Asymptotic network models of subwavelength metamaterials formed by closely packed photonic and phononic crystals. (28th November 2017)

• Record Type: Journal Article
• Authors: Vanel, Alice L.; Schnitzer, Ory; Craster, Richard V.
• Is Part Of: Europhysics letters. Volume 119:Number 6(2017:Sep.)
• Publication Date: 2017-11-28
• Subjects: 42.70.Qs -- 78.67.Pt -- 43.30.Dr; Physics -- Periodicals; Electronic journals; 530.05
• DOI: 10.1209/0295-5075/119/64002
• Languages: English
• ISSNs: 0295-5075

Abstract

Abstract: We demonstrate that photonic and phononic crystals consisting of closely spaced inclusions constitute a versatile class of subwavelength metamaterials. Intuitively, the voids and narrow gaps that characterise the crystal form an interconnected network of Helmholtz-like resonators. We use this intuition to argue that these continuous photonic (phononic) crystals are in fact asymptotically equivalent, at low frequencies, to discrete capacitor-inductor (mass-spring) networks whose lumped parameters we derive explicitly. The crystals are tantamount to metamaterials as their entire acoustic branch, or branches when the discrete analogue is polyatomic, is squeezed into a subwavelength regime where the ratio of wavelength to period scales like the ratio of period to gap width raised to the power ; at yet larger wavelengths we accordingly find a comparably large effective refractive index. The fully analytical dispersion relations predicted by the discrete models yield dispersion curves that agree with those from finite-element simulations of the continuous crystals. The insight gained from the network approach is used to show that, surprisingly, the continuum created by a closely packed hexagonal lattice of cylinders is represented by a discrete honeycomb lattice. The analogy is utilised to show that the hexagonal continuum lattice has a Dirac-point degeneracy that is lifted in a controlled manner by specifying the area of a symmetry-breaking defect.