Practical limits to spatial resolution of magnetic imaging with a quantum diamond microscope

Widefield quantum diamond microscopy is a powerful technique for imaging magnetic fields with high sensitivity and spatial resolution. However, current methods to approach the ultimate spatial resolution ( < 500 nm) are impractical for routine use as they require time-consuming fabrication or transfer techniques to precisely interface the diamond sensor with the sample to be imaged. To address this challenge, we have designed a co-axial sensor holder that enables simple, repeatable sensor-sample interfacing while being compatible with high numerical aperture (NA) optics. With our new design we demonstrate low standoffs < 500 nm with a millimeter sized sensor. We also explore the relationship between spatial resolution and NA spanning from 0.13 to 1.3. The spatial resolution shows good agreement with the optical diffraction limit at low NA but deviates at high NA, which is shown to be due to optical aberrations. Future improvements to our design are discussed, which should enable magnetic imaging with < 500 nm resolution in an accessible, easy-to-use instrument.