Complete photonic bandgap in a low-index two-dimensional quasicrystalline structure.

A bandgap in the continuum spectrum of photons in addition to its basic physical significance has strong potential for applications. Analogous to semiconductor crystals for electrons, periodic dielectric structures named photonic crystals were proposed to control photon flux propagation. In our search for low refractive index (RI) structures with a photonic bandgap, initial research efforts were focused on photonic crystal design, while aperiodic structures allow lower values of refractive index contrast to sustain a photonic bandgap. Here, we report on a two-dimensional quasicrystalline structure designed as a set of one-dimensional lattices merged into a single binary structure made of two materials with refractive index contrast 2|n1 - n2|/(n1 + n2) = 0.16 and even less in theory. We confirmed the theoretical prediction of bandgap exciting by measuring the radiation suppression of a dipole source placed in the center of the quasicrystalline structure. The full-wave numerical simulations and the experimental study appear to be in good agreement with the theoretical model.

Multifunctional and Transformative Metaphotonics with Emerging Materials.

Future technologies underpinning multifunctional physical and chemical systems and compact biological sensors will rely on densely packed transformative and tunable circuitry employing nanophotonics. For many years, plasmonics was considered as the only available platform for subwavelength optics, but the recently emerged field of resonant metaphotonics may provide a versatile practical platform for nanoscale science by employing resonances in high-index dielectric nanoparticles and metasurfaces. Here, we discuss the recently emerged field of metaphotonics and describe its connection to material science and chemistry. For tunabilty, metaphotonics employs a variety of the recently highlighted materials such as polymers, perovskites, transition metal dichalcogenides, and phase change materials. This allows to achieve diverse functionalities of metasystems and metasurfaces for efficient spatial and temporal control of light by employing multipolar resonances and the physics of bound states in the continuum. We anticipate expanding applications of these concepts in nanolasers, tunable metadevices, metachemistry, as well as a design of a new generation of chemical and biological ultracompact sensing devices.

Antenna-Based Approach to Fine Control of Supercavity Mode Quality Factor in Metasurfaces.

The utilization of photonic bound states in the continuum (BIC) is a very attractive approach for many applications requiring efficient resonators. High-Q modes related to symmetry-protected BIC are formed due to perturbation defined by an asymmetry parameter, and the smaller this parameter is, the bigger the Q factor can be achieved. Inevitable fabrication imperfectness limits precise control of the Q factor through the asymmetry parameter. Here we propose an antenna-based design of metasurfaces for accurate tailoring of the Q factor where stronger perturbation leads to the same effect in the conventional design. This approach allows the fabrication of samples with equipment having lower tolerance keeping the Q factor at the same level. Furthermore, our findings reveal two regimes of the Q factor scaling law with saturated and unsaturated resonances dependent on the ratio of antenna particles to all particles. The boundary is defined by the efficient scattering cross section of the metasurface constituent particles.

Resonant silicon waveguide with strong transverse electric field rotation for magnetic-field-induced non-reciprocity.

Non-reciprocal devices suitable for on-chip implementation are of high importance in modern photonics. In most cases, non-reciprocity is caused by the magneto-optical effect. At the same time, the external magnetic field is convenient to apply in the normal direction to the chip plane, which leads to Voigt geometry and modes with in-plane transverse rotations of the electric field. Here, we propose two resonant magnetic material-free silicon waveguides supporting such modes. The first one is a rectangular waveguide with air holes inside, whose modes have higher rotations at the telecom wavelength, but the silicon thickness is non-standard. The second one is based on a 220-nm-thick silicon waveguide compatible with commercially available silicon-on-insulator wafers. We also propose the scheme of an optical isolator based on a Mach-Zehnder interferometer with a rotation elements delay in its arms.

Lasing Action from Anapole Metasurfaces.

