Dynamic Holographic Optical Tweezers

Trapping Light

Optical tweezers use a tightly focused laser beam to trap microscopic particles — cells, beads, molecules. The radiation pressure gradient near the focus creates a stable 3D potential well that holds the particle in place. Moving the focus moves the trapped object. One beam, one trap.

The challenge: creating multiple independently positioned traps from a single laser. The solution is a spatial light modulator (SLM) — a device that reshapes the wavefront without changing amplitude. The SLM displays a computed phase mask that splits the beam into multiple focal points, each capable of trapping a particle independently.

Computing the Phase Mask

The weighted Gerchberg-Saxton algorithm computes phase masks through iterative Fourier transform cycling: start with desired trap positions, inverse FFT to the SLM plane (extract phase, replace amplitude with uniform beam), forward FFT to focal plane (extract phase, replace amplitude with desired pattern), repeat. The weighting adjusts trap intensities at each iteration to ensure uniform trapping force — without it, outer traps tend to be weaker.

Convergence typically requires 10–50 iterations. The result: a phase pattern that, when displayed on the SLM, creates multiple laser foci at precisely specified positions in 3D space.

The application provides real-time phase mask computation with WebSocket communication for low-latency browser interaction, HTML5 Canvas visualization of phase patterns and simulated focal planes, and 6 Zernike aberration modes for simulating optical system imperfections. Originally developed at CEFOP (Center for Optics and Photonics), Universidad de Concepción.