Theoretical Analysis of Terahertz Dielectric-Loaded Graphene Waveguide

Terahertz angle-independent photonic bandgap in a one-dimensional photonic crystal containing InSb-based hyperbolic metamaterials

According to the Bragg scattering theory, terahertz (THz) photonic bandgaps (PBGs) in all-dielectric one-dimensional (1-D) photonic crystals (PhCs) are strongly dependent on the incident angle. Such a strongly angle-dependent property of the PBGs not only limits the widths of omnidirectional PBGs, but also causes the strongly angle-dependent property of defect modes and optical Tamm states in multilayer structures containing all-dielectric 1-D PhCs. Until now, ways to achieve a THz angle-independent PBG have been an open problem. Herein, according to the existing phase-variation compensation theory, we achieve a THz angle-independent PBG in a 1-D PhC containing indium antimonide (InSb)-based hyperbolic metamaterials for transverse magnetic polarization. Different from conventional strongly angle-dependent PBGs, the angle-independent PBG remains almost unshifted as the incident angle changes. The relative frequency shifts of the upper and the bottom edges of the angle-independent PBG are only 1.4% and 0.4%, respectively. Besides, the angle-independent property of the PBG is robust against the disturbance of the layer thickness. The proposed 1-D PhC composes only two frequently used materials: silicon (Si) and InSb. Such a Si/InSb multilayer can be fabricated by the current ion-assisted electron beam coating or spin coating techniques. This THz angle-independent PBG would be utilized to design THz omnidirectional filters or absorbers.

Hybrid Nanowire–Rectangular Plasmonic Waveguide for Subwavelength Confinement at 1550 Nm

This paper presents a hybrid waveguide based on metal surface plasmon polaritons (SPPs) at 1550 nm comprising two silver (Ag) nanowires and a rectangular silicon (Si) waveguide. Due to the strong coupling effect observed in both the metal SPP mode and Si waveguide mode, excellent waveguide characteristics, such as a small effective modal area and long transmission length, could be achieved. The research results revealed that the proposed hybrid waveguide could achieve an ultra-long transmission distance of 270 µm and normalized effective mode area of 0.01. Furthermore, the cross-sectional size of the waveguide was 500 nm × 500 nm, which helped in achieving a subwavelength size. In addition, the hybrid waveguide was resistant to manufacturing errors. These excellent performances indicate that the proposed waveguide has great application potential in optoelectronic integrated circuits.

Sodium-Based Cylindrical Plasmonic Waveguides in the Near-Infrared

Subwavelength optical field confinement and low-loss propagation are of great significance for compact photonic integration. However, the field confinement capability of plasmonic devices is always accompanied by the inherent Ohmic loss. Although recent studies have shown that sodium (Na) exhibits lower loss than noble metals in the near-infrared band, the field confinement ability has not been adequately assessed. Meanwhile, the high chemical reactivity of Na should be regulated for practical application. Two dielectric-coated Na nanowires, consisting of cylindrical Na nanowires with one or two dielectric layers as claddings, are proposed and investigated in this paper. Based on finite element calculations, we thoroughly study the modal fields and low-loss propagation properties of dielectric-coated Na nanowires. The results demonstrate that Na exhibits lower loss and stronger field confinement than the typical plasmonic material silver. These findings indicate the performance of plasmonic devices can be considerably improved by employing the metal Na compared with devices using noble metals, which may promote the applications in subwavelength photonic devices.

Dispersion Theory of Surface Plasmon Polaritons on Bilayer Graphene Metasurfaces

Surface plasmon polaritons (SPPs) on the graphene metasurfaces (GSPs) are crucial to develop a series of novel functional devices that can merge the well-established plasmonics and novel nanomaterials. Dispersion theory on GSPs is an important aspect, which can provide a basic understanding of propagating waves and further guidance for potential applications based on graphene metamaterials. In this paper, the dispersion theory and its modal characteristics of GSPs on double-layer graphene metasurfaces consisting of the same upper and lower graphene micro-ribbon arrays deposited on the dielectric medium are presented. In order to obtain its dispersion expressions of GSP mode on the structure, an analytical approach is provided by directly solving the Maxwell’s equations in each region and then applying periodical conductivity boundary onto the double interfaces. The obtained dispersion expressions show that GSPs split into two newly symmetric and antisymmetric modes compared to that on the single graphene metasurface. Further, the resultant dispersion relation and its propagating properties as a function of some important physical parameters, such as spacer, ribbon width, and substrate, are treated and investigated in the Terahertz band, signifying great potentials in constructing various novel graphene-based plasmonic devices, such as deeply sub-wavelength waveguides, lenses, sensors, emitters, etc.

THz Multi-Mode Q-Plate with a Fixed Rate of Change of the Optical Axis Using Form Birefringence

We report the design, fabrication and experimental validation of a THz all-dielectric multi-mode q-plate having a fixed rate of change of the optical axis. The device consists of space-variant birefringent slabs manufactured by 3D printing using melt-extruded Acrylonitrile Butadiene Styrene (ABS). The desired form birefringence is analytically evaluated and experimentally measured by the THz time domain spectroscopy technique. The manufactured q-plate design is characterized using a polarization-sensitive imaging setup. The full electric field spatial maps are acquired from the beam propagating through the q-plate. The device enables the realization of both radial and azimuthal vector beams at discrete frequency intervals by controlling the space-dependent orientation of the ordinary and extraordinary axes in the transverse plane with a multi-mode sequence.

