Ultracoherent Gigahertz Diamond Spin-Mechanical Lamb Wave Resonators

At a Glance

Metadata	Details
Publication Date	2024-08-22
Journal	Nano Letters
Authors	Xinzhu Li, Ignas Lekavicius, Jens U. Noeckel, Hailin Wang
Institutions	University of Oregon
Citations	4
Analysis	Full AI Review Included

Ultracoherent GHz Diamond Spin-Mechanical Resonators: Technical Analysis and 6CCVD Solutions

This document analyzes the research paper “Ultracoherent GHz Diamond Spin-Mechanical Lamb Wave Resonators” and outlines how 6CCVD’s advanced MPCVD diamond materials and custom fabrication services meet the stringent requirements for replicating and advancing this quantum spin-mechanics platform.

Executive Summary

The research demonstrates a breakthrough in quantum phononics by achieving ultrahigh coherence in diamond nanomechanical resonators, a critical step toward scalable quantum networks.

Record Coherence: Achieved a Quality Factor ($Q$) exceeding $1.2 \times 10^{7}$ for a fundamental compression mode near 1 GHz at 7 K, comparable to state-of-the-art silicon devices.
Material Platform: Utilized electronic grade bulk diamond (Single Crystal Diamond, SCD) hosting implanted Silicon Vacancy (SiV) centers as strain-sensitive quantum defects.
Novel Excitation/Detection: Developed an all-optical approach using a temporally modulated optical gradient force for excitation and SiV phonon sidebands/sideband optical interferometry for picometer-scale vibration detection.
Phononic Protection: The Lamb Wave Resonator (LWR) geometry, embedded in a phononic crystal lattice, provides a band gap shield, significantly reducing mechanical loss.
Quantum Potential: This platform enables Phononic Cavity Quantum Electrodynamics (QED) of electron spins, with potential cooperativity ($C$) estimated to exceed 10, paving the way for mechanical quantum networks.
6CCVD Relevance: The success relies on high-purity, electronic-grade SCD thin films, a core specialization of 6CCVD, enabling precise thickness control and defect engineering (SiV implantation).

Technical Specifications

Parameter	Value	Unit	Context
Material Purity	Electronic Grade	N/A	Required for high-coherence SiV centers.
Sample Thickness	1.5	µm	LWR fabrication base material.
LWR Dimensions	9.5 x 4.5	µm	Rectangular diamond plate size.
Phononic Lattice Period	8	µm	Square lattice design for band gap.
SiV Implantation Depth	~45	nm	Below the diamond surface (²⁸Si ions).
Resonance Frequency ($f_m$)	0.977	GHz	Fundamental compression mode.
Quality Factor ($Q$)	$1.2 \times 10^{7}$	N/A	Achieved near 7 K.
Mechanical Linewidth ($\gamma_m / 2\pi$)	80 - 83	Hz	Measured via SiV fluorescence/interferometry.
Operating Temperature	Near 7	K	Closed cycle cryostat.
Estimated Vibration Amplitude	$3 \times 10^{-12}$	m (picometer)	Detected via sideband optical transitions.
Target Cooperativity ($C$)	> 10	N/A	Estimated for current LWR dimensions.
Target Cooperativity (Optimized)	> 250	N/A	Projected for reduced resonator size (4, 2, 0.3 µm).

Key Methodologies

The fabrication and measurement relied on precise material engineering and advanced optical techniques:

Material Preparation: Electronic grade bulk diamond film was used. ²⁸Si ions were implanted approximately 45 nm below the surface to create SiV centers.
PECVD Deposition: A 280 nm layer of Si₃N₄ was deposited via Plasma-Enhanced Chemical Vapor Deposition (PECVD).
Pattern Definition: A 500 nm layer of Polymethyl Methacrylate (PMMA) was deposited, followed by Electron Beam Lithography (EBL) to define the phononic crystal pattern.
Front-Side Etching: The pattern was transferred from PMMA to Si₃N₄ using CHF₃ plasma etching. The diamond film was then etched to a depth of 1.6 µm using O₂ plasma Reactive Ion Etching (RIE) at a rate of 100 nm/minute.
Back-Side Thinning and Release: The diamond film was thinned down from the backside using alternating Ar/Cl₂ and O₂ plasma RIE until the LWRs were released. A U-shaped shadow mask ensured the structure remained attached to the bulk diamond film.
All-Optical Excitation: A temporally modulated 1550 nm laser (focused beam waist 2.3 µm) was used to drive the compression mode via optical gradient force.
Detection: Mechanical vibrations were detected via strain coupling to the SiV center, observed through strong phonon sidebands in the SiV optical excitation spectrum (Photoluminescence Excitation, PLE) and sideband optical interferometry.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the high-purity diamond materials and custom processing required for ultracoherent quantum spin-mechanics research, enabling both replication and advancement of this work.

