Inverse-designed photon extractors for optically addressable defect qubits

At a Glance

Metadata	Details
Publication Date	2020-11-17
Journal	Optica
Authors	Srivatsa Chakravarthi, Pengning Chao, Christian Pederson, Sean Molesky, Andrew Ivanov
Citations	36
Analysis	Full AI Review Included

Technical Documentation & Analysis: Inverse-Designed Photon Extractors

Reference: Chakravarthi et al., “Inverse-designed photon extractors for optically addressable defect qubits,” arXiv:2007.12344v3 (2020).

Executive Summary

This research demonstrates a critical advancement in solid-state quantum technology by utilizing inverse-designed photonic structures to enhance the performance of near-surface Nitrogen-Vacancy (NV) centers in diamond.

Core Achievement: Demonstrated a compact hybrid Gallium Phosphide (GaP)-on-Diamond planar dielectric structure achieving up to a 14-fold broadband enhancement in photon extraction efficiency.
Application Focus: Optimized for optically addressable defect qubits, specifically near-surface NV centers (100 nm depth) created via implantation and annealing.
Design Methodology: Utilized topology optimization (inverse design) via 3D FDFD solvers to tailor spectral response and collection mode, while incorporating fabrication constraints (e.g., minimum feature size of 50 nm).
Material Stack: Hybrid structure consisting of a 250 nm thick GaP membrane (high index, n = 3.31) patterned onto a high-purity Single Crystal Diamond (SCD) substrate.
Scalability: The small device footprint (1.5 µm x 1.5 µm) is compatible with scalable integration and on-chip electrodes for frequency tuning.
Stability Improvement: Post-fabrication oxygen annealing at 400 °C was shown to significantly improve NV charge state stability (NV^-/NV⁰ ratio) and reduce spectral diffusion, addressing key challenges in near-surface qubit integration.

Technical Specifications

Parameter	Value	Unit	Context
Maximum PL Enhancement (Measured)	14	-fold	Broadband ZPL enhancement (575 nm to 750 nm)
Simulated PL Enhancement (z-dipole)	15.7	-fold	Relative to bare diamond-air interface (100 nm depth)
Absolute Collection Efficiency (Optimized)	20	%	Simulated total NV radiation collected
NV Center Target Depth	100 ± 20	nm	Below diamond surface (Implanted ¹⁵N at 85 keV)
GaP Refractive Index (n)	3.31	-	At NV emission energy (low loss)
GaP Membrane Thickness	250	nm	Transferred layer thickness
Device Footprint (Lateral)	1.5 x 1.5	µm²	Inverse-designed extractor structure
NV ZPL Wavelength (Negative Charge)	637	nm	Optimization target frequency
Initial Annealing Temperature (Sample B)	1200	°C	CVD diamond, ¹⁵N implantation
Post-Fabrication Annealing	400	°C	Oxygen flow, 4 hours (for stability improvement)
Minimum Feature Size (Design Constraint)	50	nm	Restricted by Electron Beam Lithography (EBL)
Single NV Linewidth (Post-Anneal, Device)	844	MHz	Average device-coupled NV linewidth

Key Methodologies

The photon extractors were designed and fabricated using advanced nanophotonic and material processing techniques:

Inverse Design Optimization: Topology optimization (TO) was employed using a 3D Finite-Difference Frequency-Domain (FDFD) solver, maximizing the minimum Poynting flux (t) through a collection surface 0.4 µm above the device, ensuring robustness across different dipole polarizations (x, y, z).
Material Preparation: High-purity Single Crystal Diamond (SCD) substrates (HPHT and CVD electronic grade) were used. NV centers were created via ¹⁴N or ¹⁵N ion implantation (20 keV or 85 keV) followed by high-temperature vacuum annealing (800 °C or 1200 °C).
Hybrid Stack Formation: A 250 nm thick Gallium Phosphide (GaP) membrane was transferred onto the diamond substrate using a wet lift-off process, forming the high-index dielectric layer.
Nanofabrication: Electron Beam Lithography (EBL, 100 kV) was used to pattern the 1.5 µm x 1.5 µm structures in a thin HSQ resist layer. A conductive Au+Pd layer was sputtered to mitigate charging on the non-conductive diamond substrate during EBL.
Pattern Transfer: Subsequent plasma Reactive Ion Etching (RIE) using Ar/Cl₂/N₂ chemistry transferred the pattern into the GaP layer, achieving near-vertical (88°) sidewalls.
Qubit Stability Enhancement: Post-fabrication oxygen annealing at 400 °C was performed to modify the diamond surface termination, resulting in a significant increase in the desired NV^-/NV⁰ charge state ratio and reduced spectral diffusion.

