  We have developed a torsion balance to detect the rotation of a cell
containing spin-polarized gaseous atoms to study angular momentum transfer from
gaseous atoms to solid. A cesium vapor cell was hung from a thin wire in a
vacuum chamber, and irradiated from the bottom with circularly polarized light
tuned to the $D_2$ transition to polarize cesium atoms in the cell. By varying
the light helicity at the resonance frequency of the torsion balance, we
induced forced rotational oscillation of the cell and detected radiation torque
exerted on the cesium vapor cell through the cesium atoms inside. The torque
was particularly large when both hyperfine levels of cesium atoms were
optically pumped with application of a longitudinal magnetic field. Further
detailed study will provide new insights into spin-transfer processes at the
gas-solid interface.
