Diffractive optical tweezers in the Fresnel regime

We demonstrate a flexible setup for holographic steering of laser tweezers in microscopy using a high resolution spatial light modulator (SLM). In contrast to other methods, hologram read-out is done in the off-axis Fresnel regime rather than in the typically used on-axis Fourier regime. The diffractive structure is calculated as a Fresnel hologram, such that after reflection at the SLM only the desired first diffraction order is guided to the input of an optical microscope, where it generates a tailored optical tweezers field. We demonstrate some advantageous features of this setup, i.e. undesired diffraction orders are suppressed, the optical traps can be easily steered in real-time by just mouse-dragging a hologram window at the SLM display, and a number of independently steerable optical traps can be generated simultaneously in a three-dimensional arrangement by displaying a corresponding number of adjacent hologram windows at the SLM screen. (C) 2004 Optical Society of America.