The gallium phosphide (GaP) photonics platform

GaP is transparent to light at NV/SiV-emission wavelengths and has a high enough refractive index to be used as a waveguiding material on diamond, making it a particularly promising platform for the creation of color-center-based quantum networks.

Our GaP is grown by our collaborators at Humboldt University​. We perform GaP-diamond membrane bonding, lithography. and etching at the UW Nanofabrication Facility, a national user facility.

Defect-integrated photonic devices

Integration of color centers with photonic structures such as cavities enables improved coupling to photons and enhancement of defect properties. These structures enable more efficient collection and routing of color-center photoluminescence forming the basis for a color-center-based qubit node. Emission captured by such devices can then be routed on-chip and interfered to generate entangled networks of color center qubits.

Inverse-designed photon extractors

Shallow-implanted nitrogen-vacancy (NV) centers often have worse coherence properties due to the imperfect crystal structure at the surface. Deeper NV centers often have better properties but are limited by their weaker coupling to photonic devices.

In collaboration with Prof. Alejandro Rodriguez's group at Princeton University, we designed and fabricated GaP photon extractor devices which enables a 14-fold enancement of the NV photoluminesence collection. The inverse-design process optimizes the performance by varying the design, thus producing the non-intuitive structures seen below.

This work is published in Optica 7, 1805 (2020).

GaP-on-diamond photonic crystal cavities coupled to silicon-vacancy (SiV) centers

Photonic crystal cavities can be engineered to have extremely small mode volumes with high quality factors which enable large enhancements of the radiative lifetime of cavity-coupled emitters. This makes them a particularly attractive architecture for defect-based qubits.

In this project, we demonstrate the integration of a GaP photonic crystal cavity with a SiV center by a stamp transfer technique, and characterize the resulting enhancement of the SiV defect emission. The stamp transfer technique minimizes the damage to the SiV center environment and enables fine tuning of the cavity prior to integration.

This work is currently posted on the arXiv.

Nonlinear GaP photonic devices

GaP is a particularly promising material for nonlinear photonics owing to a high nonlinear susceptibility 𝜒(2) ∼ 110 pm/V and high refractive index. While nonlinear photonic devices are interesting in their own right for classical applications, such devices are expected to play a significant role in the creation of large-scale quantum networks due to the necessity of telecom carrier frequencies (~1550 nm) in low-loss fiber-optic networks. GaP is well-suited for this role as it can leverage 𝜒(2) processes such as difference-frequency conversion to generate telecom photons from defect emission. Integrating these devices with defect-coupled cavities could then allow for one to realize a complete on-chip quantum network node.

Triply-resonant sum frequency generation

Nonlinear frequency conversion is usually accomplished in long waveguide structures which support broadband modes. These devices require large footprints and high laser powers which are prohibitive to on-chip scalability. Resonant structures such as ring resonator can reduce both of these constraints but require all involved frequencies to be resonant with properly phase-matched modes.

In this work we demonstrate sum frequency conversion in ring resonator which has been engineered to be simultaneously resonant and phase-matched for all three frequencies, thereby enabling efficient conversion with low powers. These results indicate a difference frequency conversion of single photons (as necessary for a quantum network) at a small-signal conversion efficiency of ~7.2%/mW in this devices, which could be as large as ~45%/mW in the same chip with optimal parameters.

This work was published in Optics Express 31, 1516 (2023).