We present spectroscopic and dynamical studies of upconversion nanoparticles (UCNP). The sample preparation and characterization were performed as described previously. Single particle nonlinear optical upconversion measurements were performed by integrated optical microscopy and spectroscopy as well as optical tweezers. The mean-square displacement from single UCNP exhibited a time-dependent diffusion coefficient which was compared with Brownian dynamics simulations of a viscoelastic model of harmonically bound spheres. Experimental time-dependent two-dimensional trajectories of individual UCNP revealed correlated two-dimensional nanoparticle motion. The measurements were compared with stochastic trajectories calculated in the presence of a non-conservative rotational force field. We used a simple model of particle shape to investigate how particle asymmetry affects particle-surface interaction, orientation, and stochastic dynamics over a planar surface. With this geometric model, we construct potential energy curves as a function of particle orientation relative to the surface, and identify the potential energy minimum for particles of various shapes ranging from symmetric to asymmetric. The calculated difference between particle centroid position and potential energy minimum location is used to define an offset, which is useful for comparison with experimental particle trajectories. For asymmetric particles the potential energy minimum location is decoupled from the center of the particle long-axis. Based on these observations, we construct a Brownian motion model of a rigid rotor with one end fixed to the planar surface. In the case of asymmetric particles, the resulting stochastic trajectories exhibit passive circular Brownian motion, thus providing one possible microscopic mechanism for the stochastic dynamics near a surface. The results are extended to molecular systems in solution by performing time-resolved ultrafast optical Kerr effect spectroscopy studies of simple liquids and comparing the results with the above mentioned model. We conclude with a comparative nonlinear optical study of UCNPs with CW and femtosecond laser excitation.

Stochastic dynamics of molecular systems and upconversion materials from experiments and simulations

2019 Fall National Meeting

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