arXiv subjects

Tony Low

Publications and source records attributed to Tony Low.

At least 127 records · Page 7Linked to original sources

arXiv2016

Broadband achromatic anomalous mirror in near-IR and visible frequency range

The anomalous achromatic mirror operating in near-IR and visible frequency range was designed using an array of metal-insulator-metal (MIM) resonators. An incident wave interacting with MIM resonator experiences phase shift that is equal to the optical path travelled by the gap plasmon, excited by the wave. The phase gradient along the mirror surface is created through the difference in plasmons optical paths in resonators of different lengths. In the frequency region well below the plasma frequency of the metal, the phase gradient is a linear function of frequency, and thus the mirror operates in achromatic regime, i.e. reflection angle does not depend on the radiation frequency. Using silver-air-silver resonators, we predicted that the mirror can steer normally incident beam to angles as large as 40$^{\circ}$ with high radiation efficiency (exceeding 98 $\%$) and small Joule losses (below 10 $\%$).

cond-mat.mes-hall↗

arXiv2016

Pumping electrons in graphene to the $\mathbf{M}$-point in the Brillouin zone: The emergence of anisotropic plasmons

We consider the existence of plasmons in a non-equilibrium situation where electrons from the valence band of graphene are pumped to states in the Brillouin zone around the $\mathbf{M}$-point by a high intensity UV electromagnetic field. The resulting out-of-equilibrium electron gas is later probed by a weak electromagnetic field of different frequency. We show that the optical properties of the system and the dispersion of the plasmons are strongly anisotropic, depending on the pumping radiation properties: its intensity, polarization, and frequency. This anisotropy has its roots in the saddle-like nature of the electronic dispersion relation around that particular point in the Brillouin zone. It is found that despite the strong anisotropy, the dispersion of the plasmons scales with the square root of the wave number but is characterized an effective Fermi energy, which depends on the properties of the pumping radiation. Our calculations go beyond the usual Dirac cone approximation taking the full band structure of graphene into account. This is a necessary condition for discussing plasmons at the $\mathbf{M}$-point in the Brillouin zone.

cond-mat.mes-hall↗

arXiv2016

Band Alignment of 2D Semiconductors for Designing Heterostructures with Momentum Space Matching

We present a comprehensive study of the band alignments of two-dimensional (2D) semiconducting materials and highlight the possibilities of forming momentum-matched type I, II and III heterojunctions; an enticing possibility being atomic heterostructures where the constituent monolayers have band edges at the zone center. Our study, which includes the Group IV and III-V compound monolayer materials, Group V elemental monolayer materials, transition metal dichalcogenides (TMD) and transition metal trichalcogenides (TMT) reveals that almost half of these materials have conduction and/or valence band edges residing at the zone center. Using first-principles density functional calculations, we present the type of the heterojunction for 903 different possible combination of these 2D materials which establishes a periodic table of heterojunctions.

cond-mat.mtrl-sci↗

arXiv2016

Anisotropic 2D materials for tunable hyperbolic plasmonics

Motivated by the recent emergence of a new class of anisotropic 2D materials, we examine their electromagnetic modes and demonstrate that a broad class of the materials can host highly directional hyperbolic plasmons. Their propagation direction can be manipulated on-the-spot by gate doping, enabling hyperbolic beams reflection, refraction and bending. The realization of these natural 2D hyperbolic media opens up a new avenue in dynamic control of hyperbolic plasmons not possible in the 3D version.

cond-mat.mes-hall↗

arXiv2015

Chiral plasmon in gapped Dirac systems

We study the electromagnetic response and surface electromagnetic modes in a generic gapped Dirac material under pumping with circularly polarized light. The valley imbalance due to pumping leads to a net Berry curvature, giving rise to a finite transverse conductivity. We discuss the appearance of nonreciprocal chiral edge modes, their hybridization and waveguiding in a nanoribbon geometry, and giant polarization rotation in nanoribbon arrays.

cond-mat.mes-hall↗

arXiv2015

Mobility anisotropy in monolayer black phosphorus due to charged impurities

We study the charged impurity limited mobility in black phosphorus, a highly anisotropic layered material. We compute the mobility within the Boltzmann transport equation under detailed balance condition, and taking into account the anisotropy in transport and electronic structure. For carrier densities accessible in experiments, we obtained an anisotropy ratio of 3 ~ 4 at zero temperature, two-folds larger than that observed in experiments on multilayers samples. We discuss also how the anisotropy depends on carrier density and impurity distribution.

