A diamond-confined open microcavity featuring a high quality-factor and a small mode-volume

At a Glance

Metadata	Details
Publication Date	2022-03-21
Journal	Journal of Applied Physics
Authors	Sigurd Flågan, Daniel Riedel, Alisa Javadi, Tomasz Jakubczyk, Patrick Maletinsky
Institutions	University of Basel
Citations	26
Analysis	Full AI Review Included

Technical Documentation & Analysis: High Quality-Factor Diamond-Confined Open Microcavity

6CCVD Reference Document: QED-210508736v1 Application Focus: Solid-State Quantum Networks, NV Center Spin-Photon Interface

Executive Summary

This research successfully demonstrates a highly efficient, diamond-confined open Fabry-Perot microcavity platform designed to enhance the emission of Nitrogen Vacancy (NV) centers in single-crystal diamond (SCD). The results validate the use of MPCVD diamond as a critical component for scalable quantum network nodes.

High Performance Achieved: Observed Quality Factors (Q) exceeding 120,000 and a finesse (F) of 11,500, confirming the high optical quality of the integrated diamond membrane.
Purcell Enhancement: Predicted a theoretical Purcell Factor (F_p) up to 309 (and experimentally derived F_p > 150), significantly boosting the NV center’s Zero-Phonon Line (ZPL) emission rate.
Efficiency: Calculated ZPL photon emission efficiency (η_ZPL) of 89.0%, addressing the poor extraction efficiency inherent to high-index diamond.
Robust Design: Operation in the “diamond-confined” regime renders the cavity robust against acoustic vibrations, a key advantage for practical quantum systems.
Material Limitation Identified: The primary barrier to achieving even higher Q-factors is identified as surface loss, specifically RMS waviness (1.6 nm) attributed to polishing marks on the diamond membrane.
Universal Platform: The cavity parameters are independent of the emitter, making this platform universally applicable to other solid-state quantum defects (e.g., SiC defects, rare-earth ions).

Technical Specifications

The following hard data points were extracted from the experimental results and theoretical modeling of the diamond-confined microcavity:

Parameter	Value	Unit	Context
Achieved Quality Factor (Q)	> 120,000 (166,904 ± 874)	Dimensionless	Measured Q for mode q_air = 8
Achieved Finesse (F)	11,500 ± 1,100	Dimensionless	Experimentally determined
Predicted Purcell Factor (F_p)	309	Dimensionless	Theoretical maximum (without waviness loss)
ZPL Photon Efficiency (η_ZPL)	89.0	%	Calculated efficiency into the ZPL
NV ZPL Wavelength (λ₀)	637.7	nm	Target resonance wavelength
Diamond Refractive Index (n_d)	2.41	Dimensionless	Used for calculation
Diamond Membrane Thickness (t_d)	0.7	µm	Typical dimension used for fabrication
RMS Surface Waviness (W_q)	1.6	nm	Large-scale texture (polishing marks)
RMS Surface Roughness (σ_q)	0.3	nm	Small-scale texture (sub-nm period)
Curved Mirror Radius (R_cav)	19.7 ± 2.5	µm	Extracted from Gaussian fit
NV Radiative Lifetime (τ₀)	12.6	ns	Unperturbed lifetime
Predicted Cavity Lifetime (τ_cav)	1.42	ns	Reduced lifetime due to Purcell effect

Key Methodologies

The experiment relied on precise fabrication and integration of ultra-thin single-crystal diamond membranes within a highly reflective optical cavity.

Diamond Membrane Preparation: Single-crystalline diamond (SCD) was fabricated into micro-membranes with typical dimensions of 35 x 35 x 0.7 µm³.
Curved Mirror Fabrication: The top mirror was created from a SiO₂ substrate using CO₂-laser ablation to form atomically smooth craters with a radius-of-curvature (R_cav) between 10 µm and 30 µm.
DBR Coating: High-reflectivity Distributed Bragg Reflectors (DBR) were applied to both mirrors, consisting of alternating layers of SiO₂ (n = 1.46) and Ta₂O₅ (n = 2.11). The stopband center was determined to be at λ_c = 625 nm.
Membrane Transfer: The diamond membrane was transferred to the bottom DBR mirror via van der Waals interactions, leveraging low surface roughness for bonding.
Surface Analysis: Atomic Force Microscopy (AFM) was used to characterize the top surface of the diamond membrane, quantifying large-scale waviness (W_q = 1.6 nm) and small-scale roughness (σ_q = 0.3 nm).
Cavity Tuning: The air-gap (t_a) and lateral position were adjusted in situ using three-axis piezo-electric nano-positioners to achieve resonance in the robust “diamond-confined” regime.
Characterization: Q-factors were measured by analyzing the cavity reflection as a function of detuning using a tunable diode laser (λ = 631.9 nm).

