176 research outputs found
Resonant Metalenses for Breaking the Diffraction Barrier
We introduce the resonant metalens, a cluster of coupled subwavelength
resonators. Dispersion allows the conversion of subwavelength wavefields into
temporal signatures while the Purcell effect permits an efficient radiation of
this information in the far-field. The study of an array of resonant wires
using microwaves provides a physical understanding of the underlying mechanism.
We experimentally demonstrate imaging and focusing from the far-field with
resolutions far below the diffraction limit. This concept is realizable at any
frequency where subwavelength resonators can be designed.Comment: 4 pages, 3 figure
Lipopeptides as dimerization inhibitors of HIV-1 protease
In AIDS therapy, attempts have been made to inhibit the virus-encoded enzymes, e.g, HIV-1 protease, using active site-directed inhibitors. This approach is questionable, however, due to virus mutations and the high toxicity of the drugs, An alternative method to inhibit the dimeric HIV protease is the targeting of the interface region of the protease subunits in order to prevent subunit dimerization and enzyme activity, This approach should be less prone to inactivation by mutation, A list of improved 'dimerization inhibitors' of HIV-1 protease is presented. The main structural features are a short `interface' peptide segment, including non-natural amino acids, and an aliphatic N-terminal blocking group. The high inhibitory power of some of the lipopeptides {[}e.g, palmitoyl-Tyr-Glu-Leu-OH, palmitoyl-Tyr-Glu-(L-thyronine)-OH, palmitoyl-Tyr-Glu-(L-biphenyl-alanine)-OH] with low nanomolar K-i values in the enzyme test suggests that mimetics with good bio-availability can be derived for AIDS therapy
Simulated slidequakes: insights from DEM simulations into the high-frequency seismic signal generated by geophysical granular flows
Geophysical granular flows generate seismic signals known as âslidequakesâ or âlandquakesâ, with low-frequency components whose generation by mean forces is widely used to infer hazard-relevant flow properties. Many more such properties could be inferred by understanding the fluctuating forces that generate slidequakesâ higher frequency components and, to do so, we conducted discrete-element simulations that examined the fluctuating forces exerted by steady, downslope-periodic granular flows on fixed, rough bases. Unlike our previous laboratory experiments, our simulations precluded basal slip. We show that, in its absence, simulated basal forcesâ power spectra have high-frequency components more accurately predicted using mean shear rates than using depth-averaged flow velocities, and can have intermediate-frequency components which we relate to chains of prolonged interparticle contacts. We develop a âminimal modelâ, which uses a flowâs collisional properties to even more accurately predict the high-frequency components, and empirically parametrize this model in terms of mean flow properties, for practical application. Finally, we demonstrate that the bulk inertial number determines not only the magnitude ratio of rapidly fluctuating and mean forces on a unit basal area, consistent with previous experimental results, but also the relative magnitudes of the high and intermediate-frequency force components
Quasi Two-dimensional Transfer of Elastic Waves
A theory for multiple scattering of elastic waves is presented in a random
medium bounded by two ideal free surfaces, whose horizontal size is infinite
and whose transverse size is smaller than the mean free path of the waves. This
geometry is relevant for seismic wave propagation in the Earth crust. We derive
a time-dependent, quasi-2D radiative transfer equation, that describes the
coupling of the eigenmodes of the layer (surface Rayleigh waves, SH waves, and
Lamb waves). Expressions are found that relate the small-scale fluctuations to
the life time of the modes and to their coupling rates. We discuss a diffusion
approximation that simplifies the mathematics of this model significantly, and
which should apply at large lapse times. Finally, coherent backscattering is
studied within the quasi-2D radiative transfer equation for different source
and detection configurations.Comment: REVTeX, 36 pages with 10 figures. Submitted to Phys. Rev.
