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

Executive Summary

Core Achievement: Demonstrated a high quality-factor (Q) open Fabry-Perot microcavity incorporating a single-crystal diamond membrane, designed for efficient spin-photon interfacing of Nitrogen-Vacancy (NV) centers.
Performance Metrics: Achieved an average Quality factor (Q) of 166,904 (for mode q_air = 8) and a Finesse (F) of 11,500, despite operating in the loss-prone diamond-confined regime.
Purcell Enhancement: Calculated a predicted Purcell factor (F_p) exceeding 150 based on measured cavity parameters, leading to a theoretical ZPL emission efficiency of 89.0%.
Limiting Factor: The primary mechanism preventing higher Q-factors is attributed to surface “waviness” (large-scale roughness, W_q = 1.6 nm) on the diamond membrane, likely caused by polishing marks.
Robustness: The diamond-confined geometry offers high robustness against acoustic vibrations (dλ/dta = 0.11), making it suitable for practical quantum network nodes.
Scalability: The platform is versatile and applicable to enhancing emission from various solid-state single-photon emitters (SPEs) embedded in crystalline hosts, provided surface losses are minimized.

Technical Specifications

Parameter	Value	Unit	Context
Measured Q-Factor (Average)	166,904	Dimensionless	For mode q_air = 8
Achieved Finesse (F)	11,500 ± 1,100	Dimensionless	Diamond-in-cavity
Bare Cavity Finesse (F_bare)	42,500 ± 4,200	Dimensionless	Without diamond membrane
Predicted Purcell Factor (F_p)	> 150	Dimensionless	Based on measured Q (F_p = 309 if waviness is eliminated)
NV ZPL Wavelength (λ₀)	637.7	nm	Target resonance
NV Free-Space Lifetime (τ₀)	~ 12.6	ns	Unperturbed radiative lifetime
Predicted Cavity Lifetime (τ_cav)	1.42	ns	Enhanced by F_p = 309
Enhanced ZPL Efficiency (η_ZPL)	89.0	%	Predicted with F_p = 309
Curved Mirror Radius (R_cav)	19.7 ± 2.5	µm	Extracted from Gaussian fit
Mirror Stopband Center (λ_c)	625	nm	DBR design center
Diamond Thickness (t_d)	0.7	µm	Typical dimension
RMS Surface Waviness (W_q)	1.6	nm	Large-scale texture (polishing marks)
RMS Surface Roughness (σ_q)	0.3	nm	Small-scale roughness
Cavity Linewidth (δν_avg)	2.86	GHz	Measured average linewidth
Diamond Refractive Index (n_d)	2.41	Dimensionless	At ZPL wavelength

Key Methodologies

Curved Mirror Fabrication: Planar-concave geometry achieved by using a CO₂-laser ablation technique on a SiO₂ substrate to create atomically smooth craters (R_cav ~ 20 µm). The profile was characterized using laser-scanning confocal microscopy.
DBR Deposition: High-reflectivity Distributed Bragg Reflectors (DBRs) were coated onto the substrates, consisting of alternating layers of SiO₂ (n=1.46) and Ta₂O₅ (n=2.11). The stopband center was confirmed via transfer-matrix calculations based on white-light transmission measurements.
Diamond Membrane Preparation: Single-crystalline diamond micro-membranes (35 x 35 x 0.7 µm³) were fabricated using established procedures.
Membrane Transfer and Characterization: The diamond membrane was transferred to the bottom DBR using a micro-manipulator, relying on van der Waals bonding. Atomic-Force Microscopy (AFM) was used to characterize the top surface, quantifying RMS roughness (0.3 nm) and waviness (1.6 nm).
Cavity Assembly and Tuning: The bottom mirror assembly was mounted on a three-axis piezo-electric nano-positioner (attocube). A soft indium layer was used as an adjustable spacer to minimize mirror tilt.
Q-Factor Measurement: A tunable diode laser (630-640 nm) was coupled into the cavity. The cavity length was scanned via the piezo, and the reflected signal was monitored. Cavity linewidth (δν) was extracted by fitting the reflected signal with triple Lorentzians, calibrated using an Electro-Optic Modulator (EOM) at ± 5 GHz.
Modeling and Analysis: Experimental results were compared against one-dimensional transfer-matrix simulations and 3D COMSOL numerical simulations to isolate loss mechanisms, including mirror transmission, surface scattering (roughness/waviness), and clipping losses.

Commercial Applications

Quantum Networking Nodes: The high-Q diamond cavity serves as a robust stationary qubit node, enabling efficient, high-rate entanglement generation necessary for large-scale distributed quantum networks.
High-Flux Coherent Photon Sources: Enhancing the Zero-Phonon Line (ZPL) emission rate of NV centers (and other defects) is critical for increasing the speed and scalability of quantum communication protocols.
Solid-State Quantum Repeaters: The enhanced spin-photon interface provides the necessary efficiency to implement quantum repeater protocols, compensating for photon loss over long distances.
Versatile Single-Photon Emitter (SPE) Platform: The cavity design is adaptable for embedding and enhancing emission from various solid-state emitters beyond NV centers, including:
- Other diamond color centers (e.g., SiV, GeV).
- Defects in Silicon Carbide (SiC).
- Rare-earth ions in crystalline hosts.
- Emitters in 2D materials.
Quantum Sensing and Metrology: While focused on quantum communication, the ability to precisely control and enhance single-photon emission rates is fundamental to advanced quantum sensing applications.

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

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