Figure 2. NMR of ²⁹Si nuclei with a strongly coupled sensor.

(a) Schematic of the experimental setup. Dilute, unpolarized nuclear spins in a silica layer interact with an electronic spin in a (100)-surface diamond, which is readout with optical microscopy. (b) Schematic of strong coupling regime. A shallow NV centre in diamond (2 nm from the surface) couples to nearby ²⁹Si nuclei in a silica layer, due to hyperfine interaction. The contour lines show the strength of the effective magnetic gradient experienced by the nuclear spins. (c) Measured ²⁹Si NMR signal as a function of applied magnetic field, using the XY8-K decoupling sequence. The detected signal has a slope of 0.847(1) kHz G⁻¹ in agreement with the gyromagnetic ratio for ²⁹Si (solid line). The systematic shift of the detected frequency is due to the finite temporal length of decoupling pulses on the NV sensor. (d) Measured echo dip for NV centres at different magnetic fields. The signal dependence on the decoupling pulse number can be seen by comparing the data sets at 1,894, 1,951 and 2,044 Gauss (64 pulse acquisition), to the data set at 1,993 Gauss (128 pulse acquisition). Higher-order pulses produce a narrower signal with a stronger dip, up to the decoherence time. Fluorescence contrast has been normalized to the level corresponding to a complete NV spin flip. Measurements at 1,894, 1,993 Gauss were performed on NV-6 with a nominal depth of 2.1 nm, measurements at 1,951, 2,044 Gauss were performed on NV-7 with a depth of 2.3 nm.