[Paper Review] Enhanced widefield quantum sensing with nitrogen-vacancy ensembles using diamond nanopillar arrays

Daniel J. McCloskey, N. Dontschuk+7 more|arXiv (Cornell University)|Feb 7, 2019

Diamond and Carbon-based Materials ResearchMaterials Science26 references3 citations

TL;DR

This paper proposes enhancing widefield quantum sensing using diamond nanopillar arrays hosting dense, near-surface nitrogen-vacancy (NV) ensembles. By patterning diamond into closely packed nanopillars, the study achieves over 11-fold improvement in photoluminescence collection and a >3× increase in measurement sensitivity due to enhanced spin and optical properties, enabling high-resolution stress tensor imaging with minimal fabrication-induced strain.

ABSTRACT

Quantum sensors based on optically active defects in diamond such as the nitrogen vacancy (NV) centre represent a promising platform for nanoscale sensing and imaging of magnetic, electric, temperature and strain fields. Enhancing the optical interface to such defects is key to improving the measurement sensitivity of these systems. Photonic nanostructures are often employed in the single emitter regime for this purpose, but their applicability to widefield sensing with NV ensembles remains largely unexplored. Here we fabricate and characterize closely-packed arrays of diamond nanopillars, each hosting its own dense, near-surface ensemble of NV centres. We explore the optimal geometry for diamond nanopillars hosting NV ensembles and realise enhanced spin and photoluminescence properties which lead to increased measurement sensitivities (greater than a factor of 3) when compared to unpatterned surfaces. Utilising the increased measurement sensitivity, we image the mechanical stress tensor in each nanopillar across the arrays and show the fabrication process has negligible impact on in-built stress compared to the unpatterned surface. Our results demonstrate that photonic nanostructuring of the diamond surface is a viable strategy for increasing the sensitivity of ensemble-based widefield sensing and imaging.

Motivation & Objective

• To improve the sensitivity of widefield quantum sensing using NV ensembles in diamond by engineering the optical and spin properties of the defect centers.
• To address the limited photon collection efficiency in bulk diamond NV systems, which restricts measurement sensitivity in widefield imaging.
• To explore whether photonic nanostructuring via diamond nanopillars can enhance both optical collection and spin coherence in ensemble-based sensing.
• To demonstrate the feasibility of using nanopillar arrays for high-resolution, widefield imaging of mechanical stress in nanoscale diamond devices.
• To evaluate the impact of fabrication on intrinsic mechanical strain in NV-hosting pillars compared to unpatterned surfaces.

Proposed method

• Fabricated closely-packed arrays of diamond nanopillars with diameters ranging from 400 nm to 1000 nm using electron beam lithography and reactive ion etching.
• Implanted nitrogen ions into the nanopillar tips to create dense, near-surface ensembles of NV centers with all four crystallographic orientations.
• Performed widefield photoluminescence (PL) imaging and optically detected magnetic resonance (ODMR) spectral imaging to map spin and optical response across the arrays.
• Used ODMR peak shifts to reconstruct the mechanical stress tensor components (σ_axial, σ_xy, σ_yz, σ_xz) with single-pillar (sub-micron) spatial resolution.
• Compared PL intensity, T₁, T₂, and T₂* relaxation times between nanopillar-hosted NV ensembles and unstructured diamond surfaces to quantify sensitivity gains.
• Applied a stress tensor reconstruction method based on fitting all eight ODMR peaks to the NV spin Hamiltonian to extract full stress components.

Experimental results

Research questions

• RQ1 Can diamond nanopillar arrays enhance the photon collection efficiency and spin coherence of NV ensembles for widefield quantum sensing?
• RQ2 What is the optimal nanopillar geometry (diameter and pitch) for maximizing NV ensemble sensitivity while preserving spin coherence?
• RQ3 To what extent does the fabrication process of nanopillars alter the intrinsic mechanical stress in the diamond material?
• RQ4 Can nanopillar-based NV ensembles enable high-resolution widefield imaging of mechanical stress at the nanoscale?
• RQ5 How do the sensitivity gains from nanostructuring compare quantitatively to unstructured diamond surfaces in terms of ODMR spectral imaging?

Key findings

• The nanopillar arrays achieved an 11.5-fold increase in collected photoluminescence intensity compared to unstructured diamond surfaces, with optimal performance in the 700–900 nm diameter range.
• The fraction of negatively charged NV centers (NV⁻) increased with decreasing pillar diameter, indicating improved optical and spin properties.
• NV ensembles in nanopillars showed a 60% increase in T₁ relaxation time, and 20% increases in both Hahn-echo T₂ and T₂* coherence times compared to unstructured surfaces.
• The measurement sensitivity for widefield ODMR spectral imaging improved by 2.8× (σ_axial), 3.3× (σ_xy), 3.7× (σ_yz), and 3.2× (σ_xz), consistent with a predicted >3.7× gain from enhanced photon collection.
• The total axial stress (σ_axial) increased with decreasing pillar diameter, while shear stress components showed no diameter dependence but varied in sign and magnitude across the tensor components.
• Stress levels in the nanopillars were comparable to those in unpatterned diamond, indicating minimal fabrication-induced strain, and measurement noise was lower in the arrays, confirming improved signal-to-noise performance.