Department Seminar of Or Lasri - Spacetime acoustic cloaking using real-time-controlled metamaterials
SCHOOL OF MECHANICAL ENGINEERING SEMINAR
Wednesday. June 1, 2022 at 14:00
Spacetime acoustic cloaking using real-time-controlled metamaterials
Or Lasri
M.Sc. Student (Direct Track) of Dr. Lea Beilkin-Sirota
Artificially manufactured structures, termed metamaterials, enable advanced wave propagation, breaking barriers in wave manipulation and control. Applications of metamaterials with space or frequency modulated parameters have been extensively addressed over the last two decades, including negative refraction, topological wave propagation, invisibility cloaking, and many more.
Recently, the new class of time-modulated metamaterials has gained an increased interest. Concepts such as Fresnel coefficients and Snell’s law have been developed analogously for the case of time-varying systems, as well as the development of unique methods for unconventional wave propagation using special relativity.
One of the most fascinating applications of such structures is the Spacetime Cloak (STC). Cloaking is usually associated with hiding objects in space from electromagnetic or acoustic detection. The STC, on the contrary, aims to conceal the occurrence of events. One of the approaches to STC is based on transformation field theory, which allows manipulating the medium dispersion in a way that a ‘hole’ in time is created. So far, realization of STCs was proposed only for electromagnetic or photonic systems, carried out using the nonlinear properties of optical fibers.
The main goal of this research was to realize an acoustic STC. This constitutes a significant challenge, as the nonlinear photonic effects are not applicable in acoustics, and new methods need to be derived. We proposed the following two independent methods:
- An alternative formulation of the acoustic STC as an active control problem, which is applied to an otherwise uniform medium. In this formulation, the required space and time dependent properties, including effective mass density and bulk modulus in the cloaked region, as well as additional cross-coupling terms of pressure and flow velocity, are created by the controllers in a real-time feedback operation of a feedback-based metamaterial.
- Creating the acoustic STC using two pairs of real-time reconfigurable programmed meta-surfaces. These surfaces eliminate the scattering of the incident beam, and simultaneously perform the uncloaked field prediction, using a targeted two-dimensional unidirectional control wave technique.
In addition, we studied general properties of time-varying waveguide systems. In these systems, under certain conditions, a temporal boundary is created, causing in-domain wave reflections at frequencies different from that of the source. We found that changing the parameters to a particular dispersive state does not cause these reflections, but produces a new wave that progresses in a negative refraction. We proposed a realization of this state by an actively-controlled metamaterial.
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