cond-mat.mtrl-sci↗

arXiv2015

Multilayer black phosphorus as a versatile mid-infrared electro-optic material

We investigate the electro-optic properties of black phosphorus (BP) thin films for optical modulation in the mid-infrared frequencies. Our calculation indicates that an applied out-of-plane electric field may lead to red-, blue-, or bidirectional shift in BP's absorption edge. This is due to the interplay between the field-induced quantum-confined Franz-Keldysh effect and the Pauli-blocked Burstein-Moss shift. The relative contribution of the two electro-absorption mechanisms depends on doping range, operating wavelength, and BP film thickness. For proof-of concept, simple modulator configuration with BP overlaid over a silicon nanowire is studied. Simulation result shows that operating BP in the quantum-confined Franz-Keldysh regime can enable improved maximal attainable absorption as well as power efficiency compared to its graphene counterpart.

cond-mat.mes-hall↗

arXiv2015

Plasmon coupling in extended structures: Graphene superlattice nanoribbon arrays

Interactions between localized plasmons in proximal nanostructures is a well-studied phenomenon. Here we explore plasmon plasmon interactions in connected extended systems. Such systems can now be easily produced using graphene. Specifically we employ the finite element method to study such interactions in graphene nanoribbon arrays with a periodically modulated electrochemical potential or number of layers. We find a rich variation in the resulting plasmonic resonances depending on the dimensions and the electrochemical potentials (doping) of the nanoribbon segments and the involvement of transverse and longitudinal plasmon interactions. Unlike predictions based of the well-known "orbital hybridization model", the energies of the resulting hybrid plasmonic resonances of the extended system can lie between the energies of the plasmons of the individual components. The results demonstrate the wide range tunability of the graphene plasmons and can help to design structures with desired spectra, which can be used to enhance optical fields in the infrared region of the electromagnetic spectrum.

cond-mat.mes-hall↗

arXiv2015

Topological currents in black phosphorus with broken inversion symmetry

We examine the nature of topological currents in black phosphorus when its inversion symmetry is deliberately broken. Here, the conduction and valence band edges are located at the $Γ$ point of the rectangular Brillouin zone, and they exhibit strong anisotropy along its two crystal axes. We will show below that these salient features lead to a linear transverse neutral topological currents, accompanied also by a non-linear transverse charge current at the Fermi surface. These topological currents are maximal when the in-plane electric field is applied along the zigzag crystal axes, but zero along the armchair direction.

cond-mat.mes-hall↗

arXiv2015

Magneto-electronic properties of multilayer black phosphorus

We examine the electronic properties of 2D electron gas in black phosphorus multilayers in the presence of a perpendicular magnetic field, highlighting the role of in-plane anisotropy on various experimental quantities such as ac magneto-conductivity, screening, and magneto-plasmons. We find that resonant structures in the ac conductivity exhibits a red-shift with increasing doping due to inter-band coupling, $γ$. This arises from an extra correction term in the Landau energy spectrum proportional to $n^2γ^2$ ($n$ is Landau index), up to second order in $γ$. We found also that Coulomb interaction leads to highly anisotropic magneto-excitons.

cond-mat.mes-hall↗

arXiv2015

Photonic and plasmonic guiding modes in graphene-silicon photonic crystals

We report systematic studies of plasmonic and photonic guiding modes in large-area chemical-vapor-deposition-grown graphene on nanostructured silicon substrates. Light interaction in graphene with substrate photonic crystals can be classified into four distinct regimes depending on the photonic crystal lattice constant and the various modal wavelengths (i.e. plasmonic, photonic and free-space). By optimizing the design of the substrate, these resonant modes can magnify the graphene absorption in infrared wavelength, for efficient modulators, filters, sensors and photodetectors on silicon photonic platforms.

physics.optics↗

arXiv2015

Tunable light-matter interaction and the role of hyperbolicity in graphene-hBN system

Hexagonal boron nitride (hBN) is a natural hyperbolic material which can also accommodate highly dispersive surface phonon-polariton modes. In this paper, we examine theoretically the mid-infrared optical properties of graphene-hBN heterostructures derived from their coupled plasmon-phonon modes. We found that the graphene plasmon couples differently with the phonons of the two Reststrahlen bands, owing to their different hyperbolicity. This also leads to distinctively different interaction between an external quantum emitter and the plasmon-phonon modes in the two bands, leading to substantial modification of its spectrum. The coupling to graphene plasmons allows for additional gate tunability in the Purcell factor, and narrow dips in its emission spectra.

physics.optics↗

arXiv2014

Multi-terminal electrical transport measurements of molybdenum disulphide using van der Waals heterostructure device platform

