Active quasi-BIC metasurfaces assisted by epsilon-near-zero materials.

Active devices play a critical role in modern electromagnetic and photonics systems. To date, the epsilon (ε)-near-zero (ENZ) is usually integrated with the low Q-factor resonant metasurface to achieve active devices, and enhance the light-matter interaction significantly at the nanoscale. However, the low Q-factor resonance may limit the optical modulation. Less work has been focused on the optical modulation in the low-loss and high Q-factor metasurfaces. Recently, the emerging optical bound states in the continuum (BICs) provides an effective way for achieving high Q-factor resonators. In this work, we numerically demonstrate a tunable quasi-BICs (QBICs) by integrating a silicon metasurface with ENZ ITO thin film. Such a metasurface is composed of five square holes in a unit cell, and hosts multiple BICs by engineering the position of centre hole. We also reveal the nature of these QBICs by performing multipole decomposition and calculating near field distribution. Thanks to the large tunability of ITO's permittivity by external bias and high-Q factor enabled by QBICs, we demonstrate an active control on the resonant peak position and intensity of transmission spectrum by integrating ENZ ITO thin films with QBICs supported by silicon metasurfaces. We find that all QBICs show excellent performance on modulating the optical response of such a hybrid structure. The modulation depth can be up to 14.8 dB. We also investigate how the carrier density of ITO film influence the near-field trapping and far-field scattering, which in turn influence the performance of optical modulation based on this structure. Our results may find promising applications in developing active high-performance optical devices.

Enhanced light-matter interaction in two-dimensional transition metal dichalcogenides.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS2, WS2, MoSe2, and WSe2, have received extensive attention in the past decade due to their extraordinary electronic, optical and thermal properties. They evolve from indirect bandgap semiconductors to direct bandgap semiconductors while their layer number is reduced from a few layers to a monolayer limit. Consequently, there is strong photoluminescence in a monolayer (1L) TMDC due to the large quantum yield. Moreover, such monolayer semiconductors have two other exciting properties: large binding energy of excitons and valley polarization. These properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak light-matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research.

On-chip low-loss all-optical MoSe2 modulator.

Monolayer transition metal dichalcogenides (TMDCs), like MoS2, MoSe2, WS2, and WSe2, feature direct bandgaps, strong spin-orbit coupling, and exciton-polariton interactions at the atomic scale, which could be harnessed for efficient light emission, valleytronics, and polaritonic lasing, respectively. Nevertheless, to build next-generation photonic devices that make use of these features, it is first essential to model the all-optical control mechanisms in TMDCs. Herein, a simple model is proposed to quantify the performance of a 35-µm-long Si3N4 waveguide-integrated all-optical MoSe2 modulator. Using this model, a switching energy of 14.6 pJ is obtained for a transverse-magnetic (TM) and transverse-electric (TE) polarized pump signals at λ = 480 nm. Moreover, maximal extinction ratios of 20.6 dB and 20.1 dB are achieved for a TM and TE polarized probe signal, respectively, at λ = 500 nm with an ultra-low insertion loss of <0.3 dB. Moreover, the device operates with an ultrafast recovery time of 50 ps, while maintaining a high extinction ratio for practical applications. These findings facilitate modeling and designing novel TMDC-based photonic devices.

Infrared all-dielectric Kerker metasurfaces.

The unidirectional scattering of electromagnetic waves in the backward and forward direction, termed Kerkers' first and second conditions, respectively, is a prominent feature of sub-wavelength particles, which also has been found recently in all-dielectric metasurfaces. Here we formulate the exact polarizability requirements necessary to achieve both Kerker conditions simultaneously with dipole terms only and demonstrate its equivalence to so-called "invisible metasurfaces". We further describe the perfect absorption mechanism in all-dielectric metasurfaces through development of an extended Kerker formalism. The phenomena of both invisibility and perfect absorption is shown in a 2D hexagonal array of cylindrical resonators, where only the resonator height is modified to switch between the two states. The developed framework provides critical insight into the range of scattering response possible with all-dielectric metasurfaces, providing a methodology for studying exotic electromagnetic phenomena.

Tailoring Second-Harmonic Emission from (111)-GaAs Nanoantennas.

