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In this paper we propose to quantify execution time variability of programs using statistical dispersion parameters. We show how the execution time variability can be exploited in mixed criticality real-time systems. We propose a heuristic to compute the execution time budget to be allocated to each low criticality real-time task according to its execution time variability. We show using experiments and simulations that the proposed heuristic reduces the probability of exceeding the allocated budget compared to algorithms which do not take into account the execution time variability parameter.

Chip-based terahertz (THz) devices are emerging as versatile tools for manipulating mm-wave frequencies in the context of integrated high-speed communication technologies for potential sixth-generation (6G) wireless applications. The characterization of THz devices is typically performed using far-field techniques that provide limited information about the underlying physical mechanisms producing them. As the library of chip-based functionalities expands, e.g., for tailoring the emission and directional propagation properties of THz antennas and waveguides, novel characterization techniques will likely be beneficial for observing subtle effects that are sensitive to a device's structural parameters. Here we present near-field measurements showing the emission properties of a broadband THz emitter placed in the vicinity of a photonic crystal (PHC) slab. These experiments reveal long-predicted emission properties, but which to our knowledge have yet to be experimentally observed at THz frequencies. We demonstrate three distinct effects between 0.3-0.5 THz: (i) field suppression at frequencies corresponding to quasi-TE bandgaps (ii) a frequency-dependent directed emission along two distinct pathways for two neighboring frequencies, resulting in a local field concentration; (iii) a re-direction of the emission, achieved by rotating the PHC with respect to the dipole orientation. Simulations reveal that the observed behavior can be predicted from the underlying band structure. Our results highlight the opportunities that PHCs can potentially provide for alignment-free, chip-based 6G technologies. Our experimental technique extends the applicability realms of THz spectroscopy and will find use for characterizing the THz modes supported by true samples, whose inherent imperfections cannot realistically be accounted for by simulations, particularly in highly dispersive frequency bands.

Despite the apparent simplicity, the problem of refraction of electromagnetic waves at the planar interface between two media has an incredibly rich spectrum of unusual phenomena. An example is the paradox that occurs when an electromagnetic wave is incident on the interface between a hyperbolic medium and an isotropic dielectric. At certain orientations of the optical axis of the hyperbolic medium relative to the interface, the reflected and transmitted waves are completely absent. In this paper, we formulate the aforementioned paradox and present its resolution by introduction of infinitesimal losses in a hyperbolic medium. We show that the reflected wave exists, but became extremely decaying as the loss parameter tends to zero. As a consequence, all the energy scattered into the reflected channel is absorbed at the interface. We support our reasoning with analytical calculations, numerical simulations, and an experiment with self-complementary metasurfaces in the microwave region. In addition to the great fundamental interest, this paradox resolution discovers a plethora of applications for the reflectors, refractors, absorbers, lenses, antennas, camouflage and holography applications., Comment: 16 pages, 4 figures

We demonstrate strongly collimated beaming, at audible frequencies, in a three-dimensional acoustic phononic crystal where the wavelength is commensurate with the crystal elements; the crystal is a seemingly simple rectangular cuboid constructed from closely-spaced spheres, and yet demonstrates rich wave phenomena acting as a canonical three-dimensional metamaterial. We employ theory, numerical simulation and experiments to design and interpret this collimated beaming phenomenon and use a crystal consisting of a finite rectangular cuboid array of $4\times 10\times 10$ polymer spheres $1.38$~cm in diameter in air, arranged in a primitive cubic cell with the centre-to-centre spacing of the spheres, i.e. the pitch, as $1.5$~cm. Collimation effects are observed in the time domain for chirps with central frequencies at $14.2$~kHz and $18$~kHz, and we deployed a laser feedback interferometer or Self-Mixing Interferometer (SMI) -- a recently proposed technique to observe complex acoustic fields -- that enables experimental visualisation of the pressure field both within the crystal and outside of the crystal. Numerical exploration using a higher-order multi-scale finite element method designed for the rapid and detailed simulation of 3D wave physics further confirms these collimation effects and cross-validates with the experiments. Interpretation follows using High Frequency Homogenization and Bloch analysis whereby the different origin of the collimation at these two frequencies is revealed by markedly different isofrequency surfaces of the sonic crystal., Comment: 10 pages, 8 figures

