Wave transport in random systems: multiple resonance character of necklace modes and their statistical behavior

Necklace-State-Mediated Anomalous Enhancement of Transport in Anderson-Localized non-Hermitian Hybrid Systems

Non-Hermiticity is known to manifest interesting modifications in the transport properties of complex systems. We report an intriguing regime of transport of hybrid quasiparticles in a non-Hermitian setting. We calculate the probability of transport, quantified by the Thouless conductance, of hybrid plasmons under varying degrees of disorder. With increasing disorder, we initially observe an expected decrease in average transmission, followed by an anomalous rise at localizing disorder. The behavior originates from the confluence of hybridization and non-Hermiticity, in which the former realizes the aggregation of eigenvalues migrating under disorder, while the latter enables energy transfer between the eigenmodes. We find that the enhanced transmission is mediated by quasiparticle hopping over various Anderson-localized states within the so-formed necklace states. We note that, in this scenario, all configurations exhibit the formation of necklace states and enhanced transport, unlike the conventionally known behavior of necklace states which only occurs in rare configurations.

Generalized Fano lineshapes reveal exceptional points in photonic molecules

The optical behavior of coupled systems, in which the breaking of parity and time-reversal symmetry occurs, is drawing increasing attention to address the physics of the exceptional point singularity, i.e., when the real and imaginary parts of the normal-mode eigenfrequencies coincide. At this stage, fascinating phenomena are predicted, including electromagnetic-induced transparency and phase transitions. To experimentally observe the exceptional points, the near-field coupling to waveguide proposed so far was proved to work only in peculiar cases. Here, we extend the interference detection scheme, which lies at the heart of the Fano lineshape, by introducing generalized Fano lineshapes as a signature of the exceptional point occurrence in resonant-scattering experiments. We investigate photonic molecules and necklace states in disordered media by means of a near-field hyperspectral mapping. Generalized Fano profiles in material science could extend the characterization of composite nanoresonators, semiconductor nanostructures, and plasmonic and metamaterial devices.

Fano lineshapes are found in many photonic systems where discrete and extended spectra interfere. Here, the authors extend this description and introduce generalized Fano lineshapes to describe the results from hyperspectral mapping around an exceptional point in a coupled-cavity system.

Fractional Transport of Photons in Deterministic Aperiodic Structures

The propagation of optical pulses through primary types of deterministic aperiodic structures is numerically studied in time domain using the rigorous transfer matrix method in combination with analytical fractional transport models. We demonstrate tunable anomalous photon transport, including the elusive logarithmic Sinai sub-diffusion in photonic systems for the first time. Our results are in excellent agreement with the scaling theory of transport in aperiodic media with fractal spectra, and additionally demonstrate logarithmic sub-diffusion in the presence of multifractality. Moreover, we establish a fruitful connection between tunable photon diffusion and fractional dynamics, which provides analytical insights into the asymptotic transport regime of optical media with deterministic aperiodic order. The demonstration of tunable sub-diffusion and logarithmic photon transport in deterministic aperiodic structures can open novel and fascinating scenarios for the engineering of wave propagation and light-matter interaction phenomena beyond the conventional diffusive regime.

Characterization of short necklace states in the logarithmic transmission spectra of localized systems

High transmission plateaus exist widely in the logarithmic transmission spectra of localized systems. Their physical origins are short chains of coupled localized states embedded inside the localized system, which are dubbed as 'short necklace states'. In this work, we define the essential quantities and then, based on these quantities, we investigate the properties of the short necklace states statistically and quantitatively. Two different approaches are utilized and their results agree very well. In the first approach, the typical plateau-width and the typical order of short necklace states are obtained from the correlation function of the logarithmic transmission. In the second approach, we investigate the statistical distribution of the peak/plateau-width measured in the logarithmic transmission spectra. A novel distribution is found, which can be exactly fitted by the summation of two Gaussian distributions. These two distributions are the results of sharp peaks of localized states and the high plateaus of short necklace states. The center of the second distribution also tells us the typical plateau-width of short necklace states. With increasing system length, the scaling property of the typical plateau-width is very special since it hardly decreases. The methods and quantities defined in this work can be widely used in Anderson localization studies.

Fano interference governs wave transport in disordered systems

Light localization in disordered systems and Bragg scattering in regular periodic structures are considered traditionally as two entirely opposite phenomena: disorder leads to degradation of coherent Bragg scattering whereas Anderson localization is suppressed by periodicity. Here we reveal a non-trivial link between these two phenomena, through the Fano interference between Bragg scattering and disorder-induced scattering, that triggers both localization and de-localization in random systems. We find unexpected transmission enhancement and spectrum inversion when the Bragg stop-bands are transformed into the Bragg pass-bands solely owing to disorder. Fano resonances are always associated with coherent scattering in regular systems, but our discovery of disorder-induced Fano resonances may provide novel insights into many features of the transport phenomena of photons, phonons, and electrons. Owning to ergodicity, the Fano resonance is a fingerprint feature for any realization of the structure with a certain degree of disorder.

