Linear acene molecules in plasmonic cavities: mapping evolution of optical absorption spectra and electric field intensity enhancements

Abstract

Understanding the plasmonic cavity induced electric field enhancement in a hybrid nanosystem is of paramount importance in the development of new optical devices. In this work, using linear response time dependent density functional theory calculations (LR-TDDFT), we demonstrate interesting modulations in the optical absorption spectra and electric field enhancements of a hybrid nanosystem consisting of linear acenes of fused benzene rings (benzene up to hexacene, arranged both in the parallel and perpendicular geometrical orientations) placed inside a sodium nanodimer gap. We show that the striking effect of placing linear acenes in a plasmonic gap is to create “asymmetric-dip” structures in the broad optical spectrum of the localized plasmon. This result is in tune with a recent experimental study [H. Imada, K. Miwa, M. Imai-Imada, S. Kawahara, K. Kimura and Y. Kim, Phys. Rev. Lett., 2017, 119, 013901] predicting that the origin of the dips is due to the energy transfer between the plasmon and molecular excitons. We also demonstrate that variations in the geometrical orientations of linear acenes in the gap region affect the absorption profile in a very moderate way with small, but visible changes in the absorption amplitudes. In addition, a particular emphasis is given to the modulations in the optical absorption spectra and electric field enhancements as a result of incorporating an external electric field in the Hamiltonian. Our theoretical findings may help in the design of hybrid nanosystems for ultra-sensitive molecular sensing applications.