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Strong coupling between resonant systems is one of the remarkable phenomena in different areas of physics such as condensed matter physics, quantum optics, nanophotonics, and plasmonics. However, the strong coupling regime is often hardly achievable because it requires special experimental conditions, which ensure an increase in the coupling strength and the reduction of losses in interacting systems. A transition from the weak coupling regime to the strong coupling regime is demonstrated in this work in the system of coupled Fabry–Pérot acoustic resonators, where the coupling strength is larger than the characteristic losses in resonators by a factor of 3.5. The strong coupling regime is demonstrated in both the spectral and time domains. The results are important for the development of modern acoustic systems implementing, in particular, the slow sound regime or regimes serving for the high-Q-factor frequency filtering and highly sensitive acoustic detection. [ABSTRACT FROM AUTHOR]

We present a comprehensive experimental and theoretical study of visible light diffraction from opal- based photonic structures of different dimensions. We also developed new methodological approaches to collection, processing, and interpretation of experimental diffraction data. The key idea is to analyze the light diffraction data represented in the "incident angle-registration angle" coordinates instead of the widely used "incident angle-registration intensity" coordinates. In such coordinates, a 2D diffraction pattern consists of ovals, while a 3D Bragg diffraction pattern consists of parallel stripes owing to the specular reflection law. Therefore this representation allows one to distinguish easily the patterns originating from 2D diffraction from the ones governed by 3D diffraction. In addition, different types of structural disorder including twinning of the face-centered cubic structure become apparent in the angle coordinates. [ABSTRACT FROM AUTHOR]

Surface waves such as plasmon, exciton, or phonon polaritons as well as guided modes supported by various planar structures attract particular attention due to their capability of capturing and carrying the optical signals in two- and one-dimensional systems. An important characteristic of such waves is their dispersion , which can be designed on demand by nanostructuring of planar layered systems to obtain photonic crystal slabs and metasurfaces. Engineering the dispersion of surface waves gives rise to such exciting optical effects as transition from positive to negative refraction or from elliptic to hyperbolic dispersion, self-collimation, and diffraction-free propagation of surface waves. Along with the real part of the wave vector responsible for the phase of a propagating surface wave, a crucially important parameter for the applications is its propagation length that is related to the intrinsic losses of the mode. We propose an experimental approach allowing for the extraction of the full complex dispersion of optical surface waves and guided modes. The method relies on angle-resolved attenuated total internal reflection spectroscopy. In such a setup, scanning the air gap between the sample and a solid immersion lens yields controllable tuning of the coupling between the surface waves and the free space. In the experiment, we identify the regime of critical coupling of light to waveguide modes of a planar silicon slab and use it to extract their intrinsic losses and thus the propagation lengths in a broad spectral range. Our approach is a powerful tool for studies of light and hybrid light-matter surface waves of various kinds and may find its applications in the development of on-chip optoelectronic and nanophotonic devices. [ABSTRACT FROM AUTHOR]

Efficient second harmonic generation (SHG) in nanophotonic designs based on all-dielectric nanostructures demands materials with large values of the quadratic nonlinear susceptibility, low dissipative losses, and high refractive index. One of the best materials meeting all these parameters is gallium phosphide (GaP). However, second-order nonlinearity requires high crystallinity and morphology quality of the GaP layer grown for further lithographic processing. Here we develop a method to fabricate high-quality crystalline GaP metasurfaces, which demonstrate pronounced linear and nonlinear optical properties. Direct growth of a GaP layer on a sapphire substrate tackles the previous problem of wafer bonding, because of high optical contrast between fabricated resonant nanoparticles and the substrate. As a result, the fabricated GaP metasurface supports bound state in continuum mode with an experimental quality factor around 100 yielding a strong enhancement of SHG in narrow spectral range. We believe that the developed approach will become a versatile platform for nonlinear all-dielectric nanophotonics. [ABSTRACT FROM AUTHOR]

