Optical stretcher

The Optical Stretcher is a dual-beam optical trap that is used for trapping and deforming ('stretching') micrometer-sized soft matter particles, such as biological cells in suspension.The forces used for trapping and deforming objects arise from photon momentum transfer on the surface of the objects, making the Optical Stretcher - unlike atomic force microscopy or micropipette aspiration - a tool for contact-free rheology measurements. The Optical Stretcher is a dual-beam optical trap that is used for trapping and deforming ('stretching') micrometer-sized soft matter particles, such as biological cells in suspension.The forces used for trapping and deforming objects arise from photon momentum transfer on the surface of the objects, making the Optical Stretcher - unlike atomic force microscopy or micropipette aspiration - a tool for contact-free rheology measurements. The trapping of micrometer-sized particles by two laser beams was first demonstrated by Arthur Ashkin in 1970,before he developed the single-beam trap now known as optical tweezers.An advantage of the single-beam design is that no two laser beams need to be exactly adjusted to make their optical axes match.From the late 1980s on, optical tweezers have been used to trap and hold biological dielectrica, such as cells or viruses. However, in order to ensure trap stability, the single beam must be highly focused, with the particle trapped close to the focus point.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. The optical stretcher, developed at the end of the 1990s by Jochen Guck and Josef A. Käs,circumvents this problem by going back to the dual-beam design originally developed by Ashkin.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 laser powers used in stretching cells are typically on the order of 1 W, generating stretch forces on the order of 100 pN.The resulting relative cellular deformation then usually lies in the range from 1% - 10%.