Electrical-Readout Microwave-Free Sensing with Diamond

At a Glance

Metadata	Details
Publication Date	2022-08-30
Journal	Physical Review Applied
Authors	Huijie Zheng, Jaroslav Hrubý, Emilie Bourgeois, Josef Souček, Petr Siyushev
Institutions	GSI Helmholtz Centre for Heavy Ion Research, Center for Integrated Quantum Science and Technology
Citations	5
Analysis	Full AI Review Included

Technical Documentation & Analysis: Microwave-Free NV Sensing

Executive Summary

This research demonstrates a significant advancement in diamond-based quantum sensing by implementing microwave-free photoelectric readout of Nitrogen-Vacancy (NV) centers. This approach addresses key limitations of conventional optical detection, paving the way for highly integrated and scalable quantum devices.

Microwave-Free Sensing: The study successfully utilized the Ground-State Level Anti-Crossing (GSLAC) feature ($\approx$ 102.4 mT) for magnetometry, eliminating the need for complex microwave control systems.
Electrical Readout Advantage: Photoelectric detection (PC) overcomes the diffraction limit and offers superior collection efficiency compared to photoluminescence (PL) readout.
Enhanced Spatial Resolution: PC detection limits the effective interrogation volume to a shallow region (0-30 µm), resulting in a 1:20 volume ratio compared to PL, which is critical for nanoscale imaging and dense sensor arrays.
Material Requirements: The experiment relied on a low-nitrogen, single-crystal [111]-cut HPHT diamond, subsequently processed via high-energy electron irradiation (14 MeV) and annealing (700 °C) to create NV centers.
Integration Potential: The electrical readout method is inherently compatible with nanoscale lithography (5 µm electrode gaps used) and integrated electronics, supporting the development of hybrid gradiometers and scalable quantum sensors.
Achieved Sensitivity: Demonstrated magnetic sensitivity of 350 nT/√Hz using PC detection at the GSLAC working point, with clear pathways identified for future optimization (e.g., two-color protocols).

Technical Specifications

The following hard data points were extracted from the experimental setup and results:

Parameter	Value	Unit	Context
Diamond Type	Single Crystal (HPHT)	N/A	[111]-cut orientation
Sample Dimensions	2.1 x 2.3 x 0.65	mm³	Sensor substrate size
Initial Nitrogen Conc.	<200	ppm	Pre-irradiation impurity level
NV Creation Method	Electron Irradiation	N/A	14 MeV, 10¹⁸ cm^-2 dose
Annealing Parameters	700 °C for 3 hours	N/A	Post-irradiation processing
Electrode Material Stack	Ti (20 nm) / Al (100 nm)	N/A	Coplanar interdigitated contacts
Electrode Gap	5	µm	Fabricated via optical lithography
Excitation Wavelength	532	nm	Green laser light source
GSLAC Resonance Field	102.4	mT	Optimal magnetic field for microwave-free sensing
PC Detection Sensitivity	350	nT/√Hz	At GSLAC working point
PL Detection Sensitivity	90	nT/√Hz	At GSLAC working point
Effective Sensing Volume Ratio	1:20 (PC:PL)	N/A	PC detection volume is significantly smaller
Applied Bias Voltage	17	V	Used for charge carrier collection

Key Methodologies

The experimental procedure focused on material preparation, defect engineering, and advanced electrical readout integration:

Material Selection: A single-crystal [111]-cut HPHT diamond with a low initial nitrogen concentration (<200 ppm) was chosen as the base material.
NV Center Generation: NV centers were created by high-energy electron irradiation (14 MeV, 10¹⁸ cm^-2 dose) followed by high-temperature annealing (700 °C for 3 hours).
Surface Cleaning: The diamond surface was cleaned using an oxidizing mixture (H₂SO₄ and KNO₃) at $\approx$ 250 °C prior to metalization.
Electrode Fabrication: Coplanar interdigitated electrodes with a 5 µm gap were fabricated on the diamond surface using optical lithography. The metal stack consisted of 20 nm Titanium (Ti) covered by 100 nm Aluminum (Al).
Optical Excitation: A 532 nm green laser was modulated via an Acousto-Optic Modulator (AOM) and focused between the electrodes.
Photoelectric Readout (PDMR): Photocurrent (PC) was detected synchronously using a lock-in amplifier referenced to the laser modulation frequency, while a static magnetic field (B_s) was swept along the NV axis.
Magnetometry Implementation: A microwave-free magnetometer was realized by applying an alternating magnetic field modulation ($\approx$ 0.1 mT depth, 3.3 kHz frequency) around the GSLAC point (102.4 mT) and demodulating the PC signal.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced diamond materials and fabrication services required to replicate, optimize, and scale the microwave-free NV sensing technology demonstrated in this paper. Our MPCVD capabilities offer superior control over material properties essential for high-performance quantum applications.

