Conical Intersections Induced by Quantum Light: Field-Dressed Spectra from the Weak to the Ultrastrong Coupling Regimes

TL;DR

This paper develops a theoretical framework to study how quantum light in optical cavities induces conical intersections in molecular spectra, revealing breakdown of the Born--Oppenheimer approximation and significant spectral changes across coupling regimes.

Contribution

The work introduces a novel theoretical approach to analyze light-induced conical intersections and their spectral signatures in cavity-molecule systems, covering weak to ultrastrong coupling.

Findings

Demonstrates breakdown of Born--Oppenheimer approximation due to cavity field

Identifies light-induced conical intersections (LICIs) affecting spectra

Shows spectral changes across different light-matter coupling strengths

Abstract

A fundamental theoretical framework is formulated for the investigation of rovibronic spectra resulting from the coupling of molecules to one mode of the radiation field in an optical cavity. The approach involves the computation of (1) cavity-field-dressed rovibronic states, which are hybrid light-matter eigenstates of the `molecule + cavity radiation field' system, and (2) the transition amplitudes between these field-dressed states with respect to a weak probe pulse. The predictions of the theory are shown for the homonuclear Na$_2$ molecule. The field-dressed rovibronic spectrum demonstrates undoubtedly that the Born--Oppenheimer approximation breaks down in the presence of the cavity radiation field. A clear fingerprint of the strong nonadiabaticity is found, which can only emerge in the close vicinity of conical intersections. In this work, the conical intersection is induced by the quantized radiation field, and it is thus called a "light-induced conical intersection" (LICI). Dependence of the cavity-field-dressed spectrum on the cavity-mode wavelength as well as on the light-matter coupling strength is investigated. Essential changes are identified in the spectra from the weak to the ultrastrong coupling regimes.