Junichiro Kono, Rice University
Recent experiments have demonstrated that light and matter can mix together to an extreme degree, and previously uncharted regimes of light-matter interactions are currently being explored in a variety of settings, where new phenomena emerge through the breakdown of the rotating wave approximation . This talk will summarize a series of experiments we have performed in such regimes. We will first describe our observation of ultrastrong light-matter coupling in a two-dimensional electron gas in a high-Q terahertz cavity in a quantizing magnetic field, demonstrating a record-high cooperativity . The electron cyclotron resonance peak exhibited splitting into the lower and upper polariton branches with a magnitude that is proportional to the square-root of the electron density, a hallmark of cooperative vacuum Rabi splitting, known as Dicke cooperativity. Additionally, we have obtained clear and definitive evidence for the vacuum Bloch-Siegert shift , a signature of the breakdown of the rotating-wave approximation. The second part of this talk will present microcavity exciton polaritons in a thin film of aligned carbon nanotubes  embedded in a Fabry-Pérot cavity. This system exhibited cooperative ultrastrong light-matter coupling with unusual continuous controllability over the coupling strength through polarization rotation . Finally, we have generalized the concept of Dicke cooperativity to demonstrate that it also occurs in a magnetic solid in the form of matter-matter interaction . Specifically, the exchange interaction of N paramagnetic erbium(III) (Er3+) spins with an iron(III) (Fe3+) magnon field in erbium orthoferrite (ErFeO3) exhibited a vacuum Rabi splitting whose magnitude is proportional to N1/2. Our results provide a route for understanding, controlling, and predicting novel phases of condensed matter using concepts and tools available in quantum optics, opening up exciting possibilities to combine the traditional disciplines of many-body condensed matter physics and cavity-based quantum optics.
1. For a review, see, e.g., P. Forn-Díaz, L. Lamata, E. Rico, J. Kono, and E. Solano, arXiv:1804.09275, to appear in Reviews of Modern Physics.
2. Q. Zhang et al., Nature Physics 12, 1005 (2016).
3. X. Li et al., Nature Photonics 12, 324 (2018).
4. X. He et al., Nature Nanotechnology 11, 633 (2016).
5. W. Gao et al., Nature Photonics 12, 362 (2018).
6. X. Li et al., Science 361, 794 (2018).
To be followed by coffee in QNC 1201