Faqiang Wang

A simple scheme for quantum networks based on orbital angular momentum states of photons

We propose a new quantum network scheme using orbital angular momentum states of photons to route the network and spin angular momentum states to encode the information. A four-user experimental scheme based on this efficient quantum network is analyzed in detail, which is particularly appealing for the free space quantum key distribution. Users can freely exchange quantum keys with each other.

Tunable Terahertz Filters Based on Graphene Plasmonic All-Dielectric Metasurfaces

A tunable terahertz filter based on graphene plasmonic all-dielectric metasurfaces is proposed and investigated numerically by using the finite-difference time-domain (FDTD) method. Especially, hybrid all-dielectric metasurfaces are used to make a whole single-sheet graphene forms two different conductivity patterns with the same gate voltage. The simulated results show that resonance wavelength is shifted significantly with the change of gate voltage. Besides, the transmittance spectra are also shifted with the change of the width of SiC, and the filter shows a polarization-dependent modulation property for the length and the width of SiC being 480 and 320xa0nm, respectively. In addition, the filter can be applied for refractive sensing because the transmittance spectra are shifted with the change of the background refractive index. The study could provide availability for versatile tunable terahertz graphene plasmonic metasurfaces.

Visible-wavelength metalenses for diffraction-limited focusing of double polarization and vortex beams

In recent years, two-dimensional functional optical devices utilizing metasurfaces to manipulate phases of light develop rapidly. In this paper, we demonstrated metalenses according to the geometric Pancharatnam–Berry phase concept based on zinc sulfide (ZnS) material. Metalenses at the visible wavelengths λ=405, 532 and 633 nm have the capacity to focus incident light down to diffraction-limited spots with corresponding transfer efficiencies of 72%, 65%, 67%, respectively. A full-hybrid metalens was developed to adapt for double-polarized light focusing, which altered the condition that metalenses designed by geometric phase concepts could not focus different polarized light into one spot. In addition, we realized the conversion of the circularly polarized plane light into a vortex beam and focusing the vortex beam simultaneously by one device.

Multifunctional Sensors and Switch in MDM Waveguide With Symmetric Dual Side-Coupled Nanodisks

A multirole waveguide with symmetric dual side-coupled nanodisk resonators, which can act as refractive index sensor, temperature sensor, and optical switch, is proposed and investigated numerically. By the finite-difference time-domain simulations, it is demonstrated that the value of the refractive index sensitivity of the nano-sensor can reach 1333 nm/RIU. And with ethanol fully filled in the resonators, it can act as a temperature sensor with temperature sensitivity of 0.52 nm/°C. Furthermore, by filling with liquid crystal material, the transmittance of the optical switch is up to 73.6% at the wavelength of 1310 nm. This letter may pave a way for multifunctional applications in the on-chip nano-sensing area.

High-Efficiency, Near-Diffraction Limited, Dielectric Metasurface Lenses Based on Crystalline Titanium Dioxide at Visible Wavelengths

Metasurfaces are planar optical elements that hold promise for overcoming the limitations of refractive and conventional diffractive optics. Previous metasurfaces have been limited to transparency windows at infrared wavelengths because of significant optical absorption and loss at visible wavelengths. Here we report a polarization-insensitive, high-contrast transmissive metasurface composed of crystalline titanium dioxide pillars in the form of metalens at the wavelength of 633 nm. The focal spots are as small as 0.54λd, which is very close to the optical diffraction limit of 0.5λd. The simulation focusing efficiency is up to 88.5%. A rigorous method for metalens design, the phase realization mechanism and the trade-off between high efficiency and small spot size (or large numerical aperture) are discussed. Besides, the metalenses can work well with an imaging point source up to ±15° off axis. The proposed design is relatively systematic and can be applied to various applications such as visible imaging, ranging and sensing systems.

Analogue of electromagnetically induced absorption with double absorption windows in a plasmonic system

We report the observation of an analog of double electromagnetically induced absorption (EIA) in a plasmonic system consisting of two disk resonators side-coupled to a discrete metal-insulator-metal (MIM) waveguide. The finite-difference time-domain (FDTD) simulation calculations show that two absorption windows are obtained and can be easily tuned by adjusting the parameters of the two resonance cavities. The consistence between the coupled-model theory and FDTD simulation results verify the feasibility of the proposed system. Since the scheme is easy to be fabricated, our proposed configuration may thus be applied to narrow-band filtering, absorptive switching, and absorber applications.

Metalenses Based on Symmetric Slab Waveguide and c-TiO2: Efficient Polarization-Insensitive Focusing at Visible Wavelengths

A key goal of metalens research is to achieve wavefront shaping of light using optical elements with thicknesses on the order of the wavelength. Here we demonstrate ultrathin highly efficient crystalline titanium dioxide metalenses at blue, green, and red wavelengths (λ0 = 453 nm, 532 nm, and 633 nm, respectively) based on symmetric slab waveguide theory. These metalenses are less than 488 nm-thick and capable of focusing incident light into very symmetric diffraction-limited spots with strehl ratio and efficiency as high as 0.96 and 83%, respectively. Further quantitative characterizations about metalenses’ peak focusing intensities and focal spot sizes show good agreement with theoretical calculation. Besides, the metalenses suffer only about 10% chromatic deviation from the ideal spots in visible spectrum. In contrast with Pancharatnam–Berry phase mechanism, which limit their incident light at circular polarization, the proposed method enables metalenses polarization-insensitive to incident light.

Angular and Wavelength Simultaneous Selection in Transparent OPVs Based on Near-Infrared Bragg Reflector and Antireflection Coating

A novel design based on genetic algorithm is proposed to construct a near-infrared Bragg reflector and an antireflection coating, which are supposed to be applied in the transparent organic photovoltaics. By incorporating the absorbing layer which composed of ClAlPc/C60 with the structure designed, it leads to a significant near-infrared absorption increase from 16.6% to 36.3% within 650–850xa0nm and high average visible-transparency of 72% within 400–580xa0nm at normal incidence. Moreover, the near-infrared absorption increases with the enlargement of the incident angle from 0° to 30°, where the average transmittance remains above 60% simultaneously. This design can further improve the balance between transparency in the visible for scenic sight view and sunlight absorption in near-infrared for high-efficiency electricity generation. The potential of adopting this novel design on transparent building-integrated photovoltaic modules, which dominate the landscape of the major cities, is tremendous.