225 research outputs found
Zero refractive index in space-time acoustic metamaterials
New scientific investigations of artificially structured materials and
experiments have exhibit wave manipulation to the extreme. In particular, zero
refractive index metamaterials have been on the front line of wave physics
research for their unique wave manipulation properties and application
potentials. Remarkably, in such exotic materials, time-harmonic fields have
infinite wavelength and do not exhibit any spatial variations in their phase
distribution. This unique feature can be achieved by forcing a Dirac cone to
the center of the Brillouin zone ( point), as previously predicted and
experimentally demonstrated in time-invariant metamaterials by means of
accidental degeneracy between three different modes. In this article, we
propose a different approach that enables true conical dispersion at with
twofold degeneracy, and generates zero index properties. We break time-reversal
symmetry and exploit a space-time modulation scheme to demonstrate a
time-Floquet acoustic metamaterial with zero refractive index. This behavior,
predicted using stroboscopic analysis, is confirmed by fullwave finite elements
simulations. Our results establish the relevance of space-time metamaterials as
a novel reconfigurable platform for wave control
Non-Reciprocal gain in non-Hermitian time-Floquet Systems
We explore the unconventional wave scattering properties of non-Hermitian
systems in which amplification or damping are induced by time-periodic
modulation. These non- Hermitian time-Floquet systems are capable of
non-reciprocal operations in the frequency domain, which can be exploited to
induce novel physical phenomena such as unidirectional wave amplification and
perfect non-reciprocal response with zero or even negative insertion losses.
This unique behavior is obtained by imparting a specific low-frequency
time-periodic modulation to the coupling between lossless resonators, promoting
only upward frequency conversion, and leading to non-reciprocal parametric
gain. We provide a full-wave demonstration of our findings in a one-way
microwave amplifier, and establish the potential of non-Hermitian time-Floquet
devices for insertion-loss free microwave isolation and unidirectional
parametric amplification.Comment: 15 pages, 4 figure
Invisibility and Cloaking: Origins, Present, and Future Perspectives
The development of metamaterials, i.e., artificially structured materials that interact with waves in unconventional ways, has revolutionized our ability to manipulate the propagation of electromagnetic waves and their interaction with matter. One of the most exciting applications of metamaterial science is related to the possibility of totally suppressing the scattering of an object using an invisibility cloak. Here, we review the available methods to make an object undetectable to electromagnetic waves, and we highlight the outstanding challenges that need to be addressed in order to obtain a fully functional coating capable of suppressing the total scattering of an object. Our outlook discusses how, while passive linear cloaks are fundamentally limited in terms of bandwidth of operation and overall scattering suppression, active and/or nonlinear cloaks hold the promise to overcome, at least partially, some of these limitations.AFOSR Award FA9550-13-1-0204NSF CAREER Award ECCS-0953311DTRA YIP Award HDTRA1-12-1-0022Electrical and Computer Engineerin
Non-reciprocal Optical Mirrors Based on Spatio-Temporal Modulation
The recent surge of interest in temporal modulation schemes to induce
magnet-free non-reciprocity has inspired several exciting opportunities for
photonic technology. Here, we investigate a scheme to realize free-space
isolators and highly non-reciprocal mirrors with weak modulation imparted by an
acoustic wave. Conventional optical mirrors are reciprocal: in a given plane of
incidence, reflection is independent of the sign of the angle of incidence,
which enables two people to simultaneously look at each other through their
reflection. In contrast, we propose a strategy to dramatically break this
symmetry by exploiting resonant interactions between a travelling acoustic wave
and highly resonant guided optical modes, inducing total reflection of an
optical beam at a given angle, and no reflection at the negative angle.
Different from conventional acousto-optic isolators, which are based on
non-resonant frequency conversion and filtering, our proposal operates at the
frequency of the optical signal by tailoring the resonant properties of the
structure as well as the acoustic wave frequency and intensity, enabling 50 dB
isolation with modest modulation requirements. Operation in reflection allows
for close-to-zero insertion loss, enabling disruptive opportunities in our
ability to control and manipulate photons
Extreme Spatial Dispersion in Nonlocally-Resonant Elastic Metamaterials
To date, the vast majority of architected materials have leveraged two
physical principles to control wave behavior, namely Bragg interference and
local resonances. Here, we describe a third path: structures that accommodate a
finite number of delocalized zero-energy modes, leading to anomalous dispersion
cones that nucleate from extreme spatial dispersion at 0 Hz. We explain how to
design such zero-energy modes in the context of elasticity and show that many
of the landmark wave properties of metamaterials can also be induced at an
extremely subwavelength scale by the associated anomalous cones, without
suffering from the same bandwidth limitations. We then validate our theory
through a combination of simulations and experiments. Finally, we present an
inverse design method to produce anomalous cones at desired locations in
momentum space
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