  We have theoretically investigated optical tweezing of gain-functionalized
microspheres using a highly focused single beam in the nonparaxial regime. We
employ the Mie-Debye theory of optical tweezers to calculate the optical force
acting on homogeneous and core-shell Mie microspheres with gain. We demonstrate
that the optical gain plays a crucial role in optical manipulation, especially
to optimize the restoring force and thus allowing for trapping of large
refractive index and plasmonic particles. Indeed we demonstrate that one can
trap such particles, which is usually not possible in the case of passive
media, by functionalizing them with an optical gain material. We show that by
varying the value of the gain, which can be realized by changing the pump
power, one can not only achieve trapping but also manipulate the equilibrium
position of the tweezed particle. Altogether our findings open new venues for
gain-assisted optomechanics, where gain functionalized systems could facilitate
optical trapping and manipulation of plasmonic nanoparticles in particular,
with potential applications in self-assembling of nanoparticle suspensions and
on a chip.