Second-harmonic generation (SHG) in resonant dielectric Mie-scattering nanoparticles has been hailed as a powerful platform for nonlinear light sources. While bulk-SHG is suppressed in elemental semiconductors, for example, silicon and germanium due to their centrosymmetry, the group of zincblende III-V compound semiconductors, especially (100)-grown AlGaAs and GaAs, have recently been presented as promising alternatives. However, major obstacles to push the technology toward practical applications are the limited control over directionality of the SH emission and especially zero forward/backward radiation, resulting from the peculiar nature of the second-order nonlinear susceptibility of this otherwise highly promising group of semiconductors. Furthermore, the generated SH signal for (100)-GaAs nanoparticles depends strongly on the polarization of the pump. In this work, we provide both theoretically and experimentally a solution to these problems by presenting the first SHG nanoantennas made from (111)-GaAs embedded in a low index material. These nanoantennas show superior forward directionality compared to their (100)-counterparts. Most importantly, based on the special symmetry of the crystalline structure, it is possible to manipulate the SHG radiation pattern of the nanoantennas by changing the pump polarization without affecting the linear properties and the total nonlinear conversion efficiency, hence paving the way for efficient and flexible nonlinear beam-shaping devices.

Reversible Image Contrast Manipulation with Thermally Tunable Dielectric Metasurfaces.

Increasing demand for higher resolution of miniaturized displays requires techniques achieving high contrast tunability of the images. Employing metasurfaces for image contrast manipulation is a new and rapidly growing field of research aiming to address this need. Here, a new technique to achieve image tuning in a reversible fashion is demonstrated by dielectric metasurfaces composed of subwavelength resonators. It is demonstrated that by controlling the temperature of a metasurface the encoded transmission pattern can be tuned. To this end, two sets of nanoresonators composed of nonconcentric silicon disks with a hole that exhibit spectrally sharp Fano resonances and forming a Yin-Yang pattern are designed and fabricated. Through exploitation of the thermo-optical properties of silicon, full control of the contrast of the Yin-Yang image is demonstrated by altering the metasurface temperature by ΔT ≈ 100 °C. This is the first demonstrated technique to control an image contrast by temperature. Importantly, the turning technique does not require manipulating the external stimulus, such as polarization or angle of the illumination and/or the refractive index of this environment. These results open many opportunities for transparent displays, optical switches, and tunable illumination systems.

Linear control of light scattering with multiple coherent waves excitation.

With the wave interferometric approach, we study how extrinsically coherent waves excitation can dramatically alter the overall scattering properties, resulting in tailoring the energy assignment between radiation and dissipation, as well as filtering multipolar resonances. As an illustration, we consider cylindrical passive systems encountered by arbitrary configurations of incident waves with various illuminating directions, phases, and intensities. With formulas for dissipation and radiation powers, we demonstrate that a coherent superposition of incident waves extrinsically interferes with the targeted channels in a desirable way. Moreover, the interferometric results can be irrespective of inherent system properties such as size, material, and structure. Our approach paves a non-invasive solution to manipulate wave-obstacle interaction at will.

Simultaneously nearly zero forward and nearly zero backward scattering objects.

With theoretical analyses and numerical calculations, we show that a passive scatterer at the sub-wavelength scale can simultaneously exhibit both nearly zero forward scattering (NZFS) and nearly zero backward scattering (NZBS). It is related to the interference of dipolar quadrupole modes of different origin, leading to coexistence of Kerker's first and second conditions at the same time. For optical frequencies, we propose two different sets of composited materials in multi-layered nano-structures, i.e., CdTe/Si/TiO2 and TiO2/Au/Si, for the experimental realization.

Highly sensitive selectively coated photonic crystal fiber-based plasmonic sensor.

