Optical bound states in the continuum (BICs) provide a way to engineer resonant response in photonic crystals with giant quality factors. The extended interaction time in such systems is particularly promising for enhancement of nonlinear optical processes and development of a new generation of active optical devices. However, the achievable interaction strength is limited by the purely photonic character of optical BICs. Here, 1 arXiv:1905.13505v1 [cond-mat.mes-hall] 31 May 2019 we mix optical BIC in a photonic crystal slab with excitons in atomically thin transition metal dichalcogenide MoSe 2 via strong coupling to form exciton-polaritons with Rabi splitting exceeding 27 meV. We experimentally show BIC-like behavior of both upper and lower polariton branches, with complete suppression of radiation into far-field at the BIC wavevector and strongly varying Q-factor in its vicinity. Owing to an effective disorder averaging through motional narrowing, we achieve small polariton linewidth of 2 meV and demonstrate linewidth control via angle and temperature tuning. Our results pave the way towards developing tunable BIC-based polaritonic devices for sensing, lasing, and nonlinear optics. Optical bound states in the continuum (BICs), supported by photonic crystal structures of certain geometries, have received much attention recently as a novel approach to generating extremely spectrally narrow resonant responses. 1,2 Since BICs are uncoupled from the radiation continuum through symmetry protection 3 or resonance trapping, 4 their high quality factors, while reaching 10 5 − 10 6 , can be robust to perturbations of photonic crystal geometric parameters. This enables a broad range of practical applications, including recently demonstrated spectral filtering, 5 chemical and biological sensing, 6,7 and lasing. 4Providing an efficient light-trapping mechanism, optical BICs are particularly attractive for enhancing nonlinear optical effects, with recent theoretical proposals discussing enhanced bistability 8 and Kerr-type focusing nonlinearity. 9 However, for practical realization of these proposals, a significantly stronger material nonlinear susceptibility is needed than generally available in dielectric-based photonic crystals.An attractive approach to the enhancement of effective nonlinearity is through the use of exciton-polaritons -hybrid quasi-particles that inherit both the coherent properties of photonic modes and interaction strength of excitons. 10,11 Hybrid nanophotonic systems incorporating atomically thin transition metal dichalcogenides (TMDs) have proven to be a particularly promising platform owing to their ease of fabrication and possibility of room temperature operation. [12][13][14] In addition to conventional microcavity-based designs, TMD

Modern nanophotonics has witnessed the rise of “electric anapoles” (EDAs), destructive interferences of electric and toroidal electric dipoles, actively exploited to resonantly decrease radiation from nanoresonators. However, the inherent duality in Maxwell equations suggests the intriguing possibility of “magnetic anapoles,” involving a nonradiating composition of a magnetic dipole and a magnetic toroidal dipole. Here, a hybrid anapole (HA) of mixed electric and magnetic character is predicted and observed experimentally via dark field spectroscopy, with all the dominant multipoles being suppressed by the toroidal terms in a nanocylinder. Breaking the spherical symmetry allows to overlap up to four anapoles stemming from different multipoles with just two tuning parameters. This effect is due to a symmetry‐allowed connection between the resonator multipolar response and its eigenstates. The authors delve into the physics of such current configurations in the stationary and transient regimes and explore new ultrafast phenomena arising at sub‐picosecond timescales, associated with the HA dynamics. The theoretical results allow the design of non‐Huygens metasurfaces featuring a dual functionality: perfect transparency in the stationary regime and controllable ultrashort pulse beatings in the transient. Besides offering significant advantages with respect to EDAs, HAs can play an essential role in developing the emerging field of ultrafast resonant phenomena.

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Gallium phosphide is a low-loss, high-refractive-index semiconductor considered as a promising material for active and passive components in modern nanophotonics. In this work, we show that nanoscale epitaxial layers of GaP with high optical quality can be formed directly on the transparent sapphire wafers despite the symmetry and lattice constant mismatch. This is achieved using a two-step growth technique through the framework of a domain matching epitaxy mechanism. Direct molecular beam epitaxial growth enables the control of material properties and layer thickness with subnanometer precision and allows us to obtain (111)-oriented epitaxial layers of GaP on high-optical-contrast sapphire wafers without the use of postgrowth layer transfer techniques. The influence of growth conditions on the structural quality of GaP-on-sapphire is revealed using Raman spectroscopy and X-ray diffraction reciprocal space mapping. We study the impact of the growth procedure employing a low-temperature seeding layer on the GaP layer morphology and structural quality. Spectroscopic ellipsometry measurements confirm that both the refractive index and the absorption coefficient of the epitaxial GaP layers are close to those of bulk GaP crystals. We also discuss how the GaP layer morphology and structural quality affect its optical density, drawing special attention to the mechanisms of optical losses. Finally, by nanostructuring the grown layer, we fabricate single GaP nanoantennas and confirm their highly resonant optical response in the visible spectral range, thus confirming the feasibility of the reported technology for various nanophotonic applications.