Single-crystal diamond low-dissipation cavity optomechanics

At a Glance

Metadata	Details
Publication Date	2016-08-24
Journal	Optica
Authors	Matthew Mitchell, Behzad Khanaliloo, David P. Lake, Tamiko Masuda, J P Hadden
Citations	74
Analysis	Full AI Review Included

6CCVD Technical Documentation: Single-Crystal Diamond Low-Dissipation Cavity Optomechanics

This technical analysis examines research demonstrating single-crystal diamond (SCD) microdisk devices engineered for hybrid photon-phonon-spin coupling, achieving record low mechanical dissipation in ambient conditions. These results validate SCD as the premier platform for coherent quantum optomechanics.

Executive Summary

The reported research successfully validates single-crystal diamond (SCD) microdisks as a powerful platform for coherent quantum hybrid systems, achieving a record combination of frequency and mechanical quality factor in ambient conditions.

Record Coherence: Achieved a mechanical quality factor-frequency product ($Q_m \cdot f_m$) of $\sim 1.9 \times 10^{13}$ Hz, the highest reported for any cavity optomechanical system operating in ambient air. This metric satisfies the minimum criteria for room-temperature single-phonon coherent behavior.
Ultra-Low Dissipation: Demonstrated high mechanical quality ($Q_m > 9000$) for 2 GHz frequency radial breathing modes (RBMs) in SCD microdisks.
High Optical Performance: Maintained high optical quality factors ($Q_o > 10^{4}$) at both telecom (1550 nm) and visible (637 nm) wavelengths, enabling resonant coupling to NV center optical transitions.
Coherent Coupling: Realized high optomechanical cooperativity ($C \sim 3$), sufficient for coherent photon-phonon coupling.
Quantum Potential: Predicted strong coupling rates ($G/2\pi \approx 0.6$ MHz) between mechanical phonons and nitrogen vacancy (NV) center electron spins via radiation pressure induced strain fields.
Material Basis: Fabrication relied on high-quality, optical-grade CVD-grown (100)-oriented SCD substrates.

Technical Specifications

The following hard metrics were achieved for the SCD microdisk devices characterized under ambient conditions (Room Temperature, Atmosphere):

Parameter	Value	Unit	Context
Mechanical Quality Factor ($Q_m$)	> 9000	Dimensionless	Fundamental Radial Breathing Mode (RBM)
Mechanical Frequency ($f_m$)	2.0 - 2.1	GHz	RBM resonance
Coherence Product ($Q_m \cdot f_m$)	$1.9 \times 10^{13}$	Hz	Record for ambient optomechanical systems
Optical Quality Factor ($Q_o$)	> $10^{4}$	Dimensionless	Measured at 1530 nm and 637 nm
Optomechanical Cooperativity ($C$)	$\sim 3$	Dimensionless	Achieved at intracavity photon number $N \sim 2.8 \times 10^{6}$
Single Photon Coupling Rate ($g_o / 2\pi$)	26 $\pm$ 2	kHz	Extracted from linewidth narrowing fit
Microdisk Diameter Range	5.0 to 6.0	µm	Device dimensions
Microdisk Thickness (Average)	$\sim 940$	nm	Device dimensions
Pedestal Waist (Minimum)	< 100	nm	Required for minimizing clamping loss
Predicted Spin Coupling Rate ($G/2\pi$)	$\approx 0.6$	MHz	Predicted at maximum self-oscillation amplitude (31 pm)
Predicted Strain ($\epsilon_{zpm}$)	$\approx 3 \times 10^{-10}$	Dimensionless	Zero point motion strain
Estimated Device Temperature Shift ($\Delta T$)	$\sim 50$	K	Due to 1.5 mW absorbed power ($P_a$)
Diamond N Concentration	$\sim 1$	ppm	Sample purity resulting in NV ensembles

Key Methodologies

The SCD microdisks were fabricated using advanced nanofabrication techniques optimized for mechanical isolation and minimal dissipation.

Material Preparation:
- Used optical grade, Chemical Vapor Deposition (CVD) grown, (100)-oriented Single Crystal Diamond (SCD) substrates (supplied by Element Six).
- Substrates were polished and cleaned in boiling piranha.
Hard Mask Definition:
- Coated with $\sim 400$ nm of PECVD Silicon Nitride (Si₃N₄) hard mask.
- A $\sim 5$ nm Titanium (Ti) layer was deposited on the Si₃N₄ to mitigate charging during Electron Beam Lithography (EBL) using ZEP 520A resist.
Pattern Transfer:
- The developed pattern was transferred to the hard mask using Inductively Coupled Plasma Reactive Ion Etching (ICPRIE) with C₄F₈/SF₆ chemistry.
Diamond Etching:
- An anisotropic ICPRIE diamond etch was performed using O₂.
- A $\sim 250$ nm conformal PECVD Si₃N₄ layer was deposited as a sidewall protection layer.
Undercut Release:
- A short C₄F₈/SF₆ ICPRIE etch cleared the bottom of the etch windows.
- A zero RF power O₂ RIE diamond undercut etch was used for partial release, crucial for minimizing the pedestal waist to < 100 nm to reduce clamping loss.
Final Clean:
- The Si₃N₄ layers were removed via wet-etching in 49% Hydrofluoric Acid (HF).
Characterization:
- Probed optomechanically using a dimpled optical fiber taper evanescently coupled to the microdisk near 1530 nm (telecom) and 637 nm (visible) wavelengths.

