Loss Control with Annealing and Lattice Kerker Effect in Silicon Metasurfaces

The resonant phenomena of metasurfaces highly depend on the scattering strength of each component and their interferences. The losses modify the phase and reduce the amplitude of all multipoles; thus, the loss control is vital for obtaining the designed properties. Amorphous (a-)Si has a higher absorption coefficient than that of the crystalline form, which limits its optical application. A simple rapid thermal annealing (RTA) path to refine the a-Si metasurfaces is found. It is applied to the sputtering-made a-Si metasurface comprising square array of nanodisks. While the large loss smears out the resonances for the as-made metasurface, the sharp and near-zero reflectance with near-perfect absorptance is achieved after RTA, satisfying the lattice Kerker condition via the interference of magnetic and electric dipoles. At the lattice Kerker condition, the forward-enhanced and backward-reduced directional photoluminescence is observed from the emitter layer deposited on the metasurface. The numerical results are all found to be in good agreement with the experimental results, and the multipole expansion analysis for the single nanodisk gives the physical background of this observation. This refinement of a-Si metasurfaces by RTA treatment paves the simple and robust way for realizing thrilling optical and optoelectrical applications, such as detectors and filters

Resonant critical coupling of surface lattice resonances with fluorescent absorptive thin film

Surface lattice resonance supported on nanoparticle arrays is a promising candidate in enhancing fluorescent effects in both absorption and emission. The optical enhancement provided by surface lattice resonance is primarily through the light confinement beyond the diffraction limit, where the nanoparticle arrays can enhance light-matter interaction for increased absorption as well as providing more local density of states for enhanced spontaneous emission. In this work, we optimize the in-coupling efficiency to the fluorescent molecules by finding the conditions to maximize the absorption, also known as the critical coupling condition. We studied the transmission characteristics and the fluorescent emission of a $TiO_2$ nanoparticle array embedded in an index-matching layer with fluorescent dye at various concentrations. A modified coupled-mode theory that describes the nanoparticle array was then derived and verified by numerical simulations. With the analytical model, we analyzed the experimental measurements and discovered the condition to critically couple light into the fluorescent dye, which is demonstrated as the strongest emission. This study presents a useful guide for designing efficient energy transfer from excitation beam to the emitters, which maximizes the external conversion efficiency.Comment: 26 pages, 10 figure

Non-Equilibrium Bose–Einstein Condensation of Exciton-Polaritons in Silicon Metasurfaces

Exciton-polaritons (EPs) are hybrid light–matter quasi-particles with bosonic character formed by the strong coupling between excitons in matter and photons in optical cavities. Their hybrid character offers promising prospects for the realization of non-equilibrium Bose–Einstein condensates (BECs), and room-temperature BECs are possible with organic materials. However, the thresholds required to create BECs of organic EPs remain still high to allow condensation with electrical injection of carriers. One of the factors behind these high thresholds is the very short cavity lifetimes, leading to a fast EP decay and the need to inject higher exciton densities in the reservoir to form the condensate. Here a BEC of EPs in organic dyes and all-dielectric metasurfaces at room temperature is demonstrated. By using dielectric metasurfaces that exhibit very low losses it is possible to achieve cavity lifetimes long enough to allow an efficient population of EP states via vibrational relaxation and radiative pumping. It is shown how polariton lasing or non-equilibrium Bose–Einstein condensation is achieved in several cavities, and one of the lowest reported thresholds for BECs in organic materials is observed.</p

Tunable Faraday rotation of ferromagnet thin film in whole visible region coupled with aluminum plasmonic arrays

To date, the plasmonic nanostructure utilized for magneto-optical (MO) enhancement has been limited to noble metals with resulted enhancement in the green-red part of visible spectrum. In this study, we fabricated a diffractive hexagonal array composed of Al nanoparticles (NPs) with a thin 7.5 nm ferromagnetic film and pushed the enhanced Faraday rotation (FR) into the blue to green range of the visible light. The freedom and ability to control the working spectral region in the whole visible range from 400 to 800 nm were also demonstrated by changing the lattice constant and the dielectric environment of plasmonic nanostructures. Particularly, in the blue range we obtained the maximum FR 0.57° at 410 nm with a broad boosting region around 0.5° from 400 to 500 nm. Moreover, the largest FR 1.66° was shown at 638 nm by tuning the dielectric environment into a higher refractive index medium. The results of our investigation demonstrate the potential of Al-based magnetoplasmonic effect and offer opportunities to push the MO spectral response out of visible range into the ultraviolet-blue range

