Light Localization Properties Research Articles

Localization of light in 2D photonic Moiré superlattices

In this paper, monolayer photonic Moiré superlattices (PMSs) are constructed by superposing two periodic sublattices with square primitive cells and tunable twist angles. The sublattices are designed by interfering four umbrella-like beams. Light localization properties and physical flat-band effects of PMSs are studied by solving the Schrödinger equation in the periodic potentials. We have investigated the influence of interference angle and twist angle of sublattices on the localization of light and physical flat bands, and demonstrated that PMSs have physical flat bands and strong light localization properties when the twist angle is less than 5°, while the interference angle does not affect the flat-bands and localization properties. Our work provides a new research idea for manipulating spatial light fields and processing light information.

Non-Hermitian high-quality-factor topological photonic crystal cavity

Non-Hermitian topological photonics possesses abundant and complicated physical effects and phenomena, which have great potential and important applications in integrated photonic devices. However, the light localization properties and approaches for active control of topological states have not been revealed sufficiently so far. Here we prove the phase transition exists in the interface between gain and loss domains of a parity-time symmetric ${C}_{6\mathrm{v}}$ topological photonic crystal from its projected bands. The non-Hermitian topological cavity formed by closed gain-loss boundary has unique resonant responses and better light localization properties compared with the Hermitian counterpart. The cavity modes are composed of edge states and corner states whose eigenfrequencies change with gain-loss quantity in the same way as the projected band. The maximum value of the quality factor of non-Hermitian topological cavity reaches ${10}^{4}$, which is enlarged by one order compared with the Hermitian counterpart. At the same time, the mode volume of non-Hermitian topological cavity is reduced by over tenfold compared with the Hermitian counterpart. The non-Hermitian induced edge states and corner states may open a new route for the design of the topologically protected and reconfigurable localized resonances. This work provides a new degree of freedom to control different topological states and offers a scheme for solving the contradiction between the flexibility of devices and the robustness of topological properties.

Subdiffusive wave transport and weak localization transition in three-dimensional stealthy hyperuniform disordered systems

The purpose of this work is to understand the fundamental connection between structural correlations and light localization in three-dimensional (3D) open scattering systems of finite size. We numerically investigate the transport of vector electromagnetic waves scattered by resonant electric dipoles spatially arranged in 3D space by stealthy hyperuniform disordered point patterns. Three-dimensional stealthy hyperuniform disordered systems are engineered with different structural correlation properties determined by their degree of stealthiness $\ensuremath{\chi}$. Such fine control of exotic states of amorphous matter enables the systematic design of optical media that interpolate in a tunable fashion between uncorrelated random structures and crystalline materials. By solving the electromagnetic multiple scattering problem using Green's matrix spectral method, we establish a transport phase diagram that demonstrates a distinctive transition from a diffusive to a weak localization regime beyond a critical scattering density that depends on $\ensuremath{\chi}$. The transition is characterized by studying the Thouless number and the spectral statistics of the scattering resonances. In particular, by tuning the $\ensuremath{\chi}$ parameter, we demonstrate large spectral gaps and suppressed subradiant proximity resonances, facilitating light localization. Moreover, consistently with previous studies, our results show a region of the transport phase diagram where the investigated scattering systems become transparent. Our work provides a systematic description of the transport and weak localization properties of light in stealthy hyperuniform structures and motivates the engineering of photonic systems with enhanced light-matter interactions for applications to both classical and quantum devices.

Effective light polarization insensitive and omnidirectional properties of Si nanowire arrays developed on different crystallographic planes

In this paper, fabrication of vertical Si nanowire arrays (SiNWAs) by a facile metal assisted chemical etching approach on different crystallographic planes of Si has been reported. A very low specular reflectance (Rspec) of 0.04% and 0.03% has been achieved in the whole visible range for SiNWAs grown on Si(100) and Si(111) oriented substrates, respectively. High broadband enhancement has been detected for vertical SiNWAs due to multiple scattering paths inside the nanowire arrays. On the other hand, inclined nanowires showed a fascinating behavior at the longer wavelength regime, where light gets the longer path to reflect back-forth and ease to reflect back outward at normal incidence. Moreover, for [100] SiNWAs, transverse electric field component demonstrates the strong polarization insensitive properties at the expense of transverse magnetic field component with a minimum reflectance of <2% up to 1200 nm. The [100] SiNWAs demonstrates extraordinary omnidirectional properties at θB ≥ 58°. Theoretical validation of COMSOL with an effective medium approach reveal the effective dipole coupling and the presence of strong absorption modes for vertical SiNWs at a typical wavelength regime. The highly bound states of the particle tunneling through classical forbidden region shows a strong dependence on the gradient in the refractive index (mi) from 1 to 3.4. The high order scattering effect is observed at ∼520 cm−1 in a disordered optical medium. This novel finding of light localization properties for SiNWAs with different orientation gives a new route to support various photonic applications.

Light localization properties of weakly disordered optical media using confocal microscopy: application to cancer detection.

We have developed a novel technique to quantify submicron scale mass density fluctuations in weakly disordered heterogeneous optical media using confocal fluorescence microscopy. Our method is based on the numerical evaluation of the light localization properties of an 'optical lattice' constructed from the pixel intensity distributions of images obtained with confocal fluorescence microscopy. Here we demonstrate that the technique reveals differences in the mass density fluctuations of the fluorescently labeled molecules between normal and cancer cells, and that it has the potential to quantify the degree of malignancy of cancer cells. Potential applications of the technique to other disease situations or characterizing disordered samples are also discussed.

