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December 23, 2021

from the University of Freiburg

We can test the quality and freshness of fruit and vegetables with our fingers, and even industrial robots have been successfully performing tactile applications for years. But how is it possible to grab and rotate objects the width of a human hair? A study on this question was carried out by Prof. Dr. Alexander Rohrbach from the Institute for Microsystem Technology at the University of Freiburg and his team have now been published in the journal Nature Communications. Her work shows how several optical tweezers made of highly focused laser light can one day grip cell clusters in a controlled manner and turn them in any direction. This enables tiny objects such as miniature tumors to be examined more precisely under the microscope.

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In the laboratory, the gripping fingers correspond to so-called optical tweezers, which are generated from highly focused laser light. The particular advantage of lightweight tweezers is that, unlike mechanical tweezers, they can exert forces or torques even when reaching through transparent objects.

Computer holographic optical tweezers that can focus laser light pixel by pixel in arbitrary and multiplied configurations used for years to control the positions of several light fingers simultaneously in 3D space. This method has existed in research laboratories for almost two decades, but is unable to exert forces and torques on larger objects, ie those with a diameter of more than about 1/10 mm. The tweezers encounter difficulties because the objects are too large and sluggish to be rotated in an arbitrary and stable manner in an aqueous solution, because the optical tweezers are either not strong enough or cannot find a good gripping position and therefore slip off. What is remarkable is that they fail to find the best gripping position because they aren’t looking for it at all, but gripping blindly and relying on the ability of the researchers trying to position the optical tweezers they take effect by measuring and analyzing the light scattered on the object, ”explains Rohrbach. “We see different objects with our eyes because sunlight or room light is scattered on them and reproduced on our retina.” Laser tweezers can reach through transparent objects. The biological research objects that scientists examine under the microscope, such as clusters of cells such as miniature tumors or small fly embryos, are not completely transparent, but behave like frosted glass in a bathroom window, where the light is diffuse after transmission and therefore difficult to analyze. The new concept for seeing where the tweezers are reaching consists of analyzing the defocused laser scattered light on a fast camera behind the object, which serves as a feedback signal. The more asymmetrical the light spots of the individual light tweezers on the camera, the more the light is scattered in the focus, which leads to a stronger change in the refractive index at the respective point in the object. The optical tweezers can efficiently grip the object at these points. Physically, a local change in the polarization of the matter leads to an increased optical dipole force.

According to Rohrbach, the amazing thing about the principle of localizing the best gripping position is that the light scattering, i.e. the change in momentum, is much stronger directly in the laser focus than in front of or behind the focus. Each of the approximately five to ten optical tweezers should use the scattered light to feel the best gripping position in order to rotate the object in different directions. However, if one of the tweezers exerts too much force, the other tweezers can lose their hold. “This is a highly complex optimization problem that we will be tinkering with for a few more years,” says Rohrbach. His vision is that, if successful, the principle of contactless specimen holding will be integrated into the microscopes of the future.

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