Microcavity physics and design will be reviewed. Following an overview of applications in quantum optics, communications and biosensing, recent advances in ultra-high-Q research will be presented.

Optical microcavities

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

Hybrid plasmonic waveguides are described that employ a high-gain semiconductor nanostructure functioning as a gain medium that is separated from a metal substrate surface by a nanoscale thickness thick low-index gap. The waveguides are capable of efficient generation of sub-wavelength high intensity light and have the potential for large modulation bandwidth >1 THz.

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Plasmon lasers at deep subwavelength scale

Nanoplasmonics — the nanoscale manipulation of surface plasmons (fluctuations in the electron density at a metallic surface) — could revolutionize applications ranging from sensing and biomedicine to imaging and information technology. But first, we need a simple and efficient method for actively generating coherent plasmonic fields. This is in theory possible with the spaser, first proposed in 2003 as a device that generates and amplifies surface plasmons in the same way that a laser generates and amplifies photons. Now Noginov et al. present the first unambiguous experimental demonstration of spasing, using 44-nm diameter nanoparticles with a gold core and dye-doped silica shell. The system generates stimulated emission of surface plasmons in the same way as a laser generates stimulated emission of coherent photons, and has been used to implement the smallest nanolaser reported to date, and the first operating at visible wavelengths. Nanoplasmonics promises to revolutionize applications ranging from sensing and biomedicine to imaging and information technology, but its full development is hindered by the lack of devices that can generate coherent plasmonic fields. In theory, this is possible with a so-called 'spaser' — analogous to a laser — which would generate stimulated emission of surface plasmons. This is now realized experimentally, and should enable many new technological developments. One of the most rapidly growing areas of physics and nanotechnology focuses on plasmonic effects on the nanometre scale, with possible applications ranging from sensing and biomedicine to imaging and information technology1,2. However, the full development of nanoplasmonics is hindered by the lack of devices that can generate coherent plasmonic fields. It has been proposed3 that in the same way as a laser generates stimulated emission of coherent photons, a ‘spaser’ could generate stimulated emission of surface plasmons (oscillations of free electrons in metallic nanostructures) in resonating metallic nanostructures adjacent to a gain medium. But attempts to realize a spaser face the challenge of absorption loss in metal, which is particularly strong at optical frequencies. The suggestion4,5,6 to compensate loss by optical gain in localized and propagating surface plasmons has been implemented recently7,8,9,10 and even allowed the amplification of propagating surface plasmons in open paths11. Still, these experiments and the reported enhancement of the stimulated emission of dye molecules in the presence of metallic nanoparticles12,13,14 lack the feedback mechanism present in a spaser. Here we show that 44-nm-diameter nanoparticles with a gold core and dye-doped silica shell allow us to completely overcome the loss of localized surface plasmons by gain and realize a spaser. And in accord with the notion that only surface plasmon resonances are capable of squeezing optical frequency oscillations into a nanoscopic cavity to enable a true nanolaser15,16,17,18, we show that outcoupling of surface plasmon oscillations to photonic modes at a wavelength of 531 nm makes our system the smallest nanolaser reported to date—and to our knowledge the first operating at visible wavelengths. We anticipate that now it has been realized experimentally, the spaser will advance our fundamental understanding of nanoplasmonics and the development of practical applications.

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Demonstration of a spaser-based nanolaser

Cavity quantum electrodynamics, a central research field in optics and solid-state physics, addresses properties of atom-like emitters in cavities and can be divided into a weak and a strong coupling regime. For weak coupling, the spontaneous emission can be enhanced or reduced compared with its vacuum level by tuning discrete cavity modes in and out of resonance with the emitter. However, the most striking change of emission properties occurs when the conditions for strong coupling are fulfilled. In this case there is a change from the usual irreversible spontaneous emission to a reversible exchange of energy between the emitter and the cavity mode. This coherent coupling may provide a basis for future applications in quantum information processing or schemes for coherent control. Until now, strong coupling of individual two-level systems has been observed only for atoms in large cavities. Here we report the observation of strong coupling of a single two-level solid-state system with a photon, as realized by a single quantum dot in a semiconductor microcavity. The strong coupling is manifest in photoluminescence data that display anti-crossings between the quantum dot exciton and cavity-mode dispersion relations, characterized by a vacuum Rabi splitting of about 140 microeV.

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E. M. Purcell

Papers

Spontaneous Emission Probabilities at Radio Frequencies