Optimized Planar Microwave Antenna for Nitrogen Vacancy Center Based Sensing Applications

At a Glance

Metadata	Details
Publication Date	2021-08-19
Journal	Nanomaterials
Authors	Oliver Opaluch, Nimba Oshnik, Richard Nelz, Elke Neu
Institutions	University of Kaiserslautern
Citations	32
Analysis	Full AI Review Included

Executive Summary

Optimized Antenna Design: Developed a planar, Q-shaped microstripline microwave (MW) antenna optimized for coherent spin control of Nitrogen Vacancy (NV) centers in diamond.
Homogeneity and Scale: Achieved highly uniform MW fields over a macroscopic area (approximately 400 x 400 µm²), critical for high-sensitivity ensemble NV sensing and wide-field imaging.
High Performance: Demonstrated high Rabi frequencies (Ω_R) up to 10 MHz, enabling fast manipulation necessary for complex pulsed protocols.
Wideband Resonance: Numerical optimization (FIT) resulted in a wide bandwidth (up to 8.2 GHz), allowing reliable NV spin manipulation across a large range of external magnetic bias fields (up to 190 mT).
Fabrication Reliability: Established a reliable microfabrication process on low-cost, transparent borosilicate glass substrates, ensuring compatibility with inverted confocal microscopy setups.
Material Selection: Confirmed that thermally evaporated gold layers significantly outperform sputtered layers, yielding higher ODMR contrast (21% ± 2%) and higher Rabi frequencies (2.4 MHz ± 0.5 MHz).
Advanced Sensing Capability: Successfully implemented the antenna for dynamical decoupling, increasing the spin coherence time (T₂) from 167.1 µs (Spin-Echo) to 638.1 µs (CPMG-8).

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	Borosilicate Glass	N/A	Relative permittivity (ε_r) = 4.82.
Substrate Thickness (s_z)	1	mm	Standard thickness used for fabrication.
Adhesion Layer Thickness	20	nm	Chromium (Cr) layer.
Conductor Layer Thickness	100	nm	Gold (Au) layer (thermally evaporated).
Optimized Gap Width (g_w)	7	µm	Determined via numerical simulation.
Optimized Radial Width (r_w)	1.151	mm	Determined via numerical simulation.
Optimized Feedline Width (f_w)	1.851	mm	Determined via numerical simulation.
Target Resonance Frequency	2.87	GHz	NV center Zero Field Splitting (ZFS).
S₁₁ Parameter (Optimized)	-47	dB	Back reflection coefficient (at 2.5 GHz resonance).
Bandwidth (50 µm Diamond)	8.2	GHz	Enables operation with bias fields up to 190 mT.
Uniform Field Area	400 x 400	µm²	Area of highly homogeneous MW radiation.
Max Experimental Rabi Frequency (Ω_R)	10	MHz	Achieved using high-power MW pulses.
Average Experimental Rabi Frequency (Ω_R)	2.1 ± 0.1	MHz	Measured over 200 µm distance to aperture center.
Spin-Echo Coherence Time (T₂)	167.1	µs	Baseline coherence time measurement.
CPMG-8 Coherence Time (T₂)	638.1	µs	Enhanced coherence time using dynamical decoupling.
ODMR Contrast (Evaporated Au)	21 ± 2	%	Measured without external magnetic field.

Key Methodologies

The antenna optimization relied on numerical simulation using the Finite Integral Technique (FIT), followed by a multi-step microfabrication process:

