Lifetime Reduction and Enhanced Emission of Single Photon Color Centers in Nanodiamond via Surrounding Refractive Index Modification

At a Glance

Metadata	Details
Publication Date	2015-06-25
Journal	Scientific Reports
Authors	Asma Khalid, Kelvin Chung, Ranjith Rajasekharan Unnithan, Desmond W. M. Lau, T. J. Karle
Institutions	RMIT University, The University of Melbourne
Citations	63
Analysis	Full AI Review Included

6CCVD Technical Documentation: Lifetime Reduction and Enhanced Single Photon Emission in Nanodiamonds

Analysis of: Lifetime Reduction and Enhanced Emission of Single Photon Color Centers in Nanodiamond via Surrounding Refractive Index Modification This technical documentation analyzes the requirements and findings of the featured research paper to highlight relevant material and engineering solutions provided by 6CCVD.

Executive Summary

The research demonstrates a crucial methodology for optimizing Nitrogen Vacancy (NV-) centers in nanodiamonds (NDs) for use as high-performance single-photon sources (SPSs), a key component in emerging quantum technologies.

Core Value Proposition: Modification of the surrounding refractive index using a polymer film (PMMA, $n \approx 1.48$) significantly enhances NV- center performance.
Performance Achievement: The strategy resulted in an average 63% reduction in the spontaneous emission lifetime ($\tau$).
Emission Enhancement: This lifetime reduction was accompanied by an average 1.6-fold enhancement in the single-photon emission rate (up to 2.1x maximum observed).
Mechanism: The enhancement is attributed to the modification of the local electromagnetic environment, specifically reducing the refractive index contrast between the high-index diamond nanoparticle and the surrounding cladding.
Application Relevance: These results directly address the need for brighter, faster, room-temperature SPSs critical for quantum information processing (QIP), quantum key distribution (QKD), and high-resolution medical imaging.
Method Validation: Experimental results were validated using rigorous numerical modeling techniques, including Finite Element Method (FEM) and Finite Difference Time Domain (FDTD) simulations.

Technical Specifications

The following hard data points define the material parameters and critical performance metrics achieved in the study.

Parameter	Value	Unit	Context
Core Emitter	NV<sup>-</sup> Center	Defect	Negatively-charged nitrogen vacancy
Excitation Wavelength ($\lambda_{exc}$)	532	nm	Continuous Wave (CW) Laser
Zero Phonon Line (ZPL) Emission ($\lambda_{ZPL}$)	637	nm	Characteristic triplet state transition
Nanodiamond (ND) Diameter (Avg)	60	nm	Commercial, non-detonation NDs
Substrate Material	Silicon (Si)	N/A	Partially reflecting substrate
Coating Material	Polymethyl Methacrylate (PMMA)	Polymer	950 PMMA A Resist, 2% solution
PMMA Refractive Index ($n_{PMMA}$)	1.479 $\pm$ 0.001	N/A	Measured at $\lambda=637 \text{ nm}$
PMMA Film Thickness ($t$)	111.05 $\pm$ 0.03	nm	Measured via Spectroscopic Ellipsometry
Average Lifetime Reduction	63	%	Across ten characterized emitters
Lifetime Reduction Ratio ($\tau_{b}/\tau_{a}$)	0.48 - 0.89	Ratio	Range of experimental results
Emission Rate Enhancement (Avg)	1.6	Factor	Average enhancement factor
Emission Rate Enhancement (Range)	1.1 - 2.1	Factor	Range of experimental results
Si Refractive Index ($n_{Si}$, at 637 nm)	3.875 + 0.0111$i$	N/A	Complex index used for simulations

Key Methodologies

The experiment successfully combined microfabrication and advanced optical characterization to achieve precise control over the NV- environment.

