Cascaded plasmonic superlens for far-field imaging with magnification at visible wavelength

Ultra-high Q-factor quasi-BIC BaTiO3 metasurface for electro-optic modulation

Metasurfaces play a crucial role in trapping electromagnetic waves with specific wavelengths, serving as a significant platform for enhancing light-matter interactions. In all kinds of dynamic modulation metasurfaces, electro-optic modulation metasurfaces have attracted much attention due to its advantages of fast, stable and high efficiency. In order to respond to the extremely weak refractive index change of the electro-optical effect of the materials, the metasurfaces are required to support optical signals with high Q values. The quasi-bound state in the continuum (Q-BIC) is often used to enhance the light-field modulation capability of metasurfaces and to improve the modulation sensitivity of electro-optic modulators due to its ability to generate high Q-factor resonances. However, the design of an electro-optic modulation metasurface that facilitates the application of voltage and achieves modulation efficiency of nearly 100% is still in urgent need of development. In this study, single-crystal BTO metasurfaces are modeled using finite-difference time-domain method, and the structural symmetry is broken to introduce a Q-BIC resonance to generate a high Q-factor optical signal of 2.45 × 104 for high-depth electro-optic modulation. By simulating an applied electric field of 143 V/mm on the metasurface, a slight refractive index change of BTO of 8 × 10-4 was produced, leading to an electro-optical intensity modulation depth of 100%. Furthermore, the nanostructure of the metasurface was carefully designed to facilitate nano-fabrication and voltage application, and it is ideal for the development of low-power, CMOS-compatible, and miniaturized electro-optic modulation devices. Although the results of this study are based on simulations, they provide a crucial theoretical basis and guidance for the realization of efficient and realistic design of dynamic metasurfaces.

Super-resolution scanning imaging based on metal-dielectric composite metamaterials

We propose super-resolution scanning imaging by using a metamaterial composed of a silver-silicon dioxide composite covered by a layer of chromium containing one slit and a silicon dioxide substrate. By simulating a distribution of energy flow in the metamaterial for an H-polarized wave, we find that the output beam exhibits focusing accompanied with good directional radiation, which is able to be designed as a super-resolution scanning probe. We also demonstrate numerically super-resolution imaging by scanning our designed metamaterial over a sub-wavelength object.

Optically transparent and microwave diffusion coding metasurface by utilizing ultrathin silver films

The past few years have witnessed the great success of artificial metamaterials with effective medium parameters to control electromagnetic waves. Herein, we present a scheme to achieve broadband microwave low specular reflection with uniform backward scattering by using a coding metasurface, which is composed of a rational layout of subwavelength coding elements, via an optimization method. We propose coding elements with high transparency based on ultrathin doped silver, which are capable of generating large phase differences (∼180°) over a wide frequency range by designing geometric structures. The electromagnetic diffusion of the coding metasurface originates from the destructive interference of the reflected waves in various directions. Numerical simulations and experimental results demonstrate that low reflection is achieved from 12 to 18 GHz with a high angular insensitivity of up to ±40° for both transverse electric and transverse magnetic polarizations. Furthermore, the excellent visible transparency of the encoding metasurface is promising for various microwave and optical applications such as electronic surveillance, electromagnetic interference shielding, and radar cross-section reduction.

Freestanding bilayer plasmonic waveguide coupling mechanism for ultranarrow electromagnetic-induced transparency band generation

Strong electromagnetic coupling among plasmonic nanostructures paves a new route toward efficient manipulation of photons. Particularly, plasmon-waveguide systems exhibit remarkable optical properties by simply tailoring the interaction among elementary elements. In this paper, we propose and demonstrate a freestanding bilayer plasmonic-waveguide structure exhibiting an extremely narrow transmission peak with efficiency up to 92%, the linewidth of only 0.14 nm and an excellent out of band rejection. The unexpected optical behavior considering metal loss is consistent with that of electromagnetic induced transparency, arising from the destructive interference of super-radiative nanowire dipolar mode and transversal magnetic waveguide mode. Furthermore, for slow light application, the designed plasmonic-waveguide structure has a high group index of approximately 1.2 × 10⁵ at the maximum of the transmission band. In sensing application, its lowest sensing figure of merit is achieved up to 8500 due to the ultra-narrow linewidth of the transmission band. This work provides a valuable photonics design for developing high performance nano-photonic devices.

Unibody microscope objective tipped with a microsphere: design, fabrication, and application in subwavelength imaging

Microsphere-based subwavelength imaging technique was first demonstrated in 2011. After nearly a decade of efforts, such technique has spawned numerous interests in fields such as laser nano-machining, imaging, sensing, and biological detection. For wider industrial-scale application of the technique, a robust and low-cost objective lens incorporating a microsphere lens is highly desired and sought by many researchers. In this work, we demonstrate a unibody microscope objective lens formed by tipping a high-index microsphere onto a plano-convex lens and subsequently fitting them into a conventional objective lens. We call this the plano-convex-microsphere (PCM) objective, which resembles the appearance and operation of an ordinary microscope objective while providing super-resolving power in discerning subwavelength 100 nm features ($\lambda /{4}.{7}$λ/4.7) in air and far-field conditions. The imaging performance of the PCM objective, along with the working distance, has been systematically investigated. It has a calibrated resolution of $\lambda /{3}$λ/3 in the far field, a numerical aperture of 1.57, and a working distance of 3.5 µm. With the assistance of a scanning process, larger-area imaging is realized. The PCM objective can be easily adapted to existing microscope systems and is appealing for commercialization.

Numerical Investigation on Multiple Resonant Modes of Double-Layer Plasmonic Grooves for Sensing Application

A high-performance multi-resonance plasmonic sensor with double-layer metallic grooves is theoretically constructed by introducing a polymethyl methacrylate groove with a numerical simulation method. Multiple resonance wavelengths can be generated at the oblique incidence, and the number and feature of resonant mode for sensing detection is different for various incident angles. Specifically, at the incident angle of 30°, the reflection spectrum exhibits two resonant dips, in which the dip at the wavelength of 1066 nm has an extremely narrow line width of ~4.5 nm and high figure of merit of ~111.11. As the incident angle increases, the electric dipole mode gradually weakens, but the surface plasmon resonance and cavity resonance mode are enhanced. Therefore, for an incident angle of 65°, three resonance dips for sensing can be generated in the reflection spectrum to realize three-channel sensing measurement. These double-layer plasmonic grooves have potential in the development of advanced biochemical surface plasmon polariton measurements.

Cascaded DBR plasmonic cavity lens for far-field subwavelength imaging at a visible wavelength

We experimentally demonstrate a novel cascaded plasmonic superlens, which can directly image subwavelength objects with magnification in the far field at a wavelength of 640nm. The lens consists of two plasmonic slabs. One is a plasmonic cavity lens used for near-field coupling, and the other one is a planar plasmonic lens for phase compensation and thus, image magnification. To tune the performance wavelength to visible and to enhance the near-field transmission, distributed Bragg reflectors are integrated to the plasmonic cavity lens around the lens center, forming additional lateral cavities for surface waves. In this article, we first show numerical results about the working principle and the performance of the lens. Then, we demonstrate the imaging performance of a fabricated superlens experimentally. The fabricated superlens exhibits a lateral resolution down to 200 nm at the wavelength of 640 nm observed in the far field. Compared to our earlier design, shift invariance is achieved with the current approach. Our results could open a way for designing and fabricating novel miniaturized plasmonic superlenses in the future.