  The ability to design, fabricate and control systems that can convert photons
with dissimilar frequencies has technological implications in classical as well
as quantum communications. Laser heating and thermal-mechanical motion in
conventional micro/nanoscale optomechanical systems hamper the use of these
systems in quantum information processing networks. In contrast, we propose an
unconventional system comprising of a bulk quartz crystal placed within a
Fabry-Perot cavity. The pumping laser is in the far-infrared region. We explore
the possibility of efficient mode conversion between two optical modes
supported by the system, mediated by the bulk acoustic phonons of the quartz
crystal. Unlike the earlier optomechanical systems, the dark mode in our
proposed system is not decoupled from the mechanical mode and yet it enables
the efficient mode conversion. The novel results found in our study can be used
to harness the dark state for quantum state transfer. The proposed system is
robust against excessive heating.
