Grating-based metasurfaces for ultra-narrow near-infrared bandpass filtering with wide out-of-band suppression.

Here, we present a straightforward strategy for designing silicon grating-based metasurfaces tailored for narrow near-infrared bandpass filtering. By selecting appropriate structural parameters for the grating and including periodic groove perturbations within each grating slit, transverse guided mode resonances (GMRs) propagating perpendicular and parallel to the grating slit are created to provide wide out-of-band suppression and high-Q filter responses, respectively. The destructive and constructive interference between radiations from groove perturbations are then introduced to eliminate all GMRs except one, producing a single-band bandpass filter. Simply adjusting the period of the groove perturbations allows precise tuning of the passband's central wavelength across the operational spectral range from 1350 nm to 1750nm, throughout which the passband exhibits a Q-factor exceeding 9,000 and the attenuation level outside the passband remains below 1%. Furthermore, our proposed narrow bandpass filters are found to be robust against the potential fabrication imperfections, such as variations in groove size and position.

Large-scale and tunable transparent displays based on silver nanoparticles metasurface.

We report a transparent display based on a metasurface of silver nanoparticles (Ag NPs), consisting of a transparent substrate and a layer of Ag NPs deposited by a dielectric film. The Ag NPs metasurface is prepared by a simple and direct annealing process. It presents a deep transmission valley at the wavelength ofλ= 468 nm and enables desired transparent display by projecting the monochromatic image onto the metasurface. We also demonstrate that the formed Ag NPs can be approximated as truncated nanospheres, which have obvious directional scattering properties, and can radiate most of the scattered energy into the backward hemisphere with a relatively large angular beamwidth (the full width at half maximum of the scattered intensity) of ∼90°. Therefore, the fabricated displays possess wide viewing angles and high brightness characteristics. Additionally, the transmission modes can be red-shifted to the wavelength ofλ= 527 nm by controlling the thickness of the deposited dielectric film. This approach using traditional thin film deposition and moderate annealing processing techniques enables simple, low-cost, and scalable fabrication in large areas for transparent displays.

High-Q filtering and dynamic modulation in all-dielectric metasurfaces induced by quasi-BIC.

The all-dielectric metasurfaces can significantly reduce the volume of optical components while having low loss and high performance, which has become a research hotspot in recent years. However, due to the complexity of metasurface geometric design, it is challenging to realize dynamic modulation on all-dielectric metasurface optical elements. Here, we propose a high quality factor (high-Q) pass-band filter designed by introducing the quasi-bound states in the continuum (quasi-BIC) into the silicon array phase-gradient metasurfaces. Our simulations show that due to the quasi-BIC effect only a high-Q resonance with the linewidth less than 1 nm and the corresponding Q value of ∼37000 could transmit along the zeroth order direction, which could be used for ultra-narrow linewidth filtering. Furthermore, our simulations present that the near-fields of the waveguide modes supported by the silicon arrays are partially distributed inside the indium tin oxide (ITO) substrate, which makes it possible to dynamically tune the central wavelength of our proposed filter by varying the ITO refractive index.

Dielectric loading method for doubly resonant enhancement of third-harmonic generation from complementary split-ring resonators.

Nonlinear optical response could be greatly enhanced when metasurfaces support plasmonic resonances at both fundamental and harmonic wavelengths. However, it is still challenging to fulfill the doubly resonant condition. Here, we propose a dielectric-loading method, which simply coats a conformal thin dielectric layer onto the plasmonic metasurfaces, to introduce an additional degree of freedom and make the doubly resonant condition easily fulfilled. We demonstrate that by simultaneously tuning the thickness of the coated dielectric layer and the geometrical parameters of the gold complementary split-ring resonators (CSRRs), the doubly resonant enhancement of third harmonic generation (THG) could be achieved for any given fundamental wavelengths. We also experimentally verify this concept and show that the THG intensity in the dielectric-loaded CSRRs under the doubly resonant condition could be further increased about 3 times as compared with the case of the conventional CSRRs.

Enhanced second-harmonic generation from gold complementary split-ring resonators with a dielectric coating.

