Surface passivation for narrowing optical linewidth of silicon T centers in nanophotonic devices

Silicon T centers are promising spin-photon interfaces for quantum networking, but their performance is limited by spectral diffusion—the broadening of optical transitions caused by fluctuating local electric fields. This study investigates whether surface passivation with atomic-layer-deposited (ALD) Al2O3 can mitigate this noise in nanophotonic silicon devices.

The researchers fabricated silicon nanobeam waveguides containing T centers and applied conformal Al2O3 coatings of varying thicknesses (7–26 nm) using ALD. They evaluated the impact on optical performance by comparing the photoluminescence excitation (PLE) linewidths of individual emitters before and after passivation. They also performed spectral hole burning and above-bandgap illumination experiments to distinguish between homogeneous broadening and residual spectral diffusion.

• Linewidth Narrowing: Al2O3 passivation consistently narrowed the optical linewidths of T centers, with an average reduction of 32% and a maximum observed reduction of 57%.
• Optimal Thickness: A thickness of 11 nm was identified as the optimal balance; thinner layers provided insufficient passivation, while thicker layers introduced strain-induced non-radiative decay, which shortened the excited-state lifetime.
• Residual Broadening: Despite the improvement, the linewidth remains significantly broader than the transform limit. Spectral hole burning measurements placed an upper bound of 75 MHz on the homogeneous linewidth, confirming that slow spectral diffusion from nearby charge traps remains the dominant source of broadening.
• Charge Stabilization: Above-bandgap illumination further narrowed the linewidth, suggesting that photogenerated carriers help stabilize the local charge environment, providing a potential path for further optimization.