  Reducing energy dissipation is a central goal of classical and quantum
technologies. Optics achieved great success in bringing down power consumption
of long-distance communication links. With the rise of mobile, quantum and
cloud technologies, it is essential to extend this success to shorter links.
Electro-optic modulators are a crucial contributor of dissipation in such
links. Numerous variations on important mechanisms such as free-carrier
modulation and the Pockels effect are currently pursued, but there are few
investigations of mechanical motion as an electro-optic mechanism in silicon.
In this work, we demonstrate electrical driving and optical read-out of a 7.2
GHz mechanical mode of a silicon photonic waveguide. The electrical driving is
capacitive and can be implemented in any material system. The measurements show
that the mechanically-mediated optical phase modulation is two orders of
magnitude more efficient than the background phase modulation in our system.
Our demonstration is an important step towards efficient
opto-electro-mechanical devices in a scalable photonic platform.
