Modulating Polarization in Second Harmonic Generation through Symmetry Evolution in Plasmonic Lattices.

We report a strategy for preparing cost-effective plasmonic square lattices with tunable unit structures of circles, crosses, and circle-cross pairs on a centimeter scale. The asymmetrical electromagnetic (EM) field distribution of the lattice enhances second harmonic generation (SHG) under oblique incidence. The SHG signals are progressively strengthened as the unit symmetry decreases from C∞v (circle) to C4v (cross) to C2v (circle-cross pair). The peak SHG signal is observed from the plasmonic lattice with a circle-cross pair, showcasing a conversion efficiency of 1.0 × 10-2, which is a 7.3-fold enhancement relative to the dielectric lattice comprised of circle units. This notably high conversion efficiency of SHG is on par with that of phase-matched bulk nanostructures under normal incidence, benefiting from the Bloch-surface plasmon polariton (Bloch-SPP) modes associated with the distribution of the photonic local density of states (LDOS). Furthermore, the SHG emission exhibits distinctive directional and polarization characteristics as the unit symmetry is reduced. This work offers valuable insights into a structural symmetry-dependent SHG in plasmonic lattices and the way forward for the design of functional nonlinear plasmonic devices.

Surface lattice resonances enhanced directional amplified spontaneous emission on plasmonic honeycomb nanocone array.

Plasmonic arrays have emerged as a promising platform for investigating light-matter interactions enhanced by surface lattice resonance (SLR) at the nanoscale, which exhibit superior properties in localized field enhancement, narrow linewidth, and effective radiation loss suppression. In this study, an Al nanocone array in a honeycomb arrangement served as an optical cavity with a tip effect to realize the directional and polarized amplified spontaneous emission (ASE) of R6G. Based on the optical feedback between the degenerated SLR mode of high local density of states (LDOS) and the emission of gain media, 140-fold enhanced ASE was observed at an emission angle of 25° under TM polarization when the pump power density exceeded the threshold of 197.8 W cm-2. Moreover, polarization-resolved iso-frequency images indicated that a specific polarization dependence of ASE was modulated by the SLR mode. This study clarifies the interaction between the gain media and plasmonic system, which is beneficial for the generation of nanolasing with directional emission and lays a foundation for the plasmonic device.

Direction- and polarization-tunable spontaneous emission beneficial from diffraction orders of a square R6G-nanopore array.

To meet the miniaturization and compatibility of current micro-nano optical devices, two-dimensional (2D) photonic crystals (PCs), which can manipulate the optical parameters and their propagation with more degree of freedom, have become more important in nano optics. In the case of 2D PCs, the specific symmetry of the microscopic lattice arrangement determines its macroscopic optical properties. In addition, beyond this key element of lattice arrangement, the unit cell of PCs is also a critical factor in modulating the far-field optical behaviors. This work explores the manipulation of spontaneous emission (SE) of rhodamine 6G (R6G) in a square lattice of anodic aluminum oxide (AAO) membrane. The directional and polarized emissions are observed to be related to the diffraction orders (DOs) of the lattice arrangement. By further tuning the size of unit cells, different DOs are triggered to overlap with the emission of R6G, resulting in an extended tuning for directions and polarizations of light emission. It exhibits the significance for the design and application of nano optics devices.

Bloch-Surface Plasmon Polariton Enhanced Amplified and Directional Spontaneous Emission from Plasmonic Hexagonal Nanohole Array.

The light-matter interactions at nanoscale can be enhanced by Bloch-surface plasmon polariton (Bloch-SPP) on the plasmonic lattice. An Ag nanohole array in hexagonal arrangement served as an optical cavity to realize the directional and polarized amplified spontaneous emission (ASE) of R6G. A 100-fold enhanced ASE was observed at 15° emission angle under TM polarization when the pump power density exceeded the threshold of 198 W/cm2 based on the degenerated high state density modes. Moreover, a specific polarization dependence of ASE was modulated by the Bloch-SPP modes, and the degree of polarization was enhanced from 1.3 to 2.1 when the pump power density exceeded the threshold of ASE. This work clarifies the interaction between the gain media and plasmonic systems, which lays a foundation for the plasmonic device designing.

Directional amplified spontaneous emissions from Ag nanohole array with high diffraction orders.

Surface plasmon excitations in metallic hole arrays have been extensively studied in the context of light-matter interaction, since the generated Bloch surface plasmon polariton (Bloch-SPP) exhibits unique properties of nanoscale light confinement, near-field enhancements, and long-range metal surface propagation. In this work, we experimentally demonstrate a plasmonic device that exhibits highly directional emission in visible light; using Ag film with a thickness of 100 nm deposited on a subwavelength porous alumina array as a plasmonic cavity, four-level rhodamine 6G (R6G) is selected as the gain material. It is suggested that a Bloch-SPP with high diffraction orders on a Ag nanohole array can generate a strong local electric field and a high Purcell factor at a nanohole edge. Moreover, directional five-fold enhanced amplified spontaneous emission (ASE) with polarization dependence is observed under a low threshold of 199.9 W/cm2 in the visible light region, which comes from the optical feedback provided by the 2D periodic nanohole array. This work opens up a wide range of applications for real-time tunable wavelength, controlled multimode laser, fluorescence detection, etc.

Strong coupling between monolayer quantum emitter WS2 and degenerate/non-degenerate surface lattice resonances.

Strong light-matter coupling manifested by Rabi splitting has drawn considerable interest owing to its fundamental significance for impressive interaction enhancement in the fields of ultrafast active plasmonic devices and quantum information. In this paper, we investigate the coherent optical properties of a plasmonic system consisting of periodic metal nanoparticle arrays covered by a WS2 thin film of atomic layer thickness. The coupling factor, energy splitting, and temporal dynamics of this coherent coupling phenomenon are quantitatively revealed by finite-difference time-domain (FDTD) simulation and a full quantum mechanical model proves that the exciton behavior of the fermionic quantum emitter WS2 is carefully modulated by bosonic surface lattice resonances. This work may pave the way for coherent modulation of polariton and plasmon devices and can potentially open up diverse exciting possibilities like nanoscale light sources, single-photon emitters, and all-optical transistors.

Wafer-scale plasmonic metal-dielectric-metal structural color featuring high saturation and low angular dependence.

Structural color has been studied through various methods due to its distinguished features of stability, durability, high information storage density and high integration. However, the artificial structural color samples do not exhibit superior performance in color saturation and low angular dependence. Here, we present an approach to acquire additive reflective color based on a metal-dielectric-metal (MDM) stack. The upper layer composed of Ag particles is perforated in a hexagonal arrangement which profits from the dielectric anodic aluminium oxide (AAO) membrane. The size and shape of the Ag particles are getting inhomogeneous as the deposition thickness of the upper layer increasing, which expands the desired absorption range of surface plasmons. The residual non-anodized Al foil serves as a highly reflective substrate for efficient color presenting through the thin-film interference in this plasmonic MDM system. As a result, the color gamut area of this MDM stack is extended 8 times in CIE chromaticity coordinates. Finally, a wafer-scale (diameter of 83 mm) badge of Harbin Engineering University (HEU) with highly saturated colors and a pattern characterized with low angle-dependent property (up to 60°) are presented, which exhibit promising prospects in commercial coloring and imaging.