Optical Tweezing

The typical colloidal length scale is well above the limitations of optical imaging and their time scales can be easily captured with readily available cameras, even a phone camera is sufficient to capture a colloid’s random walk. Here, in the Dullens Lab we can tune the properties of our colloidal particles such that we can control and manipulate both individual and groups of colloids with infrared lasers (optical tweezers) whilst combining this with simultaneous optical imaging.

Optical tweezing is a technique where the light’s momentum is exploited to trap and manipulate micron sized objects. Since a single optical trap acts as Hookean spring, we can measure the displacement of a trapped object, and calibrate the force exerted by the optical trap. This system now can act as a force measurement device, where unknown forces, such as the migration of a grain boundary, can be studied.

Reconfigurable diffractive optical elements are used in our lab to create and control single and multiple optical traps. Holographic Optical Tweezers use a Spatial Light Modulator (SLM), to dynamically reshape the laser beam. This can steer, defocus or split the laser beam to produce multiple optical traps in arbitrary 3D positions. Another way we control our optical landscapes is with an Acoustic-Optic Deflectors, these devices can reposition a single optical trap significantly faster than the colloids equilibrium time, effectively generating multiple optical traps that, to the colloid(s), appear to be static.

Optical Trapping Simulation


Trap Strength

The simplest optical trapping experiement consists of a highly focused Gaussian laser beam, and a single colloid suspended in a solvent. For simplicity we can assume our colloid is one micron in diamter and its interaction with the laser is well described by Ray Optics with an acting force described by Hokes Law. We are neglecting interia as our colloid is in the overdamped regime.

So the equation of motion reduces from the full Langevin equation to dx = ζ(x) - κ(x).

In the simulation you can play with the temperature (ζ(x)) and the trap stiffness (κ(x)) to see how they effect the colloids motion.