Page "Optical tweezers" Paragraph 40

Expanding the beam emitted from the laser to fill the aperture of the objective will result in a tighter, diffraction-limited spot.

While lateral translation of the trap relative to the sample can be accomplished by translation of the microscope slide, most tweezer setups have additional optics designed to translate the beam to give an extra degree of translational freedom.

This can be done by translating the first of the two lenses labelled as " Beam Steering " in the figure.

For example, translation of that lens in the lateral plane will result in a laterally deflected beam from what is drawn in the figure.

If the distance between the beam steering lenses and the objective are chosen properly, this will correspond to a similar deflection before entering the objective and a resulting lateral translation in the sample plane.

The position of the beam waist, that is the focus of the optical trap, can be adjusted by an axial displacement of the initial lens.

Such an axial displacement causes the beam to diverge or converge slightly, the end result of which is an axially displaced position of the beam waist in the sample chamber.