A single-beam gradient force trap or an optical laser tweezer has been a powerful tool to accurately measure pico-newton to femto-newton forces in a variety of fields, such as biology, colloid/polymer physics, and rheology. When an appropriate refractive index mismatch between an object and a medium is provided, the laser beam can hold the object of which its size ranges from a few nanometers to tens of micrometer scale.
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