We observe a room-temperature low-threshold transition to a coherent polariton state in bulk GaN microcavities in the strong-coupling regime. Nonresonant pulsed optical pumping produces rapid thermalization and yields a clear emission threshold of 1 mW, corresponding to an absorbed energy density of $29\mu \mathrm{J}{\mathrm{cm}}^{-2}$, 1 order of magnitude smaller than the best optically pumped (In,Ga)N quantum-well surface-emitting lasers (VCSELs). Angular and spectrally resolved luminescence show that the polariton emission is beamed in the normal direction with an angular width of $\pm 5°$ and spatial size around $5\mu \mathrm{m}$.

Conventional surface plasmons have a wave vector exceeding that of light in vacuum, and therefore cannot be directly excited by light that is simply incident on the surface. However, we propose that a plasmon-polariton state can be formed at the boundary between a metal and a dielectric Bragg mirror that can have a zero in-plane wave vector and therefore can be produced by direct optical excitation. In analogy with the electronic states at a crystal surface proposed by Tamm, we call these excitations Tamm plasmons, and predict that they may exist in both the TE and TM polarizations and are characterized by parabolic dispersion relations.

Long-lived polariton condensates can propagate well beyond the area of their initial excitation while still maintaining spatial coherence. This enables direct and controllable manipulation of the condensate wavefunction. Cavity exciton-polaritons1,2 (polaritons) are bosonic quasi-particles offering a unique solid-state system for investigating interacting condensates3,4,5,6,7,8,9,10. Up to now, disorder-induced localization and short lifetimes4,6,11 have prevented the establishment of long-range off-diagonal order12 needed for any quantum manipulation of the condensate wavefunction. In this work, using a wire microcavity with polariton lifetimes much longer than in previous samples, we show that polariton condensates can propagate over macroscopic distances outside the excitation area, while preserving their spontaneous spatial coherence. An extended condensate wavefunction builds up with a degree of spatial coherence larger than 50% over distances 50 times the polariton de Broglie wavelength. The expansion of the condensate is shown to be governed by the repulsive potential induced by photogenerated excitons within the excitation area. The control of this local potential offers a new and versatile method to manipulate extended polariton condensates. As an illustration, we demonstrate synchronization of extended condensates by controlled tunnel coupling13,14 and localization of condensates in a trap with optically controlled dimensions.

The rapid development of microfabrication and assembly of nanostructures has opened up many opportunities to miniaturize structures that confine light, producing unusual and extremely interesting optical properties. This book addresses the large variety of optical phenomena taking place in confined solid state structures: microcavities. Microcavities represent a unique laboratory for quantum optics and photonics. They exhibit a number of beautiful effects, including lasing, superfluidity, super-radiance, and entanglement. The book is written by four practitioners strongly involved in experiments and theories of microcavities. The introductory chapters present the semi-classical and quantum approaches to description of light-matter coupling in various solid state systems, including planar cavities, pillars, and spheres; introduce exciton-polaritons, and discuss their statistics and optical properties. The weak and strong exciton-light coupling regimes are discussed further with emphasis on the Purcell effect, lasing, optical parametric oscillations, and Bose-Einstein condensation of exciton polaritons. The last chapter discusses polarization and spin properties of cavity polaritons. The book also contains portraits of scientists who gave key contributions to classical electromagnetism, quantum optics, and exciton physics.

A remarkable analogy is established between the well-known spin Hall effect and the polarization dependence of Rayleigh scattering of light in microcavities. This dependence results from the strong spin effect in elastic scattering of exciton polaritons: if the initial polariton state has a zero spin and is characterized by some linear polarization, the scattered polaritons become strongly spin polarized. The polarization in the scattered state can be positive or negative dependent on the orientation of the linear polarization of the initial state and on the direction of scattering. Very surprisingly, spin polarizations of the polaritons scattered clockwise and anticlockwise have different signs. The optical spin Hall effect is possible due to strong longitudinal-transverse splitting and finite lifetime of exciton polaritons in microcavities.

The spin Hall effect consists of the generation of a spin current perpendicular to the charge current flow. Thirty-five years after its prediction by Dyakonov and Perel’¹, it is the focus of experimental and theoretical investigations and constitutes one of the most remarkable effects of spintronics. Owing to scattering and dephasing in electronic gases, it is difficult to observe and has only been demonstrated for the first time a few years ago^2,3,4,5. Recently, three of us have predicted the optical spin Hall effect⁶, which consists of a separation in real space and momentum space of spin-polarized exciton–polaritons generated by a laser in a semiconductor microcavity⁷. The separation takes place owing to a combination of elastic scattering of exciton–polaritons by structural disorder and an effective magnetic field coming from polarization splitting of the polariton states. The excitonic spin current is controlled by the linear polarization of the laser pump. Here, we report the first experimental evidence for this effect and demonstrate propagation of polariton spin currents over 100 μm in a high-quality GaAs/AlGaAs quantum microcavity. By rotating the polarization plane of the exciting light, we were able to switch the directions of the spin currents.