Optical Tweezers
A biophysical instrument using a strongly focused laser beam to trap, manipulate, and exert piconewton forces on microscopic dielectric particles or biological molecules.
Governing formula
F_gradient = ½ α ∇(E²), Hooke's Law F = -k_trap x
SI unit
pN (Piconewton Forces ~0.1–100 pN)
In depth
Pioneered by Arthur Ashkin (Nobel Prize 2018). Refraction of laser light through a microscopic bead transfers photon momentum, creating a gradient force pulling the bead toward the laser focus waist.
Examples in the real world
Measuring single-molecule mechanical stall forces of RNA polymerase during DNA transcription.