Overview
Non-contact and sensitive measurement of the smallest magnetic fields and currents opens up new possibilities for failure analysis in microelectronics, the exploration of rare raw materials, the magnetic characterisation of materials and biomedicine. Optical quantum sensors based on nitrogen-vacancy (NV) centres in diamond can detect weak magnetic fields with very high sensitivity and high spatial resolution – at room temperature and without complex cooling.
The NV centre is a point defect in the diamond lattice, consisting of a nitrogen atom and an adjacent lattice vacancy. In its negatively charged state (NV⁻), its electron-spin ground state is a triplet with the sublevels ms = 0 and ms = ±1, which are separated by the zero-field splitting D ≈ 2.87 GHz in the absence of an external magnetic field.
Under optical excitation with green laser light (532 nm), the NV centre fluoresces in the red to near-infrared range (approx. 637–800 nm). Optical pumping preferentially polarises the spin into the ms = 0 state. In the ms = 0 state, the fluorescence is stronger than in the ms = ±1 states. This spin-dependent fluorescence contrast enables optical readout of the spin state.
In optically detected magnetic resonance (ODMR), the microwave frequency is swept. When it matches a spin transition, the fluorescence decreases measurably. An external magnetic field splits the resonances via the Zeeman effect by about 28 MHz/mT, so that the magnetic field can be determined quantitatively from the resonance frequencies. Because the NV axes occur in four crystallographic orientations, the full magnetic field vector can be reconstructed from the resonances of all orientations.
For wide-field imaging a dense ensemble of NV centres in a near-surface diamond layer is used. A lock-in camera records the fluorescence pixel by pixel with high sensitivity, so that each pixel delivers an ODMR spectrum. A single measurement measures a magnetic field map of the entire field of view, from which the current distribution in the sample can be obtained. In addition to continuous-wave ODMR, pulsed measurement schemes can be used to further increase sensitivity.
We developed a 3D wide-field magnetometer with tailored signal processing, including noise suppression. It captures the 3D components of the magnetic field across a field of view of 3 × 3 mm² (500 × 500 pixels) with a sensitivity of ~0.1 µT/√Hz per pixel. The system is suitable for failure analysis of integrated circuits and printed circuit boards, for example to locate short circuits and defects through non-contact current imaging, for non-destructive testing of metallic and magnetic components and, in the longer term, for biomedical measurement technology, for example to detect weak magnetic fields from cardiac and muscle activity.
