Control of Power in Parity-Time -Symmetric Lattices

We investigate wave transport properties of Parity-Time (PT) symmetric lattices that are periodically modulated along the direction of propagation. We demonstrate that in the regime of unbroken PT-symmetry the system Floquet-Bloch modes may interfere constructively leading to either controlled oscillations or linear power absorption and amplification occurring exactly at the phase transition point. The differential power response is effected by the overlap of the gain and loss system distribution with wave intensity pattern that is formed through Rabi oscillations engaging the coupled Floquet-Bloch modes

Breather lattices as pseudospin glasses

We study the thermodynamics of discrete breathers by transforming a lattice of weakly coupled nonlinear oscillators into an effective Ising pseudospin model. We introduce a replica ensemble and investigate the effective system susceptibilities through the replica overlap distribution. We find that a transition occurs at a given temperature to a new phase characterized by a slow decay of the relevant correlation functions. Comparison of long time pseudospin correlation functions to maximal replica overlap demonstrates that the high temperature phase has glassy-like properties induced by short range order found in the system.Comment: 19 pages, 6 figure

PT-Symmetric Nonlinear Metamaterials and Zero-Dimensional Systems

A one dimensional, parity-time (${\cal PT}$)-symmetric magnetic metamaterial comprising split-ring resonators having both gain and loss is investigated. In the linear regime, the transition from the exact to the broken ${\cal PT}$-phase is determined through the calculation of the eigenfrequency spectrum for two different configurations; the one with equidistant split-rings and the other with the split-rings forming a binary pattern (${\cal PT}$ dimer chain). The latter system features a two-band, gapped spectrum with its shape determined by the gain/loss coefficient as well as the inter-element coupling. In the presense of nonlinearity, the ${\cal PT}$ dimer chain with balanced gain and loss supports nonlinear localized modes in the form of novel discrete breathers below the lower branch of the linear spectrum. These breathers, that can be excited from a weak applied magnetic field by frequency chirping, can be subsequently driven solely by the gain for very long times. The effect of a small imbalance between gain and loss is also considered. Fundamendal gain-driven breathers occupy both sites of a dimer, while their energy is almost equally partitioned between the two split-rings, the one with gain and the other with loss. We also introduce a model equation for the investigation of classical ${\cal PT}$ symmetry in zero dimensions, realized by a simple harmonic oscillator with matched time-dependent gain and loss that exhibits a transition from oscillatory to diverging motion. This behaviour is similar to a transition from the exact to the broken ${\cal PT}$ phase in higher-dimensional ${\cal PT}-$ symmetric systems. A stability condition relating the parameters of the problem is obtained in the case of piecewise constant gain/loss function that allows for the construction of a phase diagram with alternating stable and unstable regions.Comment: 10 pages, 11 figs, subm. to Applied Physics A for the Proceedings of the META13 Conferenc

Multistable Dissipative Breathers and Novel Collective States in SQUID Lieb Metamaterials

A SQUID (Superconducting QUantum Interference Device) metamaterial on a Lieb lattice with nearest-neighbor coupling supports simultaneously stable dissipative breather families which are generated through a delicate balance of input power and intrinsic losses. Breather multistability is possible due to the peculiar snaking flux ampitude - frequency curve of single dissipative-driven SQUIDs, which for relatively high sinusoidal flux field amplitudes exhibits several stable and unstable solutions in a narrow frequency band around resonance. These breathers are very weakly interacting with each other, while multistability regimes with different number of simultaneously stable breathers persist for substantial intervals of frequency, flux field amplitude, and coupling coefficients. Moreover, the emergence of chimera states as well as novel temporally chaotic states exhibiting spatial homogeneity within each sublattice of the Lieb lattice is demonstrated.Comment: 11 pages, 9 figures, submitted to Physical Review

Superconducting Metamaterials

Metamaterials (MMs), i.e. artificial media designed to achieve properties not available in natural materials, have been the focus of intense research during the last two decades. Many properties have been discovered and multiple designs have been devised that lead to multiple conceptual and practical applications. Superconducting MMs have the advantage of ultra low losses, a highly desirable feature. The additional use of the Josephson effect and SQUID configurations produce further specificity and functionality. SQUID-based MMs are both theoretically investigated but also fabricated and analyzed experimentally in many labs and exciting new phenomena have been found both in the classical and quantum realms. The SQUID is a unique nonlinear oscillator that can be manipulated through multiple external means. This flexibility is inherited to SQUID-based MMs, i.e. extended units that contain a large arrangement of SQUIDs. Such an assembly of weakly coupled nonlinear oscillators presents a nonlinear dynamics laboratory where numerous complex spatio-temporal phenomena may be explored. We focus primarily on SQUID-based MMs and present basic properties related to their individual and collective responses to external drives. We start by showing how a SQUID-based system acts as a genuine MM, demonstrate that the Josephson nonlinearity leads to wide-band tunability, intrinsic nonlinear as well as flat band localization. We explore further properties such as multistability and self-organization and the emergence of chimera states. We then dwell into the truly quantum regime and explore the interaction of electromagnetic pulses with superconducting qubits where the coupling between the two yields self-induced transparency and superradiance. We thus attempt to present the rich behavior of coupled superconducting units and point to their basic properties and practical utility.Comment: review article, to appear in Physics Reports: 84 pages, 36 figures, 350 reference

