Graphene-based surface plasmon waveguide with the anisotropic clad

A graphene-based surface plasmon (SP) waveguide switch is designed at near infrared frequencies. First, graphene is modeled in electronic and electromagnetic solvers, in order to use graphene strips for designing the waveguide. Because of finite size of these strips, propagating surface plasmon polaritons (SPPs) along them exhibit spatial dispersion. By considering this and shifting interband losses to higher frequencies (by doping the graphene strips), the waveguide switch is designed to operate at 30 THz. In this paper, chemical potential in the graphene strips is controlled through applying back and top gate voltages which provide a reconfigurable waveguide switch and the chemical potentials of the strips are changed so that ultra-confinement and extremely low-loss propagation are achieved. The designed switch provides extremely low return loss and ultra-bandwidth in 30--40 THz frequency range. Due to the spatial dispersion in the graphene strips, new procedures are proposed to calculate the return and insertion losses of the switch and also absorbed power in the graphene strips.

We present theoretically the transport of plasmonic waves in doped graphene tube, which is made by rolling planar graphene sheet into a cylinder and periodic doping is applied on it. It is shown that periodic modulation of the Fermi level along the tube can open gaps in the dispersion relations of graphene plasmons and eventually create plasmonic band structures. The propagation of graphene plasmons is forbidden within the bandgaps; while within the band, the plasmonic waves present axially-extended field distributions and propagate along the tubes, yet well confined around the curved graphene surface. Furthermore, the bandgaps, propagation constants and propagation lengths of the modes in plasmonic band structures are significantly tuned by varying the Fermi level of graphene, which provides active controls over the plasmonic waves. Our proposed structures here may provide an approach to dynamically control the plasmonic waves in graphene-based subwavelength waveguides.

Advancement in technology has opened the doors for the terahertz (THz) frequency range to be applied in different fields for various applications. The future communication technology, especially 6G, will also intend to utilize the THz frequency band due to its large bandwidth that has the capabilities to achieve a high data rate. Great losses are presented in the early research into terahertz transmission medium. It is critical to design an appropriate waveguide that can integrate the THz waves into the system efficiently with minimum loss and provides the ease of transmission of data and overcomes the free space loss issues. Communication, sensing, and other application parameters are highly affected by transmission losses; therefore, low transmission loss and dispersion loss waveguide designs are required for proper utilization. In this paper, the review on reduction in the transmission loss in different types of waveguides operating at the Terahertz frequency range is studied. The design and the experimental setup for several classes of THz waveguides for minimizing transmission loss are also discussed. The review study shows that these waveguides can be a promising transmission medium for future 6G communication.

In this paper, the real parts of the effective refractive indexes and the propagating lengths of five low-order modes of the terahertz waveguides based on three graphene-coated dielectric nanowires are analyzed by using the multipole method. The formation of these five lowest order modes can be attributed to the five combinations between the two lowest order modes supported when three nanowires exist alone. Therefore they are named Mode 1, Mode 2, Mode 3, Mode 4, and Mode 5 in sequence. The results show that the mode characteristics of the waveguide can be effectively tuned by changing the operating frequency, the radius of the intermediate nanowire, the gap distance between the nanowires and the Fermi energy of graphene. As the operating frequency increases from 30 THz to 40 THz, the real part of each of the effective refractive indexes increases and the propagation length decreases, and the crossover phenomenon occurs in the process of change. In addition, the real parts of the effective refractive indexes and the propagation lengths of Modes 3 and 4 are basically the same. When the radius of the middle nanowire increases from 25 nm to 75 nm, the real parts of the effective refractive indexes of Modes 1 and 2 increase, and the propagation length of Mode 1 decreases and then increases. Besides the real parts of the effective refractive indexes and the propagation lengths of Modes 3 and 4 are basically not affected by the change of radius, and the values of these two modes are basically the same. For Mode 5, the real part of the effective refractive index and propagation length slowly increase. When the spacing between the nanowires increases from 10 nm to 50 nm, Modes 3 and 4 are basically unaffected by the change of spacing, and the values of these two modes are basically the same. The real parts of the effective refractive indexes of the other modes decrease and the propagation lengths increase and eventually stabilize, and the crossover phenomenon occurs in the process of change. As the Fermi energy of graphene increases from 0.4 eV to 1.2 eV, the real part of the effective refractive index decreases and the propagation length increases. The calculation shows that the result obtained by the multipole method is exactly the same as that obtained by the finite element method. To date, no one has analyzed the terahertz waveguides based on three graphene-coated dielectric nanowires. This work can provide a theoretical basis for the design, fabrication and application of terahertz waveguide based on graphene-coated dielectric nanowires. Such waveguides have potential applications in the field of mode-division multiplexing.

