatial and temporal distortion of the pulse.

  Extending the strategy to cylindrical coordinates, a wave rotator is designed. It is demonstrated experimentally how a pulsed plane wave is twisted inside the rotator, while its wavefront is recovered behind the rotator and the pulse shape is preserved, with no extra time delay. The realization of the dynamical mirage effect is proposed, where an obstacle appears oriented in a deceptive direction

Pairwise Mode Locking in Dynamically Coupled Parametric Oscillators

Leon Bello, Marcello Calvanese Strinati, Shai Ben-Ami, and Avi Pe’er  Phys. Rev. Lett. 126, 083601 – Published 22 February 2021

ABSTRACT

Mode locking in lasers is a collective effect, where due to a weak coupling a large number of frequency modes lock their phases to oscillate in unison, forming an ultrashort pulse in time. We demonstrate an analogous collective effect in coupled parametric oscillators, which we term “pairwise mode locking,” where many pairs of modes with twin frequencies (symmetric around the center carrier) oscillate simultaneously with a locked phase sum, while the phases of individual modes remain undefined. Thus, despite being broadband and multimode, the emission is not pulsed and lacks first-order coherence, while possessing a very high degree of second-order coherence. Our configuration comprises two coupled parametric oscillators within identical multimode cavities, where the coupling between the oscillators is modulated in time at the repetition rate of the cavity modes, with some analogy to active mode locking in lasers. We demonstrate pairwise mode locking in a radio-frequency experiment, covering over an octave of bandwidth with approximately 20 resonant mode-locked pairs, filling most of the available bandwidth between dc and the pump frequency. We accompany our experiment with an analytic model that accounts for the properties of the coupled parametric oscillators near threshold.

Faceting and Flattening of Emulsion Droplets: A Mechanical Model (PRL Editors' Suggestion)

Ireth García-Aguilar, Piermarco Fonda, Eli Sloutskin, and Luca Giomi Phys. Rev. Lett. 126, 038001 – Published 21 January 2021

ABSTRACT

When cooled down, emulsion droplets stabilized by a frozen interface of alkane molecules and surfactants have been observed to undergo a spectacular sequence of morphological transformations: from spheres to faceted liquid icosahedra, down to flattened liquid platelets. While generally ascribed to the interplay between the elasticity of the frozen interface and surface tension, the physical mechanisms underpinning these transitions have remained elusive, despite different theoretical pictures having been proposed in recent years. In this Letter, we introduce a comprehensive mechanical model of morphing emulsion droplets, which quantitatively accounts for various experimental observations, including the size scaling behavior of the faceting transition. Our analysis highlights the role of gravity and the spontaneous curvature of the frozen interface in determining the specific transition pathway.

An article was written about this article in Physics Magazine

Fractional Advection-Diffusion-Asymmetry Equation

Wanli Wang and Eli Barkai

PHYSICAL REVIEW LETTERS

Fractional kinetic equations employ noninteger calculus to model anomalous relaxation and diffusion in many systems. While this approach is well explored, it so far failed to describe an important class of transport in disordered systems. Motivated by work on contaminant spreading in geological formations, we propose and investigate a fractional advection-diffusion equation describing the biased spreading packet. While usual transport is described by diffusion and drift, we find a third term describing symmetry breaking
which is omnipresent for transport in disordered systems. Our work is based on continuous time random walks with a finite mean waiting time and a diverging variance, a case that on the one hand is very common and on the other was missing in the kaleidoscope literature of fractional equations. The fractional space
derivatives stem from long trapping times, while previously they were interpreted as a consequence of spatial L ́evy flights.