Color centers in diamond such as the nitrogen-vacancy (NV) and silicon-vacancy (SiV) defects are leading candidates for the creation of long-range quantum networks due to their strong interactions with optical-frequency photons and inherent scalability. However, in order to realize a color-center-based quantum network, it is necessary to (1) create an efficient photon-defect interface to route photons to and from the defect and (2) convert the photons to telecom wavelengths for long-range transmission between nodes.
To address these challenges we study gallium phosphide (GaP) as a platform for photonic devices. You can find some recent examples below!
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.
(a) Optical microscope image of bonded 1mm x 1mm GaP membrane (gold square) on 2mm x 2mm diamond chip (larger gray square). (b) Schematic cross-section of fabricated device layer. Exposed HSQ (electron-beam resist) is very similar to oxide and can be left on top of 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.
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).
(a) A render of the optimized inverse design structure showing increased collection of photoluminescence from a NV center located 100 nm below the surface. (b) False-color SEM images of the fabricated GaP-on-diamond devices.
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.
(a) Image of the photonic crystal cavity on an intermediate SiO2 substrate. (b) Diagram of the spacing between the holes: the tapered region forms a high-Q optical cavity. (c,d) Simulated structure of the defect mode.
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.
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).
(a) Schematic of the triply resonant device and SEM image of the actual device with input and output signals labeled. (b) Schematic of the microscope setup. (c) Transmission spectra of the three resonant modes used for sum-frequency conversion, with their mode profile inset.
Srivatsa Chakravarthi, Nicholas S. Yama, Alex Abulnaga, Ding Huang, Christian Pederson, Karine Hestroffer, Fariba Hatami, Nathalie P. de Leon, Kai-Mei C Fu, Hybrid integration of GaP photonic crystal cavities with silicon-vacancy centers in diamond by stamp transfer https://arxiv.org/abs/2212.04670 (2022)
Alan D. Logan, Shivangi Shree, Srivatsa Chakravarthi, Nicholas Yama, Christian Pederson, Karine Hestroffer, Fariba Hatami and Kai-Mei C. Fu, Triply-Resonant Sum Frequency Conversion with Gallium Phosphide Ring Resonators Optics Express 31, 1516 (2023)
Lillian Thiel, Alan D. Logan, Srivatsa Chakravarthi, Shivangi Shree, Karine Hestroffer, Faribar Hatami and Kai-Mei C. Fu, Target-wavelength-trimmed second harmonic generation with gallium phosphide-on-nitride ring resonators Optics Express 30, 6921 (2022)
Srivatsa Chakravarthi, Pengning Chao, Christian Pederson, Sean Molesky, Karine Hestroffer, Fariba Hatami, Alejandro W. Rodriguez and Kai-Mei C. Fu, Inverse-designed photon extractors for optically addressable defect qubits Optica 7, 1805 (2020)