  We address the textbook problem of dynamics of a spin placed in a dc magnetic
field and subjected to an ac drive. If the drive is polarized in the plane
perpendicular to the dc field, the drive photons are resonantly absorbed when
the spacing between the Zeeman levels is close to the photon energy. This is
the only resonance when the drive is circularly polarized. For linearly
polarized drive, additional resonances corresponding to absorption of three,
five, and multiple odd numbers of photons is possible. Interaction with the
environment causes the broadening of the absorption lines. We demonstrate that
the interaction with environment enables the forbidden two-photon absorption.
We adopt a model of the environment in the form of random telegraph noise
produced by a single fluctuator. As a result of the synchronous time
fluctuations of different components of the random field, the shape of the
two-photon absorption line is non-Lorentzian and depends dramatically on the
drive amplitude. This shape is a monotonic curve at strong drive, while, at
weak drive, it develops a two-peak structure reminiscent of an induced
transparency on resonance.
