  Microengines have shown promise for a variety of applications in
nanotechnology, microfluidics and nanomedicine, including targeted drug
delivery, microscale pumping, and environmental remediation. However, achieving
precise control over their dynamics remains a significant challenge. In this
study, we introduce a microengine that exploits both optical and thermal
effects to achieve a high degree of controllability. We find that in the
presence of a strongly focused light beam, a gold-silica Janus particle becomes
confined at the stationary point where the optical and thermal forces balance.
By using circularly polarized light, we can transfer angular momentum to the
particle breaking the symmetry between the two forces and resulting in a
tangential force that drives directed orbital motion. We can simultaneously
control the velocity and direction of rotation of the particle changing the
ellipticity of the incoming light beam, while tuning the radius of the orbit
with laser power. Our experimental results are validated using a geometrical
optics phenomenological model that considers the optical force, the absorption
of optical power, and the resulting heating of the particle. The demonstrated
enhanced flexibility in the control of microengines opens up new possibilities
for their utilization in a wide range of applications, encompassing microscale
transport, sensing, and actuation.

Optics;Soft Condensed Matter