  Hybrid spin-mechanical systems are a promising platform for future quantum
technologies. Usually they require application of additional microwave fields
to project integer spin to a readable state. We develop a theory of optically
detected spin-mechanical resonance associated with half-integer spin defects in
silicon carbide (SiC) membranes. It occurs when a spin resonance frequency
matches a resonance frequency of a mechanical mode, resulting in a shortening
of the spin relaxation time through resonantly enhanced spin-phonon coupling.
The effect can be detected as an abrupt reduction of the photoluminescence
intensity under optical pumping without application of microwave fields. We
propose all-optical protocols based on such spin-mechanical resonance to detect
external magnetic fields and mass with ultra-high sensitivity. We also discuss
room-temperature nonlinear effects under strong optical pumping, including
spin-mediated cooling and heating of mechanical modes. Our approach suggests a
new concept for quantum sensing using spin-optomechanics.
