From Superfluorescence to Ultrastrong Light-Matter Coupling: Dicke Physics in Solid-State Quantum Matter

Ultrafast and ultrastrong light–matter interaction in solids can reveal collective optical phenomena that are traditionally studied in quantum optics [1]. Solid-state systems provide new platforms with giant dipole moments, highlighting the pivotal role of many-body enhancement of light-matter coupling. We explore Dicke phenomena in solids, including cooperative emission and collective decay. We demonstrated superfluorescence in electron–hole ensembles [2], a burst of emission where peak intensity scales as $N^2$ [3]. We also observe cooperative radiative damping of cyclotron resonance in high-mobility electron systems, where the decoherence rate scales linearly with carrier density [4]. Motivated by the Dicke superradiant phase transition (SRPT), we first explored the ultrastrong coupling regime in a 2D electron gas in a THz cavity [5,6], showing the breakdown of the rotating-wave approximation via the vacuum Bloch–Siegert shift. We also demonstrated Dicke cooperativity [7] and the magnonic SRPT [8] in the $Er^{3+}-Fe^{3+}$ exchange interaction within the antiferromagnet $ErFeO_3$. This matter-matter coupling avoids the no-go theorem, enabling the experimental observation of the magnonic SRPT, characterized by the simultaneous kink and softening of the hybrid spin-magnon modes [8]. These studies demonstrate cooperative light–matter interactions in solids are not perturbative; they emerge naturally and profoundly shape the dynamics of correlated electrons. I will discuss implications for realizing novel nonequilibrium quantum phases and engineering functional quantum devices through cavity control.

1. K. Cong et al., J. Opt. Soc. Am. B 33, C80 (2016); 2. G. T. Noe II et al., Nat. Phys. 8, 219 (2012); 3. R. H. Dicke, Phys. Rev. 93, 99 (1954); 4. Q. Zhang et al., Phys. Rev. Lett. 113, 047601 (2014); 5. Q. Zhang et al., Nat. Phys. 12, 1005 (2016); 6. X. Li et al., Nat. Photon. 12, 324 (2018); 7. X. Li et al., Science 361, 794 (2018); 8. D. Kim et al., Sci. Adv. 11, eadt1691 (2025).