  The realization of microscopic heat engines has gained a surge of research
interest in statistical physics, soft matter, and biological physics. A typical
microscopic heat engine employs a colloidal particle trapped in a confining
potential, which is modulated in time to mimic the cycle operations. Here, we
use a lanthanide-doped upconverting particle (UCP) suspended in a passive
aqueous bath, which is highly absorptive at 975 nm and converts NIR photons to
visible, as the working substance of the engine. When a single UCP is optically
trapped with a 975 nm laser, it behaves like an active particle by executing
motion subjected to an asymmetric temperature profile along the direction of
propagation of the laser. The strong absorption of 975 nm light by the particle
introduces a temperature gradient and results in significant thermophoretic
diffusion along the temperature gradient. However, the activity of the particle
vanishes when the trapping wavelength is switched to 1064 nm. We carefully
regulate the wavelength-dependent activity of the particle to engineer all four
cycles of a Stirling engine by using a combination of 1064 nm and 975 nm
wavelengths. Since the motion of the particle is stochastic, the work done on
the particle due to the stiffness modulation per cycle is random. We provide
statistical estimation for this work averaged over 5 cycles which can be
extended towards several cycles to make a Stirling engine. Our experiment
proposes a robust set-up to systematically harness temperature which is a
crucial factor behind building microscopic engines.
