Midrefractive Dielectric Modulator for Broadband Unidirectional Scattering and Effective Radiative Tailoring in the Visible Region

Room-Temperature Observation of Near-Intrinsic Exciton Linewidth in Monolayer WS2

The homogeneous exciton linewidth, which captures the coherent quantum dynamics of an excitonic state, is a vital parameter in exploring light-matter interactions in 2D transition metal dichalcogenides (TMDs). An efficient control of the exciton linewidth is of great significance, and in particular of its intrinsic linewidth, which determines the minimum timescale for the coherent manipulation of excitons. However, such a control is rarely achieved in TMDs at room temperature (RT). While the intrinsic A exciton linewidth is down to 7 meV in monolayer WS₂ , the reported RT linewidth is typically a few tens of meV due to inevitable homogeneous and inhomogeneous broadening effects. Here, it is shown that a 7.18 meV near-intrinsic linewidth can be observed at RT when monolayer WS₂ is coupled with a moderate-refractive-index hydrogenated silicon nanosphere in water. By boosting the dynamic competition between exciton and trion decay channels in WS₂ through the nanosphere-supported Mie resonances, the coherent linewidth can be tuned from 35 down to 7.18 meV. Such modulation of exciton linewidth and its associated mechanism are robust even in presence of defects, easing the sample quality requirement and providing new opportunities for TMD-based nanophotonics and optoelectronics.

Highly efficient unidirectional forward scattering induced by resonant interference in a metal-dielectric heterodimer

We demonstrate that a metal-dielectric heterodimer structure can satisfy a nearly ideal first Kerker condition at a wavelength close to the resonance peak of the dimer, yielding efficient unidirectional forward scattering with a high forward-to-backward scattering ratio (≈48 dB) and remarkable enhancement of the forward scattering intensity (∼2.68 times compared to a single dielectric nanoparticle). Using a rigorous analytical dipole-dipole interaction model, the underlying mechanism is revealed, in which the originally weak electric dipole moment of the dimer is significantly enhanced owing to the strong resonant interference between the localized surface plasmon resonance of the metal and the Mie resonances of the dielectric material, which could up-match the magnetic dipole moment of the dimer at a wavelength close to the resonance peak, boosting the forward scattering efficiency. To achieve the optimal conditions, the sizes of the metal and dielectric constituents as well as the gap distance of the dimer have to be physically and delicately tuned to ensure a perfect match in both the amplitudes and phases of the electric and magnetic dipole moments of the dimer. On top of that, the loss of the heterodimer can be effectively suppressed to a level well below that of a pure metal nanoparticle, which further benefits the forward scattering efficiency. The flexibility in designing the dimer geometry and choosing metal-dielectric material combinations enables efficient unidirectional forward scattering in a broadband spectrum (UV to visible) with an intermediate gap distance (10-20 nm), greatly expanding the application scope. The proposed hybrid dimer could serve as a powerful and versatile building block in many emergent fields such as metasurfaces, nanoantennae, etc.

Asymmetric structural colors based on monodisperse single-crystal Cu2O spheres

Structural colors have attracted broad attention owing to their anti-photobleaching capability and brilliant metallic color. In particular, the asymmetric structural colors generated by a simple material will have great practical significance in the fields of biomimetic materials, double-side display and anti-counterfeiting. The asymmetric optical effects were usually achieved by the plasmonic effects of Ag or Au nanocrystals. Here, for the first time, we realized the asymmetric structural colors based on the asymmetric scattering of Cu₂O single-crystal spheres. By spray-coating Cu₂O spheres on a glass slide, different structural colors were viewed from the Cu₂O film side and the glass slide side. The FDTD simulations confirmed that the asymmetric colors were ascribed to the inhomogeneous distribution of the electric field intensity. The film built by 200 nm Cu₂O spheres on a glass slide shows green and cyan structural colors from the front and back sides, respectively. The colors on both sides of the Cu₂O films were proved to be tuned by changing the diameters of the Cu₂O single-crystal spheres. Different substrates were used to examine the influence of substrates on the asymmetric colors. Finally, inspired by different brilliant colors from the front and back of natural creatures, the patterns of butterfly and petals were fabricated by Cu₂O spheres. Impressively, similar to nature, the patterns show completely different colors viewed from the front and back sides. The asymmetric structural colors of Cu₂O single-crystal spheres will open up new avenues to realize multi-mode color output and pave their applications in display, biomimetic materials and anti-counterfeiting materials.

Absorption avoided resonance crossing of hybridization of silicon nanoparticles and gold nanoantennas

The near-field coupling between a high-refractive-index nanoparticle and gold nanoantennas is investigated theoretically. The absorption enhancement and also avoided resonance crossing in the absorption cross section spectra were observed with the hybridization system due to the coupling between the localized surface plasmon resonance of the gold nanoantennas and the magnetic dipole resonance of the silicon nanoparticle. By controlling the nanoparticle size or the separation distance, the near-field coupling can be tuned from the weak to the strong regime.

