Efficient Single-Photon Coupling from a Nitrogen-Vacancy Center Embedded in a Diamond Nanowire Utilizing an Optical Nanofiber

At a Glance

Metadata	Details
Publication Date	2017-10-05
Journal	Scientific Reports
Authors	Yuya Yonezu, Kentaro Wakui, K. Furusawa, Masahiro Takeoka, Kouichi Semba
Institutions	National Institute of Information and Communications Technology, Waseda University
Citations	26
Analysis	Full AI Review Included

Technical Analysis: Efficient Single-Photon Coupling in Diamond Nanowires

6CCVD Technical Documentation based on Efficient Single-Photon Coupling from a Nitrogen-Vacancy Center Embedded in a Diamond Nanowire Utilizing an Optical Nanofiber

Executive Summary

This paper presents a rigorous numerical simulation demonstrating a highly efficient hybrid system for coupling single photons from a solid-state Nitrogen-Vacancy (NV) emitter into a Single-Mode Fiber (SMF), a critical component for scalable quantum networks.

Maximum Efficiency Achieved: Numerical simulations predict a maximum coupling efficiency of 75% for the sum of both fiber ends, achieved through geometric optimization of a cylindrical diamond nanowire integrated with a standard optical nanofiber.
Geometric Optimization: Peak performance required tight control over dimensions: Nanofiber radius ($r_f$) 240 nm, Nanowire radius ($r_a$) 85 nm, and Nanowire length ($L_a$) 3.6 µm.
Critical Physical Effects: Maximizing efficiency depends on optimizing modal interference between nanowire-based and nanofiber-based supermodes, and accounting for Fabry-Perot resonance caused by reflection at the nanowire end facets.
Material Requirement: The system relies exclusively on high-quality, high-refractive-index diamond ($n_d = 2.41$) suitable for NV center integration and precise nanofabrication into nanoscale structures.
Fabrication Tolerance Benchmark: High coupling efficiencies (> 70%) require the NV center dipole to be placed axially within $\pm 0.2$ µm of the nanowire center, and the nanowire must be aligned to the nanofiber with a relative angle of $\le 5^\circ$.
Core Application: This simple, yet efficient, interface technique advances solid-state quantum emitters for practical use in SMF-based quantum networks.

Technical Specifications

Parameter	Value	Unit	Context
Maximum Coupling Efficiency ($\eta$)	75	%	Sum of both fiber ends, radial polarization
Optimal Nanofiber Radius ($r_f$)	240	nm	-
Optimal Nanowire Radius ($r_a$)	85	nm	Cylindrical structure
Optimal Nanowire Length ($L_a$)	3.6	µm	Determined by beat length/Fabry-Perot resonance
NV Center Emission Wavelength ($\lambda$)	637	nm	Modeled as point dipole
Diamond Refractive Index ($n_d$)	2.41	-	Assumed value for single crystal diamond (SCD)
Silica Refractive Index ($n_f$)	1.46	-	Assumed value for nanofiber
Required Axial Dipole Position Accuracy ($z_{NV}$)	$\pm 0.2$	µm	Required for $\eta > 70$%
Required Nanowire Alignment Tolerance ($\phi$)	$\le 5$	°	Required for $\eta > 70$%
Highest Efficiency Polarization	Radial	%	75%
Second Highest Efficiency Polarization	Azimuthal	%	69% (at $r_f = 200$ nm)
Lowest Efficiency Polarization	Axial	%	9% (at $r_f = 160$ nm)

Key Methodologies

The research utilizes highly specific numerical simulations to determine the optimal geometry and material requirements for efficient coupling.

