Superdirectivity from arrays of strongly coupled meta-atoms

Abstract

Enhanced thermal stability of dielectric, energy storage, and discharge efficiency in a structurally frustrated piezoelectric system: Erbium modified Na 0.5 Bi 0.5 TiO 3 -BaTiO 3

Superdirective dimers of coupled self-resonant split ring resonators: Analytical modelling and numerical and experimental validation

“…Moreover, based on a coupled mode formalism, general expressions for the coupling coefficient κ T valid for both conducting and dielectric resonators have been derived that make it possible to estimate the frequencies and fields of the coupled modes for arbitrarily oriented and spaced resonators [39][40][41] . These approaches enable the calculation of the characteristic parameters of a coupled system; in principle, they could be used to tailor the coupling and quality factor Q of the two SRR elements forming a meta-dimer to realize the array excitation prescribed for superdirectivity, given that these latter requirements can be translated into equivalent conditions on T κ and Q, as anticipated in 20,22 and discussed in detail in 28 . However, if engineering the inter-element coupling is crucial to the design of a superdirective meta-dimer, this would actually suffice achieving superdirectivity only for dimers formed by simple dipole radiators.…”

“…As shown in 28 , by equating (6) to the ratio a a / 02 01 , an insightful relation can be established between requirements for superdirectivity and the coupling and quality factor of the constituent elements of a meta-dimer. In fact, from the expansion in Taylor www.nature.com/scientificreports www.nature.com/scientificreports/ By comparing (6) with the first two terms of (9), we deduce that for two SRRs to behave as a superdirective array requires…”

“…In order for a subwavelength device to achieve superdirectivity, it is required that the constituent resonators have high quality factors Q and strongly couple to each other with a high coupling constant κ. In fact it has been shown that conditions on excitation coefficients of superdirective arrays can be translated in the figure-of-merit Q κ having to be proportional to the ratio of the wavelength λ and the unit cell size d 20,28 . However, this condition alone is sufficient to ensure a superdirective emission only for simple dipole arrays, but not in the case of coupled SRR dimers.…”

Superdirective Antennas of Coupled Helical Elements

“…The required phase difference between the currents in the resonators and their magnitudes provide the strong negative magnetic coupling between them [17]. High values of D were shown for both MHz [18,19] and GHz [20] frequency ranges using dimers formed of two coplanar split ring resonators (SRRs), with the first being driven and the second passively excited. It was theoretically demonstrated [14,19] that the directivity of such a structure depends on several parameters: the coupling coefficient, κ, the centre-to-centre distance, a, the operating wavenumber, k, and the quality factor, Q.…”

“…High values of D were shown for both MHz [18,19] and GHz [20] frequency ranges using dimers formed of two coplanar split ring resonators (SRRs), with the first being driven and the second passively excited. It was theoretically demonstrated [14,19] that the directivity of such a structure depends on several parameters: the coupling coefficient, κ, the centre-to-centre distance, a, the operating wavenumber, k, and the quality factor, Q. To achieve the theoretical maximum value of directivity, the following relation between them need be satisfied, (1) where α is a dimensionless parameter that depends on the geometry of the elements.…”