Exciton-polariton condensates are considered as a deterministic source of bright, coherent non-Gaussian light. Exciton-polariton condensates emit coherent light via photoluminescence through the microcavity mirrors due to the spontaneous formation of coherence. Unlike conventional lasers which emit coherent Gaussian light, polaritons possess a natural nonlinearity due to the interaction of the excitonic component. This produces light with a negative component to the Wigner function at steady-state operation when the phase is stabilized via a suitable method such as injection locking. In contrast to many other proposals for sources of non-Gaussian light, in our case, the light typically has an average photon number exceeding one and emerges as a continuous wave. Such a source may have uses in continuous-variable quantum information and communication.
Exciton-polariton condensates are considered as a deterministic source of bright, coherent non-Gaussian light. Exciton-polariton condensates emit coherent light via photoluminescence through the microcavity mirrors due to the spontaneous formation of coherence. Unlike conventional lasers which emit coherent Gaussian light, polaritons possess a natural nonlinearity due to the interaction of the excitonic component. This produces light with a negative component to the Wigner function at steady-state operation when the phase is stabilized via a suitable method such as injection locking. In contrast to many other proposals for sources of non-Gaussian light, in our case, the light typically has an average photon number exceeding one and emerges as a continuous wave. Such a source may have uses in continuous-variable quantum information and communication.
Bose-Einstein condensate of exciton polaritons in a semiconductor microcavity is a macroscopically populated coherent quantum state subject to concurrent pumping and decay. Debates about the fundamental nature of the condensed phase in this open quantum system still persist. Here, we gain a new insight into the spontaneous condensation process by imaging long-lifetime exciton polaritons in a high-quality inorganic microcavity in the single-shot optical excitation regime, without averaging over multiple condensate realisations. In this highly non-stationary regime, a condensate is strongly influenced by the `hot' incoherent reservoir, and reservoir depletion is critical for the transition to the ground energy and momentum state. Condensates formed by more photonic exciton polaritons exhibit dramatic reservoir-induced density filamentation and shot-to-shot fluctuations. In contrast, condensates of more excitonic quasiparticles display smooth density and are second-order coherent. Our observations show that the single-shot measurements offer a unique opportunity to study formation of macroscopic phase coherence during a quantum phase transition in a solid state system.
We review, aiming at an audience of final year undergraduates, the phenomena observed in, and properties of, microcavity exciton–polariton condensates. These are condensates of mixed light and matter, consisting of superpositions of photons in semiconductor microcavities and excitons in quantum wells. Because of the imperfect confinement of the photon component, exciton–polaritons have a finite lifetime, and have to be continuously re-populated. Therefore, exciton–polariton condensates lie somewhere between equilibrium Bose–Einstein condensates and lasers. We review in particular the evidence for condensation, the coherence properties studied experimentally, and the wide variety of spatial structures either observed or predicted to exist in exciton–polariton condensates, including quantised vortices and other coherent structures. We also discuss the question of superfluidity in a non-equilibrium system, reviewing both the experimental attempts to investigate superfluidity to date, and the theoretical suggestions of how it may be further elucidated.
We analyze elementary properties of exciton and polariton lasers --- devices that generate coherent optical and matter waves using final-state stimulation of exciton-phonon scattering. First we discuss the relation between the conditions for the onset of equilibrium and nonequilibrium excitonic condensates. Provided that the thermal de Broglie wavelength ${\ensuremath{\lambda}}_{\mathit{T}}$ exceeds the exciton Bohr radius aB, an exciton laser operates without electronic population inversion. In contrast to previous proposals, this is a different type of laser without inversion which utilizes many-body coherences. When the excitonic character of the polariton branch vanishes, a polariton laser becomes indistinguishable from a photon laser. \textcopyright{} 1996 The American Physical Society.
