Enhancing the Purcell Factor and Stability of Nitrogen-Vacancy Centers Coupled to Plasmonic Nanocavities through Dielectric Encapsulation

Quantum emitters coupled to nanophotonic and plasmonic cavities are critical for advancing on-chip quantum photonics and serve as key elements for quantum information processing, communication, and computing. Plasmonic-enhanced single photon emitters, such as nitrogen-vacancy (NV) centers coupled to plasmonic nanopatch antennas (NPAs), demonstrate record-high Purcell enhancement, essential for improving photon quality and efficiency. However, plasmonic NPAs degrade rapidly after exposure to air, limiting their practical applicability. In this work, we demonstrate that atomic layer deposition of an Al₂O₃ encapsulation layer on the NV-NPA system extends its lifetime from three days to over three months. Moreover, the encapsulation fine-tunes the dielectric environment, enhances the Purcell factor by shortening the emitter lifetime by a factor of 1.5, increases saturation intensity, enables emission wavelength tunability, and significantly improves radiation directivity by reducing substrate leakage. Numerical simulations highlight the critical role of optimizing the dielectric environment for plasmonic nanocavities in emerging quantum photonic applications.