Dexterous holographic trapping of dark-seeking particles with Zernike holograms

Optical traps projected with computer-generated holograms can be optimized for compatibility with the physical properties of individual target particles. Dynamic optimization is especially desirable for manipulating dark-seeking particles that are repelled by conventional optical tweezers, and even more so when dark-seeking particles coexist in the same system as light-seeking particles. We address the need for dexterous manipulation of dark-seeking particles by introducing a class of “dark” traps created from the superposition of two focused Gaussian modes. The constituent modes have different waist diameters and opposite phases and therefore create a dark central core that is completely surrounded by light. These difference-of-Gaussian (DoG) traps confine dark-seeking particles rigidly in three dimensions and can be combined with conventional optical tweezers and other types of traps for use in heterogeneous samples. The ideal hologram for a DoG trap being purely real-valued, we introduce a method based on the Zernike phase-contrast principle to project real-valued holograms with the phase-only diffractive optical elements used in standard holographic optical trapping systems.