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Optical tweezers are instruments that use a tightly focused laser beam to hold and move microscopic objects, typically micron-sized dielectric beads, without touching them. The trap exerts forces in the piconewton range and resolves displacements in nanometres, which is why labs use it to stretch single molecules and manipulate single cells. This page is general educational information about the technique, not instrument-purchasing or experimental advice.
Short answer
An optical tweezer focuses a laser through a microscope objective. Near the focus, light pushes and pulls on a small transparent particle so that it settles at the point of highest light intensity and is held there in three dimensions. Researchers attach such beads to molecules or cells and use the trap to apply and measure tiny forces.
Definition and scope
Arthur Ashkin shared half of the 2018 Nobel Prize in Physics “for the optical tweezers and their application to biological systems” (Physics Today). His group observed in 1969 that a sphere was pulled toward the high-intensity centre of a beam, reported the first stable three-dimensional optical trap in 1970, and published the tweezer design in 1986. The technique belongs to the field of biophysics, and it rests on the physics of light described in optics and photonics.
Traps can hold particles ranging from micron-sized spheres down to silica particles of about 25 nm (Physics Today). Variants include split beams and holographic multi-trap arrangements that hold several particles at once.
How optical tweezers work in practice
According to the manufacturer documentation from Lumicks, a laser is focused by a microscope objective to form a trap at a spot in the sample. Light carries momentum, and when it is refracted by a bead that momentum is transferred to the bead. Two effects result:
- Scattering force: pushes the bead along the direction of the beam.
- Gradient force: pulls the bead toward the region of highest intensity, which for a Gaussian beam is the focal centre.
A single-beam trap works near the focus, where these two forces balance (Physics Today). If the bead drifts away from the centre, a restoring force pulls it back, and that force behaves like a spring. The trap therefore doubles as a force sensor: forces are in the piconewton range and displacements in nanometres (Lumicks).
A typical system has a laser source, an objective, the sample, and dielectric beads coated so that they bind DNA, RNA, proteins or filaments (Lumicks).
Why beads act as handles
Individual macromolecules are much smaller than the wavelength of light, so they cannot be trapped directly. Researchers instead attach them to micron-sized beads that serve as handles (Physics Today). In the two-bead arrangement described by Lumicks, a single molecule is held between two trapped beads, stretched, and the force is measured.
What it has been used for, by target scale
| Target | Documented use | Source |
|---|---|---|
| Whole bacteria and yeast | Ashkin trapped and moved bacteria and yeast | Physics Today |
| Organelles and chromosomes inside cells | Berns moved a chromosome in a cell and developed laser microsurgery | Physics Today |
| Filaments and motor proteins | In 1994 Finer and Spudich stretched an actin filament with two traps to study myosin; Block’s lab measured kinesin steps | Physics Today |
| Enzymes acting on nucleic acids | Block’s lab measured RNA polymerase steps of 3.4 angstroms | Physics Today |
| DNA-binding proteins and protein mechanics | Organisation, replication, transcription and repair; energy landscapes and elasticity of proteins | Lumicks |
| Viruses, bacteria, single cells | Trapping and holding | Lumicks |
Where it matters for labs
The two sources point to a short planning checklist for any group considering or using a trap:
- Wavelength: a 515 nm green argon laser killed bacteria in early experiments, an effect nicknamed “opticution”. Switching to a 1064 nm infrared laser, to which most biological material is transparent, solved the problem (Physics Today).
- Handle chemistry: molecular targets need beads coated to bind the specific DNA, RNA, protein or filament under study (Lumicks).
- Optics: the trap is formed by a microscope objective, so the objective is part of the instrument rather than an add-on (Lumicks). Related optics background: oil immersion lenses.
- Imaging context: trapping is a force and manipulation method; for comparison with an imaging method, see light-sheet microscopy.
Common misunderstandings
- It is not a mechanical grip. The bead is held by light momentum balancing scattering and gradient forces, and it behaves like a bead on a spring.
- It is not only for molecules. The documented uses run from nanometre-scale steps up to whole bacteria, yeast and chromosomes.
- The forces are small. They are measured in piconewtons, matched to molecular and cellular mechanics.
FAQ
What are optical tweezers used for?
They hold and move microscopic objects and measure tiny forces, for example motor-protein steps, DNA-binding protein behaviour, protein elasticity, and the manipulation of bacteria, yeast and single cells (Lumicks; Physics Today).
Who invented optical tweezers?
Arthur Ashkin, who shared half of the 2018 Nobel Prize in Physics for the optical tweezers and their application to biological systems (Physics Today).
Why do biologists use infrared lasers in them?
Most biological material is transparent to 1064 nm infrared light, which avoided the photodamage seen with a 515 nm green laser (Physics Today).
What size of object can a trap hold?
Micron-sized spheres down to silica particles of about 25 nm; smaller macromolecules are attached to beads (Physics Today).
Sources
- Lumicks, “Optical tweezers” documentation, lumicks.com/documentation/optical-tweezers, read 2026-11-08.
- Physics Today, “Father of optical trapping awarded a share of the Nobel Prize in Physics”, physicstoday.aip.org, read 2026-11-08.
