Plasmonic hot-electron generation has recently come into focus as a new scheme for solar energy conversion. So far, however, due to the relatively narrow bandwidth of the surface plasmon resonances and the insufficient resonant light absorption, most of plasmonic photocatalysts show narrow-band spectral responsivities and small solar energy conversion efficiencies. Here we experimentally demonstrate that a three-layered nanostructure, consisting of a monolayer gold-nanoparticles and a gold film separated by a TiO₂ gap layer (Au-NPs/TiO₂/Au-film), is capable of near-completely absorbing light within the whole visible region. We show that the Au-NPs/TiO₂/Au-film device can take advantage of such strong and broadband light absorption to enhance the generation of hot electrons and thus the photocurrent under visible irradiation. As compared to conventional plasmonic photocatalysts such as Au-NPs/TiO2 nanostructures, a 5-fold-enhanced incident photon-to-current conversion efficiency is achieved within the entire wavelength range 450–850 nm in the Au-NPs/TiO₂/Au-film device. Simulations show good agreements with the experimental results, demonstrating that only the plasmon-induced losses contribute to the enhanced photocurrent generation of the Au-NPs/TiO₂/Au-film device.

Efficient control of optical radiation at subwavelength scales plays important roles for various applications. Dielectric nanoparticles or dielectric shells with a large refractive index of n ~ 3–4, which are only achievable for limited semiconductors, are involved in most designs so far to control the scattering by overlapping the electric and magnetic dipolar modes of the same magnitude. Here we propose a new mechanism based on the interplay between dipolar and quadrupolar resonances of different amplitudes, both magnetic and electric, to suppress the backward scattering or the forward scattering by using metallo-dielectric core-shell nanoparticles with a dielectric shell layer having a refractive index of n = 2.0. We demonstrate that broadband zero-backward or near-zero-forward scattering can be achieved by optimizing the structural parameters. We also demonstrate that the core-shell nanoparticles with identical dielectric shells but metal cores with various sizes are able to suppress the backward or forward scattering at the same wavelength, thus revealing a large tolerance to fabrication errors induced by the size distributions in the metal cores. These features make the proposed core-shell nanoparticles beyond the dipole limit more easily realized in practical experiments.

We propose a novel type of plasmonic lasing nanostructure consisting of a metallic shell and a gain core. We demonstrate numerically that highly localized void modes of such metallodielectric core–shell nanoparticles have a very high quality factor. We found that the dipole void mode has a lasing threshold as low as $128{\mathrm{cm}}^{-1}$ at 800 nm as a result of the unique mode distribution within the shell, due to a maximum field enhancement around the void center. The lasing condition for a symmetry-reduced silver nanocup is also investigated and the low plasmonic lasing threshold is sustained provided that the opening angle of the nanocup is smaller than 10°. Our proposal presents a new path toward plasmonic lasers with low gain threshold.

Exceptional points (EPs) are degeneracies in open wave systems with coalescence of at least two energy levels and their corresponding eigenstates. In higher dimensions, more complex EP physics not found in two-state systems is observed. We consider the emergence and interaction of multiple EPs in a four coupled optical waveguides system by non-Hermitian coupling showing a unique EP formation pattern in a phase diagram. In addition, absolute phase rigidities are computed to show the mixing of the different states in definite parameter regimes. Our results could be potentially important for developing further understanding of EP physics in higher dimensions via generalized paradigm of non-Hermitian coupling for a generation of parity-time devices.

We theoretically propose a design of a spaser based on spherical hyperbolic metamaterial cavities consisting of a dielectric core wrapped by several stacks of alternating layers of metal and dielectric, which can support the multipolar whispering-gallery modes (WGMs) with resonance wavelengths much larger than the cavity size. Depending on the mode order of the excited WGMs with the same angular momentum, we show that such cavities are capable of strongly confining the electric fields within different dielectric shell layers. By introducing gains into the different dielectric layers, we demonstrate that the hyperbolic cavities can act as multi-wavelength spasers with a lasing threshold as low as 811 cm⁻¹, operating on the WGMs with the corresponding mode order. Furthermore, we also demonstrate that the slight variation on either the number of stacks or the core radius allows for easily tuning the lasing wavelengths over a wide spectral range.