title: Constant power optical tweezers with controllable torque.
abstract: We describe a means for controlling the spin angular-momentum flux of a laser beam at constant power, without introducing any elliptical or linear polarization. This allows a controllable torque, acting to spin the particle uniformly, to be exerted on a birefringent particle in optical tweezers. The constant power means that transverse and axial trapping, and heating due to absorption, are unaffected by changing the torque. The torque can be computer controlled and rapidly changed. In addition, the lateral trapping is kept constant. Very low torques can be obtained such that rotational Brownian motion of birefringent particles can be observed. This has the potential to greatly extend the quantitative applications of the rotation of birefringent objects in optical tweezers.
authors.affiliation: Centre for Biophotonics and Laser Science, School of Physical Sciences, The University of Queensland, Brisbane, Queensland, Australia. funk@physics.uq.edu.au
authors.forename: Maren
authors.initials: M
authors.lastname: Funk
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Parkin
authors.forename: Alexander B
authors.initials: AB
authors.lastname: Stilgoe
authors.forename: Timo A
authors.initials: TA
authors.lastname: Nieminen
authors.forename: Norman R
authors.initials: NR
authors.lastname: Heckenberg
authors.forename: Halina
authors.initials: H
authors.lastname: Rubinsztein-Dunlop