Optical trapping

Optical forces are a light-matter interaction phenomena originated by a momentum exchange between a light beam and a sample. When the light beam passes through a sample undergoes a global momentum change which is transferred to the sample resulting in a net force in the range of pN.

If a high numerical apperture light beam is used over a microscopic particle (from tenths of nm to tenths of um) with a refraction index greater than the surrounding medium,  the resulting force field tends to move the particle towards an equilibrium position near the light focus.

This kind of optical trap is usually called Optical Tweezer, which can be easily integrated with a conventional microscope allowing micro-manipulation of the sample in the microscope stage (view Setup description)

Thanks to digital holography it is possible to sculpt the light beam in order to get arbitrary light patterns at the sample plane. This is achieved by introducing a Spatial Light Modulator (SLM), in the beam optical path (see detailed information about Spatial Light Modulators). Using this technique, the light spot can be moved with just a mouse click, multiple traps can be created from a single beam, and arbitrary light distributions can be obtained.

For any desired trap pattern, the corresponding hologram must be computed and sent to the SLM (optically conjugated with the back focal plane of the objective lens). The calculation of the holograms can be done by means of different algorithms.