Isotropic Graphene Research Articles

Anisotropic perfect absorber based on black phosphorus-graphene

Two-dimensional materials have garnered extensive attention in the realm of optical devices due to their unique physical properties. In this paper, we propose a tunable and anisotropic perfect absorber composed of anisotropic black phosphorus (BP) and isotropic graphene. The plasmon resonance of this anisotropic material and isotropic material achieves a strong anisotropic plasma response. The proposed structure demonstrates absorption of 99.73 % and 98.35 % in the x- and y-polarization directions, respectively. By adjusting the geometric parameters and the levels of doping with graphene and BP, the resonance characteristics can be effectively controlled. In addition, the robustness to incident angle and polarization sensitivity is also illustrated. Consequently, our research offers broad prospects for applications in polarization-selective and tunable high-performance devices, such as polarizers, modulators, and mid-infrared spectral imaging.

Active Tuning and Anisotropic Strong Coupling of Terahertz Polaritons in Van der Waals Heterostructures.

Electromagnetic field confinement is significant in enhancing light-matter interactions as well as in reducing footprints of photonic devices especially in Terahertz (THz). Polaritons offer a promising platform for the manipulation of light at the deep sub-wavelength scale. However, traditional THz polariton materials lack active tuning and anisotropic propagation simultaneously. In this paper, we design a graphene/α-MoO3 heterostructure and simulate polariton hybridization between isotropic graphene plasmon polaritons and anisotropic α-MoO3 phonon polaritons. The physical fundamentals for polariton hybridizations depend on the evanescent fields coupling originating from the constituent materials as well as the phase match condition, which can be severely affected by the α-MoO3 thickness and actively tuned by the gate voltages. Hybrid polaritons propagate with in-plane anisotropy that exhibit momentum dispersion characterized by elliptical, hyperboloidal and even flattened iso-frequency contours (IFCs) in the THz range. Our results provide a tunable and flexible anisotropic polariton platform for THz sensing, imaging, and modulation.

Unconventional and Dynamically Anisotropic Thermal Conductivity in Compressed Flexible Graphene Foams.

Although a variety of methods to predict the effective thermal conductivity of porous foams have been proposed, the response of such materials under dynamic compressive loading has generally not been considered. Understanding the dynamic thermal behavior will widen the potential applications of porous foams and provide insights into methods of modifying material properties to achieve desired performance. Previous experimental work on the thermal conductivity of a flexible graphene composite under compression showed intriguing behavior: the cross-plane thermal conductivity remained approximately constant with increasing compression, despite the increasing mass density. In this work, we use molecular dynamics (MD) simulations and finite element analysis to study the variation in both the cross-plane and in-plane thermal conductivities by compressing isotropic graphene foams. We have found that, interestingly, the cross-plane thermal conductivity decreases with compression while the in-plane thermal conductivity increases; hence, the dynamic thermal transport of the graphene foam becomes anisotropic with a significant anisotropy ratio. Such observations cannot be explained by the conventional effective medium theory, which describes the increase of thermal conductivity to be proportional to mass density. Thus, we propose a model that can describe such anisotropic effective thermal conductivity of highly porous open-cell media during compression. The model is validated against the MD simulations as well as a larger-scale finite element simulation of an open-cell foam geometry.

Nonlinear optical properties of anisotropic two-dimensional layered materials for ultrafast photonics

Abstract The discovery of graphene has intrigued the significant interest in exploring and developing the two-dimensional layered materials (2DLMs) for the photonics application in recent years. Unlike the isotropic graphene, a number of 2DLMs possess the in-plane anisotropic crystal structure with low symmetry, enabling a new degree of freedom for achieving the novel polarization-dependent and versatile ultrafast photonic devices. In this review article, we focus on the typical anisotropic 2DLMs including BP, ReS2, ReSe2, SnS, and SnSe and summarize the recent development of these anisotropic 2DLMs in the pulsed laser and the optical switch applications. First, we introduce the fabrication methods as well as the material characterization of the anisotropic 2DLMs by analyzing the polarized Raman configuration. Second, we discuss the anisotropic nonlinear optical properties of the anisotropic 2DLMs and concentrate on the anisotropic nonlinear absorption response. Next, we sum up state of the art of the anisotropic 2DLMs in the application of pulse lasers and optical switches. This review ends with perspectives on the challenge and outlook of the anisotropic 2DLMs for ultrafast photonics applications.

Plasmonically induced transparency in in-plane isotropic and anisotropic 2D materials.

