Light exerts radiation pressure, but under normal conditions the force is too weak to notice. Ashkin focused on the high radiation pressure and intensity gradient produced by lasers and designed a method that draws microparticles toward the beam center, trapping them in three dimensions. When the beam is tightly focused by a lens, a particle is confined at the point of highest intensity and can be held stably even in water. Because nothing physically touches the sample, heat and chemical damage are minimal, letting researchers study living bacteria or red blood cells directly. Optical tweezers have become indispensable for measuring DNA elasticity, tracking molecular motors, and many other nano-bio experiments. The technique is now moving toward practical medical uses such as sorting malaria-infected blood cells, demonstrating its large societal impact.

An optical tweezer confines a particle in a potential well created by the balance between the transverse gradient force—proportional to the spatial intensity gradient—and the axial scattering force. Typical trap stiffness is on the order of tens to hundreds of fN nm⁻¹, allowing extraction of mechanical parameters such as Young’s modulus and membrane tension by analyzing Brownian motion. Using infrared light minimizes absorption-induced heating, enabling real-time observation of cell division, endocytosis, and other dynamic processes. By adding multiple beams or spatial light modulators, holographic traps can generate thousands of independent tweezers for high-throughput manipulation inside microfluidic channels. Beyond biophysics, optical trapping plays a role in quantum technologies, including ultracold atom confinement and the site-selective control of Bose–Einstein condensates.

Optical confinement stems from the induced dipole interaction with the electromagnetic field gradient; in the Rayleigh regime (particle radius < λ) the pseudo-potential is given by U = −αE² / 2. High-resolution position detection employs interferometric back-focal-plane methods or quadrant photodiodes to suppress momentum dissipation while achieving nanometer accuracy. The apodized PSF of a high-NA objective enhances trap stiffness, and recent metasurface lenses further mitigate chromatic aberration. With piconewton-scale force resolution, researchers have dissected hand-over-hand mechanisms of kinesin and dynein and characterized DNA polymerase pausing kinetics. Hybrid setups that combine force spectroscopy and fluorescence spectroscopy now reveal the coupling between chemical and mechanical reaction coordinates. In the quantum regime, sideband cooling of optically levitated nanobeads in photonic-crystal cavities has reached μK center-of-mass temperatures, pushing force sensitivity toward the yoctonewton limit and opening avenues in precision measurements.