Zhengji Xu

Unidirectional surface plasmon-polariton excitation by a compact slot partially filled with dielectric

We propose a new scheme on unidirectional surface plasmon-polariton (SPP) excitation with the following advantages: ultracompact size, working at arbitrary incidence angle and over a wide spectrum. The proposed structure utilizes a partially filled metallic slot with dielectric to realize unidirectional SPP excitation via direct field manipulation. We theoretically and numerically show that unidirectional SPP excitation with a ratio of 93% can be achieved by a structure with a 50 nm slot. The proposed structure keeps its functional capability over incident angles from -80° to 80°, and has a broadband working spectrum of more than 70 nm.

Waveguide devices with homogeneous complementary media

We report the design of microwave waveguide devices based on complementary media. Various kinds of waveguide devices that possess nearly 100% transmission efficiency are proposed, such as waveguide bends, splitters, connectors, and shifters. Compared with previous work on waveguide devices of low reflection and minimized distortion based on transformation optics, our transform media are homogeneous metamaterials. Electromagnetic simulations by a finite-element method on detailed examples have been performed to validate the designs and these functionalities can be close to the practical.

Groove-structured metasurfaces for modulation of surface plasmon propagation

An alternative metasurface design based on a nonresonant mechanism is proposed and demonstrated. Using the effective medium theory and finite element calculation, we show the relationship between the effective refractive index and the metasurface geometrical parameters, which potentially can be used for tuning the wavelength and wavefront of the surface plasmon polariton (SPP). Experimental studies of our metasurface were conducted by near-field scanning optical microscopy of subwavelength gold grooves fabricated using a focused ion beam (FIB). The metasurfaces give us an alternative method for manipulating SPP propagation.

Metal nanorod-based metamaterials for beam splitting and a subdiffraction-limited dark hollow light cone

We report beam splitting in a metamaterial composed of a metal nanorod array embedded in a dielectric covered by a layer of chromium containing a hole. By simulating distributions of energy flow in the metamaterial, we find that the beam splitting occurs for H-polarized incidence, which is conducive to producing a beam splitter. We also find that a dark hollow light cone can be formed for circularly polarized incidence, which has a potential application in manipulating atoms and molecules.

Study of dual color infrared photodetection from n-GaSb/n-InAsSb heterostructures

We report detailed investigation of n-GaSb/n-InAsSb heterostructure photodetectors for infrared photodetection at different temperatures and biases. Our results show that the heterostructure photodetectors are capable of dual color photodetections at a fixed forward bias with its highest responsivity occurred at room temperature; With the decrease of the forward bias, a turning point, at which the photocurrent changes its direction, exist and the corresponding voltage values increases with the decrease of temperature; At all reverse biases, the photocurrents flow in the same direction but the maximum current occurs at about 205 K. A new model is proposed, which can well explain all the observations.

Surface plasmon induced direct detection of long wavelength photons

Millimeter and terahertz wave photodetectors have long been of great interest due to a wide range of applications, but they still face challenges in detection performance. Here, we propose a new strategy for the direct detection of millimeter and terahertz wave photons based on localized surface-plasmon-polariton (SPP)-induced non-equilibrium electrons in antenna-assisted subwavelength ohmic metal–semiconductor–metal (OMSM) structures. The subwavelength OMSM structure is used to convert the absorbed photons into localized SPPs, which then induce non-equilibrium electrons in the structure, while the antenna increases the number of photons coupled into the OMSM structure. When the structure is biased and illuminated, the unidirectional flow of the SPP-induced non-equilibrium electrons forms a photocurrent. The energy of the detected photons is determined by the structure rather than the band gap of the semiconductor. The detection scheme is confirmed by simulation and experimental results from the devices, made of gold and InSb, and a room temperature noise equivalent power (NEP) of 1.5 × 10−13 W Hz−1/2 is achieved.The detection of terahertz and millimeter waves has many applications, but there are still limitations in their technical performance. Here, Tong et al. demonstrate the direct detection of long-wavelength radiation through surface plasmon excitation and a corresponding improvement in detection performance.

A multi-layered split ring metamaterial for a multiwavelength and tunable lasing spaser

We report a multiwavelength and tunable lasing spaser realized by a gain-assisted metamaterial. The metamaterial consists of three regularly spaced parallel arrays of weakly asymmetric metallic split rings, with the first-layered array located on the gain medium surface and the other two-layered arrays embedded in it. Our simulations demonstrate that the three-layered metamaterial can radiate three-wavelength electromagnetic waves with high Q factors and transmission enhancement, which can be tuned by varying the gain coefficient.

Actively tunable Fano resonances based on colossal magneto-resistant metamaterials

In this Letter, a periodic structure in which each unit cell consists of one manganese oxide (La(0.7)Ca(0.3)MnO(3)) strip and two gold strips is designed. By simulating the electromagnetic responses of the structure, we confirm that Fano resonances can be actively controlled in the infrared region by modulating the intensity of the external magnetic field applied to the structure. This is due to the colossal magneto-resistance of the La(0.7)Ca(0.3)MnO(3) material. Furthermore, a transmission phase can also be effectively tuned. The phase has a shift of ΔΦ=1.05  rad at a frequency of 130 THz when the intensity of the external magnetic field varies from 5083 to 5193  kA/m. Such a tunable method has potential applications in controllable photoelectric elements.

Aluminum based structures for manipulating short visible wavelength in-plane surface plasmon polariton propagation

We report aluminum based structures for manipulation of surface plasmon polariton (SPP) propagation at short wavelength range. Our simulation shows that aluminum is a good metal to excite and propagate SPPs with blue light and that the SPP wavelength can be reduced from about 465 nm to about 265 nm by monitoring the thickness of a coated Si(3)N(4) layer above the aluminum film. It is also shown that the damping becomes more significant with the increase of the thickness of the Si(3)N(4) layer. We also experimentally demonstrated the SPP wavelength tuning effect for 20nm Si(3)N(4) layer covered Al, which can be explained by the variation of effective permittivity. The proposed Metal-Insulator-Air (MIA) structures with SPP wavelength tuning ability have potential applications in 2D optics.

Beam splitting with subwavelength resolution using combined metallodielectric films

We report on a set of metamaterials composed of combined metallodielectric films as regards beam splitting use. The energy flow distributions of the structures are investigated by using the finite element method. It is found that the beams split and propagate along the direction normal to the films. After propagation to the far field, the resolution of the beams is about a sixth of the wavelength. Two-beam symmetrical and non-symmetrical splitting and also multi-beam splitting are obtained by changing the structures.