Going Green with Nanophotonics Plasmons are optically induced collective electronic excitations tightly confined to the surface of a metal, with silver being the metal of choice. The subwavelength confinement offers the opportunity to shrink optoelectronic circuits to the nanometer scale. However, scattering processes within the metal lead to losses. Lu et al. (p. 450 ) developed a process to produce atomically smooth layers of silver, epitaxially grown on silicon substrates. A cavity in the silver layer is capped with a SiO insulating layer and an AlGaN nanorod was used to produce a low-threshold emission at green wavelengths.

We report on the first demonstration of broadband tunable, single-mode plasmonic nanolasers (spasers) emitting in the full visible spectrum. These nanolasers are based on a single metal-oxide-semiconductor nanostructure platform comprising of InGaN/GaN semiconductor nanorods supported on an Al2O3-capped epitaxial Ag film. In particular, all-color lasing in subdiffraction plasmonic resonators is achieved via a novel mechanism based on a property of weak size dependence inherent in spasers. Moreover, we have successfully reduced the continuous-wave (CW) lasing thresholds to ultrasmall values for all three primary colors and have clearly demonstrated the possibility of "thresholdless" lasing for the blue plasmonic nanolaser.

Using atomically smooth epitaxial silver films, new optical permittivity highlighting significant loss reduction in the visible frequency range is extracted. Largely enhanced propagation distances of surface plasmon polaritons are measured, confirming the low intrinsic loss in silver. The new permittivity is free of extrinsic spectral features associated with grain boundaries and localized plasmons inevitably present in thermally deposited films.

The subject of topological materials has attracted immense attention in condensed-matter physics because they host new quantum states of matter containing Dirac, Majorana, or Weyl fermions. Although Majorana fermions can only exist on the surface of topological superconductors, Dirac and Weyl fermions can be realized in both 2D and 3D materials. The latter are semimetals with Dirac/Weyl cones either not tilted (type I) or tilted (type II). Although both Dirac and Weyl fermions have massless nature with the nontrivial Berry phase, the formation of Weyl fermions in 3D semimetals require either time-reversal or inversion symmetry breaking to lift degeneracy at Dirac points. Here we demonstrate experimentally that canted antiferromagnetic BaMnSb 2 is a 3D Weyl semimetal with a 2D electronic structure. The Shubnikov-de Hass oscillations of the magnetoresistance give nearly zero effective mass with high mobility and the nontrivial Berry phase. The ordered magnetic arrangement (ferromagnetic ordering in the ab plane and antiferromagnetic ordering along the c axis below 286 K) breaks the time-reversal symmetry, thus offering us an ideal platform to study magnetic Weyl fermions in a centrosymmetric material.topological material | nontrivial Berry phase | 3D semimetal T he observation of the quantum Hall effect has led to the discovery of new phases associated with topological ordering. In the past decade, topological materials have emerged as a new frontier of condensed matter physics due to new physical concepts and potential applications. Theory has played a major role in predicting new topological materials, which have been realized experimentally. Topological insulators, which are metallic at the surface but insulating in bulk (for a review, see ref. 1), have been extensively investigated. Surface states in these materials can be described by the Dirac equation with the Fermi surface formed by Dirac points. Dirac fermions are effectively massless because their dispersion is linear in energy. If the bulk is superconducting, the surface hosts Majorana fermions (for a review, see ref.2). More recently, a new class of topological materials, namely, Dirac or Weyl semimetals, has appeared. Such topological semimetals are characterized by the presence of electron and hole pockets touching with either degeneracy (Dirac) or nondegeneracy (Weyl). If both time reversal and inversion symmetries are preserved, the system is a Dirac semimetal (3). If either time-reversal or inversion symmetry is broken, the Dirac points split and turn into Weyl points, making the system a Weyl semimetal (3). If both timereversal and inversion symmetries are broken, the system may become a Weyl superconductor (4).The above framework makes the search of topological semimetals possible. For example, Dirac semimetals should have centrosymmetry to preserve the inversion symmetry. However, a noncentrosymmetric crystal structure would favor Weyl semimetal configuration (5). If a system is centrosymmetric and magnetically ordered, whether time-reversal sy...

We report on a study of epitaxially grown ultrathin Pb films that are only a few atoms thick and have parallel critical magnetic fields much higher than the expected limit set by the interaction of electron spins with a magnetic field, that is, the Clogston-Chandrasekhar limit. The epitaxial thin films are classified as dirty-limit superconductors because their mean-free paths, which are limited by surface scattering, are smaller than their superconducting coherence lengths. The uniformity of superconductivity in these thin films is established by comparing scanning tunneling spectroscopy, scanning superconducting quantum interference device (SQUID) magnetometry, double-coil mutual inductance, and magneto-transport, data that provide average superfluid rigidity on length scales covering the range from microscopic to macroscopic. We argue that the survival of superconductivity at Zeeman energies much larger than the superconducting gap can be understood only as the consequence of strong spin-orbit coupling that, together with substrate-induced inversionsymmetry breaking, produces spin splitting in the normal-state energy bands that is much larger than the superconductor's energy gap.superconductivity is a topic of growing interest in contemporary condensed matter physics. Early experimental work in this field used granular thin films to study phase transitions to insulating normal states driven by weakened superfluid rigidity in the ultrathin film regime. Recent experimental progress (1-12) in epitaxial growth of uniform 2D superconductors whose properties are largely intrinsic has opened up new possibilities for the design of superconducting systems with specific desirable physical properties. Indeed, these almost ideal 2D systems have yielded (6, 12, 13) surprisingly robust superconductivity in films that are only one or two atomic layers thick, and very recently the observation of an astonishingly high T c in singlelayer FeSe on SrTiO 3 (14-17). In addition, because 2D superconductors must be placed on a substrate, they necessarily have broken inversion symmetry and Rashba-type spin-orbit interactions that break the spin degeneracy of quasiparticle levels in the normal state, and enable the possibility of achieving topological (18-21) superconducting states.Here, we investigate the superconducting properties of strong spin-orbit coupling 2D superconductors using epitaxially grown, ultrathin Pb films on Si. By establishing that the superfluid rigidity vanishes at essentially the same T c when measured on different length scales, from atomic to macroscopic (greater than millimeter), we demonstrate the uniformity of the superconductivity in our films and obtain highly reliable quantitative superfluid density (SFD) values. We then perform magneto-transport measurements in parallel fields, which clearly establish that the Clogston-Chandrasekhar (CC) limit does not apply to our films. Superconductivity at Zeeman fields well in excess of the superconducting energy gap can be understood only as a consequence of strong...