We study active dielectric metasurfaces composed of two-dimensional arrays of split-nanodisk resonators fabricated in InGaAsP membranes with embedded quantum wells. Depending on the geometric parameters, such split-nanodisk resonators can operate in the optical anapole regime originating from an overlap of the electric dipole and toroidal dipole Mie-resonant optical modes, thus supporting strongly localized fields and high-Q resonances. We demonstrate room-temperature lasing from the anapole lattices of engineered active metasurfaces with low threshold and high coherence.

Optical coupling of overlapping nanopillars.

Engineering of nanophotonic devices for controlling light requires deep understanding of the interaction between their subwavelength structure elements. Theoretical approaches based on the multiple scattering theory provide simple analytics valuable for design. However, they consider different elements separated by the surrounding medium. Here, we develop an approach to study wave coupling in the case of overlapping particles. We consider the simplest system-a dimer of nanopillars-and find that it can be described by a three-oscillator model. Two modes correspond to the multipole response of isolated particles that interact through radiating and evanescent waves in accordance with the conventional multiple scattering theory, but there exists a third effective non-resonant oscillator supporting a direct mode coupling via the intersecting part. Our simple model yields results with a reliable agreement with numerical simulations and allows insight into the physical processes underlying the collective response of a cluster of overlapped subwavelength particles.

All-Dielectric Nanostructures with a Thermoresponsible Dynamic Polymer Shell.

We suggest a new strategy for creating stimuli-responsive bio-integrated optical nanostructures based on Mie-resonant silicon nanoparticles covered by an ensemble of similarity negatively charged polyelectrolytes (heparin and sodium polystyrene sulfonate). The dynamic tuning of the nanostructures' optical response is due to light-induced heating of the nanoparticles and swelling of the polyelectrolyte shell. The resulting hydrophilic/hydrophobic transitions significantly change the shell thickness and reversible shift of the scattering spectra for individual nanoparticles up to 60 nm. Our findings bring novel opportunities for the application of smart nanomaterials in nanomedicine and bio-integrated nanophotonics.

Disorder-Immune Photonics Based on Mie-Resonant Dielectric Metamaterials.

When the feature size of photonic structures becomes comparable or even smaller than the wavelength of light, the fabrication imperfections inevitably introduce disorder that may eliminate many functionalities of subwavelength photonic devices. Here we suggest a novel concept to achieve a robust band gap which can endure disorder beyond 30% as a result of the transition from photonic crystals to Mie-resonant metamaterials. By utilizing Mie-resonant metamaterials with high refractive index, we demonstrate photonic waveguides and cavities with strong robustness to position disorder, thus providing a novel approach to the band-gap-based nanophotonic devices with new properties and functionalities.

Dielectric metamaterials with electric response.

Dielectric metamaterials are usually studied as low-loss media with magnetic response. However, control over the electric response is more promising for applications in photonics. Here we report an all-dielectric metamaterial with electric response. The structure consists of high-index dielectric rods arranged in a square lattice. We present a phase diagram that includes regions of metamaterials with magnetic and electric response. A metamaterial behavior is demonstrated for homogeneous ϵ-near zero modes, which are observed regardless of a lattice orientation and a structure boundary. The ϵ-near zero modes make it possible to enhance electric field intensity by two orders of magnitude, which can be used for applications exploiting light-matter interactions.

High-Q Supercavity Modes in Subwavelength Dielectric Resonators.

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 a 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. In contrast to the previously suggested multilayer structures with zero permittivity, we reveal strong mode coupling and Fano resonances in homogeneous 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.

FeAu mixing for high-temperature control of light scattering at the nanometer scale.

Control of the optical properties of a nanoparticle (NP) through its structural changes underlies optical data processing, dynamic coloring, and smart sensing at the nanometer scale. Here, we report on the concept of controlling the light scattering by a NP through mixing of weakly miscible chemical elements (Fe and Au), supporting a thermal-induced phase transformation. The transformation corresponds to the transition from a homogeneous metastable solid solution phase of the (Fe,Au) NP towards an equilibrium biphasic Janus-type NP. We demonstrate that the phase transformation is thermally activated by laser heating up to a threshold of 800 °C (for NPs with a size of hundreds of nm), leading to the associated changes in the light scattering and color of the NP. The results thereby pave the way for the implementation of optical sensors triggered by a high temperature at the nanometer scale via NPs based on metal alloys.

