Rotational dynamics of optically trapped nanofibers

Abstract

The optical trapping of polymeric nanofibers and the characterization of the rotational dynamics are reported. A strategy to apply a torque to a polymer nanofiber, by tilting the trapped fibers using a symmetrical linear polarized Gaussian beam is demonstrated. Rotation frequencies up to 10 Hz are measured, depending on the trapping power, the fiber length and the tilt angle. A comparison of the experimental rotation frequencies in the different trapping configurations with calculations based on optical trapping and rotation of linear nanostructures through a T-Matrix formalism, accurately reproduce the measured data, providing a comprehensive description of the trapping and rotation dynamics.PACS numbers: 87.80.Cc; 45.20.da; 42.50.Wk; 07.10.Pz * Corresponding author: antonio.neves@unisalento.it † marago@its.me.cnr.it 2 Optical forces are currently employed to study a range of chemical, physical and biological problems, by trapping microscale objects and measuring sub pico-Newton forces [1,2]. In particular, optical trapping of elongated nanoparticles, including nanowires [3] and nanotubes [4], is gaining an increasing interest because of the high shape anisotropy and unique physical properties of these systems. Among linear nanostructures, polymeric nanofibers are novel nanomaterials with many strategic applications ranging from scaffolding for tissue-engineering to integrated photonics [5,6] and electronics [7,8]. However, optical trapping and manipulation of polymer nanofibers has never been reported, despite the understanding of the optical forces and torques acting on these objects as well as their trapping dynamics might open a new range of applications, exploiting the polymeric fibers as local probes or active elements in microrheology [9] and microfluidics [10], and in next generation Photonic Force Microscopy [3]. Furthermore, the nanofibers are characterized by subwavelength diameters and lengths in the range 10-100 m, therefore constituting ideal systems for studying effects occurring in the intermediate regime between the Rayleigh scattering and geometrical optics.A laser beam can carry intrinsic (spin) or extrinsic (orbital) angular momentum, associated to the polarization and to the light beam phase structure, respectively [11]. Either trapping beams with elliptical polarization or with a rotating linear polarization can be exploited to apply a torque to trapped objects. Rotation in trapped particles can also be induced by exploiting the phase structure or the astigmatism of the trapping laser beam [12 and references therein]. The rotatable object can be spherical, exhibiting a birefringence or a slight absorption, or it can have more complex shapes, as in microfabricated propellers by two-photon polymerization [13] or cylinders with inclined faces [14].In this work we trap polymeric nanofibers and characterize their rotational dynamics in different trapping configurations by a different method. We employ a strategy to rotate a dielectric cylinder with flat end faces, based on a non-rotat...

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