180 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
Far-field subwavelength acoustic imaging by deep learning
Seeing and recognizing an object whose size is much smaller than the
illumination wavelength is a challenging task for an observer placed in the far
field, due to the diffraction limit. Recent advances in near and far field
microscopy have offered several ways to overcome this limitation; however, they
often use invasive markers and require intricate equipment with complicated
image post-processing. On the other hand, a simple marker-free solution for
high-resolution imaging may be found by exploiting resonant metamaterial lenses
that can convert the subwavelength image information contained in the
near-field of the object to propagating field components that can reach the far
field. Unfortunately, resonant metalenses are inevitably sensitive to
absorption losses, which has so far largely hindered their practical
applications. Here, we solve this vexing problem and show that this limitation
can be turned into an advantage when metalenses are combined with deep learning
techniques. We demonstrate that combining deep learning with lossy metalenses
allows recognizing and imaging largely subwavelength features directly from the
far field. Our acoustic learning experiment shows that, despite being thirty
times smaller than the wavelength of sound, the fine details of images can be
successfully reconstructed and recognized in the far field, which is crucially
enabled by the presence of absorption. We envision applications in acoustic
image analysis, feature detection, object classification, or as a novel
noninvasive acoustic sensing tool in biomedical applications
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