Substrate Marking: A Silicon (Si) substrate was marked using Focused Ion Beam (FIB) milling (30 keV, 7 nA current) to create markers (approx. 3 $\mu$m deep) allowing the exact relocation of individual NV- centers before and after coating.
Nanodiamond Deposition: Commercial nanodiamonds (60 nm average diameter) were drop-cast onto the marked Si substrate.
Baseline Optical Characterization: Confocal fluorescence microscopy (532 nm excitation) was used to locate potential single-photon emitters. Single-photon emission was confirmed using intensity autocorrelation ($\text{g}^{(2)}(\Delta t=0) < 0.2$), and spontaneous emission lifetime ($\tau_{b}$) and emission rate ($I_{b}$) were measured.
Surface Metrology: Atomic Force Microscopy (AFM) was used to measure the height of the selected NDs (average 65 nm) prior to coating.
Polymer Coating: The sample was spin-coated at 2000 RPM using a 2% solution of Polymethyl Methacrylate (PMMA) in anisole, creating a thin film cladding.
Cladding Metrology: Spectroscopic Ellipsometry (SE) measured the resulting film properties, confirming a PMMA thickness of $111.05 \text{ nm}$ and a refractive index of $1.48$ at the emission wavelength.
Post-Coating Analysis: Steps 3 and 4 were repeated, using FIB markers to precisely re-locate the same ten NV- centers, measuring the new lifetime ($\tau_{a}$) and emission rate ($I_{a}$).
Numerical Validation: Emission enhancement calculations were performed using Finite Element Method (FEM) (COMSOL) and Finite Difference Time Domain (FDTD) (RSoft) for orthogonal and parallel dipole orientations relative to the Si surface.

6CCVD Solutions & Capabilities (Quantum Material )

This research demonstrates the necessity of controlling diamond properties and their surrounding electromagnetic environment for optimized quantum applications. 6CCVD provides the foundational single-crystal diamond (SCD) materials and advanced engineering services required to replicate and significantly extend this type of research, moving beyond commercial nanodiamonds to engineered structures.

Research Requirement	6CCVD Applicable Materials & Services	Customization and Sales Driver
High-Performance Quantum Emitters	Optical Grade Single Crystal Diamond (SCD): We supply high-ppurity, low-strain SCD plates (up to 500 $\mu$m thickness). This material is essential for repeatable, high-coherence NV center formation via ion implantation or specific growth doping.	Foundation for QIP: Our SCD eliminates the material impurity variations inherent in commercial NDs, ensuring maximum coherence time stability and quantum efficiency necessary for next-generation quantum devices.
Precision Nanophotonic Structures	Custom Dimensions and Advanced Shaping: We offer SCD and PCD wafers up to 125mm. Laser cutting and mechanical processes provide micron and sub-micron dimensional accuracy, ideal for creating precise structures like nanobeams or solid immersion lenses (SILs) to integrate NV centers.	Engineering Beyond NDs: We enable the transition from drop-cast NDs to fully engineered, fixed, and oriented NV centers within large, high-quality SCD templates, essential for reproducible performance.
Interface Management & Coatings	Ultra-Low Roughness Polishing: SCD polishing capability achieves $\text{Ra} < 1 \text{ nm}$. PCD inch-size plates achieve $\text{Ra} < 5 \text{ nm}$. This is critical for minimizing scattering and optimizing the refractive index contrast at the diamond-cladding interface.	Optimized for Refractive Index Control: A pristine, low-roughness surface is non-negotiable for accurately applying and modeling high-index thin-film coatings (like the PMMA used) or for integrating metal contacts.
Custom Dipole Environment Control	Internal Metalization Services: We offer custom deposition of noble and refractory metals (Au, Pt, Pd, Ti, W, Cu). This allows researchers to integrate high-reflectivity structures or electrical contacts necessary for advanced quantum device architectures.	Full Device Integration: Control the photonic environment not just with polymers, but with metallic interfaces, supporting advanced Purcell enhancement or electrode placement for Stark tuning of the NV centers.
Replicating Substrate Interaction	Substrate Engineering Expertise: While Si was used here, 6CCVD can assist researchers requiring SCD growth, bonding, or handling on specific non-native substrates (Si, Quartz, Sapphire) required for integration with CMOS or MEMS platforms.	Platform Compatibility: Ensure your high-quality diamond material is ready for integration into your desired quantum device architecture.

Engineering Support: 6CCVD’s in-house PhD material science team can assist with defining optimal material parameters, including target nitrogen concentration or specific growth recipes, for projects focused on spontaneous emission modification and maximizing room-temperature NV- center brightness.

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

Tech Support

Original Source

DOI: https://doi.org/10.1038/srep11179