  Magnetic resonance techniques provide a powerful tool for controlling spin
systems, with applications ranging from quantum information processing to
medical imaging. Nevertheless, the behavior of a spin system under strong
excitation remains a rich dynamical problem. In this paper, we examine spin
resonance of the nitrogen-vacancy center in diamond under conditions outside
the regime where the usual rotating wave approximation applies, focusing on
effects of multifrequency excitation and excitation with orientation parallel
to the spin quantization axis. Strong-field phenomena such as multiphoton
transitions and coherent destruction of tunneling are observed in the spectra
and analyzed via numerical and analytic theory. In addition to illustrating the
response of a spin system to strong multifrequency excitation, these
observations may inform techniques for manipulating electron-nuclear spin
quantum registers.