Mie scattering as a cascade of Fano resonances.

We reveal that the resonant Mie scattering by high-index dielectric nanoparticles can be presented through cascades of Fano resonances. We employ the exact solution of Maxwell's equations and demonstrate that the Lorenz-Mie coefficients of the Mie problem can be expressed generically as infinite series of Fano functions as they describe interference between the background radiation originated from an incident wave and narrow-spectrum Mie scattering modes that lead to Fano resonances.

Light-Induced Color Switching of Single Metal-Organic Framework Nanocrystals.

Photoinduced modulation of the optical parameters of nanomaterials underlies the operating principles of all-optical nanodevices. Here, we demonstrate the laser-induced 10% modulation of the refractive index and 16-fold modulation of the extinction coefficient of the dynamic metal-organic framework (HKUST-1) nanocrystals within the whole visible range. Using the laser-induced water sorption/desorption process inside HKUST-1, we have achieved size-dependent reversible tuning of brightness and color of its nanocrystals over the different spatial directions and color palette. The numerical analysis also confirmed the detected optical tuning through the evolution of optical spectra and directivity of the scattered light. The results of the work demonstrate the promising nature of the dynamic metal-organic frameworks for nonlinear optics and expand the library of chemically synthesized hybrid materials with light-controlled optical properties.

Special scattering regimes for conical all-dielectric nanoparticles.

All-dielectric nanophotonics opens a venue for a variety of novel phenomena and scattering regimes driven by unique optical effects in semiconductor and dielectric nanoresonators. Their peculiar optical signatures enabled by simultaneous electric and magnetic responses in the visible range pave a way for a plenty of new applications in nano-optics, biology, sensing, etc. In this work, we investigate fabrication-friendly truncated cone resonators and achieve several important scattering regimes due to the inherent property of cones-broken symmetry along the main axis without involving complex geometries or structured beams. We show this symmetry breaking to deliver various kinds of Kerker effects (generalized and transverse Kerker effects), non-scattering hybrid anapole regime (simultaneous anapole conditions for all the multipoles in a particle leading to the nearly full scattering suppression) and, vice versa, superscattering regime. Being governed by the same straightforward geometrical paradigm, discussed effects could greatly simplify the manufacturing process of photonic devices with different functionalities. Moreover, the additional degrees of freedom driven by the conicity open new horizons to tailor light-matter interactions at the nanoscale.

All-Dielectric Active Terahertz Photonics Driven by Bound States in the Continuum.

The remarkable emergence of all-dielectric meta-photonics governed by the physics of high-index dielectric materials offers a low-loss platform for efficient manipulation and subwavelength control of electromagnetic waves from microwaves to visible frequencies. Dielectric metasurfaces can focus electromagnetic waves, generate structured beams and vortices, enhance local fields for advanced sensing, and provide novel functionalities for classical and quantum technologies. Recent advances in meta-photonics are associated with the exploration of exotic electromagnetic modes called the bound states in the continuum (BICs), which offer a simple interference mechanism to achieve large quality factors (Q) through excitation of supercavity modes in dielectric nanostructures and resonant metasurfaces. Here, a BIC-driven terahertz metasurface with dynamic control of high-Q silicon supercavities that are reconfigurable at a nanosecond timescale is experimentally demonstrated. It is revealed that such supercavities enable low-power, optically induced terahertz switching and modulation of sharp resonances for potential applications in lasing, mode multiplexing, and biosensing.

Purcell effect and Lamb shift as interference phenomena.

