Xiaoyong He

Graphene-supported tunable near-IR metamaterials

By integrating the metallic metamaterials (MMs) with a graphene layer, the resonant properties of an active tunable device based on the metal-SiO(2)-graphene (MSiO(2)G) structure have been theoretically investigated in the near-IR spectral region. The results manifest that the influences of the graphene layer on the propagation properties are significant. Owing to the tunability of the Fermi level of graphene, the resonance of transmitted or reflected curves can be tuned in a wide range (160-193 THz). To an original metal unit cell structure, an elevated Fermi level of graphene layer enhances the resonance dips and shifts it to the higher frequency. Compared with the original structure, the corresponding complementary MMs structure shows a much sharper spectral curve and can be used to fabricate a switcher or filters. The results are very helpful for designing graphene plasmonic devices.

Numerical analysis of the propagation properties of subwavelength semiconductor slit in the terahertz region

The propagation properties of terahertz (THz) waves passing through heavily doped semiconductor slit have been numerically investigated by using the transfer matrix method. The effects of geometrical parameters, carrier concentration, and dielectric materials filling in the slit have been considered. The contour for carrier concentration and slit width show that as slit width and carrier concentration decreases, the effective indices increase and the propagation lengths decrease. For the case of water filling in the slit, temperature has more effect on the imaginary part of propagation constant than the real part. Most of the energy stored in the slit is in the form of electric energy, which firstly decreases and then increases with the decreasing of slit width. It is expected that the semiconductor slit structure is very useful for the practical applications of THz waves in the fields of biological specimen analysis and medical diagnosis.

Analysis of graphene TE surface plasmons in the terahertz regime

Unlike common metals, graphene can support transverse electric (TE) surface modes when the imaginary part of its conductivity is negative. We have theoretically investigated and numerically simulated plasmonic properties of graphene TE surface plasmons (SPs) in the terahertz regime. The influence of the external magnetic field, gate voltage and temperature as the tuning schemes of the SPs have been investigated. The results show that graphene TE modes can be realized by tuning the magnetic fields or gate voltage. If the permeability of the dielectrics on both sides of the graphene layer differs enough, the graphene TE modes can still be achieved. The work presented here has the potential for application to graphene-based plasmonic devices in photonics and optoelectronics, such as sensors, polarizers and modulators.

Numerical Study of Gain-Assisted Terahertz Hybrid Plasmonic Waveguide

A numerical transfer matrix method (TMM) is applied to investigate hybrid surface plasmon polaritons (HySPPs) waveguide structure, which consists of a high permittivity dielectric fiber separated from a metal surface with a low permittivity dielectric gap. The results obtained from the TMM agree well with those from the finite element method but with a faster calculation speed. As a demonstration example, we have systematically investigated the propagation properties of the gain-assisted HySPPs waveguide in the terahertz regime by using this method, studying the influences of structure parameters, frequency, temperature, and material gain. The results manifest that the effective index and the propagation loss decrease with the increase of temperature. In addition, as the frequency increases, the effective index increases and the propagation loss shows a peak. Furthermore, lossless propagation can be achieved when certain gain materials are applied into the HySPPs structure. Our method provides an efficient approach to investigate HySPPs waveguide and other plasmonic devices.

Comparison of Graphene-Based Transverse Magnetic and Electric Surface Plasmon Modes

Dispersion properties and field distributions of graphene supported transverse magnetic (TM) and transverse electric (TE) surface plasmon (SP) modes in air-graphene-SiO2-Si structures have been investigated. The results show that graphene-based TM (TE) SPs are bound (lossy) modes, which decay into the air in the range of tens of micrometers (several thousand micrometers). In addition, when the thickness of the SiO2 layer is in the range of 200-300 nm, the influence of the Si substrate on the dispersion property is significant (negligible) for the TM (TE) modes. Furthermore, the effective indexes of the graphene TM (TE) modes increase with the increase (decrease) of the frequency. Compared with the traditional metal-based structures, graphene-based TM mode exhibits a better confinement but with a larger loss. The presented results are useful for the design of compact graphene-based optoelectronic devices.