Reconfigurable Intelligent Surfaces vs. Relaying: Differences, Similarities, and Performance Comparison
Reconfigurable intelligent surfaces (RISs) have the potential of realizing
the emerging concept of smart radio environments by leveraging the unique
properties of meta-surfaces. In this article, we discuss the potential
applications of RISs in wireless networks that operate at high-frequency bands,
e.g., millimeter wave (30-100 GHz) and sub-millimeter wave (greater than 100
GHz) frequencies. When used in wireless networks, RISs may operate in a manner
similar to relays. This paper elaborates on the key differences and
similarities between RISs that are configured to operate as anomalous
reflectors and relays. In particular, we illustrate numerical results that
highlight the spectral efficiency gains of RISs when their size is sufficiently
large as compared with the wavelength of the radio waves. In addition, we
discuss key open issues that need to be addressed for unlocking the potential
benefits of RISs.Comment: Submitted for journal publication (revised version
Time-resolved dynamics of electron wave packets in chaotic and regular quantum billiards with leads
We perform numerical studies of the wave packet propagation through open
quantum billiards whose classical counterparts exhibit regular and chaotic
dynamics. We show that for t less or similar to tau (tau being the Heisenberg
time), the features in the transmitted and reflected currents are directly
related to specific classical trajectories connecting the billiard leads. In
contrast, the long-time asymptotics of the wave packet dynamics is
qualitatively different for classical and quantum billiards. In particularly,
the decay of the quantum system obeys a power law that depends on the number of
decay channels, and is not sensitive to the nature of classical dynamics
(chaotic or regular).Comment: 5 pages, 4 figure
Diffuse field cross-correlations: Scattering theory and electromagnetic experiments
The passive estimation of impulse responses from ambient noise correlations arouses increasing interest in seismology, acoustics, optics and electromagnetism. Assuming the equipartition of the noise field, the cross-correlation function measured with non-invasive receiving probes converges towards the difference of the causal and anti-causal Greenâs functions. Here, we consider the case when the receiving field probes are antennas which are well coupled to a complex medium â a scenario of practical relevance in electromagnetism. We propose a general approach based on the scattering matrix formalism to explore the convergence of the cross-correlation function. The analytically derived theoretical results for chaotic systems are confirmed in microwave measurements within a mode-stirred reverberation chamber. This study provides new fundamental insights into the Greenâs function retrieval technique and paves the way for a new technique to characterize electromagnetic antennas.French Agence Nationale de la Recherche (reference ANR-17- ASTR-0017); European Union (European Regional Development Fund (ERDF)), French region of Brittany and Rennes MĂ©tropole (CPER Project SOPHIE/STIC & Ondes); Institut Universitaire de France (IUF)
Wireless coils based on resonant and nonresonant coupled-wire structure for small animal multinuclear imaging
Earlier work on RF metasurfaces for preclinical MRI has targeted applications such as wholeâbody imaging and dualâfrequency coils. In these studies, a nonresonant loop was used to induce currents into a metasurface that was operated as a passive inductively powered resonator. However, as we show in this study, the strategy of using a resonant metasurface reduces the impact of the loop on the global performance of the assembled coil. To mitigate this deficiency, we developed a new approach that relies on the combination of a commercial surface coil and a coupledâwire structure operated away from its resonance. This strategy enables the extension of the sensitive volume of the surface coil while maintaining its local high sensitivity without any hardware modification. A wireless coil based on a two parallel coupledâwire structure was designed and electromagnetic field simulations were carried out with different levels of matching and coupling between both components of the coil. For experimental characterization, a prototype was built and tested at two frequencies, 300 MHz for 1H and 282.6 MHz for 19F at 7 T. Phantom and in vivo MRI experiments were conducted in different configurations to study signal and noise figures of the structure. The results showed that the proposed strategy improves the overall sensitive volume while simultaneously maintaining a high signalâtoânoise ratio (SNR). Metasurfaces based on coupled wires are therefore shown here as promising and versatile elements in the MRI RF chain, as they allow customized adjustment of the sensitive volume as a function of SNR yield. In addition, they can be easily adapted to different Larmor frequencies without loss of performance
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