Optical tweezers (OT) have been revealed as a versatile tool that enable the manipulation of microand nano-objects at the single particle level [1]. It also offers the high spatio-temporal resolution for
the detection of the dynamics of the trapped particle by exploiting interferometry-based detection
assays, not to mention the application of sub-Pico newton forces simultaneously. In particular, optical
trapping of single upconverting particles (UCPs) has recently attracted great interest as it constitutes
a new window for a wide range of applications such as cellular imaging, sensing of temperature and
chemicals, solar cell development and luminescent probes in a medium [2]. Since an upconversion
particle absorbs near infrared light predominantly and emits almost all possible spectral lines in visible
range through multiphoton processes, it can heat up the environment and mimic an ‘active’ particle
by continuous dissipation of energy into its surroundings [3]. This active behaviour calls up for the
studies on non-equilibrium stationary states of an active microparticle experimentally.
In our work, we show the synthesis and characterisations of ferromagnetic upconversion (iron
doped) particles which are optically trappable and exhibit high upconversion efficiencies at the same
time. It allows us to magnetically manipulate the particles in an optical trap by applying large range
of forces, varying from femto newtons to nano newtons. These particles fall under the category of
multifunctional micro particles by finding their applications being opto-magnetic probes to the
environment, photothermal/photodynamic agents, contactless micro thermometers as well as large
scale magnetic actuators.
We also show the opto-plasmonic, thermophoretic flow assisted confinement of upconversion
particles of different geometries and sizes. This trapping scheme also enables us to manipulate living
cells and amoeboid spores. When a conductive thin film such as gold is being irradiated by laser, it
delivers heat energy to the surroundings as the resonant condition for the surface plasmons is
achieved. The transfer of heat flux is continuous at its boundary and it induces the convective flows
in water where the particles are immersed. Such a point of irradiation is referred to as a hotspot and
two such hotspot sufficiently confines the particle at an equilibrium position along the line joining
them [4].
In addition to this, we exploit the active behaviour of upconversion particle upon illumination by
975 nm to create a microscopic Stirling cycle. The active upconversion particle instantly gets
converted to a normal Brownian particle when it is trapped with any other wavelength, e.g., 1064
nm. The controlled illumination and trapping of a single upconversion particle using both these
lasers enables us to generate all the four phases of a Stirling engine.
References:
[1] Neuman, K.C.; Block, S.M. Optical trapping. Rev. Sci. Instrum. (2004) 75, 2787–2809.
[2] Rodriguez-Sevilla et al. Optical trapping for biosensing: Materials and applications. J. Mater.
Chem. B (2017) 5, 9085–9101.
[3] Kumar, S. et al. Trapped in out-of-equilibrium stationary state: Hot Brownian motion in optically
trapped upconverting nanoparticles. Front. Phys. 8, 429 (2020)
[4] Nalupurackal. G, et al. "A hydro-thermophoretic trap for microparticles near a gold-coated
substrate." Soft matter (2022) 18, 6825-6835.