Huaiqing Liu

Analytical model for plasmon modes in graphene-coated nanowire

An analytical model for plasmon modes in graphene-coated dielectric nanowire is presented. Plasmon modes could be classified by the azimuthal field distribution characterized by a phase factor exp(imφ) in the electromagnetic field expression and eigen equation of dispersion relation for plasmon modes is derived. The characteristic of plasmon modes could be tuned by changing nanowire radius, dielectric permittivity of nanowire and chemical potential of graphene. The proposed model provides a fast insight into the mode behavior of graphene-coated nanowire, which would be useful for applications based on graphene plasmonics in cylindrical waveguide.

Tunable subwavelength terahertz plasmon- induced transparency in the InSb slot waveguide side-coupled with two stub resonators

We numerically investigated the realization of electromagnetically induced transparency (EIT) at the terahertz (THz) region in an InSb slot waveguide side-coupled with two stub resonators. The mechanism of the EIT phenomenon is theoretically analyzed and numerically studied by using coupled mode theory and the finite element method, respectively, and the theoretical results are in good agreement with the simulation results. The simulation results reveal that the EIT-like response is strongly dependent on the coupling separation between the two stub resonators, and we derived the best separation between the two stub resonators to get the most obvious EIT-like spectra. More importantly, the central wavelength of the EIT-like spectra can be actively controlled by tuning the temperature. This plasmonic waveguide system may have potential applications for ultracompact THz integrated circuits, such as thermo-tunable filters, THz switching, slow-light components, and THz sensitive sensors.

Tunable Terahertz Plasmonic Perfect Absorber Based on T-Shaped InSb Array

High absorption efficiency is particularly desirable for various microtechnological applications. In this paper, a nearly perfect terahertz absorber for transverse magnetic (TM) polarization based on T-shaped InSb array is proposed and numerically investigated. Incident wave at the Fabry-Perot resonant frequency can be totally absorbed into the narrow grooves between the two adjacent T-shaped InSb arms. The absorption mechanism is theoretically and numerically studied by using the Fabry-Perot model and the finite element method (FEM), respectively. It is found that the proposed absorber has large angle tolerance. Moreover, the absorption peak can be controlled by varying the temperature. Furthermore, a new absorption peak will emerge while breaking the symmetry of the T-shaped InSb array. This tunable and angle-independent THz perfect absorber may find important applications in THz devices such as microbolometers, coherent thermal emitters, solar cells, photo detectors, and sensors.

Nanomechanical Plasmonic Filter Based on Grating-Assisted Gap Plasmon Waveguide

We propose and numerically analyze a tunable nanomechanical plasmonic filter based on Bragg-grating-assisted gold-air-gold gap plasmon (GP) waveguide. Bragg grating is formed by introducing periodical grooves on lower gold layers, and the air-gap size is mechanically changeable by electrostatically actuating the upper gold layer. Using the sensitivity of the GP mode to the gap size, the tunability of the proposed filter is realized by changing the gap size. By introducing a phase shift in the Bragg grating, sharp transmission peak with a quality factor of 35 is achieved within the stopband, which could be tuned over a wide wavelength range by a small change in the gap size. The proposed filter can be applied in the tunable plasmonic devices for future integrated optical circuits.

Ultracompact electro-optical logic gates based on graphene–silica metamaterial

Abstract. We propose and simulate numerically a permittivity-tunable metamaterial channel, which is composed of alternating layers of graphene and silica. The real part of the permittivity of the proposed metamaterial can be tuned from a positive value to a negative one for a broadband width. Furthermore, optical waves can pass through the metamaterial channel only when its permittivity is tuned to zero. Inspired by this intriguing property of the graphene–silica metamaterial, three basic electro-optical logic gates, including NOT, NOR, and NAND gates, were proposed and numerically investigated by using the finite element method. Taking advantage of the permittivity-tunable property of graphene, the working wavelength of the proposed electro-optical logic gates can be actively controlled by tuning the external voltage applied on the graphene–silica metamaterial. These tunable and ultracompact electro-optical logic gates could benefit the development of nanoscale optical devices for highly integrated photonic circuits.

Multilayer-core fiber with a large mode area and a low bending loss

We present a single-mode multilayer-core fiber with a large mode area (LMA) and a low bending loss in this Letter. A low equivalent core-cladding refractive index difference is achieved by exploiting the multilayer structure. The multilayer structure has a better bending performance than a traditional step-index core and this structure also contributes to realizing different curved refractive index profiles that have a better bending performance. An index trench is also introduced to dramatically reduce the bending loss. The experimental results show that, at a wavelength of 1550 nm, the mode area of the fabricated fiber is about 215.5 μm2 and the bending loss is 0.58 dB/turn at a 10 mm bending radius. The LMA and excellent bending performance can be obtained simultaneously with the proposed fiber.

Numerical investigation of a multi-functional optical device based on graphene-silica metamaterial

Abstract We propose a permittivity-tunable metamaterial channel, which is composed of alternative layers of graphene and silica. Optical waves can pass through the metamaterial channel only if its permittivity is tuned to zero. Taking advantage of the permittivity tunable property of the metamaterial, a multi-functional optical device, which can act as a wavelength demultiplexer, switch, and optical splitter without changing the geometric parameters has been proposed and numerically investigated by using the Finite Element Method. Owing to the permittivity tunable property of graphene, the working wavelength of the multi-functional device can be flexibly controlled by tuning the gate voltage applied on the metamaterial. This tunable ultracompact multi-functional optical device may find potential applications in highly integrated photonic circuits.

Nanomechanical Plasmonic Switch Based on Multimode Interference

We propose and investigate a 2 × 2 optical switch based on multimode interference in Au-Air-Au gap plasmonic waveguide. The propagation of surface plasmon wave in gap plasmonic waveguide could be tuned by changing the size of air gap through a nanomechanical actuator. By tailoring the dispersion of the gap plasmonic waveguide, the multimode region only supports two lowest order modes with the opposite field symmetry. Switching between two states of gap size in the multimode region, the phase difference of two modes at output port could be tuned from 0 to π leading to switching plasmonic wave energy between two output ports. The proposed structure is simple in design and compact in size, which may find promising applications in active plasmonic devices.