Radiofrequency field inhomogeneity is a significant issue in imaging large fields of view in high- and ultrahigh-field MRI. Passive shimming with coupled coils or dielectric pads is the most common approach at 3 T. We introduce and test light and compact metasurface, providing the same homogeneity improvement in clinical abdominal imaging at 3 T as a conventional dielectric pad. The metasurface comprising a periodic structure of copper strips and parallel-plate capacitive elements printed on a flexible polyimide substrate supports propagation of slow electromagnetic waves similar to a high-permittivity slab. We compare the metasurface operating inside a transmit body birdcage coil to the state-of-the-art pad by numerical simulations and in vivo study on healthy volunteers. Numerical simulations with different body models show that the local minimum of B1+ causing a dark void in the abdominal domain is removed by the metasurface with comparable resulting homogeneity as for the pad without noticeable SAR change. In vivo results confirm similar homogeneity improvement and demonstrate the stability to body mass index. The light, flexible, and cheap metasurface can replace a relatively heavy and expensive pad based on the aqueous suspension of barium titanate in abdominal imaging at 3 T., Comment: 18 pages, 6 figures, 4 supplementary figures

We study, both theoretically and experimentally, tunable metasurfaces supporting sharp Fano-resonances inspired by optical bound states in the continuum. We explore the use of arsenic trisulfide (a photosensitive chalcogenide glass) having optical properties which can be finely tuned by light absorption at the post-fabrication stage. We select the resonant wavelength of the metasurface corresponding to the energy below the arsenic trisulfide bandgap, and experimentally control the resonance spectral position via exposure to the light of energies above the bandgap., Comment: 7 pages, 5 figures

Photonic cavities are gathering a large interest to enhance the energy transfer between two dipoles, with far-reaching consequences for applications in photovoltaics, lighting sources and molecular biosensing. However, experimental difficulties in controlling the dipoles' positions, orientations and spectra have limited the earlier work in the visible part of the spectrum, and have led to inconsistent results. Here, we directly map the energy transfer of microwaves between two dipoles inside a resonant half-wavelength cavity with ultrahigh control in space and frequency. Our approach extends F\"orster resonance energy transfer (FRET) theory to microwave frequencies, and bridges the gap between the descriptions of FRET using quantum electrodynamics and microwave engineering. Beyond the conceptual interest, we show how this approach can be used to optimize the design of photonic cavities to enhance dipole-dipole interactions and FRET., Comment: Accepted for publication in PRX

Eliciting semantic similarity between concepts in the biomedical domain remains a challenging task. Recent approaches founded on embedding vectors have gained in popularity as they risen to efficiently capture semantic relationships The underlying idea is that two words that have close meaning gather similar contexts. In this study, we propose a new neural network model named MeSH-gram which relies on a straighforward approach that extends the skip-gram neural network model by considering MeSH (Medical Subject Headings) descriptors instead words. Trained on publicly available corpus PubMed MEDLINE, MeSH-gram is evaluated on reference standards manually annotated for semantic similarity. MeSH-gram is first compared to skip-gram with vectors of size 300 and at several windows contexts. A deeper comparison is performed with tewenty existing models. All the obtained results of Spearman's rank correlations between human scores and computed similarities show that MeSH-gram outperforms the skip-gram model, and is comparable to the best methods but that need more computation and external resources., Comment: 6 pages, 2 tables

Acoustic flat lensing is achieved here by tuning a phononic array to have indefinite medium behaviour in a narrow frequency spectral region along the acoustic branch. This is confirmed by the occurrence of a flat band along an unusual path in the Brillouin zone and by interpreting the intersection point of isofrequency contours on the corresponding isofrequency surface; coherent directive beams are formed whose reflection from the array surfaces create lensing. Theoretical predictions are corroborated by time-domain experiments, airborne acoustic waves generated by a source with a frequency centered about $10.6$ kHz, placed at three different distances from one side of a finite phononic crystal slab, constructed from polymeric spheres, yield distinctive focal spots on the other side. These experiments evaluate the pressure field using optical feedback interferometry and demonstrate precise control of the three-dimensional wave trajectory through a sonic crystal., Comment: 6 pages, 5 figures

In this paper we propose, design and test a new dual-nuclei RF-coil inspired by wire metamaterial structures. The coil operates due to resonant excitation of hybridized eigenmodes in multimode flat periodic structures comprising several coupled thin metal strips. It was shown that the field distribution of the coil (i.e. penetration depth) can be controlled independently at two different Larmor frequencies by selecting a proper eigenmode in each of two mutually orthogonal periodic structures. The proposed coil requires no lumped capacitors for tuning and matching. In order to demonstrate the performance of the new design, an experimental preclinical coil for $^{19}$F/$^{1}$H imaging of small animals at 7.05T was engineered and tested on a homogeneous liquid phantom and in-vivo. The presented results demonstrate that the coil was well tuned and matched simultaneously at two Larmor frequencies and capable of image acquisition with both the nuclei reaching large homogeneity area along with a sufficient signal-to-noise ratio. In an in-vivo experiment it has been shown that without retuning the setup it was possible to obtain anatomical $^{1}$H images of a mouse under anesthesia consecutively with $^{19}$F images of a tiny tube filled with a fluorine-containing liquid and attached to the body of the mouse.