  We study the force of light on a two-level atom near an ultrathin optical
fiber using the mode function method and the Green tensor technique. We show
that the total force consists of the driving-field force, the
spontaneous-emission recoil force, and the fiber-induced van der Waals
potential force. Due to the existence of a nonzero axial component of the field
in a guided mode, the Rabi frequency and, hence, the magnitude of the force of
the guided driving field may depend on the propagation direction. When the
atomic dipole rotates in the meridional plane, the spontaneous-emission recoil
force may arise as a result of the asymmetric spontaneous emission with respect
to opposite propagation directions. The van der Waals potential for the atom in
the ground state is off-resonant and opposite to the off-resonant part of the
van der Waals potential for the atom in the excited state. Unlike the potential
for the ground state, the potential for the excited state may oscillate
depending on the distance from the atom to the fiber surface.