Atomically thin two-dimensional (2D) semiconductors such as molybdenum disulphide (MoS2) hold great promise in electrical, optical, and mechanical devices and display novel physical phenomena such as coupled spin-valley physics and the valley Hall effect. However, the electron mobility of mono- and few-layer MoS2 has so far been substantially below theoretically predicted limits, particularly at low temperature (T), which has hampered efforts to observe its intrinsic quantum transport behaviors. Potential sources of disorder and scattering include both defects such as sulfur vacancies in the MoS2 itself, and extrinsic sources such as charged impurities and remote optical phonons from oxide dielectrics. To reduce extrinsic scattering and approach the intrinsic limit, we developed a van der Waals (vdW) heterostructure device platform where MoS2 layers are fully encapsulated within hexagonal boron nitride (hBN), and electrically contacted in a multi-terminal geometry using gate-tunable graphene electrodes. Multi-terminal magneto-transport measurements show dramatic improvements in performance, including a record-high Hall mobility reaching 34,000 cm2/Vs for 6-layer MoS2 at low T. Comparison to theory shows a decrease of 1-2 orders of magnitude in the density of charged impurities, indicating that performance at low T in previous studies was limited by extrinsic factors rather than defects in the MoS2. We also observed Shubnikov-de Haas (SdH) oscillations for the first time in high-mobility monolayer and few-layer MoS2. This novel device platform therefore opens up a new way toward measurements of intrinsic properties and the study of quantum transport phenomena in 2D semiconducting materials.

cond-mat.mtrl-sci↗

arXiv2014

Gate-controlled mid-infrared light bending with aperiodic graphene nanoribbons array

Graphene plasmonic nanostructures enable subwavelength confinement of electromagnetic energy from the mid-infrared down to the terahertz frequencies. By exploiting the spectrally varying light scattering phase at vicinity of the resonant frequency of the plasmonic nanostructure, it is possible to control the angle of reflection of an incoming light beam. We demonstrate, through full-wave electromagnetic simulations based on Maxwell equations, the electrical control of the angle of reflection of a mid-infrared light beam by using an aperiodic array of graphene nanoribbons, whose widths are engineered to produce a spatially varying reflection phase profile that allows for the construction of a far-field collimated beam towards a predefined direction.

physics.optics↗

arXiv2014

Graphene Side Gate Engineering

Various mesoscopic devices exploit electrostatic side gates for their operation. In this paper, we investigate how voltage-biasing of graphene side gates modulates the electrical transport characteristics of graphene channel. We explore myriads of typical side gated devices such as symmetric dual side gates and asymmetric single side gate biasing, in monolayer and bilayer graphene. The side gates modulate the electrostatic doping in the graphene channel whose effect is reflected in transport measurement. This modulation efficiency is systematically characterized for all our devices and agrees well with the modeling presented.

cond-mat.mes-hall↗

arXiv2014

Nonlocal Electromagnetic Response of Graphene Nanostructures

Nonlocal electromagnetic effects of graphene arise from its naturally dispersive dielectric response. We present semi-analytical solutions of nonlocal Maxwell's equations for graphene nano-ribbons array with features around 100 nm, where we found prominent departures from its local response. Interestingly, the nonlocal corrections are stronger for light polarization parallel to the ribbons, which manifests as additional broadening of the Drude peak. For the perpendicular polarization case, nonlocal effects lead to blue-shifts of the plasmon peaks. These manifestations provide a physical measure of nonlocal effects, and we quantify their dependence on ribbon width, doping and wavelength.

physics.optics↗

arXiv2014

Valley Splitting and Polarization by the Zeeman Effect in Monolayer MoSe2

We have measured circularly polarized photoluminescence in monolayer MoSe2 under perpendicular magnetic fields up to 10 T. At low doping densities, the neutral and charged excitons shift linearly with field strength at a rate of $\mp$ 0.12 meV/T for emission arising, respectively, from the K and K' valleys. The opposite sign for emission from different valleys demonstrates lifting of the valley degeneracy. The magnitude of the Zeeman shift agrees with predicted magnetic moments for carriers in the conduction and valence bands. The relative intensity of neutral and charged exciton emission is modified by the magnetic field, reflecting the creation of field-induced valley polarization. At high doping levels, the Zeeman shift of the charged exciton increases to $\mp$ 0.18 meV/T. This enhancement is attributed to many-body effects on the binding energy of the charged excitons.

cond-mat.mes-hall↗

arXiv2014

Substrate Sensitive Mid-Infrared Photoresponse in Graphene

We report mid-infrared photocurrent spectra of graphene nanoribbon arrays on SiO2 dielectrics showing dual signatures of the substrate interaction. First, hybrid polaritonic modes of graphene plasmons and dielectric surface polar phonons produce a thermal photocurrent in graphene with spectral features that are tunable by gate voltage, nanoribbon width, and light polarization. Secondly, phonon-polaritons associated with the substrate are excited, which indirectly heat up the graphene leading to a graphene photocurrent with fixed spectral features. Models for other commonly used substrates show that the responsivity of graphene infrared photodetectors can be tailored to specific mid-IR frequency bands by the choice of the substrate.

cond-mat.mtrl-sci↗