  Single-layer graphene exhibits exceptional mechanical properties attractive
for optomechanics: it combines low mass density, large tensile modulus, and low
bending stiffness. However, at visible wavelengths, graphene absorbs weakly and
reflects even less, thereby inadequate to generate large optical forces needed
in optomechanics. Here, we numerically show that a single-layer graphene sheet
is sufficient to produce strong optical forces under terahertz or infrared
illumination. For a system as simple as graphene suspended atop a uniform
substrate, high reflectivity from the substrate is crucial in creating a
standing-wave pattern, leading to a strong optical force on graphene. This
force is readily tunable in amplitude and direction by adjusting the suspension
height. In particular, repellent optical forces can levitate graphene to a
series of stable equilibrium heights above the substrate. One of the key
parameters to maximize the optical force is the excitation frequency: peak
forces are found near the scattering frequency of free carriers in graphene.
With a dynamically controllable Fermi level, graphene opens up new
possibilities of tunable nanoscale optomechanical devices.
