Both rich fundamental physics of microcavities and their intriguing potential applications are addressed in this book, oriented to undergraduate and postgraduate students as well as to physicists and engineers. We describe the essential steps of development of the physics of microcavities in their chronological order. We show how different types of structures combining optical and electronic confinement have come into play and were used to realize first weak and later strong light–matter coupling regimes. We discuss photonic crystals, microspheres, pillars and other types of artificial optical cavities with embedded semiconductor quantum wells, wires and dots. We present the most striking experimental findings of the recent two decades in the optics of semiconductor quantum structures. We address the fundamental physics and applications of superposition light-matter quasiparticles: exciton-polaritons and describe the most essential phenomena of modern Polaritonics: Physics of the Liquid Light. The book is intended as a working manual for advanced or graduate students and new researchers in the field.

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The relaxation bottleneck present in the dispersion relation of exciton polaritons in semiconductor microcavities has prevented the realization of low threshold lasing based on exciton-polariton condensation. Here we show theoretically that the introduction of a cold electron gas into such structures induces efficient electronpolariton scattering. This process allows the condensation of the polaritons accumulated at the bottleneck to the final emitting state with a transition time of a few picoseconds, opening the way to a new generation of low-threshold light-emitting devices. DOI: 10.1103/PhysRevB.65.153310 PACS number͑s͒: 78.47.ϩp, 42.50.Ϫp, 42.65.Ϫk, 71.36.ϩc The observation of the strong coupling of light with excitons in semiconductor microcavities 1 has generated much speculation regarding the possibility for low-threshold optical devices.2,3 Realization of such devices based on the Bosonic character of the optical eigenmodes ͑exciton polaritons 4 ͒ of these structures would be a revolutionary step in semiconductor optics. However, Bose condensation of exciton polaritons has not yet been observed. One of the main obstacles is set by a bottleneck in the polariton relaxation rate.5-7 As a result, the emission of the microcavity under nonresonant excitation remains weak and nondirectional.Here we propose an original relaxation mechanism based on the scattering of polaritons with free electrons. This allows polaritons to relax efficiently from the bottleneck region to the lowest energy state once free electrons are introduced into the active region either via doping or by photoexcitation. This opens the way to realization of lowthreshold laserlike devices based on cavity polaritons.The basic principle of a polariton laser is illustrated in Fig. 1͑a͒ using the dispersion curve of the lower branch exciton polaritons in a typical microcavity. The strong-coupling regime creates a trap containing a small number of polariton states at energies below all other states in the semiconductor. This polariton trap is sharp with a depth equal to nearly half the splitting ⍀ between the two polariton modes. Polaritons in the trap are half photon and half exciton, and have properties suitable for the Bose condensation of exciton polaritons once sufficiently populated. Recombination from this state in the Bose condensation regime is coherent, monochromatic, and sharply directed, characteristic of laser emission. The relaxation of polaritons into the kϭ0 state is found to be stimulated if the population of the final state is larger than 1. The amplification process proposed here is physically very different from the classical lasing process.9 In particular, the threshold to lasing which in conventional lasers is conditional on the inversion of population is only dependent on the lifetime of the ground state in the polariton laser. As soon as relaxation to the ground state of the trap becomes faster than the radiative recombination from this state, optical amplification is achieved. Note that the absorption and re-absorption...

We present the quantum theory of momentum and spin relaxation of exciton-polaritons in microcavities. We show that giant longitudinal-transverse splitting of the polaritons mixes their spin states, which results in beats between right- and left-circularly polarized photoluminescence of microcavities, as was recently experimentally observed [Phys. Rev. Lett. 89, 077402 (2002)]]. This effect is strongly sensitive to the bosonic stimulation of polariton scattering.

Spin-orbit coupling is a fundamental mechanism that connects the spin of a charge carrier with its momentum 1 . Likewise, in the optical domain, a synthetic spin-orbit coupling is accessible, for instance, by engineering optical anisotropies in photonic materials 2 . Both, akin, yield the possibility to create devices directly harnessing spin-and polarization as information carriers 3 . Atomically thin layers of transition metal dichalcogenides provide a new material platform which promises intrinsic spin-valley Hall features both for free carriers, two-particle excitations (excitons), as well as for photons 4 . In such materials, the spin of an exciton is closely linked to the high-symmetry point in reciprocal space it emerges from (K and K' valleys) 5,6 . Here, we demonstrate, that spin, and hence valley selective propagation is accessible in an atomically thin layer of MoSe2, which is strongly coupled to a microcavity photon mode. We engineer a wire-like device, where we can clearly trace the flow, and the helicity of exciton-polaritons expanding along a channel. By exciting a coherent superposition of K and K' tagged polaritons, we observe valley selective expansion of the polariton cloud without neither any applied external magnetic fields nor coherent Rayleigh scattering. Unlike the valley Hall effect for TMDC excitons 7 , the observed optical valley Hall effect (OVHE) 8 strikingly occurs on a macroscopic scale, and clearly reveals the potential for applications in spin-valley locked photonic devices.Spin-valley locking is a striking feature of free charge carriers and excitons emerging in monolayers of transition metal dichalcogenides (TMDCs) 6,9 . It originates form the strong spin-orbit interaction, which arises from the heavy transition metals in TMDCs and the broken inversion symmetry of the crystal lattice. This leads to inverted spin orientations at opposite K points at the corners of the hexagonal Brillouin zone, for both conduction band electrons and valence band holes. As a result, the K and K' valleys can be selectively addressed by σ + and σcircular polarized light 10,11 , which is referred to as valley-polarization. Likewise, coherent superpositions of both valleys can be excited by linear polarized light, which is referred to as valley coherence. The outstanding control of the valley pseudospin has attracted great interest in exploiting this degree of freedom to encode and process information by manipulating free charge carriers 12 and excitons 7,13,14 , which has led to the emerging field of valleytronics 4 . However, exciton spin-valley applications are strongly limited by the depolarization mechanisms due to the strong Coulomb exchange interaction of electrons and holes, as well as by the limited exciton diffusion and propagation lengths.