Applicable Materials for Replication and Optimization

Research Requirement	6CCVD Material Recommendation	Technical Rationale & Advantage
Low-Nitrogen SCD Substrate	Optical Grade Single Crystal Diamond (SCD)	Our MPCVD SCD offers ultra-low nitrogen content (typically <5 ppb), significantly lower than the <200 ppm HPHT material used. This allows for precise, controlled creation of NV centers and minimizes background noise from P1 centers, directly improving spin contrast and sensitivity.
High Spatial Resolution Sensing	Thin SCD Plates (0.1 µm - 50 µm)	The paper notes PC detection is limited to the top 30 µm. 6CCVD can supply ultra-thin SCD plates, maximizing the ratio of active sensing volume to total material, improving photon utilization efficiency, and enabling integration onto heat sinks or complex structures.
Future Hybrid Gradiometers	Polycrystalline Diamond (PCD) Substrates	For large-area, cost-effective background PL detection (the large sensing volume component of the hybrid gradiometer), 6CCVD offers PCD wafers up to 125 mm diameter with excellent uniformity.

Customization Potential for Integrated Devices

The successful implementation of this technology relies heavily on precise material geometry and electrode integration. 6CCVD provides comprehensive customization services:

Custom Dimensions and Geometry: The paper used a specific 2.1 x 2.3 x 0.65 mm³ sample. 6CCVD provides custom laser cutting and shaping for SCD and PCD plates up to 125 mm in diameter, ensuring exact fit for experimental setups or integrated device packaging.
Precision Surface Finishing: The 5 µm electrode gap requires an extremely flat surface. We guarantee atomic-scale polishing (Ra < 1 nm for SCD; Ra < 5 nm for inch-size PCD), essential for high-resolution optical lithography and minimizing surface leakage currents.
Advanced Metalization Stacks: The Ti/Al stack used for ohmic contacts can be optimized. 6CCVD offers in-house metalization using Ti, Pt, Au, Pd, W, and Cu, allowing researchers to design robust, low-resistance contacts tailored for specific annealing and wire bonding protocols. We can provide standard Ti/Pt/Au stacks for enhanced chemical stability.
Defect Engineering Support: While the paper used 14 MeV irradiation and 700 °C annealing, 6CCVD’s engineering team can consult on optimal post-growth processing parameters (irradiation dose, energy, and annealing temperature/duration) to achieve the desired NV density and NV^-/NV⁰ charge state ratio for maximum PDMR contrast.

Engineering Support

6CCVD’s in-house PhD team specializes in the physics and engineering of diamond quantum materials. We can assist researchers in optimizing material selection for similar electrical-readout NV magnetometry projects, focusing on:

P1 Center Mitigation: Consulting on ultra-low nitrogen substrates and two-color protocols (as suggested in the paper) to reduce the background photocurrent caused by P1 center ionization.
Charge State Control: Advising on surface termination and doping strategies to stabilize the NV^- charge state, crucial for GSLAC sensing.
Scalable Fabrication: Providing materials and processing advice compatible with large-scale fabrication of integrated quantum sensor arrays.

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

View Original Abstract

While nitrogen-vacancy (N-V & minus;) centers have been extensively investigated in the context of spin -based quantum technologies, the spin-state readout is conventionally performed optically, which may limit miniaturization and scalability. Here, we report photoelectric readout of ground-state cross-relaxation fea-tures, which serves as a method for measuring electron-spin resonance spectra of nanoscale electronic environments and also for microwave-free sensing. As a proof of concept, by systematically tuning N -V centers into resonance with the target electronic system, we extract the spectra for the P1 electronic spin bath in diamond. Such detection may enable probing optically inactive defects and the dynamics of local spin environment. We also demonstrate a magnetometer based on photoelectric detection of the ground -state level anticrossings (GSLACs), which exhibits a favorable detection efficiency as well as magnetic sensitivity. This approach may offer potential solutions for determining spin densities and characterizing local environment. <comment>Superscript/Subscript Available</comment

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevapplied.18.024079