We experimentally and theoretically investigate the influence of alumina coating on the second-harmonic generation (SHG) from split-ring resonator shaped air apertures engraved in a gold film, which are also termed as complementary split-ring resonators (CSRRs). By coating the CSRR arrays with alumina film of certain thickness, we precisely tune their electric diploe resonances (EDRs) to overlap the fundamental wavelength (FW) and realize the EDR enhanced SHG process. On this basis, by shortening the arm length of the CSRRs and then coating them with a certain thickness of the alumina film, we have achieved an SHG enhancement of nearly 1.2-fold in experiment and 8-fold in simulation compared to the CSRR array with an unshortened arm length. We attributed it to the improvement of the magnitude of the effective nonlinear source due to the realization of a doubly-resonant condition. As a flexible method, dielectric coating not only is beneficial to precisely and dynamically optimize the linear and nonlinear properties of the as-fabricated nanoscale devices but also can play the role of a protective layer, which can partially improve the damage threshold of these plasmonic nanoscale devices.

Highly tunable multiple narrow emissions of dyed dielectric-metal core-shell resonators: towards efficient fluorescent labels.

We report a potential efficient fluorescent label based on the dyed dielectric-metal core-shell resonators (DMCSRs). By utilizing the near-field coupling between the dyes and the multipolar sharp cavity plasmon resonances, the dyed DMCSRs with diameter of 1.02 µm are demonstrated to be capable of supporting multiple spontaneous emission peaks with the linewidths as narrow as âˆ¼ 10 nm in visible range, and these reshaped fluorescent emissions are insensitive to the surrounding dielectric environment. Furthermore, these multiple narrow emission peaks show a precise tunability on the spectrum by simply separating a nanometric dielectric layer between the dielectric core and the metallic shell, which may provide an attractive spectral multiplexing strategy in the fields of cell biology and medical sciences.

Dual plasmonic nanostructures for switching polarity of hot electron-induced photocurrent.

We report on the experimental investigation of polarity-switchable hot electron-induced photocurrents in dual-plasmonic nanostructures, consisting of two layers of gold nanoparticles (AuNPs) separated by a TiO2 film. Hot electrons generated through the non-radiative decay of the localized surface plasmon resonances supported by the top and bottom layers of AuNPs can be simultaneously injected into the TiO2 film in opposite directions and counteract each other. As a result, the polarity and magnitude of the net photocurrents can be tailored by controlling the population of hot electrons either generated from or collected by the two layers of AuNPs. We believe the wavelength-dependent photocurrent polarity switching could be useful for biosensors with a direct electrical readout and photoconversion applications.

5.1% efficiency of Si photoanodes for photoelectrochemical water splitting catalyzed by porous NiFe (oxy)hydroxide converted from NiFe oxysulfide.

A porous NiFe (oxy)hydroxide catalyst fabricated on n+pp+-Si/Ni/NiOx, which is converted from an electrodeposited NiFe oxysulfide, allows a silicon photoanode for water splitting to hit a record 5.1% efficiency with good stability of up to 135 h under 40 mA cm-2 in 1.0 M NaOH.

UV-visible photocurrent enhancement using metal-semiconductor-metal with symmetric and asymmetric double Schottky barriers.

We experimentally report improved photocurrent performance under the illumination of both UV and visible light in the metal-semiconductor-metal nanostructures with double Schottky barriers, consisting of a monolayer of Au nanoparticles and a planar Au film separated by a thin WO3 layer. In addition to the near-unit broadband optical absorption causing the increased population of hot electrons, we demonstrate that the occurrence of the electron trapping effect at the bottom Schottky barrier is also responsible for the photocurrent enhancement under visible light illumination. Under the direct UV bandgap excitation of WO3, the electron trapping effect at the bottom barrier plays a crucial role in the anodic photocurrent enhancement. Furthermore, we demonstrate that by replacing the Au film with the metal Pt of higher work function to increase the barrier height of the bottom Schottky barrier, and hence strengthening its ability for electron trapping, the photocurrent is found to gain a further significant enhancement under the illumination of both visible and UV light.