Terahertz Strong-Field Physics without a Strong External Terahertz Field

摘要

Traditionally, strong-field physics explores phenomena in laser-driven matter (atoms, molecules, and solids)that cannot be understood by treating the laser field as a small perturbation. Therefore, the presence of anextremely strong external field is usually a prerequisite for observing strong-field phenomena. However, evenin the complete absence of an external electromagnetic field, strong-field phenomena can arise when matterstrongly couples with the zero-point field of the quantum vacuum state, i.e., uctuating electromagnetic waveswhose expectation value is zero. This can occur in free space where the matter strongly interacts with acontinuum of photon modes, but some of the most striking examples of strong-field physics without an externalfield occur in a cavity setting, in which an ensemble of two-level atoms resonantly interacts with a single photonicmode of vacuum fields, producing vacuum Rabi splitting. In particular, the nature of the matter{vacuum-fieldcoupled system fundamentally changes when the coupling rate (equal to one half of the vacuum Rabi splitting)becomes comparable to, or larger than, the resonance frequency. In this so-called ultrastrong coupling regime,a non-negligible number of photons exist in the ground state of the coupled system. Furthermore, the couplingrate can be cooperatively enhanced (via so-called Dicke cooperativity) when the matter is comprised of a largenumber of identical two-level particles, and a quantum phase transition is predicted to occur as the coupling ratereaches a critical value. Low-energy electronic or magnetic transitions in many-body condensed matter systemswith large dipole moments are ideally suited for searching for these predicted phenomena. Here, we discusstwo condensed matter systems that have shown cooperative ultrastrong interactions in the terahertz frequencyrange: a Landau-quantized two-dimensional electron gas interacting with high-quality-factor cavity photons, andan Er~(3+) spin ensemble interacting with Fe~(3+) magnons in ErFeO_3.