Recent progress in nanoscale optical physics is associated with the development of a new branch of nanophotonics exploring strong Mie resonances in dielectric nanoparticles with high refractive index. The high-index resonant dielectric nanostructures form building blocks for novel photonic metadevices with low losses and advanced functionalities. However, unlike extensively studied cavities in photonic crystals, such dielectric resonators demonstrate low quality factors (Q-factors). Here, we uncover a novel mechanism for achieving giant Q-factors of subwavelength nanoscale resonators by realizing the regime of bound states in the continuum. We reveal strong mode coupling and Fano resonances in high-index dielectric finite-length nanorods resulting in high-Q factors at the nanoscale. Thus, high-index dielectric resonators represent the simplest example of nanophotonic supercavities, expanding substantially the range of applications of all-dielectric resonant nanophotonics and meta-optics.Trapping of light in localized modes is extremely important for various applications in optics and photonics including lasing [1], sensing [2,3], harmonic generation [4,5], Raman scattering [6], and photovoltaics [7,8]. For many optical devices, it becomes critical to localize electromagnetic fields in small subwavelength volumes. Plasmonic structures based on metals allow subwavelength localization of light by means of surface plasmon polaritons [9]. However, metals impose inevitable losses and heating, which limit the device performance and efficiency. In contrast, dielectric nanoparticles with high refractive index offer a novel way for the subwavelength localization of light by employing the Mie resonances being limited only by the radiation damping [10]. Unlike metallic nanoscale structures, dielectric nanoparticles support both electric and magnetic Mie modes that expand substantially the applications of meta-optics [11]. Also, dielectric materials with high refractive index are available in a broad spectral range. At the same time, the standard Mie theory predicts relatively low values of the quality factor (Q ≈ 10) for nanoparticles made of conventional optical materials such as Si, Ge, and AlGaAs, both in the visible and near-infrared spectral ranges.However, for many applications of all-dielectric nanophotonics it is very desirable to achieve higher values of the Q factor. One way to enhance the Q factor is to increase the size of the resonator, for example by confining waves by cavities and defects in photonic crystals [12] or by exploiting modes with high angular momentum known as whispering gallery modes (WGM) [13]. Another way is to arrange several resonators in space and excite collective modes [14,15]. An alternative approach for enhancing the Q factors is to use the so-called anapole mode with the spectrally overlapped electric and toroidal dipole modes [16,17]. As a result, the Q factor of the anapole mode realized in a dielectric resonator may exceed 30 [18]. Here we suggest a novel approach based on bound stat...

The study of resonant dielectric nanostructures with high refractive index is a new research direction in nanoscale optics and metamaterial-inspired nanophotonics. Because of the unique opticallyinduced electric and magnetic Mie resonances, high-index nanoscale structures are expected to complement or even replace different plasmonic components in a range of potential applications. Here we study strong coupling between modes of a single subwavelength high-index dielectric resonator and analyse the mode transformation and Fano resonances when resonator's aspect ratio varies. We demonstrate that strong mode coupling results in resonances with high quality factors, which are related to the physics of bound states in the continuum when the radiative losses are almost suppressed due to the Friedrich-Wintgen scenario of destructive interference. We explain the physics of these states in terms of multipole decomposition and show that their appearance is accompanied by drastic change of the far-field radiation pattern. We reveal a fundamental link between the formation of the high-quality resonances and peculiarities of the Fano parameter in the scattering cross-section spectra. Our theoretical findings are confirmed by microwave experiments for the scattering of a high-index cylindrical resonators with a tunable aspect ratio. The proposed mechanism of the strong mode coupling in single subwavelength high-index resonators accompanied by resonances with high quality factor helps to extend substantially functionalities of all-dielectric nanophotonics that opens new horizons for active and passive nanoscale metadevices. arXiv:1805.09265v2 [physics.optics] 1 Dec 2018

Optical bound states in the continuum (BIC) are localized states with energy lying above the light line and having infinite lifetime. Any losses taking place in real systems result in transformation of the bound states into resonant states with finite lifetime. In this Letter, we analyze properties of BIC in CMOS-compatible one-dimensional photonic structure based on silicon-on-insulator wafer at telecommunication wavelengths, where the absorption of silicon is negligible. We reveal that a high-index substrate could destroy both off-Γ BIC and in-plane symmetry protected at-Γ BIC turning them into resonant states due to leakage into the diffraction channels opening in the substrate. We show how two concurrent loss mechanisms, scattering due to surface roughness and leakage into substrate, contribute to the suppression of the resonance lifetime and specify the condition when one of the mechanisms becomes dominant. The obtained results provide useful guidelines for practical implementations of structures supporting optical bound states in the continuum.