The Purcell effect and Lamb shift are two well-known physical phenomena which are usually discussed in the context of quantum electrodynamics, with the zero-point vibrations as a driving force of those effects in the quantum approach. Here we discuss the classical counterparts of these quantum effects in photonics, and explain their physics trough interference wave phenomena. As an example, we consider a waveguide in a planar photonic crystal with a side-coupled defect, and demonstrate a perfect agreement between the results obtained on the basis of quantum and classic approaches and reveal their link to the Fano resonance. We find that in such a waveguide-cavity geometry the Purcell effect can modify the lifetime by at least 25 times, and the Lamb shift can exceed 3 half-widths of the cavity spectral line.

Transition from two-dimensional photonic crystals to dielectric metasurfaces in the optical diffraction with a fine structure.

We study experimentally a fine structure of the optical Laue diffraction from two-dimensional periodic photonic lattices. The periodic photonic lattices with the C4v square symmetry, orthogonal C2v symmetry, and hexagonal C6v symmetry are composed of submicron dielectric elements fabricated by the direct laser writing technique. We observe surprisingly strong optical diffraction from a finite number of elements that provides an excellent tool to determine not only the symmetry but also exact number of particles in the finite-length structure and the sample shape. Using different samples with orthogonal C2v symmetry and varying the lattice spacing, we observe experimentally a transition between the regime of multi-order diffraction, being typical for photonic crystals to the regime where only the zero-order diffraction can be observed, being is a clear fingerprint of dielectric metasurfaces characterized by effective parameters.

Switching from visibility to invisibility via Fano resonances: theory and experiment.

Subwavelength structures demonstrate many unusual optical properties which can be employed for engineering of a new generation of functional metadevices, as well as controlled scattering of light and invisibility cloaking. Here we demonstrate that the suppression of light scattering for any direction of observation can be achieved for a uniform dielectric object with high refractive index, in a sharp contrast to the cloaking with multilayered plasmonic structures suggested previously. Our finding is based on the novel physics of cascades of Fano resonances observed in the Mie scattering from a homogeneous dielectric rod. We observe this effect experimentally at microwaves by employing high temperature-dependent dielectric permittivity of a glass cylinder with heated water. Our results open a new avenue in analyzing the optical response of high-index dielectric nanoparticles and the physics of cloaking.

Phase diagram for the transition from photonic crystals to dielectric metamaterials.

Photonic crystals and dielectric metamaterials represent two different classes of artificial media but are often composed of similar structural elements. The question is how to distinguish these two types of periodic structures when their parameters, such as permittivity and lattice constant, vary continuously. Here we discuss transition between photonic crystals and dielectric metamaterials and introduce the concept of a phase diagram, based on the physics of Mie and Bragg resonances. We show that a periodic photonic structure transforms into a metamaterial when the Mie gap opens up below the lowest Bragg bandgap where the homogenization approach can be justified and the effective permeability becomes negative. Our theoretical approach is confirmed by microwave experiments for a metacrystal composed of tubes filled with heated water. This analysis yields deep insight into the properties of periodic structures, and provides a useful tool for designing different classes of electromagnetic materials with variable parameters.

Fano interference governs wave transport in disordered systems.

Light localization in disordered systems and Bragg scattering in regular periodic structures are considered traditionally as two entirely opposite phenomena: disorder leads to degradation of coherent Bragg scattering whereas Anderson localization is suppressed by periodicity. Here we reveal a non-trivial link between these two phenomena, through the Fano interference between Bragg scattering and disorder-induced scattering, that triggers both localization and de-localization in random systems. We find unexpected transmission enhancement and spectrum inversion when the Bragg stop-bands are transformed into the Bragg pass-bands solely owing to disorder. Fano resonances are always associated with coherent scattering in regular systems, but our discovery of disorder-induced Fano resonances may provide novel insights into many features of the transport phenomena of photons, phonons, and electrons. Owning to ergodicity, the Fano resonance is a fingerprint feature for any realization of the structure with a certain degree of disorder.