Optically controllable coupling between edge and topological waveguide modes of certain graphene metasurfaces

In this paper, optically controllable and topologically protected plasmon transport is implemented via a topological nanohole plasmonic waveguide coupled to a standard edge mode of a graphene metasurface. By introducing nanoholes with different sizes in the unit cell, one breaks the spatial-inversion symmetry of a graphene metasurface in which the topological waveguide is constructed, leading to the emergence of topologically protected modes located in a nontrivial band-gap. Based on the strong Kerr effect and tunable optical properties of graphene, the coupling between the edge and topological interface modes can be efficiently controlled by optical means provided by an optical pump beam injected in a bulk mode. In particular, by tuning the power inserted in the bulk mode, one can control the difference between the wave-vectors of the topological and edge modes and consequently the optical power coupled in the topological mode. Our results show that when the pump power approaches a specific value, the edge and topological modes become phase-matched and the topological waveguide mode can be efficiently excited. Finally, we demonstrated that the optical coupling is strongly dependent on the group-velocity of the pump mode, a device feature that can be important in practical applications

All-optically Control of Light Propagation in Valley-Hall Topological Waveguides of Graphene Metasurfaces

We study the influence of graphene Kerr effect on valley-Hall topological modes of a graphene plasmonic crystal waveguide. Extra air holes are introduced to break the spatial-inversion symmetry of the plasmonic metasurface, which can be performed using e-beam lithography. As a result, a gapless Dirac cone and topologically protected edge modes form inside the nontrivial frequency bandgap. Taking advantage of the fact that graphene is a nonlinear optical material possessing an extremely large Kerr coefficient, we demonstrate that an all-optical switch can be implemented in this topological photonic system by controlling an optical signal propagating in the waveguide via a pump beam injected into the bulk modes of the metasurface. This work may lead to new graphene-based active topological photonic nanodevices

Valley-Hall Topological Transport in Graphene Plasmonic Crystal Waveguides

Due to immunity to disorder and structural imperfections, topologically-protected plasmonic modes have recently attracted increasing attention. Here, we introduce two different mechanisms to construct valley-Hall domain-wall interface waveguides in graphene plasmonic crystal to mimic the quantum valley-Hall effect. In the first case, we break the in-plane spatial inversion symmetry of a single-layer graphene plasmonic crystal waveguide to achieve valley-Hall topological characteristics, whereas in the second case, we break the out-of-plane spatial inversion symmetry of a bi-layer graphene plasmonic crystal waveguide to implement the analog quantum valley-Hall effect. A molecular sensor based on this valley-Hall topological transport phenomenon is also be presented

All-optical control of topological valley transport in graphene metasurfaces

We demonstrate that the influence of Kerr effect on valley-Hall topological transport in graphene metasurfaces can be used to implement an all-optical switch. In particular, by taking advantage of the large Kerr coefficient of graphene, the index of refraction of a topologically-protected graphene metasurface can be tuned via a pump beam, which results in an optically controllable frequency shift of the photonic bands of the metasurface. This spectral variation can in turn be readily employed to control and switch the propagation of an optical signal in certain waveguide modes of the graphene metasurface. Importantly, our theoretical and computational analysis reveals that the threshold pump power needed to optically switch ON/OFF the signal is strongly dependent on the group velocity of the pump mode, especially when the device is operated in the slow-light regime. This study could open up new routes towards active photonic nanodevices whose underlying functionality stems from their topological characteristics

Enhanced Second-Harmonic Generation in Monolayer MoS2 Driven by a BIC-based Nonlinear Metasurface

Dielectric metasurfaces have opened novel routes for nonlinear optics in recent years. In this work, we integrate a nonlinear metasurface with monolayer molybdenum disulfide (MoS2) to enhance second-harmonic generation (SHG) from atomically thin MoS2. By utilizing bound states in the continuum, we achieve about 600× of SHG enhancement from monolayer MoS2 on a resonant metasurface relative to suspended monolayer MoS2. Moreover, an eigenmode expansion approach is exploited to express second-harmonic power and the corresponding analytical results agree well with the rigorous calculations

Optically and Chemically Controllable Light Flow in Topological Plasmonic Waveguides Based on Graphene Metasurfaces

In this work, topologically-protected plasmon transport is demonstrated in graphene-based plasmonic crystal waveguides, the main ideas being subsequently applied to optically and chemically controllable nanodevices. In two configurations of topological graphene metasurfaces created by breaking their inversion symmetry, symmetry-protected Dirac cones associated to the underlying metasurfaces are gapped out, which leads to the formation of topological valley modes inside the nontrivial bandgap. The propagation of the corresponding topological modes shows unidirectional characteristics in both cases. Based on the proposed plasmonic topological waveguides, an active optical nanoswitch and a gas molecular sensor are designed by optically and chemically tuning the frequency dispersion of graphene metasurfaces via Kerr effect and gas molecular absorption, respectively. Specifically, the variation of the frequency dispersion of graphene can switch the topological mode into the region of leaky bulk modes, resulting in a dramatic variation of the plasmon transmission. Our work may contribute to the development of new ultracompact and ultrafast active photonic nanodevices based on graphene

MORPH-II : Inconsistencies and cleaning whitepaper

This paper presents a detailed summary of the inconsistencies in the non-commercial releaseof the MORPH-II dataset and covers the steps and strategy taken to clean it. In addition,examples of prior research that made use of the uncleaned data are briefly introduced and thepotential implications on their results are discussed

MORPH-II: Feature vector documentation

Four different subsets of the MORPH-II database were selected for a wide range of purposes, including age estimate, gender and race classification, and facial recognition. Due to the variety of poses and lighting conditions present in the MORPH-II database, certain preprocessing techniques were used to standardize the images and make them more suitable for machine learning tasks

MORPH-II: A proposed subsetting scheme

In this paper, we propose a new subsetting scheme for the longitudinal face aging databaseMORPH-II. Our subsetting scheme is intended to overcome the unbalanced racial and genderdistributions of MORPH-II, while ensuring independence between training and testing sets.Our subsetting scheme can be used for various face analysis tasks, including gender classica-tion, age prediction, and race classication

Probing the switching mechanism in ZnO nanoparticle memristors

We investigate the resistance switching mechanism in memristors based on colloidal ZnO nanoparticles using electroabsorption (EA) spectroscopy. In this EA experiment, we incorporate a small amount of low-bandgap polymer, poly(9,9-dioctylfluorene-cobenzothiadiazole) (F8BT), as a probe molecule in ZnO-nanoparticle memristors. By characterizing this polymer, we can study the change of built-in potential (VBI) in the device during the resistance switching process without disturbing the resistance state by the EA probe light. Our results show that VBI increases when the device is switched to the high resistance state, suggesting a shift of effective workfunction of the electrode. Thus, we attribute the resistance switching to the field-dependent migration of oxygen vacancies associated with the adsorption and desorption of oxygen molecules at the Al/ZnO interface. This process results in the modulation of the interfacial injection barrier which governs the resistance state of the device.This work was supported by the Engineering and Physical Sciences Research Council [Grant Number EP/G060738/1]Copyright 2014 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. This article "Probing the switching mechanism in ZnO nanoparticle memristors" has been accepted by Journal of Applied Physics. After it is published, it will be found at http://scitation.aip.org/content/aip/journal/ja