Metasurfaces based on resonant subwavelength photonic structures enable novel ways of wavefront control and light focusing, underpinning a new generation of flat-optics devices. Recently emerged all-dielectric metasurfaces exhibit high-quality resonances underpinned by the physics of bound states in the continuum that drives many interesting concepts in photonics. Here we suggest a novel approach to explain the physics of bound photonic states embedded into the radiation continuum. We study dielectric metasurfaces composed of planar periodic arrays of Mie-resonant nanoparticles ("meta-atoms") which support both symmetry protected and accidental bound states in the continuum, and employ the multipole decomposition approach to reveal the physical mechanism of the formation of such nonradiating states in terms of multipolar modes generated by isolated meta-atoms. Based on the symmetry of the vector spherical harmonics, we identify the conditions for the existence of bound states in the continuum originating from the symmetries of both the lattice and the unit cell. Using this formalism we predict that metasurfaces with strongly suppressed spatial dispersion can support the bound states in the continuum with the wavevectors forming a line in the reciprocal space. Our results provide a new way for designing high-quality resonant photonic systems based on the physics of bound states in the continuum.

Being motivated by recent achievements in the rapidly developing fields of optical bound states in the continuum (BICs) and excitons in monolayers of transition metal dichalcogenides, we analyze strong coupling between BICs in Ta2O5 periodic photonic structures and excitons in WSe2 monolayers. We demonstrate that giant radiative lifetime of BICs allow to engineer the exciton-polariton lifetime enhancing it three orders of magnitude compared to a bare exciton. We show that maximal lifetime of hybrid light-matter state can be achieved at any point of k-space by shaping the geometry of the photonic structure. Our findings open new route for the realization of the moving exciton-polariton condensates with non-resonant pump and without the Bragg mirrors which is of paramount importance for polaritonic devices.Monolayers of transition metal dichalcogenides (TMDCs) are a certain class of post-graphene two-dimensional materials [1], attracting vast research interest in recent years. TMDC are direct-gap semiconductors, exhibiting strong light-matter coupling [2]. Moreover, these structures support excitons characterized by both large binding energies and sufficiently large Bohr radii [3]. While the former leads to the existence of strong excitonic response at room temperature, the latter provides strong optical nonlinearity due to the exciton-exciton interactions [4]. Another important property of the TMDC excitons is the large oscillator strength leading to the substantial exciton-photon interaction in these structures. These properties allow the observation of the so-called strong coupling regime, leading to the emergence of the new quasiparticles, exciton-polaritons [5] at room temperatures in structures comprising TMDC monolayer and an optical cavity.Excitons-polaritons have been extensively studied in last two decades both due to their fascinating fundamental properties, such as high-temperature Bose condensation and superfluidity [6,7] as well as well as emerging applications such as virtually thresholdless polariton lasers [8] and energy effective all optical logic gates [9]. Strong coupling of TMDC excitons to light has been observed in the structures resembling the conventional microcavities, where the monolayer was sandwiched between two Bragg mirrors [10][11][12][13]. At the same time, since fabrication of high quality TMDC monolayers is based on the mechanical exfoliation techniques, and thus not compatible with the standard epitaxial techniques used for the Bragg mirror fabrication, the realization of the structures considered in [10-13] is quite technologically demanding. It would be thus extremely useful to realize high quality optical resonances without the requirement for the growth of the upper mirror. As an alternative, a whispering gallery mode (WGM) in a disc resonator could be used for the realization of the strong coupling [14]. However, this approach also requires sophisticated fabrication techniques and the precise positioning of the monolayer over the region of the disc resonator where the WGM mo...