Strong Localization of Surface Plasmon Polaritons with Engineered Disorder

Localization of plasmon modes in a 2D photonic nanostructure with a controlled disorder

In this paper, we focus on the optical properties of disordered hole arrays etched in a gold thin film. The disorder is induced and controlled using hole displacements following a Gaussian distribution and starting from a periodic array. The nanostructures present a transition from ordered arrays to short-range ordered arrays and random arrays by increasing the disorder amount. The associated optical properties are characterized in far and near fields by complementary approaches (absorption spectroscopy, classical scanning near field optical microscopy (SNOM) and Finite Difference Time Domain (FDTD) simulations). By increasing the disorder, a broadened absorption up to 30% in the far-field is achieved. Experiments in agreement with FDTD simulations point out the energy localization induced by the disorder and the dependence on the amount of disorder and on the excitation wavelength. By using a controlled disorder, we also show that the effect of these two parameters is also closely linked.

Broadband mirrors for surface plasmon polaritons using integrated high-contrast diffraction gratings

We propose and numerically investigate integrated high-contrast gratings (HCGs) for surface plasmon polaritons (SPPs) propagating along metal-dielectric interfaces, which consist of periodically arranged silicon pillars located on the gold surface. We demonstrate that such on-chip HCGs can be used as broadband plasmonic mirrors, which have subwavelength footprint in the SPP propagation direction and mean reflectance exceeding 85% in a 200-nm-wide spectral range for both the cases of normal and oblique SPP incidence. In order to increase the HCG efficiency and design practically feasible structures, we utilize a parasitic scattering suppression technique based on the use of two-layer grating pillars. The presented results may find application in two-dimensional optical circuits for steering the SPP propagation.

Tuning Anderson localization of edge-mode graphene plasmons in randomly gated nanoribbons

Edge-mode graphene plasmons (EGPs) supported by graphene nanoribbons are highly confined, and they can show versatile tunability under electrostatic bias. In order to efficiently enhance and actively control the near-field intensity in integrated plasmonic devices, we theoretically study Anderson localization of EGPs in a graphene nanoribbon with an underlying electrode array in this work. By randomly arranging the electrodes in the array, positional disorder is introduced in the graphene nanoribbon system. Consequently, the Anderson localization of EGPs occurs with an exponentially decreased electric field, reduced propagation length, and rapid disappearance of the cross-correlation coefficient. Physically, inhomogeneous gating effectively creates a disordered distribution of Fermi levels in the graphene nanoribbon, which provides adequate fluctuation of the effective refractive index and results in strong localization of the EGPs at mid-infrared regime. By changing electrode array arrangements, the EGPs can be trapped at distinct locations in the nanoribbon. Further considering that the Fermi-level disorder can be introduced by randomly modulating the electrostatic bias, we apply different gate voltages at different electrodes in the array. Electrically tunable Anderson localization of EGPs are eventually realized in those randomly gated nanoribbons. Moreover, by combining both the positional and Fermi-level disorders in the system, the Anderson localization becomes more actively controlled in this electrically gated graphene nanoribbons. It is shown that the local field can be selectively trapped at single distinct location, or even several locations along the graphene nanoribbon. This investigation extends the Anderson localization to the EGPs in the mid-infrared range and enriches the graphene-based active plasmonic devices.

Field localization of hexagonal and short-range ordered plasmonic nanoholes investigated by cathodoluminescence

Plasmonic nanoholes have attracted significant attention among nanoplasmonic devices, especially as biosensing platforms, where nanohole arrays can efficiently enhance and confine the electromagnetic field through surface plasmon polaritons, providing a sensitive detection. In nanohole arrays, the optical resonances are typically determined by the inter-hole distance or periodicity with respect to the surface plasmon wavelength. However, for short-range ordered (SRO) arrays, the inter-hole distance varies locally, so the plasmon resonance changes. In this study, we investigate the local resonance of SRO nanoholes using a cathodoluminescence technique and compare it with hexagonally ordered nanoholes. The cathodoluminescence photon maps and resonance peak analysis reveal that the electric fields are confined at the edges of holes and that their resonances are determined by inter-hole distances as well as by their distributions. This demonstrates the Anderson localization of the electromagnetic waves showing locally enhanced electromagnetic local density of states in SRO nanoholes.

Light trapping structures and plasmons synergistically enhance the photovoltaic performance of full-spectrum solar cells

A full-spectrum solar cell exhibits potential as an effective strategy to enhance the absorption of incident solar light. To ensure the absorption capability of solar cells, trapping structures or plasmons have emerged as two main ways of utilizing the full spectrum of solar energy. First, recent progress in the full-spectrum solar cells based on NCs was reviewed from the aspects of trapping structures and plasmon design. Moreover, the effects of light trapping and surface plasmon resonance on light absorption and photoelectronic conversion were emphasized and discussed. Finally, the application prospect of their combination in the field of full-spectrum solar cells was examined. It was pointed out that the deep exploration of the physical mechanism of photoelectric conversion, controllable preparation of the interface and stability of composite structures will become the main directions of future research.

Microscopic and Macroscopic Signatures of 3D Anderson Localization of Light

Apart from the difficulty of producing highly scattering samples, a major challenge in the observation of Anderson localization of 3D light is identifying an unambiguous signature of the phase transition in experimentally feasible situations. In this Letter, we establish a clear correspondence between the collapse of the conductance, the increase in intensity fluctuations at the localization transition and the scaling analysis results based on the Thouless number, thus connecting the macroscopic and microscopic approaches of localization. Furthermore, the transition thus inferred is fully compatible both with the results based on the eigenvalue analysis of the microscopic description and with the effective-medium Ioffe-Regel criterion.

Manipulation of transmission by engineered disorder in one-dimensional photonic crystals

Uncovering how disorder affects light propagation is a key step to manipulating light transportation and localization utilizing engineered disorder in photonics. Herein, we study the disorder-induced localization, delocalization and inter-transition between them in the regions of photonic band gaps (PBGs) and their associated passbands of one-dimensional (1D) Photonic crystals (PhCs) under different types of disorder effect. We introduce two parameters to quantitatively evaluate how disorder effect can be exploited to tailor the localization and delocalization of light in 1D PhCs. Such parameters can clearly indicate the inter-transition between extended states and localized states. It is shown that both nontrivial and trivial disorder-induced transport scenarios exist depending on the types of disorder introduced to the otherwise periodic system. The disorder-induced band tailing, which is correlated with the disorder-induced redistribution of electric energy compared with periodic case, is crucial for observing these interesting disorder-induced light transportation scenarios. Our results might provide positive insight to the manipulation of light transmission in nanophotonics by engineered disorder.

Disorder-Induced Phase Transitions in the Transmission of Dielectric Metasurfaces

Light interaction with disordered materials is both complex and fascinating at the same time. Here, we reveal disorder-induced phase transitions in a dielectric Huygens' metasurface made from silicon nanocylinders that simultaneously support an electric and magnetic dipole resonance. Depending on the degree of positional disorder and the spectral detuning of the two resonances, the phase angle of the transmission coefficient exhibits a clear phase transition from normal to anomalous dispersion. Combined with the considerations of whether the resonances of spectrally detuned particles appear as separated or overlapping, we distinguish four different phase states. We study this phenomenon analytically by employing dipole particles and disclose the entire phase diagram, support our insights with full-wave simulations of actual structures, and corroborate the findings with experimental results. Unveiling this phenomenon is a milestone simultaneously in the growing fields of metamaterial-inspired silicon nanophotonics, photonics in disordered media, and the fundamental physics of phase transitions.