General two-dimensional (2D) material-based systems that achieve plasmonically induced transparency (PIT) are limited to isotropic graphene only through unidirectional bright-dark mode interaction. Moreover, it is challenging to extend these devices to anisotropic 2D films. In this study, we exploit surface plasmons excited at two crossed grating layers, which can be formed either by dielectric gratings or by the 2D sheet itself, to achieve dynamically tunable PIT in both isotropic and anisotropic 2D materials. Here, each grating simultaneously acts as both bright and dark modes. By taking isotropic graphene and anisotropic black phosphorus (BP) as proofs of concept, we reveal that this PIT can result from either unidirectional bright-dark or bidirectional bright-bright and bright-dark mode hybridized couplings when the incident light is parallelly/perpendicularly or obliquely polarized to the gratings, respectively. Identical grating parameters in isotropic (crossed lattice directions in anisotropic) layers produce polarization-independent single-window PIT, whereas different grating parameters (coincident lattice directions) yield polarization-sensitive double-window PIT. The proposed technique is examined by a two-particle model, showing excellent agreement between the theoretical and numerical results. This study provides insight into the physical mechanisms of PIT and advances the applicability and versatility of 2D material-based PIT devices.

Partial positive refraction in asymmetric Veselago lenses of uniaxially strained graphene

Asymmetric Veselago lenses (AVLs) can be created from ballistic $p\text{\ensuremath{-}}n$ and $n\text{\ensuremath{-}}p\text{\ensuremath{-}}n$ homojunctions of uniaxially strained graphene. This atypical converging electron flow emerges by applying uniaxial tension out of the device's symmetry axes. Part of the electron flow needs to be positively refracted to focus on an asymmetric spot, whose location is tunable with the strain. In AVLs, Klein tunneling is angularly shifted with regard to the normal incidence. This perfect transmission occurs in the straight line that connects the point source and focus, which is unaffected by variation of the Fermi level and voltage gate. Moreover, the mirror symmetry breaking by the strain also causes the asymmetry in Fabry-P\'erot interference. The novel electron optical laws allow us to evidence that reflected and refracted electrons in AVLs lie on the same straight line with opposite group velocities and pseudospins. Unlike isotropic graphene, electrons under normal incidence present backscattering, angles of reflection, and refraction different from zero. The average particle transmission is higher (lower) than the isotropic case when the tensile strain is increased near (far away from) the normal direction. These results may be useful for designing strain-bendable probing tips in scanning tunneling microscopes.

Analysis of a graphene-based silicon electro-absorption modulator in isotropic and anisotropic graphene models

We theoretically investigated a graphene-based silicon electro-absorption modulator in both isotropic and anisotropic graphene models. Regardless of the graphene model, the optical transmission increases with the chemical potential of graphene because of the Pauli blocking principle. However, we found that the modulator based on the isotropic graphene exhibits an abrupt decrease in transmission for the transverse-magnetic (TM) polarization mode due to the epsilon-near-zero effect in the isotropic graphene at a certain chemical potential. Conversely, the anisotropic graphenebased modulator exhibits no such transmission dip. These particular behaviors provide us with a simple and easy experimental way to confirm whether graphene is an isotropic or anisotropic material. To further enhance the transmission dip in the isotropic graphene model, we suggest a simply modified modulator structure that increases the transmission depth by ~ 0.5 dB/μm for the TM polarization mode.

Silicon Electro-Absorption Modulator Based on Graphene-Hexagonal Boron Nitride Heterostructure

Graphene, a monolayer of carbon atoms, is one of most interesting optical materials in nanoscale photonics because its unique optoelectronic properties have allowed us to develop fast and broadband photonic devices. Electro-optic modulators have been designed with graphene to improve their performance and to more precisely understand their operating principles. To expand the applications of photonic device design, here we propose a silicon electro-absorption modulator, based on a graphene-hexagonal boron nitride heterostructure. The optical characteristics of the proposed modulator were numerically investigated considering an anisotropic and an isotropic graphene model. In the anisotropic graphene model, the light absorption by graphene is based on the Pauli blocking principle, while graphene's epsilon-near-zero effect is dominant in the optical power modulation under the isotropic graphene model. We performed optimization of the key structural parameters to achieve satisfactory modulation characteristics.

