Metal-dielectric antennas for efficient photon collection from diamond color centers

At a Glance

Metadata	Details
Publication Date	2018-01-31
Journal	Optics Express
Authors	Amir Karamlou, Matthew E. Trusheim, Dirk Englund
Citations	35
Analysis	Full AI Review Included

Technical Documentation & Analysis: Metal-Dielectric Antennas for Efficient Photon Collection

Executive Summary

This analysis focuses on optimizing the photon collection rate (CPR) from diamond Nitrogen-Vacancy (NV) centers using advanced nanoantenna designs, a critical step for high-fidelity quantum sensing and computing.

Core Achievement: Numerical optimization of metal-dielectric hybrid nanoantennas recessed into a diamond substrate to maximize the collected photon rate (CPR).
Highest Performance Design: The Hybrid Hourglass (MD2) structure achieved the maximum average CPR enhancement.
Magnitude of Improvement: The MD2 design demonstrated an average CPR enhancement factor of 25.6, representing a 400-fold increase in collected photons compared to a bare NV emitter in bulk diamond.
Mechanism: High CPR is achieved by balancing a large radiative Purcell factor (Fp $\approx$ 199) with improved Collection Efficiency (CE $\approx$ 13%), minimizing non-radiative losses inherent in purely metallic designs.
Material Requirement: The research necessitates high-purity, low-strain Single Crystal Diamond (SCD) substrates suitable for NV center creation and nanoscale fabrication.
Application Impact: The resulting efficient spin-photon interface enables single-shot electron spin measurements of NV centers at room temperature, crucial for quantum information processing.

Technical Specifications

The following table summarizes the key performance metrics and optimized parameters derived from the simulation of the best-performing hybrid structure (MD2) and comparative data.

Parameter	Value	Unit	Context
Highest Avg. CPR Enhancement	25.6	Unitless	Hybrid Hourglass (MD2)
Total CPR Increase (MD2 vs. Bare)	400	Fold	Enhancement over bare emitter (CPR $\approx$ 0.06)
Corresponding Purcell Factor (Fp)	199	Unitless	MD2 design
Corresponding Collection Efficiency (CE)	13	%	MD2 design
Target Emitter	Nitrogen-Vacancy (NV) Center	Defect	Diamond quantum emitter
Target Wavelength Range	620 to 800	nm	NV broad emission spectrum
Collection Numerical Aperture (NA)	0.95	Unitless	Microscope objective
Metallic Tip Gap Constraint (Minimum)	20	nm	Required distance to avoid quenching
Hybrid Hourglass (MD2) Tip Angle ($\alpha$)	91	°	Optimized geometric parameter
Hybrid Hourglass (MD2) Aperture ($D$)	400	nm	Optimized geometric parameter
Hybrid Hourglass (MD2) Thickness ($d$)	250	nm	Optimized geometric parameter

Key Methodologies

The research utilized advanced numerical simulation and optimization techniques to design the nanoantenna structures.

Simulation Platform: Finite Difference Time Domain (FDTD) methods were employed using the commercial software package Lumerical.
Material Selection: High-index diamond was used as the substrate, and Silver (Ag) was selected for metallic components due to its low optical absorbance across the NV emission spectrum (620 nm to 800 nm).
Emitter Modeling: The NV center was modeled as a dipole source located at the center of the design and oriented in the plane of the inner bowtie.
Optimization Figure of Merit: The primary optimization goal was maximizing the Collected Photon Rate (CPR = Fp $\times$ CE) within a high Numerical Aperture (NA = 0.95).
Design Classes Investigated: Three primary classes were analyzed:
- Metallic Structures (M-series): Bowties, hourglass, and capped hourglass designs, often incorporating concentric gratings (G-variants).
- Dielectric Structures (D-series): Air-filled and raised diamond bowties.
- Hybrid Structures (MD-series): Combining metallic bases with dielectric tips/bridges to balance high Purcell enhancement and low ohmic loss.
Fabrication Constraints: Simulations incorporated practical fabrication limits, including a minimum metallic tip gap of 20 nm (to mitigate quenching) and an 8 nm radius of curvature smoothing applied to all corners.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-quality diamond materials and custom processing required to replicate and advance the quantum nanophotonics research presented in this paper.

Applicable Materials

To achieve stable, high-coherence NV centers and support the required nanoscale fabrication, the following 6CCVD materials are essential:

Material Specification	6CCVD Offering	Relevance to Research
Single Crystal Diamond (SCD)	Optical Grade SCD	Required for low-strain, high-purity host material necessary for high-fidelity NV center spin coherence and optical stability.
Substrate Thickness	SCD plates from 0.1 µm up to 500 µm	Provides the necessary bulk material for etching recessed antenna structures (e.g., 250 nm deep features) while maintaining mechanical stability.
Surface Quality	SCD Polishing: Ra < 1 nm	Ultra-smooth surfaces are critical for subsequent high-resolution electron-beam lithography and etching steps used to define the nanoscale antenna features (20 nm gaps).

Customization Potential

The fabrication of these complex metal-dielectric hybrid structures requires precise material engineering beyond standard CVD growth. 6CCVD offers comprehensive customization services:

Custom Metalization Services: The paper utilized Silver (Ag) for its low loss. 6CCVD offers in-house deposition of high-purity metals, including Au, Pt, Pd, Ti, W, and Cu. Our engineering team can assist researchers in optimizing hybrid designs using these alternative, highly stable metals, which are often preferred for long-term device integration and stability.
Custom Dimensions: 6CCVD supplies SCD plates in custom dimensions, ensuring compatibility with various lithography and processing tools used in cleanroom environments.
Precision Etching Preparation: We provide substrates optimized for deep reactive ion etching (DRIE) or inductively coupled plasma (ICP) etching, necessary for creating the recessed diamond features and bullseye gratings described in the paper.

Engineering Support

6CCVD’s in-house team of PhD material scientists and engineers specializes in diamond quantum applications. We offer authoritative support for projects involving:

Qubit Fabrication: Assistance with material selection and surface preparation protocols optimized for NV creation (e.g., controlled implantation depth and annealing cycles).
Material Optimization: Consultation on selecting the optimal diamond grade (e.g., low nitrogen concentration for NV centers) to maximize spin coherence time and minimize background fluorescence.
Advanced Integration: Support for integrating diamond devices with external optical components, including optimizing substrate thickness and surface orientation for efficient light coupling.

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

View Original Abstract

A central challenge in quantum technologies based on atom-like defects is the efficient collection of the emitter’s fluorescence. Optical antennas are appealing as they offer directional emission together with spontaneous emission rate enhancement across a broad emitter spectrum. In this work, we introduce and optimize metal-dielectric nanoantenna designs recessed into a diamond substrate and aligned with quantum emitters. We analyze tradeoffs between external quantum efficiency, collection efficiency, radiative Purcell factor, and overall collected photon rate. This analysis shows that an optimized metal-dielectric hybrid structure can increase the collected photon rate from a nitrogen vacancy center by over two orders of magnitude compared to a bare emitter.

Tech Support

Original Source

DOI: https://doi.org/10.1364/oe.26.003341