Abstract
Microcavity exciton polariton systems can have a wide range of macroscopic quantum effects that may be turned into better photonic technologies. Polariton Bose-Einstein condensation and photon lasing have been widely accepted in the limits of low and high carrier densities, but identification of the expected Bardeen-Cooper-Schrieffer (BCS) state at intermediate densities remains elusive, as the optical-gain mechanism cannot be directly inferred from existing experiments. Here, using a microcavity with strong polarization selectivity, we gain direct experimental access to the reservoir absorption in the presence of polariton condensation and lasing, which reveals a fermionic gain mechanism underlying the polariton laser. A microscopic many-particle theory shows that this polariton lasing state is consistent with an open-dissipative-pumped system analog of a polaritonic BCS state.
- Received 10 November 2019
- Revised 30 October 2020
- Accepted 2 December 2020
DOI:https://doi.org/10.1103/PhysRevX.11.011018
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Two of the earliest discoveries of collective quantum effects are Bose-Einstein condensates (BECs)—where ultracold bosons collectively behave as a macroscopic quantum entity—and Bardeen-Cooper-Schrieffer (BCS) states—in which electron pairs trigger superconductivity. Since their discovery, these concepts have been generalized to open systems such as semiconductor microcavities, where excitons (bound electron-hole pairs) may interact strongly with a light field to create polaritons: hybrid entities of electrons, holes, and photons. While a polariton BEC has been widely reported, the long-sought BCS phase remains elusive. Here, we report signatures of a polariton BCS phase.
A main challenge is the difficulty in distinguishing between the BCS phase and the BEC phase. Both phases result in laserlike coherent light emission. In our work, we use a specially designed anisotropic microcavity that allows us to probe not only the coherent emission of the condensed phase but also the underlying excitons directly. We reveal a polariton laser formed in the BCS regime, where excitons have dissociated. Our theory corroborates the experimental results and reveals the electron-hole-pairing mechanism consistent with a BCS-like state.
This work opens a new pathway to study BCS-like states in semiconductors and provides new insight into the microscopic picture of lasing in high-quality microcavities. It may help to lay the foundation for future generations of highly efficient semiconductor lasers that harness complex many-body effects.