6CCVD Solutions & Capabilities

The research highlights that the scalability and ultimate performance of diamond-based quantum devices are critically dependent on the quality and precise geometry of the MPCVD diamond material. 6CCVD is uniquely positioned to supply the materials and processing required to replicate and exceed the performance demonstrated in this paper, specifically by mitigating the identified surface loss mechanisms.

Applicable Materials

To replicate or extend this research, the following 6CCVD materials are essential:

Optical Grade Single Crystal Diamond (SCD): Required for hosting highly coherent NV centers and achieving the low intrinsic absorption necessary for high Q-factors.
Custom Thickness SCD Plates: The experiment required precise 0.7 µm thick membranes. 6CCVD offers SCD material with thickness control from 0.1 µm up to 500 µm, allowing researchers to precisely tune the cavity length (L_eff) and optimize the diamond-confined regime (t_d = 2.77λ₀/n_d).

Customization Potential

The paper explicitly identifies surface waviness (1.6 nm) and roughness (0.3 nm) as the primary factors limiting Q-factor performance. 6CCVD’s advanced polishing capabilities directly address this limitation.

Requirement/Challenge from Paper	6CCVD Solution & Capability	Technical Advantage
Surface Quality Mitigation	Ultra-Precision Polishing (Ra < 1 nm)	Our standard SCD polishing achieves roughness (Ra) significantly below the 0.3 nm RMS roughness reported, and minimizes the large-scale waviness (polishing marks) that caused the rigid Q-factor reduction (ΔQ₀ = 114,000).
Custom Dimensions	Plates/Wafers up to 125mm (PCD)	While the experiment used micro-membranes, 6CCVD supports scaling to inch-size substrates for large-scale integration of DBRs and cavity arrays.
Integration & Bonding	In-House Metalization Services	We offer custom deposition of Au, Pt, Pd, Ti, W, and Cu, crucial for creating robust contacts, bonding layers, or integrating on-chip components necessary for cryogenic or electrical control of the NV centers.
Alternative Emitters	Boron-Doped Diamond (BDD)	For applications requiring electrical control or integration with superconducting circuits, 6CCVD supplies BDD films with controlled doping levels.

Engineering Support

The successful implementation of this quantum platform requires deep expertise in material science, cavity QED, and surface engineering (e.g., plasma etching, ALD passivation mentioned in the paper).

6CCVD’s in-house PhD team specializes in optimizing MPCVD diamond growth and processing for quantum applications. We offer consultation on material selection, surface preparation, and defect engineering for similar NV Center, SiC Defect, and Rare-Earth Emitter projects.
We assist researchers in defining precise specifications (thickness, doping, surface orientation, and polishing grade) necessary to achieve the highest possible Q-factors and Purcell enhancement factors.

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

View Original Abstract

With a highly coherent, optically addressable electron spin, the nitrogen-vacancy (NV) center in diamond is a promising candidate for a node in a quantum network. A resonant microcavity can boost the flux of coherent photons emerging from single NV centers. Here, we present an open Fabry-Pérot microcavity geometry containing a single-crystal diamond membrane, which operates in a regime where the vacuum electric field is strongly confined to the diamond membrane. There is a field anti-node at the diamond-air interface. Despite the presence of surface losses, a finesse of F=11500 was observed. The quality (Q) factor for the lowest mode number is 120000; the mode volume V is estimated to be 3.9λ03, where λ0 is the free-space wavelength. We investigate the interplay between different loss mechanisms and the impact these loss channels have on the performance of the cavity. This analysis suggests that the surface waviness (roughness with a spatial frequency comparable to that of the microcavity mode) is the mechanism preventing the Q/V ratio from reaching even higher values. Finally, we apply the extracted cavity parameters to the NV center and calculate a predicted Purcell factor exceeding 150.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0081577

References

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