Highly sensitive and miniaturized sensors are highly desirable for real-time analyte/sample detection. In this Letter, we propose a highly sensitive plasmonic sensing scheme with the miniaturized photonic crystal fiber (PCF) attributes. A large cavity is introduced in the first ring of the PCFs for the efficient field excitation of the surface plasmon polariton mode and proficient infiltration of the sensing elements. Due to the irregular air-hole diameter in the first ring, the cavity exhibits the birefringence behavior which enhances the sensing performance. The novel plasmonic material gold has been used considering the chemical stability in an aqueous environment. The guiding properties and the effects of the sensing performance with different parameters have been investigated by the finite element method, and the proposed PCFs have been fabricated using the stack-and-draw fiber drawing method. The proposed sensor performance was investigated based on the wavelength and amplitude sensing techniques and shows the maximum sensitivities of 11,000 nm/RIU and 1,420 RIU-1, respectively. It also shows the maximum sensor resolutions of 9.1×10-6 and 7×10-6 RIU for the wavelength and amplitude sensing schemes, respectively, and the maximum figure of merits of 407. Furthermore, the proposed sensor is able to detect the analyte refractive indices in the range of 1.33-1.42; as a result, it will find the possible applications in the medical diagnostics, biomolecules, organic chemical, and chemical analyte detection.

Ultimate Absorption in Light Scattering by a Finite Obstacle.

Based on fundamental properties of the light scattering by a particle under a plane, linearly polarized wave illumination, we rigorously prove the existence of the ultimate upper limit for the light absorption by any partial mode and calculate this limit explicitly. The limit is a certain simple universal function of the incident light wave number, and the multipolarity of the corresponding partial mode solely. It does not depend on the optical constants of the scatterer, its size, or even its shape. First, we obtain this result for the scattering by a spherical particle. Then, we generalize it to an arbitrary finite obstacle. The results are valid for any polarization of the incident wave, any angle of its incidence, and any type of the scatterer (homogeneous, stratified, or with smoothly variable refractive index). We also prove that the maximal partial absorption cross section for any finite scatterer cannot exceed the corresponding value for a homogeneous sphere in 3D and circular cylinder in 2D. As an example, the results are applied to maximize the absorption cross section of a spherical core-shell structure.

Ideal Magnetic Dipole Scattering.

We introduce the concept of tunable ideal magnetic dipole scattering, where a nonmagnetic nanoparticle scatters light as a pure magnetic dipole. High refractive index subwavelength nanoparticles usually support both electric and magnetic dipole responses. Thus, to achieve ideal magnetic dipole scattering one has to suppress the electric dipole response. Such a possibility was recently demonstrated for the so-called anapole mode, which is associated with zero electric dipole scattering. By spectrally overlapping the magnetic dipole resonance with the anapole mode, we achieve ideal magnetic dipole scattering in the far field with tunable strong scattering resonances in the near infrared spectrum. We demonstrate that such a condition can be realized at least for two subwavelength geometries. One of them is a core-shell nanosphere consisting of a Au core and silicon shell. It can be also achieved in other geometries, including nanodisks, which are compatible with current nanofabrication technology.

Multifold Enhancement of Third-Harmonic Generation in Dielectric Nanoparticles Driven by Magnetic Fano Resonances.

Strong Mie-type magnetic dipole resonances in all-dielectric nanostructures provide novel opportunities for enhancing nonlinear effects at the nanoscale due to the intense electric and magnetic fields trapped within the individual nanoparticles. Here we study third-harmonic generation from quadrumers of silicon nanodisks supporting high-quality collective modes associated with the magnetic Fano resonance. We observe nontrivial wavelength and angular dependencies of the generated harmonic signal featuring a multifold enhancement of the nonlinear response in oligomeric systems.

Q-factor and absorption enhancement for plasmonic anisotropic nanoparticles.

We investigate the scattering and absorption properties of anisotropic metal-dielectric core-shell nanoparticles. It is revealed that the radially anisotropic dielectric layer can accelerate the evanescent decay of the localized resonant surface modes, leading to Q-factor and absorption rate enhancement. Moreover, the absorption cross section can be maximized to reach the single resonance absorption limit. We further show that such artificial anisotropic cladding materials can be realized by isotropic layered structures, which may inspire many applications based on scattering and absorption of plasmonic nanoparticles.

Ultrafast All-Optical Switching with Magnetic Resonances in Nonlinear Dielectric Nanostructures.

We demonstrate experimentally ultrafast all-optical switching in subwavelength nonlinear dielectric nanostructures exhibiting localized magnetic Mie resonances. We employ amorphous silicon nanodisks to achieve strong self-modulation of femtosecond pulses with a depth of 60% at picojoule-per-disk pump energies. In the pump-probe measurements, we reveal that switching in the nanodisks can be governed by pulse-limited 65 fs-long two-photon absorption being enhanced by a factor of 80 with respect to the unstructured silicon film. We also show that undesirable free-carrier effects can be suppressed by a proper spectral positioning of the magnetic resonance, making such a structure the fastest all-optical switch operating at the nanoscale.

