The electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties are reviewed. Recent advances in the development of atomically thin layers of van der Waals bonded solids have opened up new possibilities for the exploration of 2D physics as well as for materials for applications. Among them, semiconductor transition metal dichalcogenides, MX2 (M = Mo, W; X = S, Se), have bandgaps in the near-infrared to the visible region, in contrast to the zero bandgap of graphene. In the monolayer limit, these materials have been shown to possess direct bandgaps, a property well suited for photonics and optoelectronics applications. Here, we review the electronic and optical properties and the recent progress in applications of 2D semiconductor transition metal dichalcogenides with emphasis on strong excitonic effects, and spin- and valley-dependent properties.

Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides

In recent years, notions drawn from non-Hermitian physics and parity–time (PT) symmetry have attracted considerable attention. In particular, the realization that the interplay between gain and loss can lead to entirely new and unexpected features has initiated an intense research effort to explore non-Hermitian systems both theoretically and experimentally. Here we review recent progress in this emerging field, and provide an outlook to future directions and developments. This Review Article outlines the exploration of the interplay between parity–time symmetry and non-Hermitian physics in optics, plasmonics and optomechanics.

Non-Hermitian physics and PT symmetry

The ability to control the modes oscillating within a laser resonator is of fundamental importance. In general, the presence of competing modes can be detrimental to beam quality and spectral purity, thus leading to spatial as well as temporal fluctuations in the emitted radiation. We show that by harnessing notions from parity-time (PT) symmetry, stable single–longitudinal mode operation can be readily achieved in a system of coupled microring lasers. The selective breaking of PT symmetry can be used to systematically enhance the maximum attainable output power in the desired mode. This versatile concept is inherently self-adapting and facilitates mode selectivity over a broad bandwidth without the need for other additional intricate components. Our experimental findings provide the possibility to develop synthetic optical devices and structures with enhanced functionality.

http://science.sciencemag.org/content/sci/346/6212/975.full.pdf

Parity-time–symmetric microring lasers

Engineering the electromagnetic environment of a nanometre-scale light emitter by use of a photonic cavity can significantly enhance its spontaneous emission rate, through cavity quantum electrodynamics in the Purcell regime. This effect can greatly reduce the lasing threshold of the emitter, providing a low-threshold laser system with small footprint, low power consumption and ultrafast modulation. An ultralow-threshold nanoscale laser has been successfully developed by embedding quantum dots into a photonic crystal cavity (PCC). However, several challenges impede the practical application of this architecture, including the random positions and compositional fluctuations of the dots, extreme difficulty in current injection, and lack of compatibility with electronic circuits. Here we report a new lasing strategy: an atomically thin crystalline semiconductor--that is, a tungsten diselenide monolayer--is non-destructively and deterministically introduced as a gain medium at the surface of a pre-fabricated PCC. A continuous-wave nanolaser operating in the visible regime is thereby achieved with an optical pumping threshold as low as 27 nanowatts at 130 kelvin, similar to the value achieved in quantum-dot PCC lasers. The key to the lasing action lies in the monolayer nature of the gain medium, which confines direct-gap excitons to within one nanometre of the PCC surface. The surface-gain geometry gives unprecedented accessibility and hence the ability to tailor gain properties via external controls such as electrostatic gating and current injection, enabling electrically pumped operation. Our scheme is scalable and compatible with integrated photonics for on-chip optical communication technologies.

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Monolayer semiconductor nanocavity lasers with ultralow thresholds

We provide a self-consistent electromagnetic theory of the coupling between dipole emitters and dissipative nanoresonators. The theory that relies on the concept of quasinormal modes with complex frequencies provides an accurate closed-form expression for the electromagnetic local density of states of any photonic or plasmonic resonator with strong radiation leakage, absorption, and material dispersion. It represents a powerful tool to calculate and conceptualize the electromagnetic response of systems that are governed by a small number of resonance modes. We use the formalism to revisit Purcell's factor. The new formula substantially differs from the usual one; in particular, it predicts that a spectral detuning between the emitter and the resonance does not necessarily result in a Lorentzian response in the presence of dissipation. Comparisons with fully vectorial numerical calculations for plasmonic nanoresonators made of gold nanorods evidence the high accuracy of the predictions achieved by our semianalytical treatment.

https://hal-iogs.archives-ouvertes.fr/hal-00850459/document

Theory of the spontaneous optical emission of nanosize photonic and plasmon resonators.

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.

Thresholdless nanoscale coaxial lasers

Metal-clad subwavelength lasers have recently become excellent candidates for light sources in densely packed chip-scale photonic circuits. In this review, we summarize recent research efforts in the theory, design, fabrication, and characterization of such lasers. We detail advancements of both the metallo-dielectric and the coaxial type lasers: for the metallo-dielectric type, we discuss operation with both optical pumping and electrical pumping. For the coaxial type, we discuss operation with all spontaneous emission coupled into the lasing mode, as well as the smallest metal-clad lasers to date operating at room temperature. A formal treatment of the Purcell effect, the modification of the spontaneous emission rate by a subwavelength cavity, is then presented to assist in better understanding the quantum effects in these nanoscale semiconductor lasers. This formalism is developed for the transparent medium condition, using the emitter-field-reservoir model in the quantum theory of damping. We show its utility through the analysis and design of subwavelength lasers. Finally, we discuss future research directions toward high-efficiency nanolasers and potential applications, such as creating planar arrays of uncoupled lasers with emitter densities near the resolution limit.

Subwavelength semiconductor lasers for dense chip-scale integration

Electrically pumped metallo-dielectric nanolasers are demonstrated. Employing a two-step InP chemical etching, we obtain straight pedestal sidewalls and preferentially reduce the diameter of the n-doped InP cladding more than the p-doped one for optimized performance.

Electrically pumped metallo-dielectric pedestal nanolasers

In this work we present a design optimization of the nanopatch semiconductor laser. Geometry parameters are optimized for the best combination of quality factor (photonic lifetime) and stimulated emission (gain).

Geometry Optimization of Nanopatch Semiconductor Lasers: the Trade-off between Quality Factor and Gain

High mode confinement and modal sparsity in subwavelength metallic-plasmonic coaxial cavities enable the demonstration of continuous-wave and room-temperature lasing at telecommunication wavelengths in the smallest cavities to date.

Michael Katz

Papers

Thresholdless nanoscale coaxial lasers

Subwavelength semiconductor lasers for dense chip-scale integration

Electrically pumped metallo-dielectric pedestal nanolasers

Geometry Optimization of Nanopatch Semiconductor Lasers: the Trade-off between Quality Factor and Gain

Room-temperature and continuous-wave lasing with nanoscale coaxial cavities