Optical Macro-Tweezers:

With increasing particle size increases, stable 3D trapping in a single-beam trap requires scaling up the optical power which soon represents a problem for biological samples in the laser focus. Moreover, the restricted field of view of standard optical tweezers is dictated by the use of high NA objectives, which is particularly unfavorable for catching actively moving specimens.

Both problems can be overcome by traps with counter-propagating beams - our optical 'macro-tweezers' are especially designed to trap highly motile organisms: it is a mirrortrap with an axially-displaced double focus. Axial trapping is achieved by balancing the scattering forces in a region between the foci and transverse trapping by field gradients.

The low numerical apperture (NA = 0.1-0.2) of the system gives rise to a large field of view, a large manipulation volume of 4 × 4 × 2 mm3 and the option to insert a prism for side-view imaging.

Fig. 1 Basic setup of the optical "macro-tweezers".

Movie (340KB)

Fig. 2 Catching Euglena gracilis protists.

We have successfully trapped large (50-100 µm) objects, including actively swimming organisms with swimming speed around 200 µm/s. The optical forces typically are 200 pN .

Movie (373KB)

Movie (864KB)
Fig. 3 Trapping of absorbing specimens (iodine-stained starch): impossible in the focus of a single-beam trap (left), but feasible in a double-focus mirror-trap, the "macro-tweezers" (right)

Adverse bio-effects are kept low since trapping occurs outside high intensity regions, e.g., focal spots.

Movie (426KB)

Movie (3.7MB)
Fig. 4 A tool for taxonomy: turning the trapped specimen by slight misalignments for a view of all sides (left) and selective harvesting e.g. for single-specimen PCR (right).

We expect our approach to open various possibilities in the contact-free handling of 50-100 µm sized objects that could hitherto not be envisaged, for instance all-optical holding of individual micro-organisms for taxonomic identification, selective collecting or tagging.


M. Pitzek, R. Steiger, G. Thalhammer, S. Bernet, and M. Ritsch-Marte: Optical mirror trap with a large field of view, Opt. Express 17, 19414-19423 (2009).

G. Thalhammer, R. Steiger, S. Bernet, and M. Ritsch-Marte: Optical macro-tweezers: trapping of highly motile micro-organisms, J. Opt. 13, 044024 (6p) (2011).
[Selected for Nature Research Highlights, Nature, Vol. 471, p. 551, March 2011; selected for Faculty 1000]