Haisu Li

Temperature-Independent and Strain-Independent Twist Sensor Based on Structured PM-CFBG

In this letter, we present a structured polarization-maintaining chirped fiber Bragg grating (PM-CFBG) for temperature-independent and strain-independent twist measurement. The structured PM-CFBG, which has two transmission peaks, is created by tapering directly on the PM-CFBG. The grating transmissivity of structured PM-CFBG changes linearly with twist while it is insensitive to strain and temperature. The wavelength interval changes proportionally to the temperature, but remains the same as the strain increases. This novel sensor can be used to measure temperature, strain, and twist simultaneously.

Tunable Plasmonic Filter Based on Graphene Split-Ring

We propose in this paper a tunable plasmonic filter based on graphene split-ring (GSR) resonator. It is found the resonances could be classified into two categories, i.e., even-parity and odd-parity mode according to the symmetry of field profile in GSR. The coupling between graphene nanoribbon and GSR is GSR-orientation sensitive, and the odd-parity mode presents a greater sensitivity due to its asymmetric field profile. The transmission spectrum of the proposed filter could be efficiently modified by tuning the shape, orientation, and Fermi level of GSR. The proposed structure can be applied in the tunable ultra-compact graphene plasmonic devices for future nanoplasmonic applications.

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.

Flexible single-mode hollow-core terahertz fiber with metamaterial cladding

A key requirement for achieving high-density integration of terahertz (THz) systems is a strongly confining single-mode and low-loss waveguide. Several waveguide solutions based on technologies from both electronics and photonics have been proposed; among these, hollow-core waveguides are one of the best options for guiding THz radiation due to their very low material absorption of air. However, to minimize reflection losses, hollow-core waveguides typically have a core diameter larger than the operating wavelength, and as a consequence are multimode. Here, we report on a single-mode, single-polarization hollow-core THz fiber with a metamaterial cladding, consisting of subwavelength-diameter metal wires embedded in a dielectric host. The idea of using metal–dielectric hybrid cladding relies on the extreme anisotropy of wire metamaterials, which reflects transverse magnetic (TM) waves and transmits transverse electric waves, leading to a waveguide structure that only confines TM modes—thus halving the number of modes from the outset. Numerical simulations and experimental measurements confirm a wide single-mode single-polarization window ranging from 0.31 to 0.44 THz, with a wavelength-sized core (0.88 mm diameter). Our work overcomes a stumbling block for achieving compact and flexible single-mode THz waveguides, which may be important for future THz systems with high-density integration.

Linearly polarized orbital angular momentum mode purity measurement in optical fibers

We presented a simple method for measuring the mode purity of linearly polarized orbital angular momentum (OAM) modes in optical fibers. The method is based on the analysis of OAM beam projections filtered by a polarizer. The amplitude spectrum and phase spectrum of a data ring derived from the beam pattern are obtained by Fourier transform. Then the coefficients of the mixed electric field expression can be determined and the mode purity can be obtained. The proposed method is validated and it is experimentally demonstrated in a two-mode fiber.

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.

Bending losses of trench-assisted few-mode optical fibers.

A semianalytical method based on the perturbation theory is developed to calculate the bending losses of individual modes of few-mode fibers (FMFs); it is applicable for optical fibers with arbitrary circularly symmetric index profile, especially for trench-assisted fibers. The bending performance of trench-assisted step-index FMFs and parabolic-index FMFs are investigated with three key parameters (i.e., the refractive index difference of trench-cladding, the width of the trench, and the distance of the core-trench). Then, a performance index is defined to estimate the bending performance for FMFs. It is shown that changing the distance of the trench-core, for each order of mode, there is a minimum bending loss, which can be used for fiber optimization. This optimization position (core-trench distance) decreases as the mode order increases. It is found that the bending performance of parabolic-index FMFs is better than that of step-index FMFs with fixed core radius and cutoff wavelength. The conclusions are helpful for understanding the mechanism of bending loss for FMFs, and make contributions to designing and manufacturing of few-mode bend-insensitive fibers.

Linearly polarized single TM mode terahertz waveguide

We design a hollow-core terahertz (THz) waveguide guiding a single linearly polarized mode. This is achieved using a hybrid cladding, where we introduce a ring of subwavelength structures, including metal wires and air-holes. The wire-based cladding is extremely anisotropic, reflecting only transverse magnetic (TM) modes. The polarization of TM modes is further manipulated by replacing some wires with air-holes. Numerical simulations confirm the guidance of only an x-polarized TM2 mode over 0.36-0.46 THz in a wavelength-scale core (diameter of 1 mm). The propagation losses are of the order 0.25  dB/cm, with low bend losses <0.3  dB/cm at 0.4 THz for a bend radius of 5 cm.

Nanofocusing of hybrid plasmons-phonons-polaritons in a graphene-hexagonal boron nitride heterostructure

In this Letter, we investigate the nanofocusing of hybrid plasmons-phonons-polaritons (SPP-HPhPs) in a graphene-hexagonal boron nitride (h-BN) heterostructure with a graphene coating on a tapered h-BN slab. Compared with the hyperbolic phonon polariton (HPhP) in h-BN, the hybrid SPP-HPhP in a heterostructure exhibits much smaller losses, which is validated through both analytical and numerical methods. Furthermore, the thickness dependent dispersions of a hybrid SPP-HPhP enable the tapered heterostructure to achieve a function of focusing electromagnetic waves. Compared with the tapered h-BN slab, the field enhancement in a tapered heterostructure is improved evidently with a maximum enhancement of the amplitude of normalized electric field over 60. This nanofocusing scheme may have potential in applications such as nonlinear optics.