Optical Tweezers are scientific instruments that use a highly focused laser beam to provide an attractive or repulsive force (typically on the order of pN), depending on the refractive index mismatch to physically hold and move microscopic dielectric objects.
Photonic force microscopy (PFM) is an optical-tweezers-based microscopy technique. A small dielectric particle (20 nm to several µm) is held by a strongly focused laser beam. The forward scattered light, i.e. the light whose orientation is slightly changed while passing through the particle, and unscattered light are collected by a lens and projected onto a Quadrant Photodiode (QPD). These two components interfere in the detector and produce signals, which permit the detection of the bead's position in three dimensions. The precision is very good (as low as 1 nm) and the recording speed is very high (up to 1 MHz).