When a nanorod of typically d=100's nm diameter and h=1-3 micrometers length trapped in the optical tweezers, its orientation is along the trapping beam axis for h/d > 2 and is normal to beam axis for h/d < 2. We report the preliminary experimental observation that some anisotropic single crystal nanorod was stably trapped at a tiled angle to the beam axis. We explain the observation with the T-matrix calculation. In the anisotropic media, as the divergence of is non zero, the conventional vector spherical wave functions (VSWFs) do not individually satisfy the anisotropic vector wave equation. Some new bases, such as the modified VSWFS and qVSWF, have been proposed. Notice that the anisotropic nanorod is floating in the aquatic isotropic medium, we make the VSWF expansions of the incident and scattered fields in terms of, and the VSWF expansion of internal field in the anisotropic nanorod in terms of. Both expansions are therefore legitimate. The boundary condition was chosen as for the normal components of. The internal field is represented as a sum of a set of compoment VSWF expansions to gave better description with more expansion coefficients and to help the convergence of the T-matrix solver. Our calculation showed that when the optical axes of an anisotropic nanorod are not aligned to the nanorod axis, the nanorod may be trapped at a tilted angle position where the lateral torque is zero and its derivative is negative.
Paul Brule-Bareil and Yunlong Sheng, "Tilted optical trapping of anisotropic single crystal nanorod (Presentation Recording)," Proc. SPIE 9548, Optical Trapping and Optical Micromanipulation XII, 95481Y (Presented at SPIE Nanoscience + Engineering: August 13, 2015; Published: 5 October 2015); https://doi.org/10.1117/12.2188458.4519370486001.
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