Seeking to control free rotations of a microsphere in a laser trap, we have created a "Maxwell's demon" that identifies and captures a preferred "up-or-down" polarity of the microsphere. Breaking rotational symmetry, we attach a single "Raleigh-size" nanoparticle to a micron-size sphere, which establishes a "nanodirector" defining microsphere orientations in a trap. With radius <10% of the NIR trapping wavelength (1.064 μm), a polystyrene nanoparticle appended to a ∼1.3 μm glass sphere adds negligibly to scattering of the trapping beam and imperceptibly to forces trapping a doublet probe. Yet, constrained to a large orbit (∼1.5 μm diameter), the weak Raleigh dipole force induced in the nanoparticle imparts significant pole-attracting torques to the probe. At the same time, Brownian-thermal excitations contribute torque fluctuations to the probe randomizing orientations. Thus, we have combined demon control and Boltzmann thermodynamics to examine the intense competition between photonic torques aligning the nanodirector to the optical axis and the entropy confinement opposing alignment when equilibrated over long times for an order of magnitude span in laser powers. To reveal orientation, we developed novel multistep pattern-processing software to expose and enhance weak-diffuse visible light scattered from the nanoparticle. Processing a continuous stream of doublet images offline at ∼700 fps, the final step is to super resolve the transverse XY origin of the scattering pattern relative to the synchronous probe center, albeit limited to "up" state segments because of intensity. Transforming the dense histograms (∼104-105) of radial positions to polar angle (θ) distributions, we plot the results on a natural log scale versus sin(θ) to quantify the photonic potentials aligning the nanodirector to the optical axis. Then guided by principles of canonical thermodynamics, we invoke self-consistent methodology to reveal photonic potentials in the "down" state.
Vaclav Beranek, Igor R. Kuznetsov, and Evan A. Evans, "Using a Maxwell's demon to orient a microsphere in a laser trap and initiate thermodynamic assays of photonic nanofields," Proc. SPIE 9922, Optical Trapping and Optical Micromanipulation XIII, 992203 (Presented at SPIE Nanoscience + Engineering: August 28, 2016; Published: 16 September 2016); https://doi.org/10.1117/12.2239377.
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