Based on the transformation acoustic (TA) methodology, an innovative approach for designing arbitrary shape concentrators is proposed. Unlike previous works, which utilized inhomogeneous and anisotropic materials to localize the incident acoustic waves in an arbitrary domain, the same functionality will be attained by introducing only one homogeneous anisotropic medium, which is called Acoustic nihility media (ANM). A great advantage of this method is that the attained materials are not dependent on the shape of the concentrator. That is regardless of the device geometry, a constant ANM will be used for each new shape and the output results do not alter. This will circumvent the conventional transformation acoustics' sophisticated and tedious calculations and could be easily implemented in real-life scenarios.

AbstractIn this paper, generation of optical vortices with time-varying orbital angular momentum (OAM) and topological charge is theoretically demonstrated based on time-modulated metasurfaces with a linearly azimuthal frequency gradient. The topological charge of such dynamic structured light beams is shown to continuously and periodically change with time evolution while possessing a linear dependence on time and azimuthal frequency offset. The temporal variation of OAM yields a self-torqued beam exhibiting a continuous angular acceleration of light. The phenomenon is attributed to the azimuthal phase gradient in space-time generated by virtue of the spatiotemporal coherent path in the interference between different frequencies. In order to numerically authenticate this newly introduced concept, a reflective dielectric metasurface is modelled consisting of silicon nanodisk heterostructures integrated with indium-tin-oxide and gate dielectric layers on top of a mirror-backed silicon slab which renders an electrically tunable guided mode resonance mirror in near-infrared regime. The metasurface is divided into several azimuthal sections wherein nanodisk heterostructures are interconnected via nanobars serving as biasing lines. Addressing azimuthal sections with radio-frequency biasing signals of different frequencies, the direct dynamic photonic transitions of leaky-guided modes are leveraged for realization of an azimuthal frequency gradient in the optical field. Generation of dynamic twisted light beams with time-varying OAM by the metasurface is verified via performing several numerical simulations. Moreover, the role of modulation waveform and frequency gradient on the temporal evolution and diversity of generated optical vortices is investigated which offer a robust electrical control over the number of dynamic beams and their degree of self-torque. Our results point toward a new class of structured light for time-division multiple access in optical and quantum communication systems as well as unprecedented optomechanical manipulation of objects.

Multi emission meta-radiating structures have been an important area of focus due to their potential applications in multi-user scenarios. Thereby, based on the concept of multi-folded transformation optics, a general approach for enabling the customizable multiple beam radiation of a planar antenna is propounded. The presented method is competent not only in creating any desired number of beams but also in adjusting their directivities. Antithetical to phased array antennas, with their complex feeding, heavyweight and costly elements, this goal was achieved via utilizing simple homogeneous materials elaborately designed by multiple folding of the virtual spaces to physical ones with an affine transformation. Several illustrative numerical simulations were performed in order to highlight the capability of the presented method in manipulating the planar source radiation. A meta-antenna structure consisting of a multi-folded layer and the patch antenna underneath, was designed, fabricated, and measured to authenticate the concept. The structure divides the radiated wave into two beams with arbitrary directions and different directivities. The experimental results of the realized meta-antenna structure exhibited good agreement with numerical simulations and theoretical predictions. The proposed approach could find applications in several fields of engineering where multiple emission is of utmost importance such as multiinput-multi-output systems, or could be used in waveguide applications like power dividers.

In this paper, based on the recently introduced concept of time-varying media, an alternative approach is presented for controlling the compression ratio of an optical pulse in the near-infrared regime via two all-dielectric transmissive metasurfaces consisting of a zigzag array of silicon-based elliptical nanodisks. Upon introducing in-plane asymmetries and under normal incidence, the supported symmetry-protected bound-state in the continuum resonant mode collapses into two Fano resonances, which can be spectrally overlapped to satisfy the first Kerker's condition. To acquire an amplified signal, the desired chirp is applied to the incident pulse via a purely temporal waveform that modulates the optical response of the first layer, while the required group delay dispersion is imparted to the phase-modulated pulse by the second metasurface in order to compress its temporal distribution. Following such a configuration, the temporal duration of the output pulse decreases from 25 to 15 ps, leading to a peak intensity enhancement of 50%. On account of time-varying features of the first metasurface, the instantaneous frequency of the chirped light can be controlled dynamically, giving rise to the active tuning of the peak intensity from 0% up to 200%.