Numerical Simulation (CST Microwave Studio):
- Used the time domain solver (FIT) to solve Maxwell equations in the 2-4 GHz range.
- Optimization goals included minimizing the S₁₁ parameter (back reflection) at 2.87 GHz and maximizing the magnetic field amplitude 10 nm below the diamond surface.
- The titanium sample holder was included in the simulation as a ground plane to reduce back reflections.
Substrate Cleaning and Preparation:
- Borosilicate glass substrates were cleaned sequentially in acetone and isopropanol using an ultrasonic bath.
- Substrates were baked at 120 °C for 10 min to remove absorbed water.
Thin Film Deposition (PVD):
- A 20 nm Chromium (Cr) adhesion layer was deposited.
- A 100 nm Gold (Au) conductor layer was deposited, preferably using thermal evaporation due to its superior electrical performance compared to sputtering.
Photoresist Application:
- TI Prime adhesion promoter sub-monolayer was spin-coated.
- AZ1518 photoresist was spin-coated (6000 rpm, 1 min) and prebaked (100 °C, 50 s).
UV Lithography and Development:
- Contact UV lithography was performed using a laser-written binary intensity amplitude chromium photomask (Area dose: 33.6 mJ/cm²).
- The exposed resist was developed by stirring in 2.5% TMAH solution.
Wet Chemical Etching:
- The antenna structure was formed by sequentially stirring the substrate in gold etchant, followed by chromium etchant.
Final Assembly:
- Residual photoresist was removed using acetone and isopropanol.
- SMT ultra-miniature coaxial connectors (U.FL-R-SMT(01)) were attached using electrically conductive epoxy adhesives (EPO-TEK H20E).

Commercial Applications

Industry/Application	Description
Quantum Magnetometry	High-sensitivity AC magnetometry using NV ensembles, enabled by the homogeneous, high-amplitude MW field which maximizes coherence time (T₂).
Wide-Field Quantum Imaging	The macroscopic (400 x 400 µm²) and uniform MW field allows for position-independent, high-contrast imaging of magnetic fields over large sample areas.
Nanoscale Sensing (SPM/AFM)	The planar design is easily integrated into scanning confocal and Atomic Force Microscopy (AFM) setups for high-resolution magnetic imaging of samples.
Quantum Control Systems	The high Rabi frequency (up to 10 MHz) supports the implementation of fast, complex pulse sequences required for advanced quantum optimal control theory and dynamical decoupling protocols (e.g., CPMG).
Biophysics and Microstructure Analysis	Suitable for investigating biological samples or microstructures with characteristic dimensions of several hundred µm, requiring large, homogeneous MW irradiation volumes.
RF/Microwave Component Manufacturing	Provides a validated design and fabrication recipe for broadband, low-reflection (S₁₁ = -47 dB) microstripline antennas operating in the 2.5-3.5 GHz range.

View Original Abstract

Individual nitrogen vacancy (NV) color centers in diamond are versatile, spin-based quantum sensors. Coherently controlling the spin of NV centers using microwaves in a typical frequency range between 2.5 and 3.5 GHz is necessary for sensing applications. In this work, we present a stripline-based, planar, Ω-shaped microwave antenna that enables one to reliably manipulate NV spins. We found an optimal antenna design using finite integral simulations. We fabricated our antennas on low-cost, transparent glass substrate. We created highly uniform microwave fields in areas of roughly 400 × 400 μm2 while realizing high Rabi frequencies of up to 10 MHz in an ensemble of NV centers.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano11082108

References

2017 - Superresolution optical magnetic imaging and spectroscopy using individual electronic spins in diamond [Crossref]
2017 - Nanomechanical sensing using spins in diamond [Crossref]
2016 - NMR technique for determining the depth of shallow nitrogen-vacancy centers in diamond [Crossref]
2015 - Fourier magnetic imaging with nanoscale resolution and compressed sensing speed-up using electronic spins in diamond [Crossref]
2014 - Subnanometre resolution in three-dimensional magnetic resonance imaging of individual dark spins [Crossref]
2014 - Electronic properties and metrology applications of the diamond NV- center under pressure [Crossref]
2014 - All-optical thermometry and thermal properties of the optically detected spin resonances of the NV-center in nanodiamond [Crossref]
2013 - Nanometre-scale thermometry in a living cell [Crossref]
2013 - The nitrogen-vacancy colour centre in diamond [Crossref]
2015 - State-selective intersystem crossing in nitrogen-vacancy centers [Crossref]