Optical Tweezers: Trapping Particles with Light

Radiation pressure and the gradient force

Light carries momentum: a photon of frequency f carries p = hf/c, and absorbing or scattering it transfers force to a particle — a scattering force that pushes along the beam's propagation direction (F = P/c, so 100 mW gives ≈333 pN). Ashkin's insight in 1986 was that a tightly focused Gaussian beam also exerts a restoring gradient force pulling a dielectric particle toward the region of highest intensity from all directions:

F_grad = (2π·n_m·r³/c) · [(m²−1)/(m²+2)] · ∇I     (Rayleigh regime, r ≪ λ)
m = n_particle / n_medium     ∇I = intensity gradient toward focus

When the gradient force exceeds the axial scattering force — which requires a high numerical-aperture objective (NA > 1.0) to steepen the intensity gradient — stable 3D trapping results with no physical contact at all.

live demo · a focused beam trapping a dielectric bead● LIVE

Rayleigh vs Mie, and measuring piconewtons

In the Rayleigh regime (r ≪ λ — nanoparticles, quantum dots), the particle behaves as a point dipole and the gradient-force formula applies directly. In the Mie regime (r ≫ λ — beads and cells), geometric ray optics works instead: each refracted ray's momentum change contributes to a net restoring force. Most real experiments use 0.5–5 μm beads that sit in between, requiring full electromagnetic (T-matrix) calculations. Near-infrared wavelengths (800–1064 nm) minimise photodamage, letting living bacteria and cells be trapped safely.

An optical trap behaves as a linear spring, F = κ·Δx, with trap stiffness κ typically 0.01–1 pN/nm, calibrated via the equipartition theorem (κ = k_BT/⟨x²⟩), the power spectrum's Lorentzian corner frequency, or a known Stokes drag. This lets researchers measure forces from ~0.1 pN up to ~200 pN — enough to reveal kinesin's 8 nm stepping (~5–7 pN per step, discovered with optical tweezers in 1993), stretch DNA through its B-to-S transition at ~65 pN, and probe cell membrane tension. Ashkin shared the 2018 Nobel Prize in Physics for the invention.

Frequently asked questions

How do optical tweezers trap a particle without touching it?

A tightly focused laser beam creates a steep intensity gradient at the focal point. A dielectric particle experiences a gradient force proportional to that intensity gradient, pulling it toward the highest-intensity region. When this force balances the scattering force pushing the particle along the beam axis, the result is a stable three-dimensional trap with no physical contact.

What is the difference between the Rayleigh and Mie trapping regimes?

In the Rayleigh regime (particle radius r ≪ wavelength λ), the particle acts as a point dipole and the gradient force formula applies directly — used for nanoparticles and small proteins. In the Mie regime (r ≫ λ), particles are large enough to treat with geometric ray optics, where each refracted ray's momentum change contributes to the trapping force — used for beads and cells.

Who invented optical tweezers and what applications did they enable?

Arthur Ashkin demonstrated the single-beam optical trap in 1986 and received the 2018 Nobel Prize in Physics for it. Optical tweezers now measure the stepping forces of molecular motors like kinesin (~5–7 pN), stretch individual DNA molecules, and probe cell mechanics — all at piconewton force resolution.

Try it live

Everything above runs in your browser — open Optical Tweezers and drag the laser focus to watch a bead follow, jittering from Brownian kicks proportional to temperature. Nothing is installed, nothing is uploaded.

▶ Open Optical Tweezers simulation