  Electronic spins associated with the Nitrogen-Vacancy (NV) center in diamond
offer an opportunity to study spin-related phenomena with extremely high
sensitivity owing to their high degree of optical polarization. Here, we study
both single- and double-quantum transitions (SQT and DQT) in NV centers between
spin-mixed states, which arise from magnetic fields that are non-collinear to
the NV axis. We demonstrate the amplification of the ESR signal from both these
types of transition under laser illumination. We obtain hyperfine-resolved
X-band ESR signal as a function of both excitation laser power and misalignment
of static magnetic field with the NV axis. This combined with our analysis
using a seven-level model that incorporates thermal polarization and double
quantum relaxation allows us to comprehensively analyze the polarization of NV
spins under off-axis fields. Such detailed understanding of spin-mixed states
in NV centers under photo-excitation can help greatly in realizing NV-diamond
platform's potential in sensing correlated magnets and biological samples, as
well as other emerging applications, such as masing and nuclear
hyperpolarization.
