Single photon emission and single spin coherence of a nitrogen vacancy center encapsulated in silicon nitride

At a Glance

Metadata	Details
Publication Date	2020-03-30
Journal	Applied Physics Letters
Authors	Joe Smith, Jorge Monroy Ruz, John G. Rarity, Krishna C. Balram, Joe Smith
Institutions	Bristol Robotics Laboratory, University of Bristol
Citations	20
Analysis	Full AI Review Included

Executive Summary

Platform Validation: Nitrogen-rich amorphous silicon nitride (aSiN_x) is demonstrated as a viable quantum photonics platform for integrating Nitrogen Vacancy (NV) centers hosted in nanodiamonds.
Background Noise Suppression: By increasing the nitrogen content (NH₃/SiH₄ ratio R=3), the intrinsic auto-fluorescence (PL background) of the aSiN_x film was reduced by nearly two orders of magnitude compared to stoichiometric Si₃N₄.
Quantum Optical Preservation: Single photon emission (antibunching) was confirmed post-encapsulation, with the second-order correlation function g²(0) increasing from 0.22 (bare) to 0.43 (capped), remaining well below the single-emitter threshold of 0.5.
Spin Coherence Preservation: The electron spin coherence (T₂*) of the NV center was preserved post-encapsulation, decreasing from 0.45 µs (bare) to 0.29 µs (capped), still long-lived enough for dynamical decoupling techniques.
Refractive Index Control: The refractive index (n) was maintained above 1.9 (high index contrast) across the tested nitrogen concentrations, ensuring strong modal confinement and overlap necessary for efficient coupling to integrated waveguides.
Scalability: The use of PECVD-deposited aSiN_x, compatible with mature silicon foundry processes, opens a path toward large-scale quantum integration (LSQI) of solid-state emitters.

Technical Specifications

Parameter	Value	Unit	Context
NV Center Size	10 - 20	nm	Nanodiamond host size
Film Thickness (Encapsulation)	100	nm	Low auto-fluorescence aSiN_x (R=3)
Deposition Temperature	300	°C	PECVD process parameter
Deposition Pressure	1.0	Torr	PECVD process parameter
Gas Flow Ratio (R)	0.6 to 3.0	N/A	NH₃ / SiH₄ ratio tested
Refractive Index (n)	~1.9 to ~2.0	N/A	Measured at 637 nm (NV ZPL)
PL Background Reduction	~2 orders of magnitude	N/A	Reduction achieved by increasing N content
Uncapped g²(0) (NV D)	0.22	N/A	Confirms single photon emission
Capped g²(0) (NV D)	0.43	N/A	Confirms single photon emission post-processing
Uncapped Fluorescence Lifetime (T₂)	19.35	ns	Characteristic of NVs in nanodiamonds
Capped Fluorescence Lifetime (T₂)	6.84	ns	Reduced by factor of 3 due to slab mode coupling
Uncapped Spin Decay Time (T₂*)	0.45	µs	Free induction decay measurement
Capped Spin Decay Time (T₂*)	0.29	µs	Spin coherence preserved post-encapsulation
Capped Rabi Frequency (Ω)	3.82	MHz	Increased by factor of 1.5 post-capping

Key Methodologies

Substrate Preparation: Amorphous silicon nitride (aSiN_x) films (300 nm thick) were deposited on fused silica substrates using Plasma Enhanced Chemical Vapour Deposition (PECVD).
Nitrogen Content Variation: The nitrogen content was controlled by varying the NH₃ / SiH₄ gas flow ratio (R) from 0.6 to 3.0, while maintaining the total gas flow, chamber pressure (1.0 Torr), and substrate temperature (300 °C) constant.
Film Characterization: Refractive index (n) and extinction coefficient (k) were measured using ellipsometry, and background photoluminescence (PL) was measured using a standard confocal microscope.
NV Center Localisation: High-purity nanodiamonds containing single NV centers were spin-coated onto the substrate. Fiduciary markers (crosses) were fabricated using electron-beam lithography to precisely locate and track individual NV centers.
Encapsulation: A 100 nm layer of low auto-fluorescence aSiN_x (R=3) was deposited over the pre-characterized NV centers.
Optical Characterization: Single photon statistics were quantified by measuring the intensity autocorrelation function g²(τ). Fluorescence lifetime (T₂) was measured using time-resolved photoluminescence.
Spin Characterization: Electron spin coherence was measured using a free induction decay sequence (microwave and laser pulses) to extract the spin dephasing time (T₂*).

Commercial Applications

Integrated Quantum Photonics: Enabling scalable, chip-based quantum circuits by integrating solid-state single photon emitters (NV centers) with high-performance silicon nitride waveguides and cavities.
Deterministic Single Photon Sources: Development of reliable, on-demand single photon generators operating at visible wavelengths (637 nm) for quantum communication and computation protocols.
On-Chip Quantum Memories: Utilizing the long-lived spin coherence (T₂*) of the encapsulated NV centers to build integrated quantum memory elements compatible with photonic circuits.
Quantum Sensing and Metrology: The preserved spin properties allow for the development of integrated quantum sensors (e.g., magnetometers, thermometers) based on NV centers, leveraging the high-connectivity and miniaturization offered by the SiN_x platform.
Hybrid Quantum Systems: The methodology is extendable to other visible-regime solid-state emitters, including perovskite quantum dots and 2D transition metal dichalcogenides, accelerating the development of various hybrid quantum technologies.

View Original Abstract

Finding the right material platform for engineering efficient photonic interfaces to solid state emitters has been a long-standing bottleneck for scaling up solid state quantum systems. In this work, we demonstrate that nitrogen rich silicon nitride, with its low auto-fluorescence at visible wavelengths, is a viable quantum photonics platform by showing that nitrogen vacancy centers embedded in nanodiamonds preserve both their quantum optical and spin properties post-encapsulation. Given the variety of high-performance photonic components already demonstrated in silicon nitride, our work opens up a promising avenue for building integrated photonic platforms using solid state emitters.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0002709

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