Dual Control of Enhanced Quasi-Bound States in the Continuum Emission from Resonant c-Si Metasurfaces.

Optical bound states in the continuum (BICs) offer strong interactions with quantum emitters and have been extensively studied for manipulating spontaneous emission, lasing, and polariton Bose-Einstein condensation. However, the out-coupling efficiency of quasi-BIC emission, crucial for practical light-emitting devices, has received less attention. Here, we report an adaptable approach for enhancing quasi-BIC emission from a resonant monocrystalline silicon (c-Si) metasurface through lattice and multipolar engineering. We identify dual-BICs originating from electric quadrupoles (EQ) and out-of-plane magnetic dipoles, with EQ quasi-BICs exhibiting concentrated near-fields near the c-Si nanodisks. The enhanced fractional radiative local density of states of EQ quasi-BICs overlaps spatially with the emitters, promoting efficient out-coupling. Furthermore, coupling the EQ quasi-BICs with Rayleigh anomalies enhances directional emission intensity, and we observe inherent opposite topological charges in the multipolarly controlled dual-BICs. These findings provide valuable insights for developing efficient nanophotonic devices based on quasi-BICs.

Collective Mie Resonances for Directional On-Chip Nanolasers.

A highly efficient nanocavity formed by optically coupled nanostructures is achieved by optimization of the collective Mie resonances in a one-dimensional array of semiconductor nanoparticles. Analysis of quasi-normal multipole modes enables us to reveal the close relation between the collective Mie resonances and Van Hove singularities. On the basis of these concepts, we experimentally demonstrate a directional GaAs nanolaser at cryogenic temperatures with well-defined, in-plane emission, which, moreover, can be controlled by selective excitation. The lasing threshold is shown to be significantly reduced by optimizing the interparticle gap such that the optimal near-field confinement is achieved at a resonant wavelength corresponding to the highest gain of GaAs. We show that the lasing performance of this nanolaser is orders of magnitude better than a nanowire-based laser of the same dimensions. The present work provides design guidelines for high performance in-plane emission nanolasers, which may find applications in future photonic integrated circuits.

Optical Processes behind Plasmonic Applications.

Plasmonics is a revolutionary concept in nanophotonics that combines the properties of both photonics and electronics by confining light energy to a nanometer-scale oscillating field of free electrons, known as a surface plasmon. Generation, processing, routing, and amplification of optical signals at the nanoscale hold promise for optical communications, biophotonics, sensing, chemistry, and medical applications. Surface plasmons manifest themselves as confined oscillations, allowing for optical nanoantennas, ultra-compact optical detectors, state-of-the-art sensors, data storage, and energy harvesting designs. Surface plasmons facilitate both resonant characteristics of nanostructures and guiding and controlling light at the nanoscale. Plasmonics and metamaterials enable the advancement of many photonic designs with unparalleled capabilities, including subwavelength waveguides, optical nanoresonators, super- and hyper-lenses, and light concentrators. Alternative plasmonic materials have been developed to be incorporated in the nanostructures for low losses and controlled optical characteristics along with semiconductor-process compatibility. This review describes optical processes behind a range of plasmonic applications. It pays special attention to the topics of field enhancement and collective effects in nanostructures. The advances in these research topics are expected to transform the domain of nanoscale photonics, optical metamaterials, and their various applications.

Tuning Collective Plasmon Resonances of Femtosecond Laser-Printed Metasurface.

The optical response of properly excited periodically arranged plasmonic nanostructures is known to demonstrate sharp resonance features associated with high-Q collective modes demanding for various applications in light-matter interaction, filtering and sensing. Meanwhile, practical realization and replication of plasmonic platforms supporting high-Q modes via scalable inexpensive lithography-free approach is still challenging. Here, we justify direct ablation-free irradiation of Si-supported thin Au film by nanojoule-energy femtosecond laser pulses as a single-step and scalable technology for realization of plasmonic metasurfaces supporting collective plasmonic response. Using an adjustable aperture to control and upscale the size of the fabricated nanostructures, nanobumps and nanojets, we demonstrated plasmonic metasurface supporting collective resonances with a moderately high Q-factor (up to 17) and amplitude (up to 45%) within expanded spectral range (1.4-4.5 µm). Vacuum deposition of thin films above the as-fabricated nanostructure arrays was demonstrated to provide fine tuning of the resonance position, also expanding the choice of available materials for realization of plasmonic designs with extended functionality.

Super-resolution Mapping of Enhanced Emission by Collective Plasmonic Resonances.

Plasmonic particle arrays have remarkable optical properties originating from their collective behavior, which results in resonances with narrow line widths and enhanced electric fields extending far into the surrounding medium. Such resonances can be exploited for applications in strong light-matter coupling, sensing, light harvesting, nonlinear nanophotonics, lasing, and solid-state lighting. However, as the lattice constants associated with plasmonic particle arrays are on the order of their resonance wavelengths, mapping the interaction between point dipoles and plasmonic particle arrays cannot be done with diffraction-limited methods. Here, we map the enhanced emission of single fluorescent molecules coupled to a plasmonic particle array with ∼20 nm in-plane resolution by using stochastic super-resolution microscopy. We find that extended lattice resonances have minimal influence on the spontaneous decay rate of an emitter but instead can be exploited to enhance the outcoupling and directivity of the emission. Our results can guide the rational design of future optical devices based on plasmonic particle arrays.