MENP: An Open-Source MATLAB Implementation of Multipole Expansion for Applications in Nanophotonics

In modern nanophotonics, multipolar interference plays an indispensable role to realize novel optical devices represented by metasurfaces with unprecedented functionalities. Not only to engineer sub-wavelength structures that constitute such devices but also to realize and interpret unnatural phenomena in nanophotonics, a program that efficiently carries out multipole expansion is highly demanded. MENP is a MATLAB program for computation of multipole contributions to light scattering from current density distributions induced in nanophotonic resonators. The main purpose of MENP is to carry out post-processing of a rigid multipole expansion for full-field simulations which in principle provide the information of all near- and far-field interactions (e.g. as a total scattering cross section). MENP decomposes total scattering cross sections into partial ones due to electric and magnetic dipoles and higher-order terms based on recently developed exact multipole expansion formulas. We validate the program by comparing results for ideal and realistic nanospheres with those obtained with the Mie theory. We also demonstrate the potential of MENP for analysis of anapole states by calculating the multipole expansion under the long-wavelength approximation which enables us to introduce toroidal dipole moments

Optical Spin Sorting Chain

Transverse spin angular momentum of light is a key concept in recent nanophotonics to realize unidirectional light transport in waveguides by spin-momentum locking. Herein we theoretically propose subwavelength nanoparticle chain waveguides that efficiently sort optical spins with engineerable spin density distributions. By arranging high-refractive-index nanospheres of different sizes in a zigzag manner, directional optical spin propagation is realized. The origin of the efficient spin transport is revealed by analyzing the dispersion relation and spin angular momentum density distributions. In contrast to conventional waveguides, the proposed asymmetric waveguide can spatially separate up- and down-spins and locate one parity inside and the other outside the structure. Moreover, robustness against bending the waveguide and its application as an optical spin sorter are presented. Compared to previous reports on spatial engineering of local spins in photonic crystal waveguides, we achieved substantial miniaturization of the entire footprint down to subwavelength scale

Supplementary document for Optical Spin Sorting Chain - 5477307.pdf

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Metal-Core/Dielectric-Shell/Metal-Cap Composite Nanoparticle for Upconversion Enhancement

We have developed an upconversion composite nanoparticle composed of a metal core, an upconversion shell, and a metal cap. Numerical simulation of the nanocomposite revealed that hybridization of the localized surface plasmon modes of the core and the cap results in the emergence of novel bonding and antibonding modes. The latter mode has wide tunability in the resonance wavelength and strong field confinement at the position of the upconversion shell. For the fabrication of the composite nanoparticle, we developed a process that combines liquid-phase synthesis and vapor deposition processes. The scattering spectra of single composite nanoparticles agreed well with those in the numerical simulation. The comparison of the upconversion intensity between the metal-core/dielectric-shell structure and the metal-core/dielectric-shell/metal-cap structure revealed that the cap formation increases the intensity several folds

Elongated Metal Nanocap with Two Magnetic Dipole Resonances and Its Application for Upconversion Enhancement

A stand-alone plasmonic nanocomposite into which a metal nanostructure and an emitting material are integrated is a promising building block for optoelectronics and biophotonics devices. Here, we present the plasmonic property of a nanocomposite composed of a Au elongated nanocap and a β-NaYF4 dielectric nanorod. We show that elongation of a Au nanocap results in splitting of the magnetic dipole resonance, and the resonance wavelengths can be controlled in a wide wavelength range by the aspect ratio. As an application of the elongated nanocap, we demonstrate strong enhancement of the near-infrared to visible upconversion of an Er3+ and Yb3+ doped β-NaYF4 nanorod by tuning the resonance wavelength of a Au nanocap placed on it to the excitation wavelength

Monolayer of Mie-Resonant Silicon Nanospheres for Structural Coloration

Structural coloration of a monolayer of Mie-resonant silicon (Si) nanospheres (NSs) produced by a solution-based process is studied. It is shown by simulation that a monolayer of hexagonal close-packed Si NSs exhibits size-dependent structural color with a peak reflectance of ∼50%. The peak reflectance can be increased to over 90% by introducing spaces between the Si NSs. The high reflectance despite the small coverage is due to the very high scattering efficiency of Mie-resonant Si NSs. Monolayers of densely packed Si NSs are produced from Si NS suspensions by the Langmuir–Blodgett method. The monolayers exhibit size-dependent structural color with a peak reflectance of 30–50%. The color is very insensitive to the viewing angle, and the angle dependence of the reflectance spectra is very small. The peak reflectance is increased by increasing the distance between the NSs by partially oxidizing the layers. The results demonstrate that iridescence-free structural coloration of a substance is possible by a layer of Si NSs much thinner than the monolayer, i.e., by sparsely scattered Si NSs

Enhanced Light Emission from Monolayer MoS₂ by Doubly Resonant Spherical Si Nanoantennas

Optical antennas provide a powerful tool to control local photonic environments and enhance light emission from two-dimensional transition-metal dichalcogenides. Dielectric nanoantennas with multipolar Mie resonances bring unique advantages for achieving simultaneous enhancement of the absorption and emission processes. Here, we achieve a strong modification of the photoluminescence (PL) behavior of monolayer MoS2 by a spherical nanoparticle (NP) of crystalline silicon (Si) that works as a double resonance nanoantenna. From theoretical calculations for in-plane dipoles placed beneath a Si NP nanoantenna with different sizes, we explore optimal conditions for the double resonances. Then, we develop a heterostructure composed of a Si NP and a monolayer MoS2 sheet with a comparable diameter and investigate the scattering, PL, and PL excitation spectra across a wide Si NP size range. We show that the spectral shape is significantly modified and PL intensity is enhanced up to ∼10-fold due to the coupling of the excitation process to the magnetic quadrupole resonance and the emission process to the magnetic dipole resonance

Visualizing the Nanoscopic Field Distribution of Whispering-Gallery Modes in a Dielectric Sphere by Cathodoluminescence

A spherical dielectric particle can sustain the so-called whispering-gallery modes (WGMs), which can be regarded as circulating electromagnetic waves, resulting in the spatial confinement of light inside the particle. Despite the wide adoption of optical WGMs as a major light confinement mechanism in salient practical applications, direct imaging of the mode fields is still lacking and only partially addressed by simple photography and simulation work. The present study comprehensively covers this research gap by demonstrating the nanoscale optical-field visualization of self-interference of light extracted from excited modes through experimentally obtained photon maps that directly portray the field distributions of the excited eigenmodes. To selectively choose the specific modes at a given light emission detection angle and resonance wavelength, we use cathodoluminescence-based scanning transmission electron microscopy supplemented with angle-, polarization-, and wavelength-resolved capabilities. Equipped with semi-analytical simulation tools, the internal field distributions of the whispering-gallery modes reveal that radiation emitted by a spherical resonator at a given resonance frequency is composed of the interference between multiple modes, with one or more of them being comparatively dominant, leading to a resulting distribution featuring complex patterns that explicitly depend on the detection angle and polarization. Direct visualization of the internal fields inside resonators enables a comprehensive understanding of WGMs that can shed light on the design of nanophotonic applications