Free-standing circular Bragg gratings enabling efficient GaAs quantum dot entangled photon pair sources

TL;DR

This paper introduces a scalable, fabrication-minimal method to create free-standing circular Bragg gratings with deterministically positioned GaAs quantum dots, significantly enhancing photon extraction efficiency and reducing strain for quantum communication applications.

Contribution

The authors demonstrate a monolithic, free-standing CBG fabrication approach that achieves high efficiency and low strain in quantum dot photon sources without complex multi-layer processing.

Findings

Achieved up to 68% free-space extraction efficiency.

Enhanced photoluminescence intensity by up to 700 times.

Reduced exciton fine-structure splitting from 7.3 μeV to 1.3 μeV.

Abstract

Deterministic and bright quantum light sources based on scalable semiconductor technologies are a crucial building block for future quantum communication networks. While circular Bragg gratings (CBGs) are highly effective for extracting light from solid-state quantum emitters, conventional architectures rely on complex multi-layer processing or flip-chip bonding, which introduce detrimental strain and limit scalability. Here, we present a fabrication-minimal approach to realize monolithic, free-standing CBG cavities with deterministically positioned single GaAs quantum dots (QDs). By utilizing aspect-ratio-dependent etching (ARDE) in a single-step top-down process, we achieve the necessary vertical structural asymmetry for directional emission without requiring bottom reflectors. Finite-difference time-domain (FDTD) simulations validate this geometry, predicting free-space extraction efficiencies up to $68 \, \%$ and coupling efficiencies of $40 \, \%$ into a lensed single-mode fiber ($\text{NA} = 0.6$). Experimentally, the deterministically coupled QD-CBG devices yield a photoluminescence intensity enhancement of up to $\times 700$ compared to unprocessed planar QDs, reaching integrated count rates of $45 \, MHz$. Furthermore, the suspended membrane architecture effectively relaxes residual strain, significantly reducing the average exciton fine-structure splitting from $7.3 \, \mu eV$ in planar QDs to $1.3 \, \mu eV$ in the CBGs. Interferometric measurements confirm that the fabrication process preserves the optical quality of the emitters, with average coherence times of $70 \, ps$. By bridging optimized FDTD design with precise nanofabrication and robust optical performance, these results establish free-standing GaAs CBGs as a highly scalable platform for bright and coherent entangled photon pair sources.