Particle transport in asymmetric scanning-line optical tweezers

We describe a scanning-line optical tweezing technique with an asymmetric beam profile in the back focal plane of the microscope objective. The motion of a trapped particle along the scan line is studied as a function of beam asymmetry, and it is shown that this technique can be used to exert a constant lateral force on the particle, realizing purely optical constant-force tweezing. The observed effect is attributed in a geometric optics model to a non-zero lateral component of the scattering force.