  Optical spin pumping of color centers in diamond is presently attracting
broad interest as a platform for dynamic nuclear polarization at room
temperature, but the mechanisms involved in the generation and transport of
polarization within the host crystal are still partly understood. Here we
investigate the impact of continuous radio-frequency (RF) excitation on the
generation of nuclear magnetization produced by optical illumination. In the
presence of RF excitation far removed from the nuclear Larmor frequency, we
witness a magnetic-field-dependent sign reversal of the measured nuclear spin
signal when the drive is sufficiently strong, a counter-intuitive finding that
immediately points to non-trivial spin dynamics. With the help of analytical
and numerical modeling, we show our observations indicate a modified form of
'solid effect', down-converted from the microwave to the radio-frequency range
through the driving of hybrid transitions involving one (or more) nuclei and
two (or more) electron spins. Our results open intriguing opportunities for the
manipulation of many-electron spin systems by exploiting hyperfine couplings as
a means to access otherwise forbidden intra-band transitions.
