Generation of active motion from optically trapped upconverting nanoparticles

Abstract

Optical trapping of janus particles has turned out to be complicated due to lack of control on the direction of orientation. Here we use an alternative strategy, where we optically trap a NaYF₄:Yb,Er upconverting nanoparticle on the pump wavelength at 975 nm and show that there is much greater visible emission in the backscatter direction than the forward scattered direction, leading to greater heating in the backscatter region of the nanoparticle. This then generates a temperature gradient across the nanoparticle to push it in the axial direction. The Mean Square Displacement (MSD) bears signature of the Hot Brownian Motion (HBM) when trapped at 975 nm, which becomes regular diffusive when the trapping wavelength is changed to non-pump wavelength 1064 nm. The effective velocity of the particle while trapped in the tweezers can be directly estimated. Thus, this is the first time that an active janus-like particle has been optically trapped in tweezers.