  The nitrogen-vacancy (NV) center in diamond has been the focus of research
efforts because of its suitability for use in applications such as quantum
sensing and quantum simulations. Recently, the electron-spin double resonance
(ESDR) of NV centers has been exploited for detecting radio-frequency (RF)
fields with continuous-wave optically detected magnetic resonance. However, the
characteristic phenomenon of ESDR under a strong RF field remains to be fully
elucidated. In this study, we theoretically and experimentally analyzed the
ESDR spectra under strong RF fields by adopting the Floquet theory. Our
analytical and numerical calculations could reproduce the ESDR spectra obtained
by measuring the spin-dependent photoluminescence under the continuous
application of microwaves and an RF field for a DC bias magnetic field
perpendicular to the NV axis. We found that anticrossing structures that appear
under a strong RF field are induced by the generation of RF-dressed states
owing to the two-RF-photon resonances. Moreover, we found that $2n$-RF-photon
resonances were allowed by an unintentional DC bias magnetic field parallel to
the NV axis. These results should help in the realization of precise MHz-range
AC magnetometry with a wide dynamic range beyond the rotating wave
approximation regime as well as Floquet engineering in open quantum systems.