Clustering of Primordial Black Holes from QCD Axion Bubbles

We study the clustering of primordial black holes (PBHs) and axion miniclusters produced in the model proposed to explain the LIGO/Virgo events or the seeds of the supermassive black holes (SMBHs) in arXiv:2006.13137. It is found that this model predicts large isocurvature perturbations due to the clustering of PBHs and axion miniclusters, from which we obtain stringent constraints on the model parameters. Specifically, for the axion decay constant $f_a=10^{16}~\mathrm{GeV}$, which potentially accounts for the seeds of the SMBHs, the PBH fraction in dark matter should be $f_\mathrm{PBH}\lesssim7\times 10^{-10}$. Assuming that the mass of PBHs increases by more than a factor of $\mathcal{O}(10)$ due to accretion, this is consistent with the observed abundance of SMBHs. On the other hand, for $f_a=10^{17}~\mathrm{GeV}$ required to produce PBHs of masses detected in the LIGO/Virgo, the PBH fraction should be $f_\mathrm{PBH}\lesssim6\times 10^{-8}$, which may be too small to explain the LIGO/Virgo events, although there is a significant uncertainty in calculating the merger rate in the presence of clustering.Comment: 18 pages, 11 figure

Optical Responses of Localized and Extended Modes in a Mesoporous Layer on Plasmonic Array to Isopropanol Vapor

Mesoporous silica features open and accessible pores that can intake substances from the outside. The combination of mesoporous silica with plasmonic nanostructures represents an interesting platform for an optical sensor based on the dependence of plasmonic modes on the refractive index of the medium in which metallic nanoparticles are embedded. However, so far only a limited number of plasmonic nanostructures are combined with mesoporous silica, including random dispersion of metallic nanoparticles and fl at metallic thin fi lms. In this study, we make a mesoporous silica layer on an aluminum nanocylinder array. Such plasmonic arrangements support both localized surface plasmon resonances (LSPRs) and extended modes which are the result of the hybridization of LSPRs and photonic modes extending into the mesoporous layer. We investigate in situ optical re fl ectance of this system under controlled pressure of isopropanol vapor. Upon exposure, the capillary condensation in the mesopores results in a gradual spectral shift of the re fl ectance. Our analysis demonstrates that such shifts depend largely on the nature of the modes; that is, the extended modes show larger shifts compared to localized ones. Our materials represent a useful platform for the fi eld of environmental sensingEspaña MINECO grant MAT2017-88584-R

Enhanced light emission by magnetic and electric resonances in dielectric metasurfaces

We demonstrate an enhanced emission of high quantum yield molecules coupled to dielectric metasurfaces formed by periodic arrays of polycrystalline silicon nanoparticles. Radiative coupling of the nanoparticles, mediated by in-plane diffraction, leads to the formation of collective Mie scattering resonances or Mie surface lattice resonances (M-SLRs), with remarkable narrow line widths. These narrow line widths and the intrinsic electric and magnetic dipole moments of the individual Si nanoparticles allow to resolve electric and magnetic M-SLRs. Incidence angle- and polarization-dependent extinction measurements and high-accuracy surface integral simulations show unambiguously that magnetic M-SLRs arise from in- and out-of-plane magnetic dipoles, while electric M-SLRs are due to in-plane electric dipoles. Pronounced changes in the emission spectrum of the molecules are observed, with almost a 20-fold enhancement of the emission in defined directions of molecules coupled to electric M-SLRs, and a 5-fold enhancement of the emission of molecules coupled to magnetic M-SLRs. These measurements demonstrate the potential of dielectric metasurfaces for emission control and enhancement, and open new opportunities to induce asymmetric scattering and emission using collective electric and magnetic resonances.Comment: 27 pages with 9 figure

Tunable emission from H-type supramolecular polymers in optical nanocavities

H-type supramolecular polymers with preferred helicity and highly efficient emission have been prepared from the self-assembly of chiral tetraphenylene-based monomers. Implementation of the one-dimensional fibers into dielectric nanoparticle arrays allows for a significant reshaping of fluorescence due to weak light-matter coupling.</p