Millimeter-scale rigid diamond probe for high sensitivity endoscopic-magnetometry applications

Magnetometry based on diamond nitrogen-vacancy (NV) centers has been extensively studied for applications requiring diverse capabilities, spanning from nanometer spatial resolution to subpicotesla sensitivity. Among various applications, diamond magnetometers can demonstrate high sensitivity magnetic sensing within millimeter-scale size for endoscopic applications. However, the trade-off between sensitivity and spatial resolution of diamond magnetometry makes it difficult to achieve such a probe. In this study, we present a millimeter-scale rigid diamond magnetometer probe with enhanced sensitivity via optimizing the optical design. By coupling the frustum diamond with the miniaturized compound parabolic concentrator (CPC) lens, we enhance the fluorescence collection efficiency by 37% within 4 mm diameter, and the achieved sensitivity is 200 pT/Hz1/2 based on the sample with the resonance linewidth of ~8 MHz. With this verified structure, endoscopes with mm-size probe and picotesla sensitivity can be projected for surgical and industrial applications in the future.

💡 Research Summary

This paper addresses the longstanding trade‑off between magnetic‑field sensitivity and spatial resolution in nitrogen‑vacancy (NV)‑based diamond magnetometers by developing a millimeter‑scale rigid probe optimized for endoscopic applications. The authors first analyze the fluorescence emission geometry of a bulk cubic diamond, noting that only photons emitted within a narrow cone defined by the critical angle escape the crystal, while the majority are trapped by total internal reflection. To increase the fraction of emitted photons, the cubic diamond is reshaped into a frustum with four inclined side faces (designated FD‑4). This geometry widens the effective escape cone, raising the emitted‑fluorescence efficiency to 81 %—a three‑fold improvement over the original cube—while accounting for the loss of NV centers caused by the reduced volume.

The second major innovation is the miniaturization and optimization of a compound parabolic concentrator (CPC) lens. Commercial CPCs are too large and have excessive divergence for a sub‑5 mm probe. The authors therefore truncate a standard CPC to half its length (short CPC) and further refine the profile using high‑borosilicate glass, achieving a 3 mm output diameter and a divergence angle of roughly 5° in air. Because high‑index glass manufacturing is challenging, a tapered rod lens (diameter 3.4 mm, divergence ≈ 7°) is also evaluated as a simpler, mass‑producible alternative, albeit with slightly lower collection efficiency.

The complete optical assembly consists of the frustum diamond, the optimized CPC (or rod lens), a rod lens for long‑distance transmission, a dichroic prism that reflects the 532 nm excitation laser while transmitting red fluorescence, and a long‑pass filter to suppress residual laser light. A microwave (MW) loop, impedance‑matched to 50 Ω and sized to the 4 mm probe diameter, delivers the MW field required for optically detected magnetic resonance (ODMR). A permanent magnet provides a 13 G bias field aligned with the crystal’s