Simplified design of a hybrid plasmonic waveguide to maximize propagation length

Silicon-based integrated photonics became a very attractive technology for guiding and manipulating of light in highly integrated structures due to the large index contrast between silicon and cladding materials allowing for very high mode confinement. Moreover, these structures can be realized by conventional planar CMOS techniques. Different passive devices based on Si nanowire waveguides have been realized using SOI technology or amorphous silicon deposition with applications in highly integrated communication systems for optical networking, bio and sensing, as well as for computer interconnects in data centers. Nevertheless for future use, especially for inter-core and intra-core computer communication, structures allowing for subwavelength light confinement based on surface plasmon waveguiding are an attractive solution. Different methods to decrease or compensate the intrinsic high losses of these structures have been proposed. Here we report our work on design and experimental realization of hybrid plasmonic waveguides and devices that allow for considerable decrease of losses, still keeping sub-wavelength light confinement.

Graphene is different from most optical materials in that it is a thin material layer with a thickness as small as one atom. Graphene layers can be incorporated into optical simulations using either a surface conductivity material model or a volumetric permittivity material model; however, introducing graphene through a volumetric permittivity is computationally inefficient because it requires very fine discretization grids. We have recently developed a more efficient approach that enables the use of comparatively coarse grids by formulating a discretization of Maxwell’s equations (in the time or frequency domains) that combines a surface conductivity description of graphene layers with a volumetric permittivity description of other optical materials. This approach includes the full dispersion characteristics of graphene as specified by the Kubo formula. This paper demonstrates how the combined material description approach can be used to efficiently model state-of-the-art devices that take advantage of the energy confinement provided by surface plasmons. We show how to efficiently model TE and TM polarized surface plasmons, a surface plasmon waveguide switch, and an electro-optical modulator. This last example also includes electrical simulations of graphene and demonstrates how both optical and electrical simulations can be combined to produce a complete model of a graphene based device. For each example, we compare with previously published results, including experimental results.

Based on the coupled-dipole analysis and finite-difference time-domain simulation, we have investigated the surface plasmon propagation in one-dimensional metallic nanoparticle (NP) chains. Our systematic studies reveal that the interplay between the localized plasmon excitation and the lattice collective behavior leads to two phases (I and II) of different electromagnetic (EM) field transport properties. In phase I, the EM field decays follow the power-law. In phase II, the EM field shows the exponential decay in the short-distance regime and the power-law decay in the long-distance regime. Moreover, universal power-law exponents have been found in the long propagation distance. Different EM field propagation behaviors (power-law decay with different exponents, exponential decay with different propagation length) can be transformed to each other by tuning the parameters of the excitation fields (wavelength, polarization) and/or those of the NP chains. The EM field transport mechanisms we have found are very useful in the design of plasmonic waveguide with both strong field confinement and efficient field/energy transfer, which has important applications in integrated nanophotonic circuits.

In this paper, we propose dynamically tunable plasmon induced transparency (PIT) in a graphene-based nanoribbon waveguide structure by changing the chemical potential of graphene. It is the direct destructive interference between the propagating plasmonic edge mode in the graphene nanoribbon waveguide and the rectangular resonators gives rise to the PIT effect. Our numerical results reveal that high tunability in the PIT transparency window can be obtained by altering the chemical potential of the graphene rectangular resonators. Moreover, a novel plasmonic refractive index sensor (RIS) has been proposed and investigated numerically based on the PIT effect in the mid-IR range. Our calculated results exhibit that large wavelength sensitivity as high as 2500 nm/RIU and a high figure of merit (FOM) of 10.50 can be achieved in this ultra-compact structure (<0.05 μm2 ). This work not only paves a new way towards the realization of graphene-based integrated nanophotonic devices, but also has important applications in multi-channel-selective filters, sensors, and slow light.