Tunable Control of Interlayer Excitons in WS2/MoS2 Heterostructures via Strong Coupling with Enhanced Mie Resonances

Strong Coulomb interactions in monolayer transition metal dichalcogenides (TMDs) produce strongly bound excitons, trions, and biexcitons. The existence of multiexcitonic states has drawn tremendous attention because of its promising applications in quantum information. Combining different monolayer TMDs into van der Waals (vdW) heterostructures opens up opportunities to engineer exciton devices and bring new phenomena. Spatially separated electrons and holes in different layers produce interlayer excitons. Although much progress has been made on excitons in single layers, how interlayer excitons contribute the photoluminescence emission and how to tailor the interlayer exciton emission have not been well understood. Here, room temperature strong coupling between interlayer excitons in the WS2/MoS2 vdW heterostructure and cavity-enhanced Mie resonances in individual silicon nanoparticles (Si NPs) are demonstrated. The heterostructures are inserted into a Si film-Si NP all-dielectric platform to realize effective energy exchanges and Rabi oscillations. Besides mode splitting in scattering, tunable interlayer excitonic emission is also observed. The results make it possible to design TMDs heterostructures with various excitonic states for future photonics devices.

Non-Mie optical resonances in anisotropic biomineral nanoparticles

The optical properties of nanoparticles have attracted continuous attention owing to their high fundamental and applied importance across many disciplines. A recently emerged field of all-dielectric nanophotonics employs optically induced electric and magnetic Mie resonances in dielectric nanoparticles with a high refractive index. This property allows obtaining additional valuable degrees of freedom to control the optical responses of nanophotonic structures. Here we propose a conceptually distinct approach towards reaching optical resonances in dielectric nanoparticles. We show that, lacking conventional Mie resonances, low-index nanoparticles can exhibit a novel anisotropy-induced family of non-Mie eigenmodes. Specifically, we investigate light interactions with calcite and vaterite nanospheres and compare them with the Mie scattering by a fused silica sphere. Having close permittivities and the same dimensions, these particles exhibit significantly different scattering behavior owing to their internal structure. While a fused silica sphere does not demonstrate any spectral features, the uniaxial structure of the permittivity tensor for calcite and the non-diagonal permittivity tensor for vaterite result in a set of distinguishable peaks in scattering spectra. Multipole decomposition and eigenmode analysis reveal that these peaks are associated with a new family of electric and magnetic resonances. We identify magnetic dipole modes of ordinary, extraordinary and hybrid polarization as well as complex electric dipole resonances, featuring a significant toroidal electric dipole moment. As both vaterite and calcite are biominerals, naturally synthesized and exploited by a variety of living organisms, our results provide an indispensable toolbox for understanding and elucidating the mechanisms behind the optical functionalities of true biological systems.

Using highly uniform and smooth selenium colloids as low-loss magnetodielectric building blocks of optical metafluids

We systematically analyzed the magnetodielectric resonances of Se colloids for the first time in an attempt to utilize them as building blocks for all-dielectric optical metafluids. By taking advantages of the synergistic properties of Se colloids, including their (i) high-refractive-index at optical frequencies, (ii) unprecedented structural uniformity, and (iii) ready availability, we were able to observe Kerker-type directional light scattering, resulting from the efficient coupling between strong electric and magnetic resonances, directly from Se colloidal suspensions. Thus, the use of Se colloids as a generic magnetodielectric building block suggests the opportunity for the production of fluidic low-loss optical antennas, which can be processed via spin-coating and painting.

Refractive Index Sensing Using Visible Electromagnetic Resonances of Supported Cu2O Particles

Plasmonic metal nanostructures, in colloidal or surface-supported forms, have been extensively studied in the context of metamaterials design and applications, in particular as refractometric sensing platforms. Recently, high refractive index (high-n) dielectric subwavelength structures have been experimentally shown to support strong Mie scattering resonances, predicted to exhibit analogous refractive index sensing capabilities. Here we present the first experimental demonstration of the use of supported high-n dielectric nano/microparticle ensembles as refractive index sensing platforms, using cuprous oxide as a model high-n material. Single-crystalline Cu₂O particles were deposited on transparent substrates using a chemical deposition scheme, showing well-defined electric and magnetic dipolar resonances (EDR and MDR, respectively) in the visible range, which change in intensity and wavelength upon changing the medium refractive index (n_m). The significant modulation of the MDR intensity when n_m is modified appears to be the most valuable empirical sensing parameter. The Mie scattering properties of Cu₂O particles, particularly the spectral dependence of the MDR on n_m, are theoretically modeled to support the experimental observations. MDR extinction changes (i.e., refractive index sensitivity) per particle are >100 times higher compared to localized surface plasmon resonance (LSPR) changes in supported Au nanoislands, encouraging the evaluation of Cu₂O and other high-n dielectric particles and sensing modes in order to improve the sensitivity in optical (bio)sensing applications.