[Paper Review] High extraction efficiency source of photon pairs based on a quantum dot embedded in a broadband micropillar cavity

Laia Ginés, Magdalena Moczała-Dusanowska+10 more|arXiv (Cornell University)|Dec 24, 2021

Semiconductor Quantum Structures and Devices36 references34 citations

TL;DR

This paper presents a broadband micropillar cavity with a low-Q factor that achieves 69.4(10)% photon pair extraction efficiency by combining moderate Purcell enhancement with destructive interference-induced suppression of emission into non-cavity modes. The design enables high-efficiency, deterministic generation of entangled photon pairs from a single quantum dot with minimal fabrication complexity and high yield.

ABSTRACT

The generation of photon pairs in single quantum dots is based on a process that is, in its nature, deterministic. However, an efficient extraction of these photon pairs from a high-index semiconductor host material requires engineering of the photonic environment. We report on a micropillar-based device featuring an extraction efficiency of 69.4(10)$\%$ that is achieved by harnessing a broadband operation suitable for extraction of photon pairs emitted from a single quantum dot. Opposing the approaches that rely solely on Purcell enhancement to realize the enhancement of the extraction efficiency, our solution exploits a suppression of the emission into the modes other than the cavity mode. Our technological implementation requires modest fabrication effort enabling higher device yields that can be scaled up to meet the growing needs of quantum technologies. Furthermore, the design of the device can be further optimized to allow for an extraction efficiency of 85$\%$.

Motivation & Objective

• To develop a scalable, high-yield source of entangled photon pairs from a single quantum dot with high extraction efficiency.
• To overcome the challenge of extracting photon pairs from biexciton-exciton cascades in broadband cavities with low Q factors.
• To demonstrate that high extraction efficiency is achievable not only through strong Purcell enhancement but also via suppression of non-cavity mode emission.
• To provide a fabrication-simple alternative to complex waveguide or hybrid Bragg grating structures for high-performance quantum light sources.

Proposed method

• Employing a micropillar cavity with a low Q factor (200–300) to achieve a broad cavity linewidth (~5 nm), suitable for simultaneous emission of biexciton and exciton photons.
• Designing an asymmetric micropillar with 5 top and 18 bottom AlAs/GaAs distributed Bragg reflector (DBR) pairs to direct emission toward the top for efficient collection.
• Using photoluminescence imaging to precisely position the quantum dot at the center of the micropillar, ensuring resonant coupling to the cavity mode.
• Leveraging destructive interference between emission pathways at specific micropillar diameters to suppress emission into non-cavity modes, thereby increasing the β factor (fraction of emission via cavity mode).
• Simulating the β factor as a function of micropillar diameter to identify optimal geometries for high internal efficiency, with peak values reaching 0.9 for 1.8 µm diameter.
• Optimizing device performance by increasing bottom DBR pairs (to 25), using a low-index SiO2 substrate, and slightly increasing top DBRs (to 7), enabling theoretical internal efficiency up to 85%.

Experimental results

Research questions

• RQ1 Can high extraction efficiency for photon pairs be achieved in a broadband micropillar cavity with a low Q factor, despite reduced Purcell enhancement?
• RQ2 To what extent does destructive interference between emission pathways suppress non-cavity mode emission and enhance the β factor in low-Q micropillars?
• RQ3 Can a simple, high-yield fabrication process replace complex waveguide or hybrid Bragg grating structures for efficient quantum dot-based photon pair sources?
• RQ4 What design modifications maximize internal efficiency while maintaining broadband operation for biexciton-exciton cascade emission?
• RQ5 Is it feasible to generate polarization-entangled photon pairs using this micropillar platform via strain tuning?

Key findings

• The device achieves an experimentally measured internal extraction efficiency of 69.4(10)% at the cavity resonance, which is exceptionally high for a broadband micropillar cavity.
• The high efficiency is not solely due to Purcell enhancement (which yields only ~55% efficiency for a 1.25× rate enhancement), but results from a combined effect of moderate Purcell enhancement and suppression of emission into non-cavity modes.
• Theoretical simulations show that the β factor can reach 0.9 for a 1.8 µm diameter micropillar, indicating near-total emission into the cavity mode.
• Lifetime measurements confirm a 25% reduction in emission lifetime for on-resonance emitters (Purcell enhancement), while off-resonance emission is suppressed by a factor of ~2 due to destructive interference.
• Further optimization—increasing bottom DBRs to 25, using a SiO2 substrate, and increasing top DBRs to 7—can theoretically boost internal efficiency to 85%.
• The platform is suitable for generating polarization-entangled photon pairs, as the micropillar structure can be strain-tuned to control the quantum dot’s emission properties.