Nanocavity-mediated Purcell enhancement of Er in TiO$_2$ thin films grown via atomic layer deposition

TL;DR

This paper demonstrates a scalable method to integrate erbium-doped TiO₂ thin films with silicon photonics, achieving significant Purcell enhancement and enabling advanced optical and quantum devices.

Contribution

It introduces a low-temperature, substrate-independent atomic layer deposition process for Er-doped TiO₂ films with controlled doping and demonstrates Purcell enhancement in nanophotonic cavities.

Findings

Achieved a Purcell factor of 300 for Er ions in TiO₂ nanocavities.

Maintained low surface roughness suitable for nanophotonic integration.

Demonstrated scalable Er doping in amorphous TiO₂ films with post-growth annealing.

Abstract

The use of trivalent erbium (Er$^{3+}$), typically embedded as an atomic defect in the solid-state, has widespread adoption as a dopant in telecommunications devices and shows promise as a spin-based quantum memory for quantum communication. In particular, its natural telecom C-band optical transition and spin-photon interface makes it an ideal candidate for integration into existing optical fiber networks without the need for quantum frequency conversion. However, successful scaling requires a host material with few intrinsic nuclear spins, compatibility with semiconductor foundry processes, and straightforward integration with silicon photonics. Here, we present Er-doped titanium dioxide (TiO$_2$) thin film growth on silicon substrates using a foundry-scalable atomic layer deposition process with a wide range of doping control over the Er concentration. Even though the as-grown films are amorphous, after oxygen annealing they exhibit relatively large crystalline grains, and the embedded Er ions exhibit the characteristic optical emission spectrum from anatase TiO$_2$. Critically, this growth and annealing process maintains the low surface roughness required for nanophotonic integration. Finally, we interface Er ensembles with high quality factor Si nanophotonic cavities via evanescent coupling and demonstrate a large Purcell enhancement (300) of their optical lifetime. Our findings demonstrate a low-temperature, non-destructive, and substrate-independent process for integrating Er-doped materials with silicon photonics. At high doping densities this platform can enable integrated photonic components such as on-chip amplifiers and lasers, while dilute concentrations can realize single ion quantum memories.