Strong coupling between J-aggregates and surface plasmon polaritons in gold nanodisks arrays

Abstract

Recently, much attention has been paid to an interesting subject, i.e., the interactions between surface plasmon polaritons (SPPs) and molecules. The interactions between SPPs and molecules often appear in two opposite cases, namely weak and strong coupling. When the interaction is weak, the absorption maximum simply coincides with the electronic transition energy of the molecule. In the weak coupling regime, the wave functions of the molecule and the SPP modes are considered to be unperturbed, only leading to enhancement of the absorption or fluorescence of the molecules. On the other hand, when the interaction is strong enough, the SPPs and molecules can form a coherent hybrid object, thus the excitation energy is shared by and oscillates between the SPPs and molecular systems (Rabi oscillations), leading to vacuum Rabi splitting of energy levels at the resonance frequency. Due to the fact that both the SPPs and the molecule components can be confined into the nanometer scale, the work on strong coupling with SPPs offers a very good opportunity to realize nanoplasmonic devices, such as thresholdless laser and room temperature B-E condensates.In this work, we investigate a hybrid system formed by strong coupled gold nanodisk arrays and J-aggregate molecules. Smooth gold nanodisk arrays are fabricated by a template-stripping process. In such an experimentally simple replicate process, mass-production of gold nanodisk arrays with the same morphology can be transferred from patterned indium tin oxides (ITO) glass. The structures on ITO glass are milled with a focused ion beam. Periodic gold nanodisk arrays have the capability of converting light into SPPs modes, resulting in a significant field confinement at the patterned metal surface. In particular, the desired SPP mode can be chosen by changing the nanodisk array period to match the absorbance peak of the J-aggregate molecule. On the other hand, J-aggregate molecule is chosen due to its large dipole moments and absorption coefficient, which makes it attractive for designing the strong exciton-plasmon interaction system. The strong coupled system is formed when the J-aggregate molecule is spin-coated on the gold nanodisk arrays. Through reflection measurements, Rabi splitting energy value 200 meV is observed when the period of the nanodisk array is 350 nm. Through tuning the coupling strength by changing the lattice period from 250 nm to 450 nm, the typical signature of strong coupling:anticrossing of energies is found in reflection spectra. This simple replicate process for strong coupling hybrid system fabrication should play an important role in designing novel ultrafast nanoplasmonic devices with coherent functionalities.