SQUID Metamaterials on a Lieb lattice: From flat-band to nonlinear localization

The dynamic equations for the fluxes through the SQUIDs that form a two-dimensional metamamaterial on a Lieb lattice are derived, and then linearized around zero flux to obtain the linear frequency spectrum according to the standard procedure. That spectrum, due to the Lieb lattice geometry, possesses a frequency band structure exhibiting two characteristic features; two dispersive bands, which form a Dirac cone at the corners of the first Brillouin zone, and a flat band crossing the Dirac points. It is demonstrated numerically that localized states can be excited in the system when it is initialized with single-site excitations; depending on the amplitude of those initial states, the localization is either due to the flat-band or to nonlinear effects. Flat-band localized states are formed in the nearly linear regime, while localized excitations of the discrete breather type are formed in the nonlinear regime. These two regimes are separated by an intermediate turbulent regime for which no localization is observed. Notably, initial single-site excitations of only edge SQUIDs of a unit cell may end-up in flat-band localized states; no such states are formed for initial single-site excitations of a corner SQUID of a unit cell. The degree of localization of the resulting states is in any case quantified using well-established measures such as the energetic participation ratio and the second moment.Comment: 11 pages, 9 figure

Extreme waves and branching flows in optical media

We address light propagation properties in complex media consisting of random distributions of lenses that have specific focusing properties. We present both analytical and numerical techniques that can be used to study emergent properties of light organization in these media. As light propagates, it experiences multiple scattering leading to the formation of light bundles in the form of branches; these are random yet occur systematically in the the medium, particularly in the weak scattering limit. On the other hand, in the strong scattering limit we find that coalescence of branches may lead to the formation of extreme waves of the "rogue wave" type. These waves appear at specific locations and arise in the linear as well as in the nonlinear regimes. We present both the weak and strong scattering limit and show that these complex phenomena can be studied numerically and analytically through simple models.Comment: This is a Chapter from the book: Quodons in Mica: Nonlinear localized traveling excitations in crystals. Springer Series in Materials Scienc

Topological split-ring resonator based metamaterials with PT\cal PT symmetry relying on gain and loss

A one-dimensional metamaterial with parity-time (${\cal PT}$) symmetry that relies on balanced gain and loss is introduced, comprising of magnetically coupled split-ring resonators (SRRs). A particular topology that combines a non-trivial (topological) dimer configuration with a trivial (non-topological) dimer configuration which are separated by a central SRR with neither gain or loss, is investigated. By focusing on the dynamical aspects of such a topological ${\cal PT}$ metamaterial (PTMM), the existence of {\em topologically protected interface states} which are localized at the central SRR is demonstrated numerically. The solution of the corresponding {\em quadratic eigenvalue problem} reveals that the protected state is actually a robust eigenmode of the topological PTMM, whose eigenvalue is isolated in the middle of the gap (mid-gap state) of the two-band frequency spectrum. Direct numerical simulations have been further used to determine the robustness and dynamic stability of these states in the parameter space of the {\em dimerization strength} and the {\em gain-loss coefficient}.Comment: 8 pages, 7 figures, 45 references. Accepted by Physical Review B as Regular Articl

SQUID Metamaterials: Tuneability and Multistability

An overview of several dynamic properties of SQUID metamaterials is given in the presence of both constant and alternating magnetic field. The total current as a function of the driving frequency exhibits hysteretic effects which are favored by low levels of disorder. Multistability in the current states leads to multiple magnetic responses with different value of magnetic permeability. SQUID metamaterials exhibit wide-band tuneability which is periodic with the applied constant magnetic field; the numerical calculations reproduce fairly well recent experimental results. Current work also reveals the possibility for wave transmission through nonlinear bands, which is briefly discussed.Comment: META 2014 Conference, 20-23 May 2014, Singapore [3 pages, 3 figures

Gain-Driven Discrete Breathers in PT-Symmetric Nonlinear Metamaterials

We introduce a one dimensional parity-time (PT)-symmetric nonlinear magnetic metamaterial consisted of split ring dimers having both gain and loss. When nonlinearity is absent we find a transition between an exact to a broken PT-phase; in the former the system features a two band gapped spectrum with shape determined by the gain and loss coefficients as well as the inter-unit coupling. In the presence of nonlinearity we show numerically that as a result of the gain/dissipation matching a novel type of long-lived stable discrete breathers can form below the lower branch of the band with no attenuation. In these localized modes the energy is almost equally partitioned between two adjacent split rings on the one with gain and the other one with loss.Comment: 5 pages, 7 figure