Quantum optical effects in semiconductor microcavities

Investigation of light–matter interactions in two-dimensional (2D) semiconductors is essential to understand many-body effects and explore their applications in photonics. 2D semiconductor microcavities can be formed by the integration of 2D semiconductor active media with planar Fabry–Perot resonant cavities. The emerging exciton–photon interactions between the strongly bound excitons in 2D semiconductors and the cavity photons allow exploring on the wealthy photonic and polaritonic physics of bosonic quasiparticles in the 2D limit. This Perspective focuses on recent advances in exciton–photon interactions of 2D semiconductor microcavities and their inspiring applications. First, we picture the research scope of 2D semiconductor microcavities, sort out the historical development from conventional semiconductor microcavities to the emerged 2D semiconductor microcavities, and illustrate mostly employed device structures. Second, we classify exciton–photon interactions in 2D semiconductor microcavities according to their coupling strengths. In the weak coupling regime, control of spontaneous emission and photon lasing are discussed together with the Purcell effect. In the strong coupling regime, we summarize experimental observations and theoretical predictions on Rabi splitting, Bose–Einstein condensation, polariton lasing, superfluidity, and superconductivity. Third, four types of leading-edge applications are outlined, including on-chip coherent light sources, microcavity-enhanced single-photon sources, topological photonics, and other nonlinear optics. Finally, we highlight the remaining challenges and future opportunities for fundamental physics of 2D semiconductor microcavities and their technological applications.

We report the observation of polaritonic bistability in semiconductor microcavities in the strong-coupling regime. The origin of bistability is the polariton-polariton interaction, which gives rise to a Kerr-like nonlinearity. The experimental results are in good agreement with a simple model taking transverse effects into account.

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In this paper we present experimental and theoretical investigations of effects of biexcitons on coupled exciton-photon excitations, or cavity polaritons, in a GaAs quantum well (QW) microcavity. Using polarization-dependent transient pump-probe spectroscopy, we show that biexcitonic effects can result in a significant increase in normal mode splitting (NMS) of cavity polaritons, in sharp contrast to effects of band filling and exciton ionization that reduce the NMS. A theoretical model based on modified optical Bloch equations is developed to describe qualitatively effects of biexcitons in semiconductor microcavities.

The excitonic polariton concept was introduced already in 1958 by J. J. Hopfield. Although its description was based on a full quantum theory including light quantization, the investigations of the optical properties of excitons developed mainly independently of quantum optics. In this chapter we shall review exciton polariton quantum optical effects by means of some recent works and results that have appeared in the literature in both bulk semiconductors and in cavity embedded quantum wells. The first manifestation of excitonic quantum-optical coherent dynamics was observed experimentally 20 years later, in 1978, exploiting resonant hyper-parametric scattering. On the other hand, the possibility of generating entangled photon pairs by means of this resonant process was theoretically pointed out only lately in 1999, whereas the experimental evidence for the generation of ultraviolet polarization-entangled photon pairs by means of biexciton resonant parametric emission in a single crystal of semiconductor CuCl was reported only in 2004. The demonstrations of parametric amplification and parametric emission in semiconductor microcavities, together with the possibility of ultrafast optical manipulation and ease of integration of these micro-devices, have increased the interest in possible realization of nonclassical cavity-polariton states. In 2005 an experiment that probes polariton quantum correlations by exploiting quantum complementarity was proposed and realized. These results unequivocally proved that quantum optical effects at single photon level, arising from the interaction of light with electronic excitations of semiconductors and semiconductor nanostructures, were possible within these solid state systems, despite being far from isolated systems. The theoretical predictions that we review are based on a microscopic quantum theory of the nonlinear optical response of interacting electron systems relying on the dynamics controlled truncation scheme extended to include light quantization. In addition, the environment decoherence is taken into account microscopically by Markov noise sources within a quantum Langevin framework.

The investigation of exciton-polaritons in semiconductor microcavities operating in the strong-coupling regime is attracting growing interest because of their extraordinary optical properties [1]. Together with the strong excitonic nonlinearities, originating from the Coulomb interaction between excitons, the peculiar dispersion relation of these quasi-particles allows a variety of fundamental effects like optical bistability, low threshold parametric oscillations [2] and formation of robust self-localized states, so-called, cavity polariton solitons (CPSs) [3,4]. It was shown that the inclination of the coherent optical pump (plane wave) endows exciton-polaritons with a kinetic momentum (k x ) along one of the orthogonal directions in the plane of the microcavity. It even allows controlling their dispersion. As a result moving 2D CPSs can exist despite the opposite dispersion signs along (x) and orthogonal (y) to the pump momentum directions [4]. In this contribution we provide theoretical evidence of self-localization of polaritons in the direction orthogonal to the pump tilt (y), which is unusual for the defocusing nonlinearity. We study the formation of 1D CPSs excising due to the parametric four-wave mixing of polaritons.

