Magnetic-field-dependent photodynamics of single NV defects in diamond: an application to qualitative all-optical magnetic imaging

Magnetometry and magnetic imaging with nitrogen-vacancy (NV) defects in\ndiamond rely on the optical detection of electron spin resonance (ESR).\nHowever, this technique is inherently limited to magnetic fields that are weak\nenough to avoid electron spin mixing. Here we focus on the high off-axis\nmagnetic field regime for which spin mixing alters the NV defect spin dynamics.\nWe first study in a quantitative manner the dependence of the NV defect optical\nproperties on the magnetic field vector B. Magnetic-field-dependent\ntime-resolved photoluminescence (PL) measurements are compared to a seven-level\nmodel of the NV defect that accounts for field-induced spin mixing. The model\nreproduces the decreases in (i) ESR contrast, (ii) PL intensity and (iii)\nexcited level lifetime with an increasing off-axis magnetic field. We next\ndemonstrate that those effects can be used to perform all-optical magnetic\nimaging in the high off-axis magnetic field regime. Using a scanning NV defect\nmicroscope, we map the stray field of a magnetic hard disk through both PL and\nfluorescence lifetime imaging. This all-optical method for high magnetic field\nimaging at the nanoscale might be of interest in the field of nanomagnetism,\nwhere samples producing fields in excess of several tens of milliteslas are\ntypical.