Appl. Opt. 51, 5522–5526 (2012)

When using an optical trap, knowledge of how the radial optical trapping potential varies with distance can be useful in, for instance, studying the extension of a single DNA with a dual optical trap system or a colloidal particle pulled by a moving trap. Previous systems require either a variable flux control or an additional beam to determine this variation, but Ignacio Martínez and Dmitri Petrov from ICFO and ICREA in Spain have developed a simple method that can be used in existing optical trapping systems with minimal changes. Essentially the scheme makes use of an acousto-optical deflector, a device that can be used to generate multiple traps and to tune electronically the trap position and its intensity. The researchers created multiple traps by rapidly switching, on a timescale typically of the order of tens of milliseconds, a single optical beam between a number of optical focuses, by changing the frequency of acoustic waves propagating in the acousto-optical deflector. This time-sharing of the deflector permits the mapping of the trapping potential over a greater range of probe displacements through the independent control of the positions and stiffnesses of two optical traps. The team applied the scheme to particles 1 and 2 μm in diameter with a displacement range of ±4 μm, and the results were consistent with previous studies.