An algorithm for improved control of trap intensities in holographic optical tweezers

The use of phase only spatial light modulators for holographic optical trapping results in the appearance of ghost orders, creating unwanted traps with uncontrolled intensity and causing variations in the intensity of the desired traps. By introducing dummy areas in the diffraction plane during the hologram optimization, the intensity in the ghost orders can be significantly reduced. By directing a variable fraction of the light to the dummy area, the optical power in the traps can be controlled independently and kept constant also while moving traps to different arrangements. We present and evaluate an algorithm for hologram generation which utilizes dummy areas and allows arbitrary spot positioning in three dimensions. The method enables the use of holographic optical trapping for applications requiring precise control of the intensity in traps, such as optical force measurement.