Observation of a single-beam gradient force optical trap for dielectric particles
A. Ashkin, J. M. Dziedzic, J. E. Bjorkholm, and Steven Chu
TL;DR Summary
Learn how to build a trap from scratch, starting with fundamental laser alignment, Gaussian beam opticals, and the balance of gradient and scattering forces.
How Levitated Optomechanics Works
Before we build the full experimental table, we need to understand the core mechanism: Optical Tweezers.
When a laser beam is tightly focused by a high numerical aperture lens, it creates an intense electromagnetic gradient. A dielectric particle, like a silica nanosphere, experiences a restoring force that pulls it toward the region of highest light intensity—the exact focal point of the laser.
However, the photons are also traveling forward, striking the particle and transferring momentum. This is the scattering force, which pushes the particle slightly downstream from the geometric focus. Try dragging the particle out of the trap in the simulation below.
Optical Trap Simulation
Drag the silica nanoparticle with your mouse. Notice how the gradient force pulls it toward the narrowest part of the beam, while the scattering force pushes it slightly forward. Turn down the laser power to watch it escape!
Notice what happens when you reduce the laser power. The trap stiffness decreases, and the particle can no longer overcome the ambient thermal energy (simulated here as random drifts when power is low) and falls out of the trap.