Exploring the Optical Nonlinearities of Plasmon-Exciton Hybrid Resonances in Coupled Colloidal Nanostructures Thomas Simon, †,‡ Dimitry Melnikau, § Ana Sa ́ nchez-Iglesias, ∥ Marek Grzelczak, ∥,# Luis M. Liz-Marza ́ n, ∥,# Yury Rakovich, §,⊥,# Jochen Feldmann, †,‡ and Alexander S. Urban* ,†,‡ † Chair for Photonics and Optoelectronics, Ludwig-Maximilians-Universitä t Mü nchen, Amalienstrasse 54, 80799 Munich, Germany ‡ Nanosystems Initiative Munich (NIM), Schellingstrasse 4, 80799 Munich, Germany § Centro de Física de Materiales (MPC, CSIC-UPV/EHU), Paseo Manuel de Lardizabal 5, Donostia-San Sebastian 20018, Spain ∥ CIC biomaGUNE, Paseo de Miramó n 182, 20009 Donostia-San Sebastia ́ n, Spain ⊥ Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, Donostia-San Sebastia ́ n, 20018, Spain # IKERBASQUE, Basque Foundation for Science, Maria Diaz de Haro 3, 48013, Bilbao, Spain * S Supporting Information ABSTRACT: Strong coupling of plasmons and excitons can form hybrid states, the so-called “plexcitons”. Although plasmons have a low quality factor, the exceptionally high coupling strength with molecular aggregates, in particular J- aggregates, allows the realization of strong interaction. Despite several studies in recent years showing the formation of plexcitonic states, their nature, especially at very short times, is still insufficiently investigated. In this article, we identify the nonlinear optical behavior of plexcitons formed on gold nanorods coated with J-aggregated cyanine molecules at short times by transient absorption spectroscopy and a simple Lorentz oscillator model. We control the spectral overlap of the two resonances and analyze the effect of detuning as well as the effect of off- and on resonance excitation on the hybrid states. We demonstrate that at ultrashort time scales plexcitons show tunable plasmonic and excitonic nonlinear performance according to the hybridization model. In a first approach, we discover a way to optically manipulate the quality factor and study the effects on the coupled hybrid states. As a second approach, we find that the coupling strength can also be influenced on an ultrashort time scale in the strong coupling regime when plexcitons are excited. ■ INTRODUCTION Electromagnetic field strengths can be greatly enhanced for frequencies at optical resonator modes. Coupling of such modes with optical transitions in matter can be distinguished into two regimes: In the so-called weak coupling regime, vacuum fluctuations of the electromagnetic field are enhanced within the spectral width of the resonator mode and suppressed outside. This respectively enhances or diminishes the spontaneous emission rates of excitations (e.g., excitons) in or off resonance of such cavity modes. 1−3 In the strong coupling regime, optical modes, and excitations in resonance with each other interact so strongly that they mix and repel in frequency space leading to two hybrid (e.g., optical-excitonic) transitions separated by the so-called Rabi-frequency Ω R . 4−6 In the time-domain, this corresponds to an oscillating behavior between purely optical and purely excitonic modes (Rabi- oscillations). 7−9 Optical excitations in atoms, 4,5 semiconductors quantum dots, 6,10,11 and molecules 12,13 have been shown to exhibit strong coupling with cavity modes even on the single particle level. 14,15 Metal structures exhibit plasmonic resonances characterized by huge local polarizations due to collective electron oscillations. 16 Depending on the geometry, either running or localized plasmons can be realized. Such plasmons can assume the part of an optical resonator mode and couple to electronic transitions. In addition, in this case both weak and strong coupling regimes can be realized. Near-field plasmonic nanoresonators have been used to manipulate and tune spontaneous emission rates of dye molecules 17 and semi- conductors. 18 In the strong coupling regime, the plasmon and the electronic transition hybridize, resulting in two states separated by a Rabi-splitting. 19−21 Excitonic transitions of organic dyes 22 with a large dipole moment, a prime example being J-aggregates, have been reported to show strong coupling with localized surface plasmons. 23−29 J-aggregates are molecular aggregates where the dipole moments of the individual molecules align along a Received: May 9, 2016 Published: May 19, 2016 Article pubs.acs.org/JPCC © 2016 American Chemical Society 12226 DOI: 10.1021/acs.jpcc.6b04658 J. Phys. Chem. C 2016, 120, 12226−12233