Yudong Lian

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.

Graphene-coated tapered nanowire infrared probe: a comparison with metal-coated probes

We propose in this paper a graphene-coated tapered nanowire probe providing strong field enhancement in the infrared regimes. The analytical field distributions and characteristic equation of the supported surface plasmons mode are derived. Based on the adiabatic approximation, analytic methods are adopted in the investigation of field enhancement along the tapered region and show well consistence with the rigorous numerical simulations. Both the numerical and analytical results have shown that the graphene-coated nanowire probe could achieve an order of magnitude larger field enhancement than the metal-coated probes. The proposed probe may have promising applications for single molecule detection, measurement and nano-manipulation techniques.

Broadband orbital angular momentum transmission using a hollow-core photonic bandgap fiber

We present the viability of exploiting a current hollow-core photonic bandgap fiber (HC-PBGF) to support orbital angular momentum (OAM) states. The photonic bandgap intrinsically provides a large refractive index spacing for guiding light, leading to OAM transmission with low crosstalk. From numerical simulations, a broad OAM±1 mode transmission window with satisfied effective index separations between vector modes (>10-4) and low confinement loss (<3  dB/km) covering 240 nm bandwidth is observed. The OAM purity (defined as normalized power weight for OAM mode) is found to be affected by the modal effective area. Simulation results also show HC-PBGF based OAM transmission is immune to fabrication inaccuracies near the hollow core. This work illustrates that HC-PBGF is a competitive candidate for high-capacity communication harnessing OAM multiplexing.

Tunable orbital angular momentum generation in optical fibers

We present a method in this Letter to generate optical vortices with tunable orbital angular momentum (OAM) in optical fibers. The tunable OAM optical vortex is produced by combining different vector modes HE2,meven (HE2,modd) and TE0,m (TM0,m) when l=1 or combining HEl+1,meven (HEl+1,modd) and EHl-1,modd (EHl-1,meven) when l>1 with a π/2 phase shift. The vortex can be regarded as a result of overlapping two orthogonal optical vortex beams of equal helicity but opposite chirality with a π/2 phase shift. We have experimentally demonstrated the smooth variation of OAM from l=-1 to l=+1 by adjusting a polarizer at the output end of the fiber.

Simultaneous Measurement of Refractive Index and Temperature Based on NFN Structure

We propose and demonstrate a compact all-fiber sensor for simultaneous measurement of temperature and refractive index (RI), which is based on a no-core fiber (NCF) and fiber Bragg grating (FBG). Two segments of NCF used as beam splitter and combiner are embedded on the two ends of an FBG, which constitute a Mach-Zehnder (MZ) interferometer susceptible to the temperature and an ambient RI. With the change of RI and temperature, MZ interference and Bragg reflection result in the variation of transmission spectrum. The experimental results show that the sensitivity of RI is -109.573 nm/RI in the range from 1.333 to 1.398, and the sensitivity of temperature ranging from 10 °C to 70 °C is 0.014 nm/°C, respectively. The feasibility of simultaneous measurement of an external RI and the temperature with this configuration has been demonstrated experimentally.

Magnetically tunable non-reciprocal plasmons resonator based on graphene-coated nanowire

In this paper we propose a magnetically tunable plasmons resonator based on a graphene-coated nanowire. Due to the magneto-optical effect under an external magnetic field, the circumferential propagation of graphene plasmons on a magneto-optical nanowire becomes non-reciprocal with the modal indices depend on plasmons traveling directions (clockwise or anti-clockwise). When coupled with a graphene sheet waveguide, the two components form a graphene plasmons filter for which the shift direction of transmittance spectrum is determined by the direction of input plasmons. The resonant wavelengths of resonator are obtained through resonant cavity theory and verified by numerical solutions. Furthermore, the non-reciprocal transmittance enables such structures to achieve the function of a plasmons isolator where the isolation could be tuned by the amplitude of an external magnetic field and the enabled plasmons propagation direction could be switched by reversing the direction of external magnetic field. Under proper structural parameters and magnetic field, an isolation ratio over 25 dB is obtained. The proposed magnetically tunable plasmons resonator may provide new inspiration to graphene plasmonics devices.

Nanofocusing in the graphene-coated tapered nanowire infrared probe

We conduct analytical and numerical investigations of graphene-coated tapered nanowire probe and determine the optimal conditions and structural parameters for achieving maximal possible local field enhancement at the probe tip. Based on the adiabatic approximation, analytic methods are adopted in the investigation of modal performances and field enhancement of the probe, showing good consistence with the rigorous numerical simulations up to the taper angle near 24°. Both the adiabatic theory and numerical simulation have shown the existence of optimal conditions and structural parameters. The dependencies of tip field enhancement as well as the optimal structure parameters on geometric parameters, chemical potential, nanowire permittivity, and tip radius have been intensively investigated with the electric field amplitude and can be enhanced as high as 24 times. The proposed probe and the corresponding discussion may have promising applications for single molecule detection, measurement, and nanomanipulation technique.

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.

Creation of Graphene Plasmons Vortex via Cross Shape Nanoantennas Under Linearly Polarized Incidence

In this paper, we propose a method of creating plasmons vortex on graphene through cross shape nanoantennas under linearly polarized incidence. For each cross shape antenna, the linearly polarized incidence can be coupled to a near-field plasmons vortex on graphene through antenna resonances. When multiple antennas are arranged into a closed circular array, the sign of topological charge of plasmons vortex can be controlled by the linearly polarized direction of incident light with the distribution of absolute field component exhibiting a non-ideal donut profile. The creating of plasmons vortex on graphene may provide new possibilities in various applications, such as the nanoparticles trapping.