The effects of cavity quantum electrodynamics (QED), caused by the interaction of matter and the electromagnetic field in subwavelength resonant structures, have been the subject of intense research in recent years. The generation of coherent radiation by subwavelength resonant structures has attracted considerable interest, not only as a means of exploring the QED effects that emerge at small volume, but also for its potential in applications ranging from on-chip optical communication to ultrahigh-resolution and high-throughput imaging, sensing and spectroscopy. One such strand of research is aimed at developing the 'ultimate' nanolaser: a scalable, low-threshold, efficient source of radiation that operates at room temperature and occupies a small volume on a chip. Different resonators have been proposed for the realization of such a nanolaser--microdisk and photonic bandgap resonators, and, more recently, metallic, metallo-dielectric and plasmonic resonators. But progress towards realizing the ultimate nanolaser has been hindered by the lack of a systematic approach to scaling down the size of the laser cavity without significantly increasing the threshold power required for lasing. Here we describe a family of coaxial nanostructured cavities that potentially solve the resonator scalability challenge by means of their geometry and metal composition. Using these coaxial nanocavities, we demonstrate the smallest room-temperature, continuous-wave telecommunications-frequency laser to date. In addition, by further modifying the design of these coaxial nanocavities, we achieve thresholdless lasing with a broadband gain medium. In addition to enabling laser applications, these nanoscale resonators should provide a powerful platform for the development of other QED devices and metamaterials in which atom-field interactions generate new functionalities.

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The interesting physics and potential memory technologies resulting from topologically protected spin textures such as skyrmions, has prompted efforts to discover new material systems that can host these kind of magnetic structures. Here we use the highly tunable magnetic properties of amorphous Fe/Gd multilayer films to explore the magnetic properties that lead to dipole-stabilized skyrmions and skyrmion lattices that form from the competition of dipolar field and exchange energy. Using both real space imaging and reciprocal space scattering techniques we determined the range of material properties and magnetic fields where skyrmions form.Micromagnetic modeling closely matches our observation of small skyrmion features (~50 to 70nm) and suggests these class of skyrmions have a rich domain structure that is Bloch like in the center of the film and more Néel like towards each surface. Our results provide a pathway to engineer the formation and controllability of dipole skyrmion phases in a thin film geometry at different temperatures and magnetic fields.

We introduce a low refractive index layer between the metal and the gain medium in metal-coated laser resonators and demonstrate that it can significantly reduce the dissipation losses. Analysis of a gain medium waveguide shows that for a given waveguide radius, the low index layer has an optimal thickness for which the lasing threshold gain is minimal. The waveguide analysis is used for the design of a novel three-dimensional cylindrical resonator that is smaller than the vacuum wavelength in all three dimensions and exhibits a low enough threshold gain to lase at room temperature.

This paper presents analytical and numerical studies demonstrating that a perfectly conducting metallic plate perforated by a periodic array of subwavelength holes and sandwiched in between two dielectric slabs permits enhanced transmission of electromagnetic plane waves in both the optical and microwave regimes, and this for both transverse magnetically and electrically polarized fields. The enhanced transmission mechanism is attributed to coupling between the incident plane wave and resonances supported by the perforated plate that are associated with the existence of grounded dielectric slab guided waves. The enhanced transmission phenomenon occurs in one of two regimes. In the single resonance regime, the incident plane wave couples to a resonance supported by only a single slab. The transmission coefficient magnitude peaks as the excitation frequency scans through the resonance; however, the peak magnitude decays exponentially with plate thickness. In the double resonance regime, the incident plane wave couples to resonances in both slabs simultaneously. The transmission coefficient magnitude exhibits twin peaks whose magnitudes typically are larger than those resulting from single resonances and that remain large even for plates of moderate thickness. It is demonstrated that double resonances may occur for symmetric as well as for asymmetric structures, i.e., when the two slabs are identical or different. For symmetric structures double resonances occur for any angle of incidence. In contrast, for asymmetric structures, special conditions on the period and slab parameters have to be satisfied for the structure to support one or more double resonances.