6CCVD Solutions & Capabilities

The research highlights the critical need for ultra-high purity, precisely engineered diamond substrates and advanced integration capabilities to realize scalable defect qubit arrays. 6CCVD is uniquely positioned to supply the foundational SCD materials and processing services required to replicate and advance this work.

Applicable Materials & Services	6CCVD Capability Match	Research Application & Advantage
Single Crystal Diamond (SCD) Substrates	Electronic Grade SCD: N < 5 ppb, B < 1 ppb (matching Sample B purity). Available in <100> orientation up to 10 mm thick.	Provides the ultra-low defect density necessary for high optical coherence and long spin coherence times in NV centers.
Custom Dimensions & Scalability	Large-Format PCD/SCD: Wafers up to 125 mm (PCD) and plates up to 10x10 mm (SCD). Substrates up to 10 mm thick.	Supports the transition from small-scale research arrays (2 mm x 2 mm) to high-volume, wafer-scale fabrication of quantum devices.
Surface Preparation & Polishing	Precision Polishing: SCD surfaces polished to Ra < 1 nm.	Essential for minimizing surface strain and defects prior to GaP membrane transfer, directly addressing the spectral diffusion and strain issues observed at the GaP-diamond interface.
Integration Support (Implantation Depth)	Custom Thickness Control (0.1 µm - 500 µm): 6CCVD provides precisely controlled SCD layer thicknesses, facilitating the required pre-implantation etching or surface preparation for accurate 100 nm NV depth placement.	Ensures optimal coupling between the near-surface NV centers and the inverse-designed photonic mode.
Integrated Circuitry & Tuning	Custom Metalization: In-house deposition of Au, Pt, Pd, Ti, W, Cu.	Enables the integration of on-chip electrodes for dynamic Stark tuning and frequency control, which is necessary to bridge the observed static inhomogeneity (30 ± 15 GHz blue shift) between coupled NV centers.
Engineering Support	In-house PhD Material Science Team: Consultation on material selection, surface termination optimization (e.g., post-annealing recipes), and strain mitigation for similar quantum emitter projects.	Assists researchers in optimizing the critical post-fabrication steps (like the 400 °C oxygen anneal) to maximize NV charge state stability and reduce the 844 MHz linewidth observed in device-coupled NVs.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Solid-state defect qubit systems with spin-photon interfaces show great promise for quantum information and metrology applications. Photon collection efficiency, however, presents a major challenge for defect qubits in high refractive index host materials. Inverse-design optimization of photonic devices enables unprecedented flexibility in tailoring critical parameters of a spin-photon interface including spectral response, photon polarization, and collection mode. Further, the design process can incorporate additional constraints, such as fabrication tolerance and material processing limitations. Here, we design and demonstrate a compact hybrid gallium phosphide on diamond inverse-design planar dielectric structure coupled to single near-surface nitrogen-vacancy centers formed by implantation and annealing. We observe up to a 14-fold broadband enhancement in photon extraction efficiency, in close agreement with simulations. We expect that such inverse-designed devices will enable realization of scalable arrays of single-photon emitters, rapid characterization of new quantum emitters, efficient sensing, and heralded entanglement schemes.

Tech Support

Original Source

DOI: https://doi.org/10.1364/optica.408611