Speaker: Johannes Feist
Affiliation: Dpto. Física Teórica de la Materia Condensada (UAM)
Date: Tuesday, 22 November 2016 at 14:00
Location: Seminar Room, Serrano 121 (CFMAC)
Strong coupling is achieved when the coherent energy exchange between a confined electromagnetic field mode and material excitations becomes faster than the decay and decoherence of either constituent. This creates a paradigmatic hybrid quantum system with eigenstates that have mixedlight-matter character (polaritons). Organic molecules are a particularly useful system to achieve strong coupling at room temperature, since they possess excitons (bound electron-hole pairs) with large transition dipole moments and binding energies.
While most models of strong coupling are based on two-level systems, this is far from a realistic description for molecules with many nuclear (rovibrational) degrees of freedom. The influence of strong coupling on these internal degrees of freedom has only come into focus recently.
Pioneering experiments have shown modifications of material properties and chemical reaction rates under strong coupling, which cannot be explained by simple two-level models. In order to address this mismatch, we developed a first-principles model combining the tools of cavity QED with well-known molecular models in order to fully take into account electronic, nuclear and photonic degrees of freedom.
I will first discuss the applicability of the Born-Oppenheimer approximation, which is challenged by the introduction of the new intermediate timescale of energy exchange between the molecule(s) and the field. We then show how photochemical reactions such as photoisomerization can be almost completely suppressed under strong coupling. Surprisingly, this suppression works more efficiently when many molecules are coupled to a single light mode due to a “collective protection” effect within the delocalized polaritonic state.