Exciton-polariton condensates (1411.6822v1)

Abstract: Recently a new type of system exhibiting spontaneous coherence has emerged -- the exciton-polariton condensate. Exciton-polaritons (or polaritons for short) are bosonic quasiparticles that exist inside semiconductor microcavities, consisting of a superposition of an exciton and a cavity photon. Above a threshold density the polaritons macroscopically occupy the same quantum state, forming a condensate. The lifetime of the polaritons are typically comparable to or shorter than thermalization times, making them possess an inherently non-equilibrium nature. Nevertheless, they display many of the features that would be expected of equilibrium Bose-Einstein condensates (BECs). The non-equilibrium nature of the system raises fundamental questions of what it means for a system to be a BEC, and introduces new physics beyond that seen in other macroscopically coherent systems. In this review we focus upon several physical phenomena exhibited by exciton-polariton condensates. In particular we examine topics such as the difference between a polariton BEC, a polariton laser, and a photon laser, as well as physical phenomena such as superfluidity, vortex formation, BKT (Berezinskii-Kosterlitz-Thouless) and BCS (Bardeen-Cooper-Schrieffer) physics. We also discuss the physics and applications of engineered polariton structures.

• The paper shows that exciton-polariton condensates form via stimulated scattering, distinguishing them from traditional photon lasing systems.
• It details experimental observations of superfluidity, vortices, and solitons, offering insights into non-equilibrium Bose-Einstein condensation.
• The analysis highlights potential applications in quantum technologies, including low-threshold lasers and quantum simulations.

Overview of Exciton-Polariton Condensates

The paper "Exciton-polariton condensates" by Tim Byrnes, Na Young Kim, and Yoshihisa Yamamoto provides a comprehensive examination of a specific type of system that exhibits spontaneous coherence: exciton-polariton condensates. Exciton-polaritons are bosonic quasiparticles formed in semiconductor microcavities as a superposition of excitons and cavity photons. These structures lead to unique physical phenomena when they undergo condensation under certain conditions.

Fundamental Characteristics

Exciton-polaritons are remarkable for their light effective mass, typically about 10⁻⁴ times the electron mass, facilitating Bose-Einstein Condensation (BEC) at higher temperatures than other types of systems. This feature, along with the polariton's short lifetime relative to thermalization time, results in a fundamentally non-equilibrium nature, diverging from other BEC systems. The dynamics of exciton-polariton condensates raise questions about the definition of a BEC in non-equilibrium systems and offer novel physics insights.

Distinctions from Lasers

A critical analysis provided in the paper is the distinction between exciton-polariton systems and traditional laser systems. Unlike a VCSEL where the gain medium facilitates population inversion leading to photon lasing, the exciton-polariton BEC forms due to the stimulated scattering of polaritons into the zero-momentum ground state. Here, the coherence builds in the polaritons rather than the photons directly. Despite similarities in phenomenology, such as spectral narrowing and threshold behavior, this fundamental difference challenges traditional views on lasing versus condensation.

Observed Phenomena

The paper discusses several salient physical phenomena associated with exciton-polariton condensates, including:

• Superfluidity: Experimental findings show evidence of superfluidity through the suppression of Rayleigh scattering and persistent currents, although theoretical frameworks like the Landau criterion need adaptation for the non-equilibrium nature of exciton-polaritons.
• Vortices and Solitons: The existence of quantized vortices and solitons has been experimentally observed, aligning with predictions of BKT (Berezinskii-Kosterlitz-Thouless) transition phenomena in two-dimensional systems.
• BCS Physics: Exciton-polariton condensates exhibit features reminiscent of BCS (Bardeen-Cooper-Schrieffer) theory as part of a BEC-BCS crossover, though distinguishable by their predominant photonic characteristics at high densities.

Implications and Future Directions

Practically, the unique properties of exciton-polariton condensates suggest potential applications in quantum technologies, such as low-threshold lasers, spintronics, and quantum simulations. The ability to manipulate these condensates through engineered structures like optical lattices or varying the material composition can facilitate novel devices and further explore many-body quantum dynamics.

Theoretically, the paper of these systems prompts reconsideration of fundamental concepts like BEC, particularly under non-equilibrium conditions, and can inspire developments in understanding coherence phenomena across different domains, including atomic physics and condensed matter.

Conclusion

The paper provides a detailed overview of the present understanding and ongoing research questions regarding exciton-polariton condensates. The potential to devise new experimental methods and theoretical models to explore this rich field holds promise for advancing both foundational physics and application-oriented technologies. As the exploration of exciton-polariton condensates continues, we anticipate deeper insights into non-equilibrium quantum phenomena and the development of novel coherent systems.