Motional narrowing in a microcavity: contribution to the lower polariton linewidth

Anisotropic polariton scattering and spin dynamics of cavity polaritons

Parametric interactions in semiconductor heterostructures in which two exciton quasiparticles mutually scatter are typically weak because they are restricted by energy–momentum conservation. Here, semiconductor microcavities in the strong coupling regime are optically pumped at specific resonant energy and angle of incidence to enormously enhance the parametric interactions. A femtosecond goniometer is devised to allow time-, wavelength- and polarisation-resolved pump–probe spectroscopy. Time-resolved images of the microcavity emission at different angles reveal a highly asymmetric luminescence distribution which switches on as the pump power is increased. Both signal and idler beams can be identified when a probe pulse initiates stimulated scattering of polaritons from a reservoir. The images show the rich variety of interactions produced by the coupling of light–matter modes, and the future promise of dispersion engineering.

We consider theoretically the spin relaxation of exciton–polaritons in semiconductor microcavities in the strong coupling regime. The dominant quantum well exciton polarization relaxation mechanism is typically due to the long-range intra-exciton electron–hole exchange. We estimate perturbatively the corresponding contribution for the lower polaritons as a function of the elastic scattering time in analogy to the Dyakonov-Perel model for electron spin relaxation. We find a strong suppression of the polariton spin flip rate due to the electron–hole exchange with respect to the quantum well exciton case.

Quantum Manipulation of Electron- and Photon-systems in Semiconductor Microcavities

Semiconductor microcavities have attracted much recent interest because they utilize simultaneously 2D confinement of both excitons and photons in the same heterostructure. Strong coupling of these two states produces unique dynamics that can be well described in a quasiparticle state, the cavity polaritons. Their dispersion relation is dramatically modified with an apparent trap in k-space offering exciting new possibilities for tailoring nonlinear optical properties in microcavities. The reduced density of states inside the trap allows the macroscopic occupancy of polaritons producing much of the new physics. This paper describes recent work on nonlinear effects in semiconductor microcavities including stimulated scattering, parametric oscillation and non-equilibrium phase transition. By teaching polaritons new tricks, both fundamental questions about their bosonic nature can be answered and practical applications in a variety of optoelectronic and interferometric devices can be found.

We present a microscopic theory of the coherent third-order $({\ensuremath{\chi}}^{(3)})$ optical response of semiconductor quantum well microcavities, specialized to the four-wave-mixing configuration in the spectral vicinity of the lowest exciton frequency. The theory is that of a quantum-mechanical many-electron system dipole coupled to a classical radiation field. The many-electron dynamics is treated within the dynamics-controlled-truncation formalism restricted to the $1s$-exciton subspace. Within this limitation, all Coulomb correlation effects are included, resulting in an effective theory of (virtual) exciton-polariton scattering. Good quantitative agreement of the theory is obtained in comparison to the experiments reported by Gonokami et al., Phys. Rev. Lett. 79, 1341 (1997). This comparison reveals the signatures of both the bound biexciton and the exciton-exciton scattering (continuum) correlations. Furthermore, a proper calculation of the scattering correlations is shown to be important: each of two common approximations, the Markov and the second Born, results in clear discrepancies from the data.

Scientific and technological issues and considerations regarding the integration of miniaturized microphotonic devices, circuits and systems in micron, submicron, and quantum scale, are presented. First, we examine the issues regarding the miniaturization of photonic devices including the size effect, proximity effect, energy confinement effect, microcavity effect, optical and quantum interference effect, high field effect, nonlinear effect, noise effect, quantum optical effect, and chaotic effect. Secondly, we examine the issues regarding the interconnection including the optical alignment, minimizing the interconnection losses, and maintaining optical modes. Thirdly, we address the issues regarding the two-dimensional or three-dimensional integration either in a hybrid format or in a monolithic format between active devices and passive devices of varying functions. We find that the concept of optical printed circuit board (O-PCB) that we propose is highly attractive as a platform for micro/nano/quantum-scale photonic integration. We examine the technological issues to be addressed in the process of fabrication, characterization, and packaging for actual implementation of the miniaturization, interconnection and integration. Devices that we have used for our study include: mode conversion schemes, micro-ring and micro-racetrack resonator devices, multimode interference devices, lasers, vertical cavity surface emitting microlasers, and their arrays. Future prospects are also discussed.

In this experimental work, we optically excite the eigenmodes of an elliptical resonator in a semiconductor microcavity. Using a pulsed excitation, we create a superposition of eigenmodes, and image the time evolution of the coherent emission pattern. Oscillations between vortex and anti-vortex states are observed, and remarkably well described within the Poincare sphere representation for an eigenmode containing an orbital angular momentum. A semiconductor quantum well is embedded in the microcavity structure. The system is operated in the strong light matter coupling regime, where the eigenmodes are hybrid half-photonic half-excitonic quasiparticles called exciton polaritons.