Abstract
Upconverting nanoparticles typically absorb low frequency radiation and emit at higher frequencies relying upon multiphoton processes. One such type of particle is NaYF$_4$:Yb,Er, which absorbs at 975 nm while emitting in visible radiation. Such particles have routinely been optically trapped. However, we find that trapping at the absorption maximum induces non-equilibrium features to the system. When we ascertain the Mean Square Displacement (MSD) of the axial motion, we find features that resemble Hot Brownian Motion (HBM) in active particles. We characterize the HBM observed here and find that the effective translational velocity of the system is 36 nm/sec, small enough to be compensated by the optical tweezers. Thus, we have a system which is optically confined and stationary but in non-equilibrium, which we can also use to study non-equilibrium fluctuations.
Highlights
The study of non-equilibrium system thermodynamics and active matter as emerged as a topic of intense interest in the recent decades [1,2,3], necessiated by the need to study the physics of microscopic systems where athermal fluctuations pay a crucial role [4, 5] and of living matter which is inherently far from equilibrium [6, 7]
We find that the Mean Square Displacement (MSD) of the axial motion of the particle bears a signature that deviates from the effective temperature picture and instead follows Hot Brownian Motion (HBM)
We describe a new kind of active motion where the particle is trapped in optical tweezers while exhibiting what we believe is HBM, simultaneously
Summary
The study of non-equilibrium system thermodynamics and active matter as emerged as a topic of intense interest in the recent decades [1,2,3], necessiated by the need to study the physics of microscopic systems where athermal fluctuations pay a crucial role [4, 5] and of living matter which is inherently far from equilibrium [6, 7]. Such study is important keeping in mind that biological systems are active in nature. In this context a key question is to use thermodynamic parameters to explain active systems [8,9,10]. One of the first active systems used an active Brownian particle moving at a constant speed in a medium with a direction determined by rotational diffusion [11]. Boltzmann distribution was used to model the position of an active swimmer while in motion, with the effective temperature being higher than the ambient temperature. We find that the MSD of the axial motion of the particle bears a signature that deviates from the effective temperature picture and instead follows HBM. There have been reports of estimation of instantaneous ballistic velocity [18] of such UCNP but active motion never reported
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