Site-Controlled Coupling Between Plasmonic Nanostructures and Strain-Localized Excitons in Nano-Indented WSe₂

Localized excitons can be deterministically created in 2D transition metal dichalcogenides (TMDs) via strain engineering, which modifies the local band structure. It has been shown that strain-localized excitons in TMDs give rise to site-controlled single photon emission, ^1-3 which is of interest for quantum information and communications applications. This is of particular technological relevance given that TMDs are sensitive to environmental perturbations and can carry spin information through their valley occupancy. These discoveries have led to recent interest in coupling these strain localized quantum emitters to photonic structures,^4.5 to influence the emission properties and create on-chip-compatible platforms. Strong coupling to plasmonic nanostructures and cavities is a pathway to coherent control at room temperature.⁶ However, all prior attempts to couple strain localized excitons in TMDs to plasmonic structures have remained in the weak coupling regime characterized by Purcell enhancement of quantum emission.^7,8<br/><br/>Here we achieve site-controlled coupling between a localized surface plasmon resonance (LSPR) and the strain-localized excitons in WSe₂. We employ a nano-indentation technique using an atomic force microscope (AFM) to create on-demand spatially localized excitons that display quantum emission.⁹ These nano-indentations are co-located above Au plasmonic discs that host LSPRs. The exciton-plasmon coupling is characterized by the familiar mode-splitting of the scattering spectra near the exciton wavelength, indicating fast energy exchange between the two systems. The LSPR wavelength is tuned via the plasmonic disc diameter in order to map the avoided crossing between the LSPR and localized exciton absorption resonance. The observed coupling strength approaches the strong coupling regime at room temperature. Our results establish this system as a potential platform for novel quantum light sources in which to study coherent control as well as nonlinear exciton-plasmon polariton behavior.<br/><br/>1. Branny, et al., Nat. Commun. 8, 15053 (2017)<br/>2. Palacios-Berraquero, et al., Nat. Commun. 8, 15093 (2017)<br/>3. Parto, et al., Nat. Commun. 12, 3585 (2021)<br/>4. Peysken, et al., Nat. Commun. 10, 4435 (2019)<br/>5. Sortino, et al., Nat. Commun. 12, 6063 (2021)<br/>6. Chikkaraddty, et al., Nature 535, 127 (2016)<br/>7. Cai, et al., ACS Photonics 5, 3466-3471 (2018)<br/>8. Lou, et al., Nat. Nanotechnol. 13, 1137-1142 (2018)<br/>9. Rosenberger, et al., ACS Nano 13, 904-912 (2019)