Engineering Strong Coupling in a Nano-Plasmonic Cavity Using Transition Metal Dichalcogenide Nanobubbles

In the interaction of light and matter, strong coupling occurs when exchange between a photon and electronic transitions exceeds the relative loss rate leading to hybridization of the optical and electronic states. The behavior is well known in cavity quantum electrodynamics (QED), and is a fundamental ingredient in single photon quantum logic gates [1]. In solid-state systems, many strong coupling phenomena have been explored between different material excitations. Plasmons in particular have attracted great interest owing to their small mode volumes [2], allowing for strong coupling of a plasmon and single quasi-particles excitations such as excitons, potentially recreating in the solid-state, at the nanoscale, and at elevated temperatures many of the phenomena previously studied in traditional trapped atom QED.<br/><br/>Strong coupling between plasmons and excitons has been observed in excitons systems: e.g., J-aggregates [3], and colloidal quantum-dots [4-5]. While these systems offer large coupling strengths, the exciton transition energies are largely fixed, and vary randomly depend on variations in growth conditions. By contrast, transition metal dichalcogenides offer strong exciton and large tunability of exciton energy by applied strain. However, to date strong coupling between plasmons in the excitons in transition metal dichalcogenides has only been achieved in multilayer stacks[6], which have significantly weaker light emission compared to monolayers.<br/><br/>Building off our previous work observing quantum-dot like exciton states in nanobubbles [7], here we show signatures of strong coupling between nanobubble-localized excitons and a gap-mode plasmon mode formed between a scanning near-field optical probe and substrate. By adjusting the gap mode distance, the coupling strength and plasmon energy can be dynamically tuned. Further, by applying pressure with the nano-optical probe the emission energies of localized excitons can be adjusted. When approaching the tip to the nanobubble surface, we observed striking emission peak splitting. Comparing the energy dependence with a coupled oscillator model, we see qualitative agreement of the gap size dependence of the coupled exciton emission. Our results show the promise of engineering room-temperature strong coupling of plasmons with excitons in monolayer semiconductors for use as a model system and for building compact quantum plasmonic-photonic devices.<br/><br/><br/>References:<br/>[1] A. Reiserer, <i>et al.</i>, “A quantum gate between a flying optical photon and a single trapped atom,” <i>Nature</i>, <b>508, </b>237 (2014)<br/>[2] J. J. Baumberg, <i>et al.</i>, “Extreme nanophtonics from ultrathin metallic gaps,” <i>Nat. Mater.</i>, <b>18</b>, 668 (2019)<br/>[3] N. T. Fofang, <i>et al.</i> “Plexcitonic Nanoparticles: Plasmon-Exciton Coupling in Nanoshell-J-Aggregate Complexes,” <i>Nano Lett, </i><b>8</b>, 3481 (2008)<br/>[4] K-D Park, <i>et al.</i>, “Tip-enhanced strong coupling spectroscopy, imaging, and control of a single quantum emitter,” <i>Sci. Adv. </i><b>5</b>, 5931 (2019)<br/>[5] H. Groß, <i>et al. </i> “Near-field strong coupling of single quantum dots,” <i>Sci. Adv., </i><b>4</b>, 4906 (2018)<br/>[6] M.-E. Kleeman, <i>et al, </i><b>“</b>Strong-coupling WSe2 in ultra-compact plasmonic nanocavities at room temperature,” <i>Nat. Comm., </i><b>8</b>, 1296 (2017)<br/>[7] T. Darlington, <i>et al. </i>“Imaging strain-localized excitons in nanoscale bubbles of monolayer WSe2 at room temperature,” <i>Nat. Nano., </i><b>15</b>, 854 (2020)