Highly sensitive on-chip magnetometer with saturable absorbers in two-color microcavities

At a Glance

Metadata	Details
Publication Date	2017-03-29
Journal	Physical review. B./Physical review. B
Authors	Olivier Gazzano, Christoph Becher
Institutions	National Institute of Standards and Technology, Joint Quantum Institute
Citations	7
Analysis	Full AI Review Included

6CCVD Technical Documentation: Two-Color Diamond Microcavity Magnetometers

Based on: “Highly Sensitive On-Chip Magnetometer with Saturable Absorbers in Two-Color Microcavities”

Executive Summary

This paper presents a robust, scalable architecture for an on-chip magnetic field sensor utilizing Nitrogen Vacancy (NV) centers embedded within a diamond waveguide. The core innovation lies in coupling the NV ensemble’s transitions to a doubly resonant microcavity system, leading to exceptional sensitivity enhancement.

Core Achievement: Demonstrated a shot-noise limited magnetic-field sensitivity as low as 290 fT/√Hz using real-world parameters.
Performance Gain: The cavity structure improves sensitivity by a factor of 520 to 820 times compared to non-cavity configurations (transmission and reflection, respectively).
Mechanism: The device leverages the non-linear absorption introduced by NV centers coupled to two nested cavities resonant at the green pump wavelength ($\lambda_{Gr} = 532 \text{nm}$) and the infrared probe wavelength ($\lambda_{IR} = 1042 \text{nm}$).
Device Structure: The nested cavities are formed by etched Distributed Bragg Reflectors (DBRs) integrated into a micrometer-scale diamond photonic waveguide.
Critical Dependence: Optimal sensitivity is highly dependent on precise control of the cavity length, mirror reflectivity (DBR pair count), and the overall loss parameters, which are functions of the input optical power and material quality.
Scalability: The architecture is inherently scalable, enabling the future implementation of single-photonic chips for 1D or 2D real-time magnetic imaging.

Technical Specifications

Parameter	Value	Unit	Context
Best Magnetic Field Sensitivity ($\delta B$)	290	fT/√Hz	Shot-noise limited (Reflection config.)
Sensitivity Improvement Factor	520 - 820	N/A	Improvement over non-cavity measurement
Green Pump Wavelength ($\lambda_{Gr}$)	532	nm	Used for spin initialization
Infrared Probe Wavelength ($\lambda_{IR}$)	1042	nm	Used for absorption measurement (ESR)
Diamond Refractive Index ($n_D$)	~2.4	N/A	Relevant at $\lambda_{IR}$
NV Center Density ($d$)	4.4 x 10²³	m^-3	Required high ensemble density
Electronic Spin Dephasing Time ($T_2$)	390	ns	Value used for primary optimization
Waveguide Cross-Section	3 x 3	µm²	Square diamond waveguide dimension
Microwave Rabi Frequency ($\Omega$)	2$\pi$ x 10	MHz	Excitation frequency for ESR
Optimal Cavity Regime	Asymmetric Reflection	Achieved maximum sensitivity with $N^{back}{IR} = 2 \times N^{front}{IR}$

Key Methodologies

The research relies on advanced simulation techniques and a complex materials structure utilizing high-quality diamond, demonstrating the need for precise CVD diamond material engineering.

Device Fabrication Principle (Theoretical): The doubly resonant structure is based on pairs of Distributed Bragg Reflectors (DBRs) designed for both 532nm and 1042nm. These DBRs are proposed to be achieved by etching a diamond film to create the necessary periodic refractive index modulation.
Multiphysics Modeling: Finite Element Method (FEM) software (COMSOL Multiphysics) was used to confirm the excellent confinement of both the green and infrared electric fields within the two-cavity system.
Transfer Matrix Calculation: A traditional transfer matrix calculation method was employed to determine the reflectivity and overall shot-noise limited sensitivity ($\delta B$). This method accounted for wave propagation through discretized layers ($Z_{step} = 10 \text{nm}$).
Rate Equation Model Integration: The model incorporated the non-linear absorption rates of the NV ensemble—dependent on the microwave field state (on/off resonance) and local optical intensity—to calculate the occupation probability ($N_i$) of the six NV energy levels.
Optimization for Reflection: The highest sensitivity was achieved in the reflection configuration, specifically when using an asymmetric infrared cavity design ($\epsilon_2 \ll \epsilon_1$) and precise tuning of the input green pump power to compensate for saturation and non-linear losses.

6CCVD Solutions & Capabilities

6CCVD provides the specialized MPCVD diamond substrates, precise material specifications, and engineering support necessary to replicate, optimize, and scale the production of this high-sensitivity, on-chip diamond magnetometer architecture.

6CCVD Capability	Application for NV Magnetometers	Strategic Advantage for Replication/Scale
High-Purity Single Crystal Diamond (SCD)	Provides the ideal low-loss, high-refractive index platform required for high-finesse, low-scatter photonic waveguides and microcavity construction.	Thickness: SCD up to 500 µm for robust waveguide layers. Polishing: Ra < 1nm SCD surfaces for minimal optical loss and superior lithographic patterning.
Material Precursor for High NV Density	The required NV density ($d = 4.4 \times 10^{23} \text{m}^{-3}$) demands precise control over nitrogen incorporation during growth and subsequent post-processing (irradiation/annealing).	6CCVD supplies custom SCD or PCD tailored specifically for optimal creation and activation of NV ensembles, supporting the required $T_2$ times in the hundreds of nanoseconds range.
Custom Dimensions & Wafer Scale	The goal of a “scalable on-chip magnetometer” requires fabrication across industrially relevant wafer sizes for integration into single-photonic chips.	Scale: We offer Polycrystalline Diamond (PCD) wafers up to 125mm, supporting large-scale arrays necessary for 1D/2D magnetic imaging.
Metalization Services	Future device integration requires microwave strip lines (MW) for Electron Spin Resonance (ESR) excitation and wire bonding for device testing.	Internal Capability: Custom metal stack deposition including Ti, Pt, Au, Pd, Cu, essential for low-loss transmission lines integrated directly onto the diamond wafer.
Boron-Doped Diamond (BDD) Availability	While the paper focuses on NV centers, BDD is vital for related electrochemistry or high-power thermal management applications often integrated onto the same platform.	We offer BDD films to facilitate multi-functional integrated diamond sensor chips.
Expert Engineering Support	The sensitivity optimization is highly non-linear, depending critically on material quality, NV creation yield, and physical structure (DBR pair counts).	6CCVD’s in-house PhD team can assist researchers and technical engineers with material selection, post-growth treatment advice, and geometry requirements for similar NV-based Quantum Sensing projects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We ship globally (DDU default, DDP available).

View Original Abstract

Interacting resonators can lead to strong non-linearities but the details can be complicated to predict. In this work, we study the non-linearities introduced by two nested microcavities that interact with nitrogen vacancy centers in a diamond waveguide. Each cavity has differently designed resonance; one in the green and one in the infrared. The magnetic-field dependence of the nitrogen vacancy center absorption rates on the green and the recently observed infrared transitions allows us to propose a scalable on-chip magnetometer that combines high magnetic-field sensitivity and micrometer spatial resolution. By investigating the system behaviors over several intrinsic and extrinsic parameters, we explain the main non-linearities induced by the NV centers and enhanced by the cavities. We finally show that the cavities can improve the magnetic-field sensitivity by up to two orders of magnitudes.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevb.95.115312