Ultralow loss graphene-based hybrid plasmonic waveguide with deep-subwavelength confinement

Compact, low-loss, and high-polarized-extinction ratio terahertz TM-pass polarizer based on a hybrid plasmonic waveguide with a graphene ridge

A compact, low-loss, and high-polarized-extinction ratio TM-pass polarizer based on a graphene hybrid plasmonic waveguide (GHPW) has been demonstrated for the terahertz band. A ridge coated by a graphene layer and the hollow HPW with a semiround arch (SRA) Si core is introduced to improve structural compactness and suppress the loss. Based on this, a TM-pass polarizer has been designed that can effectively cut off the unwanted TE mode, and the TM mode passes with negligible loss. By optimizing the angle of the ridge, the height of the ridge, air gap height, and the length of the tapered mode converter, an optimum performance with a high polarization extinction ratio of 30.28 dB and a low insert loss of 0.4 dB is achieved in the 3 THz band. This work provides a scheme for the design and optimization of polarizers in the THz band, which has potential application value in integrated terahertz systems.

3D Dirac semimetals-dielectric elliptical fiber supported tunable terahertz hybrid waveguide

Based on the proposed elliptical dielectric fiber-polyethylene gap-3D Dirac semimetal (DSM) hybrid plasmonic waveguide structure, the tunable propagation characteristics have been systematically investigated in the terahertz region, taking into account the influences of the structural parameters, the modified dielectric fiber, and the 3D DSM Fermi levels. The results show that as the ratio of the elliptical semi-axis along the y-direction a_y and the x-direction a_x (a_y/a_x) increases, the hybrid mode confinement increases. The real part of the effective mode index and propagation length increase with increasing the refractive index of the elliptical fiber. The propagation length and figure of merit of the hybrid modes reach 1.56×10⁴µm and 300, respectively. In addition, by changing the Fermi level of the 3D DSM layer, the propagation properties of the hybrid modes can also be modulated in a wide range, e.g., the modulation depth of the propagation length reaches about 71.53% if the Fermi level varies in the range of 0.03-0.15 eV. The propagation properties of the hybrid modes are enhanced significantly by utilizing the modified three elliptical fiber structures, the real part of the effective mode index, and the propagation length of the modified structure are enhanced simultaneously. The results are very helpful for understanding the tunable mechanism of the 3D DSM devices and aids the design of novel plasmonic devices, e.g., lasers, modulators, and resonators.

Terahertz hybrid plasmonic waveguides with ultra-long propagation lengths based on multilayer graphene-dielectric stacks

To develop on-chip photonic devices capable of transmitting terahertz signals beyond the propagation distance of millimeter while keeping deep subwavelength field confinement has been a challenging task. Herein, we propose a novel multilayer graphene-based hybrid plasmonic waveguide (MLGHPW) consisting of a cylindrical dielectric waveguide and hyperbolic metamaterials. The device is based on alternating graphene and dielectric layers on a rib substrate, operating in the terahertz range (f = 3 THz). We couple the fundamental dielectric waveguide mode with the fundamental volume plasmon polarition modes originated from the coupling of plasmon polaritons at individual graphene sheets. The resulting hybrid mode shows ultra-low loss compared with the conventional GHPW modes at the comparable mode sizes. The present MLGHPW demonstrated a few millimeters of propagation length while keeping the mode area of 10^-3A₀, where A₀ is the diffraction-limited area, thus possessing a thirty times larger figure of merit (FoM) compared to other GHPWs. The additional degree of freedom (the number of graphene layers) makes the proposed MLGHPW more flexible to control the mode properties. We investigated the geometry and physical parameters of the device and identified optimal FoM. Moreover, we analyzed the crosstalk between waveguides and confirmed the potential to construct compact on-chip terahertz devices. The present design might have the possible extensibility to other graphene-like materials, like silicene, germanen, stanene etc.