Investigation of terahertz Sommerfeld wave propagation along conical metal wire

The waveguide properties of a terahertz wave propagating along conical metal wire have been investigated under the framework of the Sommerfeld model. The effects of composed materials, metal wire diameter, and temperature on the waveguide characteristics have been shown and discussed. The numerical calculation agrees well with the experimental results shown by Ji et al., and it predicts that a metal wire waveguide shows better propagation properties at lower temperature. The ratio of energy density contours demonstrate that energy concentration at the end-tip increases with the decreasing of frequency, end-tip diameter, and radial distance from metal wire surface. As the temperature decreases, the local field intensity increases, which may result from the higher conductivity and smaller skin depth of metal at lower temperature. Because the energy density is very sensitive to temperature, the conical metal wire tip can be used to measure the changes of temperature accurately, which may be confirmed by experiment in the future.

Tunable Subwavelength Terahertz Plasmonic Stub Waveguide Filters

Tunable subwavelength terahertz plasmonic stub waveguide filters based on indium antimonide are proposed and numerically investigated. The transmission line theory and the Finite Different Time Domain simulation results reveal that the single-stub waveguide structure can realize a stop-band filtering function and the central wavelength of the notch is linearly dependent on the stub length while nonlinearly dependent on the stub width. The central wavelength of the notch can be actively controlled by tuning the temperature. The proposed filters may have applications in THz highly-integrated plasmonic circuits.

Investigation of Multilayer Subwavelength Metallic-Dielectric Stratified Structures

We investigate dispersion properties of n-layers unit cell metallic-dielectric stratified structures (MDSSs) for the first time to our knowledge. An efficient and flexible numerical method is applied to study optical characteristics of the MDSSs. As an example, we systematically investigate the influences of geometric parameters, operating frequency, and gain material on the dispersion properties of the n-layers unit cell MDSSs in the terahertz regime. The results show that the effective index of the n-layers unit cell MDSSs decreases with the increase of operating frequency. The full-width-half-maximum of the transmittance of the n-layers unit cell MDSSs can be designed wider than that of the binary unit cell MDSSs, which is beneficial for the design of certain optical devices, such as superlenses. Furthermore, the effective index/loss of the proposed structure increases/decreases with the increase of the material gain. Due to the high flexibility of the proposed n-layers unit cell MDSSs, we believe they would have broad applications in the fields of nanophotonics and integrated optoelectronics.

Graphene-supported tunable waveguide structure in the terahertz regime

The tunable waveguide properties of the graphene supporting structure Si–SiO2–graphene–dielectrics–graphene–SiO2–Si (SiSiO2GDGSiO2Si) have been investigated in the terahertz regime by using the finite element method (FEM) and transfer matrix method (TMM). The study shows that the numerical results obtained from FEM and TMM agree well. The contour results show that as the frequency increases, the effective index increases, and the loss shows a peak; with the increase in the Fermi level, the effective index decreases, and the loss decreases. With a smaller effective mode area, the confinement of the SiSiO2GDGSiO2Si structure is much better than that of the Si–dielectrics–graphene–dielectrics–Si structure. The propagation properties of the structure can be modulated by using the applied gate voltage. The modulation depth of the propagation losses can reach more than 90%. The results are helpful to the design of tunable graphene optoelectronic devices, such as polarizers, modulators, and metamaterial devices.

Graphene-supported tunable extraordinary transmission

By depositing a graphene layer on the metallic film with subwavelength hole arrays, the tunable extraordinary transmission property based on the metal-dielectrics-graphene (MDG) structure has been investigated in the terahertz (THz) and near-infrared (NIR) regimes. The influences of operation frequency, composed materials, and the Fermi level of the graphene layer have been taken into account. The results show that by varying the Fermi level of the graphene layer, the transmission of the MDG structure can be tuned in a wide range and the modulation depth of the peak value of the transmission can reach more than 50%. But the tunable mechanisms in the THz and NIR regimes are quite different. In the infrared (THz) regime, the graphene behaves like the dielectric (metallic) layer; its dielectric constant decreases (increases) with the increase of Fermi level, resulting in the transmission increasing (decreasing). Compared with the metallic structure, the transmission of the semiconductor structure can also be modulated by using the doping or varying temperature; its peak position can also be changed in a much broader range. The results are very useful to understand the mechanism of the graphene plasmonic devices and to design novel filters, switchers, modulators, and sensors.