Linhua Xu

Waveguide-coupled surface phonon resonance sensors with super-resolution in the mid-infrared region

A waveguide-coupled surface phonon resonance (SPhR) sensor with super-resolution based on Fano resonance (FR) by using a multilayer system within the Kretschmann configuration in the mid-infrared wavelength region is proposed. Due to the coherent interference of the waveguide and the surface phonon polariton modes, the calculated reflectivity spectrum possesses sharp asymmetric FR dips. An ultra-small linewidth is formed because of the Fano coupling, and the physical features contribute to a highly efficient nano-sensor for refractive index sensing. The bulk and surface sensitivity by intensities are greatly enhanced relative to those of conventional SPhR sensors.

Enhanced absorption of graphene monolayer with a single-layer resonant grating at the Brewster angle in the visible range.

One method for enhancement and manipulation of light absorption with monolayer graphene covered on a single-layer guided mode resonant Brewster filter surface is demonstrated. By means of the rigorous coupled-wave analysis method, the effect of geometrical parameters on the optical response of the structure is investigated. It is possible to achieve a maximum absorption of 60% at the Brewster angle; dual-band optical absorption can also be realized when the depth of the grating is increased. The situation of oblique incidence for TM polarization is studied as well; the absorption property can be controlled by adjusting the incident angle without changing the structural parameters. The proposed structure has the advantage of more free geometry parameters compared to the graphene disk and ribbon, so the absorption could be tuned more flexibly.

Enhanced ultraviolet emission from ZnO thin film covered by TiO 2 nanoparticles

A ZnO thin film covered by TiO2 nanoparticles is prepared by electron beam evaporation. The structure and surface morphology of the sample are analyzed by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. Photoluminescence is used to investigate the fluorescent property of the ample. The results show that the ultraviolet (UV) emission of the ZnO thin film is greatly enhanced after it is covered by TiO2 nanoparticles while the green emission is suppressed. The enhanced UV emission mainly results from the fluorescence resonance energy transfer (FRET) between ZnO thin film and TiO2 nanoparticles. This TiO2-ZnO composite thin film can be used to fabricate high-efficiency UV emitters.

Angle-insensitive and narrow band grating filter with a gradient-index layer.

We demonstrate the design of an efficient angle-insensitive guided mode resonance filter (GMRF), with narrow bandwidth and low sideband reflection, for TE-polarized waves. The reflection properties of the multilayer structure have been studied, and the results verify that the thin film design of the gradient-index layer is important for the realization of an angle-insensitive filter. Various gradient coefficients of the thin film have distinct effects on the reflection spectrum. For an increasing incident angle, although the line-shape symmetry becomes less perfect, the positions of the resonant peak remain the same. The GMRF proposed here has many desirable attributes that lends itself to being an excellent platform, for devices such as lasers, detectors, filters, and sensors.

Angularly dense comb-like enhanced absorption of graphene monolayer with attenuated-total-reflection configuration

A multiline absorber based on the excitation of guided-mode resonance of one-dimensional photonic crystals (1D-PhCs), including a surface graphene monolayer under the attenuated-total-reflection configuration, is proposed and demonstrated. By carefully designing the structure parameters of the 1D-PhCs, the guided mode can be modulated by the periodic distribution of the refractive index. Our results reveal that the critical coupling of the guided resonance in periodical PhCs to graphene produces the perfect absorption. The number of absorption peaks within the photonic band corresponds to the number of unit cells. An ultrahigh Q-factor value of 4.75×106 is obtained at resonance with unity absorption, which could serve as a promising replacement of metallic thin film as a sensor probe for future biosensing applications.

Polarization-insensitive and wide-incident-angle optical absorber with periodically patterned graphene-dielectric arrays

A polarization-insensitive and angle-independent graphene absorber (GA) with periodically patterned grating is demonstrated. A periodic nanocavity composed of multilayer subwavelength grating and metal substrate supports a strongly localized mode inside the cavity, where the mode helps to absorb more electromagnetic waves. The proposed GA exhibits polarization-insensitive behavior and maintains the high absorption above 90% within a wide range of incident angle (more than 80°). We attribute the high absorption to the excitation of the cavity mode resonance and magnetic resonance for the transverse electric and transverse magnetic polarizations, respectively. The proposed GA has potential applications in the design of various devices, such as optical modulators or tunable absorption filters because of its remarkable angle-insensitive absorption performance.

High-resolution surface plasmon resonance sensor with Fano resonance in waveguide-coupled multilayer structures

An ultra-high resolution refractive-index sensor with the Kretschmann configuration was proposed and experimentally demonstrated. The Fano resonance (FR) in the attenuated total reflection curve arose from the interactions between the surface plasmon polariton and planar waveguide modes. It was shown to depend strongly on the structural parameters that governed the position of the FR and to be in good agreement with the results of electromagnetic calculations. The sensitivity by intensity was estimated to be 3.56 × 102-fold higher than that of conventional surface plasmon resonance sensors.

Multiband enhanced absorption of monolayer graphene with attenuated total reflectance configuration and sensing application

An enhanced absorption of monolayer graphene is obtained in a multilayer film-based attenuated total reflectance configuration in the visible wavelength range. The enhanced absorption under transverse magnetic and electric conditions is associated with the excitation of the waveguide mode in the thin-film layer, which is verified by the numerical calculation of field profiles. The obtained results manifest that the model induces a high field enhancement at the graphene–dielectric interface with the resonant angle, which implies potential sensing applications. The magnitude of the figure of merit is found to be three times higher than that of a conventional surface plasmon sensor.

Introducing nanoresonators into a metal–dielectric–metal waveguide array to allow beam manipulation

Stub and circular ring-shaped plasmonic resonators are introduced into a metal?dielectric?metal (MDM) waveguide array to allow light transmission control. Light focusing and splitting effects are verified by the finite difference time domain method; the simulation results reveal that the resonators can be used for modulating the superposition phase of the interference between the surface plasmon wave (SPW) from the end of the resonator and the passing SPW in the waveguide array. Furthermore, a structure utilizing a stub cavity with nonlinear material to control the phase of the transmitted SPW is proposed; the deflection angle of the light can be controlled by means of the intensity of the incident light. The proposed MDM waveguide array with plasmonic resonators, with its compact size, ease of integration, and high output, certainly has potential for application in nanophotonic circuits.

Influence of SiO2 buffer layer on the crystalline quality and photoluminescence of ZnO thin films

In this work, a SiO2 buffer layer was first grown on Si substrate by thermal oxidation, and then ZnO thin films were deposited on SiO2 buffer layer and Si substrate by electron beam evaporation and sol-gel method. The influence of SiO2 buffer layer on the crystalline quality and photoluminescence of the films was investigated. The analyses of X-ray diffraction (XRD) showed that all the ZnO thin films had a hexagonal wurtzite structure and were preferentially oriented along the c-axis perpendicular to the substrate surface. The SiO2 buffer layer improved the crystalline quality and decreased the stress in ZnO thin films. The surface morphology analyses of the samples indicated that ZnO thin films deposited on SiO2 buffer layers had densely packed grains which obviously increased compared with those grown on bare Si substrate. The photoluminescence spectra of the samples showed that the ZnO thin films deposited on SiO2 buffer layers had stronger ultraviolet emission performance. The results suggest that SiO2 buffer layer can improve the crystalline quality and ultraviolet emission of ZnO thin films.