Ultrastrong and Cooperative Light-Matter Coupling in Solids

Abstract:
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 [1]. 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 [2]. 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 [3], a signature of the breakdown of the rotatingwave
approximation. The second part of this talk will present microcavity exciton polaritons in a
thin film of aligned carbon nanotubes [4] 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 [5]. 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 [6]. 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. P. Forn-Díaz, L. Lamata, E. Rico, J. Kono, and E. Solano, Reviews of Modern Physics 91,
025005 (2019).
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).