Applicable Materials

The core requirement is ultra-high purity, low-strain Single Crystal Diamond (SCD) in thin film format.

Research Requirement	6CCVD Material Solution	Technical Advantage
Electronic Grade Diamond	Optical Grade SCD (Low N)	Nitrogen concentration < 1 ppb ensures minimal background defects and maximizes SiV coherence time ($\gamma_s$).
Thin Film Structure	Custom SCD Thickness	We provide SCD films from 0.1 µm up to 500 µm. The required 1.5 µm thickness is a standard custom order, ensuring optimal starting material for RIE release.
Enhanced Coherence	Isotopically Pure ¹²C SCD	The paper notes that $C > 10^9$ requires ¹²C enriched diamond. 6CCVD supplies high-purity ¹²C SCD substrates to minimize nuclear spin bath decoherence.
Future Electrical Control	Boron-Doped Diamond (BDD)	While not used here, BDD films (SCD or PCD) are available for integrating electrical gates or superconducting qubits, essential for hybrid quantum systems.

Customization Potential

The fabrication process described is highly complex, requiring precise dimensional control and multi-step etching. 6CCVD offers services that streamline this process for researchers.

Custom Dimensions and Thickness: We supply SCD plates up to 10x10 mm and PCD wafers up to 125 mm diameter. We guarantee the precise 1.5 µm thickness required for the LWR fabrication, reducing yield loss during RIE thinning.
Advanced Metalization Services: The paper utilizes an all-optical approach, but future integration may require electrical contacts. 6CCVD offers in-house metalization (Au, Pt, Pd, Ti, W, Cu) for creating electrodes or superconducting interfaces directly on the diamond film.
Surface Quality: Achieving high $Q$ factors depends on minimizing surface loss. Our SCD polishing capability achieves Ra < 1 nm, providing an ideal starting surface for EBL and subsequent RIE patterning.
Post-Processing Support: We can assist in preparing substrates for ion implantation (e.g., pre-thinning or providing specific crystal orientations) to optimize SiV center creation and depth control.

Engineering Support

The realization of ultracoherent GHz resonators requires deep expertise in both diamond material science and quantum defect physics.

6CCVD’s in-house PhD team specializes in material selection and optimization for quantum applications. We can assist researchers in:

Material Selection: Determining the optimal nitrogen concentration and isotopic purity (¹²C enrichment) to maximize SiV coherence and cooperativity for Phononic Cavity QED projects.
Process Integration: Consulting on RIE compatibility, thin film handling, and metal stack design for integrating diamond LWRs with external circuitry (e.g., superconducting qubits).
Scaling: Providing large-area PCD films (up to 125 mm) for scaling up mechanical quantum networks, as suggested by the authors.

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

View Original Abstract

We report the development of an all-optical approach that excites the fundamental compression mode in a diamond Lamb wave resonator with an optical gradient force and detects the induced vibrations via strain coupling to a silicon vacancy center, specifically, via phonon sidebands in the optical excitation spectrum of the silicon vacancy. Sideband optical interferometry has also been used for the detection of in-plane mechanical vibrations, for which conventional optical interferometry is not effective. These experiments demonstrate a gigahertz fundamental compression mode with a <i>Q</i> factor of >10<sup>7</sup> at temperatures near 7 K, providing a promising platform for reaching the quantum regime of spin mechanics, especially phononic cavity quantum electrodynamics of electron spins.

Tech Support

Original Source

DOI: https://doi.org/10.1021/acs.nanolett.4c03071