We report a study of the magnetic field dependence of photoluminescence ofNV$^-$ centers (negatively charged nitrogen-vacancy centers) in diamond singlecrystals. In such a magnetic field dependence characteristic sharp features areobserved, which are coming from Level Anti-Crossings (LACs) in a coupledelectron-nuclear spin system. For sensitive detection of such LAC-lines we uselock-in detection to measure the photoluminescence intensity. This experimentaltechnique allows us to obtain new LAC lines. Additionally, a remarkably strongdependence of the LAC-lines on the modulation frequency is found. Specifically,upon decrease of the modulation frequency from 12 kHz to 17 Hz the amplitude ofthe LAC-lines increases by approximately two orders of magnitude. To take aquantitative account for such effects, we present a theoretical model, whichdescribes the spin dynamics in a coupled electron-nuclear spin system under theaction of an oscillating external magnetic field. Good agreement betweenexperiments and theory allows us to conclude that the observed effects areoriginating from coherent spin polarization exchange in a coupled spin systemcomprising the spin-polarized NV$^-$ center. Our results are of great practicalimportance allowing one to optimize the experimental conditions for probingLAC-derived lines in diamond crystals comprising NV$^-$ centers and forindirect detection and identification of other paramagnetic defect centers.
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