Silicon-Based Optoelectronics Enhanced by Hybrid Plasmon Polaritons: Bridging Dielectric Photonics and Nanoplasmonics

Silicon-based optoelectronics large-scale integrated circuits have been of interest to the world in recent decades due to the need for higher complexity, larger link capacity, and lower cost. Surface plasmons are electromagnetic waves that propagate along the interface between a conductor and a dielectric, which can be confined several orders smaller than the wavelength in a vacuum and offers the potential for minimizing photonic circuits to the nanoscale. However, plasmonic waveguides are usually accompanied by substantial propagation loss because metals always exhibit significant resistive heating losses when interacting with light. Therefore, it is better to couple silicon-based optoelectronics and plasmonics and bridge the gap between micro-photonics and nanodevices, especially some nano-electronic devices. In this review, we discuss methods to enhance silicon-based optoelectronics by hybrid plasmon polaritons and summarize some recently reported designs. It is believed that by utilizing the strong light confinement of plasmonics, we can overcome the conventional diffraction limit of light and further improve the integration of optoelectronic circuits.

Planar metamaterial sensor with graphene elliptical rings in transmission mode

A periodic planar metamaterial sensor in the terahertz band based on surface plasmon polariton resonances is proposed and studied. The unit cell includes four half-elliptical graphene rings located on a three-layer substrate including a SiO₂ layer, an air gap, and another SiO₂ layer. The embedded air gap between the two layers of SiO₂ improves the sensitivity of the sensor. Parametric study is performed, and the effects of the dimensions of the elliptical rings, the air gap thickness, and the Fermi energy of graphene on resonant frequency, sensitivity, and figure of merit (FoM) are investigated and graphically illustrated. The parameters of the sensor are optimized to provide a high sensitivity with a suitable FoM. By changing the refractive index of the sensing environment from 1.2 to 2, maximum sensitivity of 21.1 µm/RIU with FoM 5.14 is provided. The performance of the sensor is compared with previous works, and it is shown that a considerable improvement in sensitivity is achieved. The proposed sensor is suitable for biosensing applications.

Theoretical Analysis of Terahertz Dielectric-Loaded Graphene Waveguide

The waveguiding of terahertz surface plasmons by a GaAs strip-loaded graphene waveguide is investigated based on the effective-index method and the finite element method. Modal properties of the effective mode index, modal loss, and cut-off characteristics of higher order modes are investigated. By modulating the Fermi level, the modal properties of the fundamental mode could be adjusted. The accuracy of the effective-index method is verified by a comparison between the analytical results and numerical simulations. Besides the modal properties, the crosstalk between the adjacent waveguides, which determines the device integration density, is studied. The findings show that the effective-index method is highly valid for analyzing dielectric-loaded graphene plasmon waveguides in the terahertz region and may have potential applications in subwavelength tunable integrated photonic devices.

Graphene-based plasmonic electro-optical SR flip-flop with an ultra-compact footprint

In this paper, we present a new concept of electro-optical plasmonic Set-Reset flip-flops at mid-infrared frequencies. We use the 3D finite-difference time-domain (FDTD) method to simulate and evaluate our designed flip-flop. In the proposed structure, the propagation of surface plasmon polaritons is controlled by applying an electrostatic field and the switching actions occur in the electrical domain while the output signal is in the form of light. The energy consumed by each switch is 2.5 fJ/bit. In this flip-flop, the no-change state of the flip-flop is realized by using a Bias port. The time response diagram indicates that the minimum extinction ratio of the flip-flop is 14.61 dB. The probability of various errors in the flip-flop state occurring due to the lack of synchronization between the switches is also considered by the FDTD simulations and it is shown that the device has a great performance against errors. Furthermore, the structure has an ultra-compact footprint of 1.62 µm². Our surveys show that no plasmonic flip-flop has been reported to date.

Graphene-Coated Nanowire Waveguides and Their Applications

In recent years, graphene-coated nanowires (GCNWs) have attracted considerable research interest due to the unprecedented optical properties of graphene in terahertz (THz) and mid-infrared bands. Graphene plasmons in GCNWs have become an attractive platform for nanoscale applications in subwavelength waveguides, polarizers, modulators, nonlinear devices, etc. Here, we provide a comprehensive overview of the surface conductivity of graphene, GCNW-based plasmon waveguides, and applications of GCNWs in optical devices, nonlinear optics, and other intriguing fields. In terms of nonlinear optical properties, the focus is on saturable absorption. We also discuss some limitations of the GCNWs. It is believed that the research of GCNWs in the field of nanophotonics will continue to deepen, thus laying a solid foundation for its practical application.