Tailoring of electromagnetic field localizations by two-dimensional graphene nanostructures

Graphene has great potential for enhancing light−matter interactions in a two-dimensional regime due to surface plasmons with low loss and strong light confinement. Further utilization of graphene in nanophotonics relies on the precise control of light localization properties. Here, we demonstrate the tailoring of electromagnetic field localizations in the mid-infrared region by precisely shaping the graphene into nanostructures with different geometries. We generalize the phenomenological cavity model and employ nanoimaging techniques to quantitatively calculate and experimentally visualize the two-dimensional electromagnetic field distributions within the nanostructures, which indicate that the electromagnetic field can be shaped into specific patterns depending on the shapes and sizes of the nanostructures. Furthermore, we show that the light localization performance can be further improved by reducing the sizes of the nanostructures, where a lateral confinement of λ0/180 of the incidence light can be achieved. The electromagnetic field localizations within a nanostructure with a specific geometry can also be modulated by chemical doping. Our strategies can, in principle, be generalized to other two-dimensional materials, therefore providing new degrees of freedom for designing nanophotonic components capable of tailoring two-dimensional light confinement over a broad wavelength range.

Supersymmetric transparent optical intersections.

Supersymmetric (SUSY) optical structures provide a versatile platform to manipulate the scattering and localization properties of light, with potential applications to mode conversion, spatial multiplexing, and invisible devices. Here we show that SUSY can be exploited to realize broadband transparent intersections between guiding structures in optical networks for both continuous and discretized light. These include transparent crossing of high-contrast-index waveguides and directional couplers, as well as crossing of guiding channels in coupled resonator lattices.

Concentric Necklace Nanolenses for Optical Near-Field Focusing and Enhancement

In this paper, we design and analyze concentric necklace nanolenses (CNNLs) which consist of metal nanoparticle dimers placed in the center of one or more concentric rings of plasmonic necklaces. We use three-dimensional finite-difference time-domain simulations, electron-beam lithography fabrication, dark-field scattering analysis, and surface-enhanced Raman scattering (SERS) measurements to investigate the far-field scattering and near-field light localization properties of CNNLs. Using these methods, we show that CNNLs display far-field scattering properties that arise from coupling between the dimer and surrounding necklace(s), leading to two pronounced peaks in single-necklace CNNLs and three pronounced peaks in double-necklace CNNLs. In our near-field analysis, we find that the number of particles in the surrounding necklace is an important degree of freedom in the optimization of near-field intensity within the dimer hot-spot region. By using CNNLs where the necklace diameters have a diameter equal to an integer multiple of the resonance wavelength of the isolated dimer times a constant scaling factor, the intensity of near-fields can be optimized for all geometries over a broad-band wavelength range. Using optimized geometries, we perform SERS experiments on CNNLs coated with a pMA monolayer and demonstrate 7× Raman enhancement in the single-necklace CNNL and 18× enhancement in the double-necklace CNNL over the reference dimer antenna geometry, with an average Raman enhancement value of approximately 7 × 10(5).

Light localization in nonuniformly randomized lattices

We address Anderson localization of light in disordered optical lattices where the disorder strength varies across the transverse direction. Such variation changes the preferred domains where formation of localized eigenmodes is most probable, hence drastically impacting light localization properties. Thus, step-like disorder results in formation of modes with different decay rates at both sides of the interface, while a smoothly varying disorder yields appearance of modes that are extended within weakly disordered domains and rapidly fade away in strongly disordered domains.

Quantification of nanoscale density fluctuations using electron microscopy: Light-localization properties of biological cells

We report a study of the nanoscale mass-density fluctuations of heterogeneous optical dielectric media, including nanomaterials and biological cells, by quantifying their nanoscale light-localization properties. Transmission electron microscope images of the media are used to construct corresponding effective disordered optical lattices. Light-localization properties are studied by the statistical analysis of the inverse participation ratio (IPR) of the localized eigenfunctions of these optical lattices at the nanoscale. We validated IPR analysis using nanomaterials as models of disordered systems fabricated from dielectric nanoparticles. As an example, we then applied such analysis to distinguish between cells with different degrees of aggressive malignancy.

Numerical Study on Light Localization in Impedance-Matched Meta-Material Random Systems

We investigate the localization properties of light propagating in two-dimensional systems with impedance-matched meta-material scatterers which are randomly positioned. Numerically, the localization length ζ versus the index of meta-material is obtained first. We find that, unlike traditional random systems, the localization length of such meta-material random systems does not depend on the total scattering cross section of scatterers, but on the back-scattering cross-section of scatterers. Furthermore, our analysis shows that there are “back-scattering paths of single scatterer" in such meta-material systems, which can cause a strong localization effect. Such back-scattering paths inside single scatterers can be thought of as the supplement to the traditional back-scattering paths of multiple scatterers.

Propagation of light waves in Thue-Morse dielectric multilayers

We investigate the propagation of light waves in an aperiodic multilayer which is composed of dielectric slabs arranged following the Thue-Morse sequence. Both refractive index modulation and optical thickness modulation are considered. The recursion relations of the transfer matrix elements are derived to calculate the transmission. By introducing a localization index we explore the localization properties of light in the aperiodic system. Around a particular center frequency (the midgap frequency of a periodic quarter-wavelength multilayer), the transmission is more sensitive to the optical thickness modulation.

Localization of light in linear and nonlinear random layered dielectric systems that are periodic on average

We compute the localization properties of light in a random system of linear dielectric slabs and a random system of Kerr-like nonlinear dielectric slabs. Particular attention is paid to systems that are periodic on average, and studies are made of the localization of light in these systems at frequencies both within and outside the band gaps of the average periodic system. It is found that the average periodic properties have the greatest effect on the exponential localization of light in the linear system, and exhibit only a small transient influence on the power-law localization observed in the nonlinear system. The average periodic properties of the nonlinear system are observed to affect localization only in systems of finite extent.