Efficient excitation and tuning of toroidal dipoles within individual homogenous nanoparticles.

We revisit the fundamental topic of light scattering by single homogenous nanoparticles from the new perspective of excitation and manipulation of toroidal dipoles. It is revealed that besides within all-dielectric particles, toroidal dipoles can also be efficiently excited within homogenous metallic nanoparticles. Moreover, we show that those toroidal dipoles excited can be spectrally tuned through adjusting the radial anisotropy parameters of the materials, which paves the way for further more flexible manipulations of the toroidal responses within photonic systems. The study into toroidal multipole excitation and tuning within nanoparticles deepens our understanding of the seminal problem of light scattering, and may incubate many scattering related fundamental researches and applications.

Invisible nanowires with interfering electric and toroidal dipoles.

By studying the scattering of normally incident plane waves by a single nanowire, we reveal the indispensable role of toroidal multipole excitation in multipole expansions of radiating sources. It is found that for both p-polarized and s-polarized incident waves, toroidal dipoles can be effectively excited within homogenous dielectric nanowires in the optical spectrum regime. We further demonstrate that the plasmonic core-shell nanowires can be rendered invisible through destructive interference of the electric and toroidal dipoles, which may inspire many nanowire-based light-matter interaction studies, and incubate biological and medical applications that require noninvasive detections and measurements.

Optical Metacages.

We suggest a novel strategy for spectrally selective optical shielding of arbitrary shaped volumes by arranging specifically designed two- or three-layer nanowires around an area that needs to be protected. We show that such nanowire shields preserve their functionality for almost arbitrary geometry, and we term such structures optical metacages. We analyze several designs of such optical metacages made from either metallic or dielectric materials with experimentally measured parameters. We employ a semianalytical approach and also verify our results by numerical simulations. We further study optical properties of the introduced metacages in both near- and far-field regions, as well as analyze their frequency selectivity and the vanishing backscattering regime.

Subwavelength topological edge States in optically resonant dielectric structures.

We suggest a novel type of photonic topological edge states in zigzag arrays of dielectric nanoparticles based on optically induced magnetic Mie resonances. We verify our general concept by the proof-of-principle microwave experiments with dielectric spherical particles, and demonstrate, experimentally, the ability to control the subwavelength topologically protected electromagnetic edge modes by changing the polarization of the incident wave.

Enhanced third-harmonic generation in silicon nanoparticles driven by magnetic response.

We observe enhanced third-harmonic generation from silicon nanodisks exhibiting both electric and magnetic dipolar resonances. Experimental characterization of the nonlinear optical response through third-harmonic microscopy and spectroscopy reveals that the third-harmonic generation is significantly enhanced in the vicinity of the magnetic dipole resonances. The field localization at the magnetic resonance results in two orders of magnitude enhancement of the harmonic intensity with respect to unstructured bulk silicon with the conversion efficiency limited only by the two-photon absorption in the substrate.

Observation of Fano resonances in all-dielectric nanoparticle oligomers.

It is well-known that oligomers made of metallic nanoparticles are able to support sharp Fano resonances originating from the interference of two plasmonic resonant modes with different spectral width. While such plasmonic oligomers suffer from high dissipative losses, a new route for achieving Fano resonances in nanoparticle oligomers has opened up after the recent experimental observations of electric and magnetic resonances in low-loss dielectric nanoparticles. Here, light scattering by all-dielectric oligomers composed of silicon nanoparticles is studied experimentally for the first time. Pronounced Fano resonances are observed for a variety of lithographically-fabricated heptamer nanostructures consisting of a central particle of varying size, encircled by six nanoparticles of constant size. Based on a full collective mode analysis, the origin of the observed Fano resonances is revealed as a result of interference of the optically-induced magnetic dipole mode of the central particle with the collective mode of the nanoparticle structure. This allows for effective tuning of the Fano resonance to a desired spectral position by a controlled size variation of the central particle. Such optically-induced magnetic Fano resonances in all-dielectric oligomers offer new opportunities for sensing and nonlinear applications.