6CCVD Solutions & Capabilities

This research highlights the indispensable role of ultra-high-quality SCD material and precise fabrication control in realizing advanced quantum devices. 6CCVD is uniquely positioned to supply and support the materials required to replicate and advance this pioneering work.

Applicable Materials

To achieve high $Q_o$ and ultra-low mechanical dissipation ($Q_m$), researchers require diamond with exceptional crystalline quality, purity, and surface finishing.

6CCVD Material Recommendation	Specification	Application in Research
Optical Grade (100) SCD Wafers	Thickness: 0.1 µm - 500 µm	Direct supply of the high-quality bulk material used for microdisk patterning.
High Purity Electronic Grade SCD	Nitrogen concentration < 1 ppb	Necessary for maximizing the coherence time ($T_2$) of NV center qubits, critical for spin-optomechanics experiments.
Custom Thickness SCD/PCD	Thickness up to 500 µm	Provides the specific $940$ nm thickness layers or bulk substrates (up to 10 mm) required for nanomechanical resonator fabrication.

Customization Potential

The success of these microdisks hinges on two factors: the material quality and the sub-micron control over the pedestal geometry (waist < 100 nm). 6CCVD offers the specialized services required for engineering such complex nanostructures:

Precision Polishing (Ra < 1 nm): The observed $Q_o$ was reportedly limited by surface roughness and linear absorption. 6CCVD provides SCD polishing down to Ra < 1 nm, which is essential for minimizing scattering losses and boosting $Q_o$ towards the predicted radiation-loss limit (> 10⁷).
Custom Metalization & Thin Film Deposition: The fabrication process utilized a 5 nm Ti layer for charge dissipation during EBL. 6CCVD offers in-house deposition capabilities for Ti, Pt, Au, Pd, W, and Cu, supporting researchers who require custom masking layers or electrical contacts on their diamond substrates.
Custom Dimensions: While the microdisks were small (5-6 µm), the starting substrates were bulk. 6CCVD can supply large area substrates (up to 125 mm PCD) and utilize precision laser cutting for custom dimension plates tailored for specific research tools and processes.

Engineering Support

The paper identifies several challenges for extending this work, including minimizing clamping loss (pedestal optimization), reducing optical absorption (processing improvements), and optimizing NV placement.

6CCVD’s in-house PhD engineering team provides authoritative support for projects involving:

Material Selection for Quantum Systems: Assisting researchers in selecting the optimal SCD purity (electronic grade versus optical grade) and crystal orientation (e.g., (111) surfaces, which the paper suggests could alleviate clamping limitations) for maximizing NV center coherence and coupling efficiency.
Surface Engineering: Consulting on processing techniques to reduce surface state absorption and linear absorption rates, which limit the achievable photon number ($N$) before thermal degradation.
Nanoscale Fabrication Requirements: Providing guidance on the material requirements necessary to withstand complex ICPRIE and RIE etching recipes essential for fabricating high aspect ratio, low-dissipation structures like those used in this cavity optomechanics project.

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

View Original Abstract

Single-crystal diamond cavity optomechanical devices are a promising example of a hybrid quantum system: by coupling mechanical resonances to both light and electron spins, they can enable new ways for photons to control solid state qubits. However, realizing cavity optomechanical devices from high quality diamond chips has been an outstanding challenge. Here we demonstrate single-crystal diamond cavity optomechanical devices that can enable photon-phonon-spin coupling. Cavity optomechanical coupling to $2,\text{GHz}$ frequency ($f_\text{m}$) mechanical resonances is observed. In room temperature ambient conditions, these resonances have a record combination of low dissipation (mechanical quality factor, $Q_\text{m} > 9000$) and high frequency, with $Q_\text{m}\cdot f_\text{m} \sim 1.9\times10^{13}$ sufficient for room temperature single phonon coherence. The system exhibits high optical quality factor ($Q_\text{o} > 10^4$) resonances at infrared and visible wavelengths, is nearly sideband resolved, and exhibits optomechanical cooperativity $C\sim 3$. The devices’ potential for optomechanical control of diamond electron spins is demonstrated through radiation pressure excitation of mechanical self-oscillations whose 31 pm amplitude is predicted to provide 0.6 MHz coupling rates to diamond nitrogen vacancy center ground state transitions (6 Hz / phonon), and $\sim10^5$ stronger coupling rates to excited state transitions.

Tech Support

Original Source

DOI: https://doi.org/10.1364/optica.3.000963