Recent advances in optical studies of condensed matter have led to the emergence of a variety of phenomena that have conventionally been studied in quantum optics. These studies have not only deepened our understanding of light-matter interactions but also introduced aspects of many-body effects inherent in condensed matter. This talk will describe our recent studies of Dicke cooperativity, i.e., many-body enhancement of light-matter interaction, a concept in quantum optics, in condensed matter. This enhancement has led to the realization of the ultrastrong coupling (USC) regime, where new phenomena emerge through the breakdown of the rotating wave approximation (RWA). We will first describe our observation of USC in a 2D electron gas in a high-Q terahertz cavity in a magnetic field, and definitive evidence for the vacuum Bloch-Siegert shift, a signature of the breakdown of the RWA. Further, we have shown that cooperative USC also occurs in magnetic solids in the form of matter-matter interaction, i.e., spin-magnon and magnon-magnon interactions in rare earth orthoferrites. Particularly, the exchange interaction of N paramagnetic Er3+ spins with an Fe3+ magnon field in ErFeO3 has exhibited a vacuum Rabi splitting whose magnitude is proportional to N1/2 [6]. In the lowest temperature range, these cooperative interactions lead to a magnonic superradiant phase transition. These results provide a route for understanding, controlling, and predict novel phases of condensed matter.

Recent advances in optical studies of condensed matter have led to the emergence of a variety of phenomena that have conventionally been studied in quantum optics. These studies have not only deepened our understanding of light-matter interactions but also introduced aspects of many-body effects inherent in condensed matter. This talk will describe our recent studies of Dicke cooperativity, i.e., many-body enhancement of light-matter interaction, a concept in quantum optics, in condensed matter. This enhancement has led to the realization of the ultrastrong coupling (USC) regime, where new phenomena emerge through the breakdown of the rotating wave approximation (RWA). We will first describe our observation of USC in a 2D electron gas in a high-Q terahertz cavity in a magnetic field, and definitive evidence for the vacuum Bloch-Siegert shift, a signature of the breakdown of the RWA. Further, we have shown that cooperative USC also occurs in magnetic solids in the form of matter-matter interaction, i.e., spin-magnon and magnon-magnon interactions in rare earth orthoferrites. Particularly, the exchange interaction of N paramagnetic Er3+ spins with an Fe3+ magnon field in ErFeO3 has exhibited a vacuum Rabi splitting whose magnitude is proportional to N1/2 [6]. In the lowest temperature range, these cooperative interactions lead to a magnonic superradiant phase transition. These results provide a route for understanding, controlling, and predict novel phases of condensed matter.