3D Finite-Difference Time-Domain (3D-FDTD) Simulation (Lumerical):
- Geometry: Cylindrical silica nanofiber ($n_f = 1.46$) coupled parallel to a cylindrical diamond nanowire ($n_d = 2.41$).
- Source Modeling: NV center modeled as a point dipole oscillating at $\lambda = 637$ nm, embedded at the center of the diamond nanowire.
- Boundary Conditions: Computational domain ($3\times3\times15$ µm³) surrounded by Perfectly Matched Layers (PMLs). System symmetry was used to reduce computation size.
Supermode and Modal Analysis (Full Vectorial Finite-Element Method - FEM, Comsol Multiphysics):
- Used to analyze the formation and effective indices ($n_{eff}$) of the hybrid system’s supermodes (Nanowire-based and Nanofiber-based).
- Determined that efficient coupling relies on the dipole preferentially coupling to the nanowire-based supermodes.
Efficiency Optimization:
- Coupling efficiency ($\eta$) was mapped across varying nanofiber radius ($r_f$: 100-400 nm) and nanowire length ($L_a$: 0-10 µm) for fixed nanowire radii ($r_a$: 70-100 nm).
- Critical Insight: Efficiency maximization was achieved by balancing the beat length (interference between TM-like supermodes) and Fabry-Perot resonance (reflection from flat nanowire facets).
Tolerance Analysis:
- Evaluated robustness against fabrication error by varying dipole position ($y_{NV}, z_{NV}$) and nanowire angular alignment ($\phi$).

6CCVD Solutions & Capabilities

The findings confirm that the realization of high-efficiency NV-nanophotonics hinges on the availability of high-quality Single Crystal Diamond (SCD) material suitable for complex downstream nanofabrication, positioning, and alignment.

Applicable Materials

To replicate or extend this research, 6CCVD strongly recommends starting with:

Optical Grade Single Crystal Diamond (SCD): Required purity and lattice quality are essential for generating stable, high-coherence NV centers and ensuring the required high refractive index uniformity ($n_d=2.41$). Our high-purity SCD material minimizes scattering losses and ensures optimal NV emission properties.
Standard SCD Thicknesses: We provide SCD plates up to 500 µm thick, offering ample material depth for fabricating dense arrays of 3.6 µm long nanowires/nanopillars (the structures targeted in this paper).

Customization Potential

The experimental feasibility of this 75% efficient coupling scheme relies directly on the precision and quality of the starting diamond substrate, particularly its surface finish and ability to withstand high-precision etching and NV alignment techniques (Focused Electron Irradiation or Laser Writing).

Requirement from Paper	6CCVD Capability & Solution
Nanoscale Fabrication Base (e.g., $r_a = 85$ nm nanowires)	Ultra-Low Roughness Polishing: Achieving the required smooth nanowire facets for minimal reflection relies on the quality of the starting wafer. 6CCVD guarantees SCD surface polishing down to Ra < 1 nm, providing the optimal foundation for top-down lithography and etching processes.
Precise NV Dipole Alignment (Axial position $\pm 0.2$ µm)	Substrate Dimension Control: 6CCVD offers custom plates/wafers up to 125 mm (PCD) and highly uniform thickness control (up to 500 µm SCD), ensuring scalable, consistent starting material for high-volume device fabrication.
Alternative Emitters (e.g., SiV centers, BDD potential)	Boron-Doped Diamond (BDD): For researchers exploring integrated active components or alternative quantum emitters, 6CCVD provides custom BDD materials with tunable conductivity, ideal for integrated electrical controls in hybrid quantum systems.
Interface Layers (Future enhancements)	Custom Metalization Services: Should future hybrid designs require electrodes or reflective layers (e.g., Ti/Pt/Au contact pads for controlling NV position/field), 6CCVD offers in-house metal deposition capabilities.

Engineering Support

This research explicitly defines tight tolerances for NV center axial positioning ($\pm 0.2$ µm) and nanowire alignment ($< 5^\circ$) to achieve maximum coupling efficiency. These requirements necessitate advanced planning for material preparation.

6CCVD’s in-house team of PhD-level material scientists and engineers specializes in assisting clients with:

Material Selection for Quantum Applications: Ensuring the SCD selected has the necessary purity and defect concentration for reliable NV center creation.
Surface Preparation Optimization: Consulting on required polishing grades (Ra < 1 nm for SCD) to minimize scattering losses and enable high-resolution nanofabrication (lithography and etching) required for the $r_a=85$ nm nanowires.
Custom Diamond Dimensions: Supplying precisely specified wafer dimensions required for integration into commercial nanofabrication workflows.

For custom specifications or material consultation on efficient solid-state quantum emitter integration projects, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1038/s41598-017-13309-z