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Optical lenses are pervasive in various areas of sciences and technologies. It is well-known that the resolving power of a lens and thus optical systems is limited by the diffraction of light. Recently, various plasmonics and metamaterials based superlenses have been emerging to achieve super resolution. Here, we show that the phase compensated negative refraction lenses perform as Janus Lenses, i.e. either converging lenses or diverging lenses depending on the illumination directions. Extraordinary imaging equations and properties that are different from those of all the existing optical lenses are also presented. These new imaging properties, along with the super resolving power, significantly expand the horizon of imaging optics and optical system design.
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Surface plasmon polaritons carrying orbital angular momentum are of great fundamental and applied interest. However, common approaches for their generation are restricted to having a weak dependence on the properties of the plasmon-generating illumination, providing a limited degree of control over the amount of delivered orbital angular momentum. Here we experimentally show that by tailoring local and global geometries of vortex generators, a change in circular polarization handedness of light imposes arbitrary large switching in the delivered plasmonic angular momentum. Using time-resolved photoemission electron microscopy we demonstrate pristine control over the generation and rotation direction of high-order plasmonic vortices. We generalize our approach to create complex topological fields and exemplify it by studying and controlling a bright vortex, exhibiting the breakdown of a high-order vortex into a mosaic of unity-order vortices while maintaining the overall angular momentum density. Our results provide tools for plasmonic manipulation and could be utilized in lab-on-a-chip devices.
Based on first-principles calculations, we studied the geometric configuration, stability and electronic structure of the two-dimensional Janus MoTeB2. The MoTeB2 monolayer is semimetal, and its attractive electronic structure reveals the perfect electron-hole compensation. Moreover, the electron-type and hole-type bands of the MoTeB2 monolayer are easily adjustable by external stain and charge doping, such as the switch of carrier polarity by charge doping, and the metal-semiconductor transition under tensile stain. These properties allow the MoTeB2 monolayer to be a controllable two-dimensional material with extraordinary large magnetoresistance in magnetic field.
We comment on the macroscopic model for surface plasmons of H.-Y. Deng [New J. Phys. 21 (2019) 043055; arXiv:1712.06101] and a claim, based on energy conversion from charges to the electric field, that surface plasmons on metallic surfaces may become unstable [J. Phys.: Cond. Matt. 29 (2017) 455002; arXiv:1606.06239, 1701.01060]. The discussion revolves around the formulation of charge conservation in the bulk and the surface of a metal. We elaborate in particular on the role of a finite electric current normal to the surface. Using a scheme of Cercignani and Lampis and of Zaremba, we point out that the model chosen by Deng for the non-specular scattering of electrons needs to be amended to prevent the disappearance of charges at the surface. Different models and approaches in the literature on surface plasmons are reviewed: the interfacial excess field approach of Bedeaux and Vlieger which contains Dengs macroscopic model, the assumption of specular reflection of Ritchie and Marusak, a hydrodynamic model with a composite charge density (partially localized at the surface), the local dielectric model, and a macroscopic method with (anti)symmetric fictitious stimuli (used, e.g., by Garc{i}a-Moliner and Flores). This puts Dengs results into perspective and illustrates problems with his approach.
The properties of suspended graphene are currently attracting enormous interest, but the small size of available samples and the difficulties in making them severely restrict the number of experimental techniques that can be used to study the optical, mechanical, electronic, thermal and other characteristics of this one-atom-thick material. Here we describe a new and highly-reliable approach for making graphene membranes of a macroscopic size (currently up to 100 microns in diameter) and their characterization by transmission electron microscopy. In particular, we have found that long graphene beams supported by one side only do not scroll or fold, in striking contrast to the current perception of graphene as a supple thin fabric, but demonstrate sufficient stiffness to support extremely large loads, millions of times exceeding their own weight, in agreement with the presented theory. Our work opens many avenues for studying suspended graphene and using it in various micromechanical systems and electron microscopy.
We present a pair of optimized objective lenses with long working distances of 117~mm and 65~mm respectively that offer diffraction limited performance for both Cs and Rb wavelengths when imaging through standard vacuum windows. The designs utilise standard catalog lens elements to provide a simple and cost-effective solution. Objective 1 provides $mathrm{NA}=0.175$ offering 3~$mu$m resolution whilst objective 2 is optimized for high collection efficiency with $mathrm{NA}=0.29$ and 1.8~$mu$m resolution. This flexible design can be further extended for use at shorter wavelengths by simply re-optimising the lens separations.
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