Analysis of hybrid plasmon-phonon-polariton modes in hBN/graphene/hBN stacks for mid-infrared waveguiding

Guiding mid-infrared (mid-IR) signals provide wide-ranging applications including chemical sensing, thermal imaging, and optical waveguiding. To manipulate mid-IR signals on photonic chips, it is critical to build a waveguide that provides both sub-diffraction field confinement and low loss. We present a mid-IR waveguide made up of a multilayer graphene/hexagonal boron nitride (hBN) stacking (MLGhS) and a high-refractive index nanowire. The guided mode of the proposed waveguide structure is formed by coupling the fundamental volume plasmon polariton with the fundamental hyperbolic phonon polariton in hBN, and is then modulated by a high-index nanowire. Interestingly, we found that the effective index, propagation length, and mode area of the guided mode vary as the dependences of N^-1, N, and N^3/2, where N is the number of graphene layers. In addition, an anomalous result, which reveals Lp and Am monotonously decrease as Fermi energy increases that is not observed in conventional graphene plasmon waveguides, occurs in the present structure. The modal properties are analyzed by altering geometry effects and material parameters, and by crossing the upper Reststrahlen band of hBN from the wavevector k = 1,300 to 1,500 cm^-1. Furthermore, crosstalk between adjacent waveguides are investigated to assess the degree of integration. The proposed idea not only provides a potential approach for designing tunable and large-area photonic integrated circuits, but it also has the potential to be extended to other 2D materials such as silicone, germanene, and stanene.

Refractive Index-Based Terahertz Sensor Using Graphene for Material Characterization

In this paper, a graphene-based THz metamaterial has been designed and characterized for use in sensing various refractive index profiles. The proposed single-band THz sensor was constructed using a graphene-metal hybridized periodic metamaterial wherein the unit cell had a footprint of 1.395λ_eff × 1.395λ_eff and resonated at 4.4754 THz. The realized peak absorption was 98.88% at 4.4754 THz. The sensitivity of the proposed metamaterial sensor was estimated using the absorption characteristics of the unit cell. The performance of the sensor was analyzed under two different categories, viz. the random dielectric loading and chemical analytes, based on the refractive index. The proposed THz sensor offered a peak sensitivity of 22.75 GHz/Refractive Index Unit (RIU) for the various sample loadings. In addition, the effect of the sample thickness on the sensor performance was analyzed and the results were presented. From the results, it can be inferred that the proposed metamaterial THz sensor that was based on a refractive index is suitable for THz sensing applications.

Graphene Nanoribbon Gap Waveguides for Dispersionless and Low-Loss Propagation with Deep-Subwavelength Confinement

Surface plasmon polaritons (SPPs) have been attracting considerable attention owing to their unique capabilities of manipulating light. However, the intractable dispersion and high loss are two major obstacles for attaining high-performance plasmonic devices. Here, a graphene nanoribbon gap waveguide (GNRGW) is proposed for guiding dispersionless gap SPPs (GSPPs) with deep-subwavelength confinement and low loss. An analytical model is developed to analyze the GSPPs, in which a reflection phase shift is employed to successfully deal with the influence caused by the boundaries of the graphene nanoribbon (GNR). It is demonstrated that a pulse with a 4 μm bandwidth and a 10 nm mode width can propagate in the linear passive system without waveform distortion, which is very robust against the shape change of the GNR. The decrease in the pulse amplitude is only 10% for a propagation distance of 1 μm. Furthermore, an array consisting of several GNRGWs is employed as a multichannel optical switch. When the separation is larger than 40 nm, each channel can be controlled independently by tuning the chemical potential of the corresponding GNR. The proposed GNRGW may raise great interest in studying dispersionless and low-loss nanophotonic devices, with potential applications in the distortionless transmission of nanoscale signals, electro-optic nanocircuits, and high-density on-chip communications.

Symmetric Graphene Dielectric Nanowaveguides as Ultra-Compact Photonic Structures

A symmetric graphene plasmon waveguide (SGPWG) is proposed here to achieve excellent subwavelength waveguiding performance of mid-infrared waves. The modal properties of the fundamental graphene plasmon mode are investigated by use of the finite element method. Due to the naturally rounded tips, the plasmon mode in SGPWG could achieve a normalized mode field area of ~10⁻⁵ (or less) and a figure of merit over 400 by tuning the key geometric structure parameters and the chemical potential of graphene. In addition, results show that the modal performance of SGPWG seems to improve over its circular counterparts. Besides the modal properties, crosstalk analysis indicates that the proposed waveguide exhibits extremely low crosstalk, even at a separation distance of 64 nm. Due to these excellent characteristics, the proposed waveguide has promising applications in ultra-compact integrated photonic components and other intriguing nanoscale devices.