Trapping and correcting,

Issued: Sun, 15 Jan 2012 13:30:00 GMT

"Spatial light modulators (SLMs) are often used to create multiple optical traps in holographic optical tweezers. In such systems, aberration correction is important for maximizing performance. Richard Bowman and co-workers in the UK have now demonstrated an SLM-based Shack–Hartmann wavefront sensor that can estimate and correct for aberrations in optical tweezers without altering the experimental set-up. The SLM projects an array of spots onto the sample plane, with each spot being focused onto a particular point in the Shack–Hartmann array. Spot displacements are used to estimate the tilt of each region on the SLM and hence give a phase map through Zernike polynomial fitting. Comparing this with the reference phase map allows the wavefront to be corrected. The researchers say that aberrations of up to ten wavelengths can be recovered, and that the system is sensitive to aberrations much smaller than one wavelength. The approach provides a fourfold improvement in optical trap stiffness. Because the spot displacement is relatively large, the spot pattern can be used to align the system by eye."

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