Enhanced Raman Scattering on In-Plane Anisotropic Layered Materials

Surface-enhanced Raman scattering (SERS) on two-dimensional (2D) layered materials has provided a unique platform to study the chemical mechanism (CM) of the enhancement due to its natural separation from electromagnetic enhancement. The CM stems from the charge interactions between the substrate and molecules. Despite the extensive studies of the energy alignment between 2D materials and molecules, an understanding of how the electronic properties of the substrate are explicitly involved in the charge interaction is still unclear. Lately, a new group of 2D layered materials with anisotropic structures, including orthorhombic black phosphorus (BP) and triclinic rhenium disulfide (ReS2), has attracted great interest due to their unique anisotropic electrical and optical properties. Herein, we report a unique anisotropic Raman enhancement on few-layered BP and ReS2 using copper phthalocyanine (CuPc) molecules as a Raman probe, which is absent on isotropic graphene and h-BN. According to detailed Raman tensor analysis and density functional theory calculations, anisotropic charge interactions between the 2D materials and molecules are responsible for the angular dependence of the Raman enhancement. Our findings not only provide new insights into the CM process in SERS, but also open up new avenues for the exploration and application of the electronic properties of anisotropic 2D layered materials.

Electromagnetic energy within coated cylinders at oblique incidence and applications to graphene coatings.

We address electromagnetic (EM) wave scattering by an infinite coated cylinder at an arbitrary incidence angle. The time-averaged EM energy stored inside the core-shell cylinder is analytically calculated for TM- and TE-polarized incident plane waves. An analytical expression relating the internal energy to the absorption cross section is derived. As an application, the EM energy inside dielectric cylinders coated with isotropic graphene layers epitaxially grown on silicon carbide (SiC) is studied. We find that off-resonance field enhancement occurs in graphene SiC microshells for TM-polarized terahertz waves, a phenomenon that can be explained in terms of Fano resonances.

Stability analysis of graphene based laminated composite sheets under non-uniform inplane loading by nonlocal elasticity

Size dependent buckling of composite laminates made of isotropic graphene layers interlaid with bonding agents is considered. Nonlocal theory of elasticity is used in the buckling analysis to reflect the size scale effects on the critical buckling loads which is discussed in detail. The method is capable of predicting the relative buckling modes for non-uniform inplane loading applied through the thickness of the laminate. All modes of buckling in which the layers may displace together or opposite one another are investigated to study their scale dependent effects. Displacement or load controls are implemented through independent parameters as constraints to form special combination of buckling modes. Each graphene sheet is considered as a Kirchhoff plate model. The interlaid bonding agent is laterally treated as Winkler elastic foundation between graphene layers while neglecting their other load carrying capacities. Various numerical results are obtained reflecting the nonlocality effects. It is observed that in cases of higher load ratios and simpler buckling modes, the effect of nonlocality tends to drastically increase. The results of simpler examples studied are verified by another reference.

An atomistic and non-classical continuum field theoretic perspective of elastic interactions between defects (force dipoles) of various symmetries and application to graphene

Force multipoles are employed to represent various types of defects and physical phenomena in solids: point defects (interstitials, vacancies), surface steps and islands, proteins on biological membranes, inclusions, extended defects, and biological cell interactions among others. In the present work, we (i) as a prototype simple test case, conduct quantum mechanical calculations for mechanics of defects in graphene sheet and in parallel, (ii) formulate an enriched continuum elasticity theory of force dipoles of various anisotropies incorporating up to second gradients of strain fields (thus accounting for nonlocal dispersive effects) instead of the usual dispersion-less classical elasticity formulation that depends on just the strain (c.f. Peyla, P., Misbah, C., 2003. Elastic interaction between defects in thin and 2-D films. Eur. Phys. J. B. 33, 233–247). The fundamental Green's function is derived for the governing equations of second gradient elasticity and the elastic self and interaction energies between force dipoles are formulated for both the two-dimensional thin film and the three-dimensional case. While our continuum results asymptotically yield the same interaction energy law as Peyla and Misbah for large defect separations (∼1/ r n for defects with n-fold symmetry), the near-field interactions are qualitatively far more complex and free of singularities. Certain qualitative behavior of defect mechanics predicted by atomistic calculations are well captured by our enriched continuum models in contrast to classical elasticity calculations. For example, consistent with our atomistic calculations of defects in isotropic graphene, even two dilation centers show a finite interaction (as opposed to classical elasticity that predicts zero interaction). We explicitly find the physically consistent result that the self-energy of a defect is equivalent to half the interaction energy between two identical defects when they “merge” into each other. The atomistic, classical elastic and the enriched continuum predictions are thoroughly compared for two types of defects in graphene: Stone-Wales and divacancy.