Condensation of bosons causes spectacular phenomena such as superfluidity or superconductivity. Understanding the nature of the condensed particles is crucial for active control of such quantum phases. Fascinating possibilities emerge from condensates of light–matter-coupled excitations, such as exciton–polaritons, photons hybridized with hydrogen-like bound electron–hole pairs. So far, only the photon component has been resolved, while even the mere existence of excitons in the condensed regime has been challenged. Here we trace the matter component of polariton condensates by monitoring intra-excitonic terahertz transitions. We study how a reservoir of optically dark excitons forms and feeds the degenerate state. Unlike atomic gases, the atom-like transition in excitons is dramatically renormalized on macroscopic ground state population. Our results establish fundamental differences between polariton condensation and photon lasing and open possibilities for coherent control of condensates. Exciton–polaritons are bosonic quasi-particles resulting from strong coupling of excitons and photons but so far only their photon component had been resolved. Here, Menard et al. monitor the intra-excitonic transitions and study how a reservoir of optically dark excitons forms and feeds the degenerate state.
Bose-Einstein condensates of exciton-polaritons in inorganic semiconductor microcavities are known to possess strong interparticle interactions attributed to their excitonic component. The interactions play a crucial role in the nonlinear dynamics of such systems and can be witnessed as the high energy shift of polariton states. However, the localised nature of Frenkel excitons in strongly coupled organic microcavities precludes interparticle Coulomb exchange-interactions that affect nonlinear dynamics and change mechanisms of polariton energy shifts accordingly. We have scrutinized the origins of energy shifts in connection with nonlinear dynamics in Frenkel exciton-polariton condensates and examined the possible contributions: intracavity optical Kerr-effect, gain-induced frequency pulling, polariton interactions and effects related to saturation of optical transitions for "dark"- and "bright" molecules [1]. Unlike the conventional strongly coupled semiconductor microcavities, we have shown that nonlinear interactions within the condensate do not rely on polariton interactions but instead originated from Pauli-blocking principle forbidding double excitation of the same molecular states. We have developed a theoretical model explaining the omnipresent energy shift of the condensate wavefunction together with its spectral and polarization features at the BEC transition in a consistent way. The crucial role of intermolecular energy transfer and “dark” exciton reservoir has been demonstrated for the first time. We believe the principles explored in this work are relevant to other systems exhibiting strong coupling of Frenkel excitons with a cavity mode, regardless of a cavity type, whether one dealing with Fabry-Perot cavities or plasmonic nanocavities etc and; therefore, provide general insight on nonlinear phenomena in composite light-matter condensates. [1] T. Yagafarov, D. Sannikov, A. Zasedatelev, K. Georgiou, A. Baranikov, O. Kyriienko, I. Shelykh, L. Gai, Z. Shen, D. G. Lidzey, P. Lagoudakis, Mechanisms of blueshifts in organic polariton condensates, Commun Phys 3, 18 (2020). https://doi.org/10.1038/s42005-019-0278-6
Introduction. Polaritons and Semiconductor Microcavities As we have mentioned previously, BEC of excitons or biexcitons and lasing can be seen as two limits of the same theory. Lasing occurs in the case of strong electron-photon coupling (recombination rate fast compared with interparticle-scattering rate) while excitonic BEC occurs in the case of weak electron-photon coupling (recombination rate slow compared with the interparticle-scattering rate.) A laser can be seen as a Bose condensate in which the long-range phase coherence exists in the photon states [1], while in the exciton condensate the coherence exists in the electronic states. This is one of the reasons for some of the confusing debates in the early history of excitonic Bose condensation – in many systems there is not a sharp distinction between an excitonic condensate and supperradiance, i.e., luminescence with enhanced intensity due to stimulated emission [2-4]. In the previous chapters, we have so far considered a laser light source only as a source of excitons by means of quantum transitions. In Chapter 6, the laser radiation was taken into account as a given external factor. This chapter is dedicated to phenomena related to a strong and noticeable exciton-photon interaction, when the light significantly influences the energy spectrum of the high-density excitons [5-9]. As we will see, this leads to many fascinating effects related to instabilities. A strong exciton-photon interaction implies a strong polariton effect, in which photon and exciton states are mixed (as in Fig. 1.3). When this mixing is strong, it is not obvious how to define the ground state of the system.