Optical stretcher

Preventing damage done to biological material (see Opticution) by the high local light intensities in the focus limits the laser powers that one can use in the optical tweezers to a force range too low for rheology experiments, i.e. optical tweezers are suitable for trapping biological particles, but unsuitable for deforming them.

This allows for weakly divergent laser, thus preventing damage done by localized light intensities and increasing the possible stretching forces to a range that is sufficient for the deformation of soft matter.

The optical stretcher has since been developed into a versatile biophysical tool used by many groups worldwide for contact-free, marker-free measurements of whole-cell rheology.

[5] The authors claim that the 'optical deformability' can be used as a biomechanical marker to distinguish cancerous from healthy cells, and even that higher stages of malignancy can be detected.

These surface forces due to photon momentum change are the origin for the ability of the optical stretcher to trap and stretch objects.

However, the refractive index of biological matter is always higher than that of water or cell medium due to the additional protein content.

Because their gradient forces point in the same direction, pulling particles towards their common beam axis, they add up, and one arrives at a stable trap position.

This leads to the known fact that electric dipoles (or dielectric, polarizable media like cells) are pulled to the region of highest field intensities, i.e. to the center of the beam.

Different mathematical models have been developed to calculate the stretching forces, based on ray optics[7][8] or the solution of Maxwell's equations.