講演情報
[18a-S2_202-6]Design of Ge-based Bull’s-eye Cavity with Dual-order Circular Grating Structure for Quantum Interfaces
〇Guangtai Lu1, Sangmin Ji2, Satoshi Iwamoto1 (1.Univ. Tokyo, 2.Hiroshima Univ.)
キーワード：
Quantum interface、Bull's-eye cavity
Gate-defined quantum dots (QDs) in strained Ge/SiGe heterostructures have recently emerged as a promising platform for photon-spin quantum interfaces, offering strong spin–orbit coupling for fast all-electrical control, reduced hyperfine-induced dephasing for long coherence time, and telecom-wavelength operation through strain and band-structure engineering.
We previously reported a Ge-based second-order bull’s-eye cavity for enhancing light–matter interaction via a resonant mode. Owing to its polarization-degenerate modes and a well-confined vertical far-field profile, such a cavity is suitable for polarization-to-spin conversion. However, because the optical absorption of Ge is intrinsically weak (Qabs~990), increasing the cavity quality factor is essential to better approach critical (or near-critical) coupling, that is matching the external radiative coupling rate (Qrad~300 in our previous design) to the internal loss rate dominated by Ge absorption.
Here we propose a Ge-based dual-order bull’s-eye cavity for increasing the quality factor of the cavity. Specifically, we combine an inner second-order circular Bragg grating (Period: 480nm) with outer first-order grating (Period: 240nm). The inner second-order section provides efficient vertical coupling and a well-confined, near-Gaussian far-field profile for high free-space mode coupling, while the outer first-order section acts as a high-reflectivity mirror that suppresses radiation leakage, thereby increasing the quality factor up to 600. Compared with a pure second-order design, this dual-order structure improves the total absorption efficiency for Ge QDs by more than 20%.
We previously reported a Ge-based second-order bull’s-eye cavity for enhancing light–matter interaction via a resonant mode. Owing to its polarization-degenerate modes and a well-confined vertical far-field profile, such a cavity is suitable for polarization-to-spin conversion. However, because the optical absorption of Ge is intrinsically weak (Qabs~990), increasing the cavity quality factor is essential to better approach critical (or near-critical) coupling, that is matching the external radiative coupling rate (Qrad~300 in our previous design) to the internal loss rate dominated by Ge absorption.
Here we propose a Ge-based dual-order bull’s-eye cavity for increasing the quality factor of the cavity. Specifically, we combine an inner second-order circular Bragg grating (Period: 480nm) with outer first-order grating (Period: 240nm). The inner second-order section provides efficient vertical coupling and a well-confined, near-Gaussian far-field profile for high free-space mode coupling, while the outer first-order section acts as a high-reflectivity mirror that suppresses radiation leakage, thereby increasing the quality factor up to 600. Compared with a pure second-order design, this dual-order structure improves the total absorption efficiency for Ge QDs by more than 20%.