An optical waveguide for working in mid-infrared range is highly desirable. However, such a waveguide is required to yield a long propagation distance and deep subwavelength confinement. Although the graphene-based surface plasmon polariton (SPP) waveguides have exhibited some potential in this regard, their propagation lengths are currently insufficient (~100 μm). In this paper, we propose a graphene-based cylindrical long-range SPP (LRSPP) waveguide, which is composed of a cylindrical silicon nanowire core surrounded by an inner graphene layer, a silica layer, and an outer graphene layer from inside to out. First, the electromagnetic field equation and the dispersion equation of this waveguide are derived. Then, combining these derived equations with the numerical simulation, the propagation properties of the LRSPP and the short-range SPP modes are analyzed. The results show that the proposed waveguide with the LRSPP fundamental mode has a notable advantage for simultaneously achieving a larger propagation length (~10 μm) and a deep subwavelength confinement (~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> , where A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the diffraction-limited mode area). This waveguide has potential for application in high-density photonic integrated circuits in the mid-infrared range.

The resolution of conventional terahertz (THz) imaging techniques is limited to about half wavelength, which is not fine enough for applications of biomedical sensing and nondestructive testing. To improve the resolution, a new superlens, constructed by a monolayer graphene sheet combining with a grating voltage gate, are proposed in this paper to achieve deep super-resolution imaging in the THz frequency range. The main idea is based on the Fabry-Perot resonance of graphene edge plasmon waves. By shaping the voltage gate into a radial pattern, magnified images of subwavelength targets can be obtained. With this approach, the finest resolution can achieve up to λ/150. Besides, the superlens can be conveniently tuned to work in a large frequency band ranging from 4.3 THz to 9 THz. The proposal could find potential applications in THz near-field imaging systems.

Several optical surface sensing techniques, such as Surface Plasmon Resonance (SPR), work by imaging the base of a prism by one of its faces. However, such a fundamental optical concern has not been fully analyzed and understood so far, and spatial resolution remains a critical and controversial issue. In SPR, the propagation length L(x) of the surface plasmon waves has been considered as the limiting factor. Here, we demonstrate that for unoptimized systems geometrical aberrations caused by the prism can be more limiting than the propagation length. By combining line-scan imaging mode with optimized prisms, we access the ultimate lateral resolution which is diffraction-limited by the object light diffusion. We describe several optimized configurations in water and discuss the trade-off between L(x) and sensitivity. The improvement of resolution is confirmed by imaging micro-structured PDMS stamps and individual living eukaryote cells and bacteria on field-of-view from 0.1 to 20 mm(2).

Data and analyses are presented on the first synthetic polypeptide system to exhibit mechanochemical coupling; the mechanochemical coupling can also be demonstrated to be both polymer-based and solvent-based with respect to where the result of the change in the chemical potential is focused. Both polymer-based and solvent-based processes are the result of chemomechanical transduction in which the change in chemical potential results in a change in the temperature at which an inverse temperature transition occurs. In the polymer-based process, the contraction/relaxation occurs due to a change in the chemical nature of the polypeptide; in the solvent-based process there is no change in the chemical nature of the polypeptide on contraction or relaxation, but rather the change in chemical potential changes the state of hydration of the polypeptide. The new mechanochemical system provides an experimental system with which to clarify and to quantitate what may be called aqueous mediated apolar–polar interaction energies in polypeptides and proteins with hydrophobic groups that may be variously exposed to the aqueous solution or buried within the folded polypeptide or protein. Furthermore, it is noted that any conformational change exhibited by a polypeptide or protein that is the result of a binding of a chemical moiety, the change in chemical nature of a bound moiety or the change in chemical potential of the medium can be viewed in terms of mechanochemical coupling or chemomechanical transduction.

Tunable dynamic metamaterial for negative refraction

Ferroelectric liquid crystals (FLC's) form the thermodynamically stable Smectic C* liquid crystalline phase that is non-centrosymmetric and has macroscopic polar order. The symmetry (C2) implies that FLC materials can exhibit second order nonlinear properties such as the linear electro-optic(EO) effect and second harmonic generation (SHG). In most FLC's designed for optical modulation applications these nonlinearities are small1,2, but in principle could be made, large by specifically synthesizing liquid crystal molecules which self-assemble in such a way that functional groups with large microscopic hyperpolarizabilities, β's, are oriented along the polar axis. By adopting this approach to produce second order nonlinear material, the problems associated with processing organic single crystals are avoided. Furthermore, the flexibility of the liquid crystal technology can be used to design materials with specific refractive indices for phase matching purposes and waveguide design. Waveguides in this material system are particularly interesting as they allow a significant increase in optical propagation length with high electric field generation capability; both important attributes for nonlinear optical phenomena. Preliminary work has already been carried out on waveguide structures whose possibilities are numerous owing to the ability to successfully align FLC's on many different substrates (including Si)3.