Enhanced optical nonlinearities under strong light-matter coupling

TL;DR

This paper investigates how strong light-matter coupling in microcavities can significantly enhance optical nonlinearities, especially multiphoton absorption, in multilevel molecular systems, revealing potential for advanced photonic applications.

Contribution

It introduces a model showing large nonlinear enhancement in multilevel anharmonic systems under strong coupling, unlike two-level systems, highlighting the role of multilevel structure in nonlinear optics.

Findings

Two-photon absorption cross section can increase by several orders of magnitude.

Resonant enhancement occurs when Rabi splitting matches molecular anharmonic shift.

Two-level systems do not exhibit similar nonlinear enhancement under strong coupling.

Abstract

Optical microcavities and metallic nanostructures have been shown to significantly modulate the dynamics and spectroscopic response of molecular systems. We present a study of the nonlinear optics of a model consisting of $N$ anharmonic multilevel systems (e.g., Morse oscillators) undergoing collective strong coupling with a resonant infrared microcavity. We find that, under experimentally accessible conditions, molecular systems in microcavities may have nonlinear phenomena significantly intensified due to the high quality of polariton resonances and the enhanced microcavity electromagnetic energy density relative to free space. Particularly large enhancement of multiphoton absorption happens when multipolariton states are resonant with bare molecule multiphoton transitions. In particular, our model predicts two-photon absorption cross section enhancements by several orders of magnitude relative to free space when the Rabi splitting $\Omega_R$ is approximately equal to the molecular anharmonic shift $2\Delta$. Our results provide rough upper bounds to resonant nonlinear response enhancement factors as relaxation to dark states is treated phenomenologically. Notably, ensembles of two-level systems undergoing strong coupling with a cavity (described by the Tavis-Cummings model) show no such optical nonlinearity enhancements, highlighting the rich phenomenology afforded by multilevel anharmonic systems. Similar conclusions are expected to hold for excitonic systems that share features with our model (e.g., molecular dyes with accessible S_0 -> S_1 -> S_2 transitions) and strongly interact with a UV-visible cavity.