Graphene-Coated Elliptical Nanowires for Low Loss Subwavelength Terahertz Transmission

Graphene has been recently proposed as a promising alternative to support surface plasmons with its superior performances in terahertz and mid-infrared range. Here, we propose a graphene-coated elliptical nanowire (GCENW) structure for subwavelength terahertz waveguiding. The mode properties and their dependence on frequency, nanowire size, permittivity and chemical potential of graphene are studied in detail by using a finite element method, they are also compared with the graphene-coated circular nanowires (GCCNWs). Results showed that the ratio of the long and short axes (b/a) of the elliptical nanowire had significant influence on mode properties, they also showed that a propagation length over 200 μm and a normalized mode area of approximately 10−4~10−3 could be obtained. Increasing b/a could simultaneously achieve both long propagation length and very small full width at half maximum (FWHM) of the focal spots. When b/a = 10, a pair of focal spots about 40 nm could be obtained. Results also showed that the GCENW had a better waveguiding performance when compared with the corresponding GCCNWs. The manipulation of Terahertz (THz) waves at a subwavelength scale using graphene plasmon (GP) may lead to applications in tunable THz components, imaging, and nanophotonics.

Ultra-high light confinement and ultra-long propagation distance design for integratable optical chips based on plasmonic technology

The ever-increasing demand for faster speed, broader bandwidth, and lower energy consumption of on-chip processing has motivated the use of light instead of electrons in functional communication components. However, considerable scattering loss severely affects the performance of nanoscale photonic devices when their physical sizes are smaller than the wavelength of light. Due to the tight localization of electromagnetic energy, plasmonic waveguides that work at visible and infrared wavebands have provided a solution for the optical diffraction limit problem and thus enable downscaling of optical circuits and chips at the nanoscale. However, due to the fundamental trade-off between propagation distance and light confinement, plasmonic waveguides, including conventional hybrid plasmonic waveguides (HPWGs), cannot be used as high performance integratable optical devices all the time. To solve this problem, a novel hybrid plasmonic waveguide is proposed where a hybrid metal-ridge-slot structure based on a two-dimensional (2D) transition metal dichalcogenide is embedded into two identical cylindrical dielectric waveguides. Benefiting from both the loss-less slot region and the high-index difference between the ultra-thin 2D material and the slot region, a 10 times longer propagation length and 100 times smaller mode area than the traditional HPWG are achieved at the telecommunication band. By removing the monolayer transition metal dichalcogenide, our designed waveguide shows a higher propagation length that is at least two orders of magnitude larger than its traditional HPWG counterpart. Therefore, the proposed hybridization waveguiding approach paves the way toward truly high-performance and deep-subwavelength integratable optical circuits and chips in the future.

Tunable hybridization of graphene plasmons and dielectric modes for highly confined light transmit at terahertz wavelength

We theoretically report a novel graphene-based hybrid plasmonic waveguide (GHPW) by integrating a GaAs micro-tube on a silica spacer that is supported by a graphene-coated substrate. In comprehensive numerical simulations on guiding properties of the GHPW, it was found that the size of hybrid plasmonic mode (TM) can be reduced significantly to ~10^-4(λ²/4), in conjunction with long propagation distances up to tens of micrometers by tuning the the waveguide's key structure parameters and graphene's chemical potential. Moreover, crosstalk between two adjacent GHPWs that are placed on the same substrate has been analyzed and ultralow crosstalk can be realized. The proposed scheme potentially enables realization of the various high performance nanophotonic components-based subwavelength plasmonic waveguides in terahertz domain.

Hybrid low-permittivity slot-rib plasmonic waveguide based on monolayer two dimensional transition metal dichalcogenide with ultra-high energy confinement

A hybrid plasmonic waveguide design is proposed that incorporates a two-dimensional transition metal dichalcogenide monolayer covered slot-rib in between a cylindrical waveguide and a metal surface. A deep optical energy confinement (mode area ranging from λ²/1000000-λ²/100000) along with a reasonable propagation length (5μm-25μm) can be realized at the working wavelength of 1550 nm. In comparison with a traditional hybrid plasmonic waveguide, the proposed waveguide structure exhibits a smaller mode area as well as a higher figure of merit. Investigation on the influence of various two-dimensional materials on modal properties reveals that a larger permittivity provides a stronger field confinement. Owing to its excellent energy field confinement with low transmission loss, the proposed waveguide can be utilized in a variety of plasmonic devices such as compact plasmonic chips, high-integration plasmonic nano-lasers and high-sensitivity plasmonic detectors.