  In this paper, we present the design and implementation of multi-trap optical
tweezers based on new quadrifocal kinoform lenses. The phase distribution of
these diffractive lenses is characterized by the Silver Mean sequence. The
focusing properties of the resulting aperiodic DOEs coined Kinoform Silver Mean
Lenses (KSMLs) are numerically examined. It is shown that, under monochromatic
illumination, a KSML drives most of the incoming light into four single foci
whose focal lengths are related to the Silver ratio. In this way, a KSML
improves the diffraction efficiency of binary Fresnel Silver Mean Zone Plates.
Through experimental results, the simultaneous trapping of particles in the
four focal planes and their three-dimensional manipulation is demonstrated.
