An optical tweezer is a tool used to trap and manipulate particles using light. The name “optical tweezers” was coined by Arthur Ashkin in 1970 when he demonstrated that laser light could be used to trap small particles. However, the principles behind optical trapping were first described by Lord Rayleigh in 1871. Optical tweezers are now commonly used in research laboratories to study a wide variety of topics ranging from single-molecule biophysics to microfluidics.
How do optical tweezers work?
Optical tweezers work by using the gradient force of a laser beam to trap and manipulate small particles. The gradient force is created by the difference in intensity of the laser beam across the particle. The intensity of the laser beam decreases as it moves away from the center of the beam, creating a force that pulls particles towards the center of the beam (Figure 1).
When a particle enters an optical tweezer, it experiences two forces:
The scattering force, which pushes the particle towards the center of the laser beam, and
The gradient force, which pulls the particle towards regions of higher intensity.
These two forces combine to create a potential well around the particle, trapping it in place (Figure 2).
The strength of an optical trap is determined by several factors including:
The wavelength of light used: shorter wavelengths result in stronger traps;
The power of the laser: more powerful lasers result in stronger traps; The size and refractive index of the particle being trapped: larger or more refractive particles require stronger traps; and The shape and size ofthe laser beam: Gaussian beams result in stronger traps than flat-top beams.
Once a particle has been trapped, it can be moved by movingthe positionof eitherthe entirelaserbeamor justthe focusofthebeamwithintherangeofafewmicrometers. This movement canbequitepreciseand allowsfortheimplementationof complexexperimentalprotocols.
In additiontoallowingforthemovementofparticlesindividuallyorinensembles,opticaltweezerstechiqueshavebeenusedtocontrolandmanipulateafluid’smicroscopicstructuresand toprovideaforcesupporthydrogelsandsurfacesbeneathwhichcellularprocessescanoccur5–7]. Alloftheseapplicationsmakeuseofthefactthatlightcanexertahighlylocalizedforceonasmallscale8].