Herein, I(V) characteristics of the tunnel junction between the scanning tunneling microscopy (STM) Pt/Ir probe and atomically flat Au film on mica using ultrahigh vacuum STM is investigated. To ensure cleanness and flatness of the Au films, optimization of the substrate annealing and Ar plasma treatment are performed. The obtained technological parameters allow to drastically improve the reproducibility of I(V) measurements. The analysis of I(V), d2I/dV2 (V), and Fowler–Nordheim plots is conducted, and the presence of the features in the bias region near 1.8 V in the form of peak and minimum, peak and anomalous extra minimum, respectively is demonstrated. The direct optical measurements confirm that the features on I(V) curves are associated with the generation of photons from the STM probe‐sample gap, governed by inelastic tunneling processes. The proposed I(V) analysis approach is used for indirect sensing and investigation of the light emission in the tunnel junction offering a powerful tool for the studies of the photonic sources with deeply subwavelength dimensions. [ABSTRACT FROM AUTHOR]

Achieving efficient localization of white light at the nanoscale is a major challenge due to the diffraction limit, and nanoscale emitters generating light with a broadband spectrum require complicated engineering. Here we suggest a simple, yet highly efficient, nanoscale white-light source based on a hybrid Si/Au nanoparticle with ultrabroadband (1.3-3.4 eV) spectral characteristics. We incorporate this novel source into a scanning-probe microscope and observe broadband spectrum of photoluminescence that allows fast mapping of local optical response of advanced nanophotonic structures with submicron resolution, thus realizing ultrabroadband near-field nanospectroscopy. [ABSTRACT FROM AUTHOR]

An experimental method has been proposed to study the dispersion properties of optical surface and waveguide modes in planar structures. An experimental setup involves a microscope with a high numerical aperture objective and a hemispherical solid immersion lens made of zinc selenide in contact with the sample surface. The reflection from the sample is detected in the back focal plane of the system. Such a configuration makes it possible to study strongly localized states with an effective refractive index up to 2.25 in the visible and near infrared spectral ranges. For a thin silicon layer deposited on a glass substrate, the possibility of visualization of isofrequency contrours with polarization resolution and the reconstruction of dispersion of waveguide modes depending on the direction of their propagation has been demonstrated. [ABSTRACT FROM AUTHOR]

Surface electromagnetic waves are characterized by the intrinsic spin-orbit interaction which results in the fascinating spin-momentum locking. Therefore, directional coupling of light to surface waves can be achieved through chiral nanoantennas. Here, we show that dielectric nanoantenna provides chiral response with strong spectral dependence due to the interference of electric and magnetic dipole momenta when placed in the vicinity of the metal-air interface. Remarkably, chiral behaviour in the proposed scheme does not require elliptical polarization of the pump beam or the geometric chirality of the nanoantenna. We show that the proposed ultracompact and simple dielectric nanoantenna allows for both directional launching of surface plasmon polaritons on a thin gold film and their demultiplexing with a high spectral resolution. [ABSTRACT FROM AUTHOR]

Photonic structures offer unique opportunities for controlling light-matter interaction, including the photonic spin Hall effect associated with the transverse spin-dependent displacement of a light beam that propagates in specially designed optical media. However, due to small spin-orbit coupling, the photonic spin Hall effect is usually weak at the nanoscale. Here we suggest theoretically and demonstrate experimentally, in both optics and microwave experiments, the photonic spin Hall effect enhanced by topologically protected edge states in subwavelength arrays of resonant dielectric particles. Based on direct near-field measurements, we observe the selective excitation of the topological edge states controlled by the handedness of the incident light. Additionally, we reveal the main requirements to the symmetry of photonic structures to achieve the topology-enhanced spin Hall effect, and also analyse the robustness of the photonic edge states against the long-range coupling. [ABSTRACT FROM AUTHOR]

A broadband thin-film antireflective coating (ARC)--a regular array of nanopores in a dielectric substrate--is studied theoretically and experimentally. Tuning the geometrical parameters of the array allows for strong suppression of reflection. For a fused silica substrate, reflectivity is lower than 1% over a range with a relative bandwidth, 60%-70%, reaching 0.05% in the minimum. The underlying physics is the spatial dispersion in the porous medium which enables phase compensation for the partially reflected waves in a broad band. This allows for more broadband antireflection than any flat homogeneous single-layer ARC can provide. Importantly, the studied ARC is universal for any transparent dielectric substrate. [ABSTRACT FROM AUTHOR]