doi:10.1515/nanoph-2019-0403engCalvo, JorgeZueco, DavidMartin-Moreno, LuisUltrastrong coupling effects in molecular cavity QEDART-2020-117541The spectrum and energy dynamics for a system that comprises a molecule interacting with a cavity photon is analyzed, taking into account the effect of both molecular vibrations and counter-rotating terms (CR) in the dipole Hamiltonian. The CR terms do not have a strong effect on the spectrum even for moderately large values of the exciton-photon interaction. However, it is shown that the polariton subspace is governed by an effective Quantum-Rabi Hamiltonian, where polaritons act as a two-level system and the phonons play the role of cavity photons. The effect of the CR terms is amplified in the dynamics: as the vibrations reduce the effective photon-exciton coupling, small Bloch-Siegert energy shifts can bring the system out of resonance.2020http://zaguan.unizar.es/record/15308410.1515/nanoph-2019-0403http://zaguan.unizar.es/record/153084oai:zaguan.unizar.es:153084info:eu-repo/grantAgreement/ES/DGA/Q-MADinfo:eu-repo/grantAgreement/ES/MCIU/MAT2017-88358-C3-1-RNanophotonics 9, 2 (2020), 277-281byhttps://creativecommons.org/licenses/by/4.0/deed.esinfo:eu-repo/semantics/openAccess