Understanding localization and delocalization phenomena is important for studying wave propagation in many types of disordered photonic systems. Here, a theoretical study of one-dimensional photonic crystal structures reveals the importance of Fano interference in wave transport in the presence of disorder.

Localized mode hybridization by fine tuning of two-dimensional random media

We study numerically the interaction of spatially localized modes in strongly scattering two-dimensional (2D) media. We move eigenvalues in the complex plane by changing gradually the index of a single scatterer. When spatial and spectral overlap is sufficient, localized states couple, and avoided level crossing is observed. We show that local manipulation of the disordered structure can couple several localized states to form an extended chain of hybridized modes crossing the entire sample, thus changing the nature of certain modes from localized to extended in a nominally localized disordered system. We suggest such a chain in 2D random systems is the analog of one-dimensional necklace states, the occasional open channels predicted by Pendry [Physics 1, 20 (2008).] through which the light can sneak through an opaque medium.

Emergence of Anderson localization in plasmonic waveguides

The propagation of surface plasmon polaritons in dielectric loaded waveguides with randomly placed scatterers is studied using both numerical simulations and a simplified transfer matrix framework. Despite the importance of losses in this system, we find fingerprints of the localized behavior of one-dimensional disordered systems. Furthermore, losses amplify the impact of the necklace states on the transport properties for systems not much larger than the localization length. The system presented here also offers the possibility to use localization effects for engineering purposes by means of deliberately introduced disorder.

Anderson localization and Brewster anomalies in photonic disordered quasiperiodic lattices

A comprehensive study of the properties of light propagation through one-dimensional photonic disordered quasiperiodic superlattices, composed of alternating layers with random thicknesses of air and a dispersive metamaterial, is theoretically performed. The superlattices consist of the successive stacking of N quasiperiodic Fibonacci or Thue-Morse heterostructures. The width of the slabs in the photonic superlattice may randomly fluctuate around its mean value, which introduces a structural disorder into the system. It is assumed that the left-handed layers have a Drude-type dispersive response for both the dielectric permittivity and magnetic permeability, and Maxwell's equations are solved for oblique incidence by using the transfer-matrix formalism. The influence of both quasiperiodicity and structural disorder on the localization length and Brewster anomalies are thoroughly discussed.

Nonuniform ensembles of diverse resonances in one-dimensional layered media

The transmission spectra perform huge fluctuations even in the mostly suitable one-dimensional localized multilayer system. Fluctuations of layer thicknesses will cause random resonances between light waves and layers. We reveal that the nonuniform ensembles of the resonances are the key for the fluctuations of transmission spectra. The transmission spectra of each stack of layers are numerically calculated through the transfer-matrix method.

Coupling and level repulsion in the localized regime: from isolated to quasiextended modes

We study the interaction of Anderson localized states in an open 1D random system by varying the internal structure of the sample. As the frequencies of two states come close, they are transformed into multiply peaked quasiextended modes. Level repulsion is observed experimentally and explained within a model of coupled resonators. The spectral and spatial evolution of the coupled modes is described in terms of the coupling coefficient and Q factors of resonators.

Finite-size scaling and disorder effect on the transmissivity of multilayered structures with metamaterials

We investigate the influence of metamaterials on the scaling laws of the transmission on multilayered structures composed of random sequences of ordinary dielectric and metamaterial layers. The spectrally averaged transmission in a frequency range around the fully transparent resonant mode is shown to decay with the total number of layers as 1/N. Such thickness dependence is faster than the 1/N(1/2) decay recently reported to take place in random sequences of ordinary dielectric slabs. The interplay of strong localization and the emergence of resonant modes within the gap leads to a non-monotonous disorder dependence of the transmission that reaches a minimum at an intermediate disorder strength.

Periodic oscillations in transmission decay of anderson localized one-dimensional dielectric systems

It is well recognized that the transmittance of Anderson localized systems decays exponentially on average with sample size, showing large fluctuations brought up by extremely rare occurrences of necklaces of resonantly coupled states, possessing almost unity transmission. We show here that in a one-dimensional (1D) random photonic system with resonant layers these fluctuations appear to be very regular and have a period defined by the localization length xi of the system. We stress that necklace states are the origin of these well-defined oscillations. We predict that in such a random system efficient transmission channels form regularly each time the increasing sample length fits so-called optimal-order necklaces and demonstrate the phenomenon through numerical experiments. Our results provide new insight into the physics of Anderson localization in random systems with resonant units.