What are optical tweezers?

Part of: Optical Tweezers

Optical tweezers are based on the principle of light carrying momentum proportional to its energy and propagation direction. Stable optical trapping of dielectric particles (electrical insulators) occurs upon the interaction between light and the object itself. A laser beam that passes through an object and refracts alters its momentum by bending and changing direction.

What can they do?

Optical tweezers are based on the principle of light carrying momentum proportional to its energy and propagation direction. Stable optical trapping of dielectric particles (electrical insulators) occurs upon the interaction between light and the object itself. A laser beam that passes through an object and refracts alters its momentum by bending and changing direction.

Newton’s third law of motion tells us that:

“When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.”

So, for the for the system to conserve total momentum the object undergoes an equal and opposite momentum change, leading to a reaction force acting on the object. Figure 1 illustrates the transfer of light (photon) momentum once the beam travels through a bead.

In a typical optical tweezers configuration, the incoming light originates from a focused laser beam through a microscope objective and focuses on a spot in the sample. The spot creates a trap able to hold a small dielectric object at place. The total forces experienced by the object, or bead in most experimental settings, consist of a scattering force and a gradient force8. The scattering force arises when an incident light beam is scattered by the surface of the bead.

This scattering produces a net momentum transfer from the light photons to the object and causes the bead to be pushed towards the beam propagation. The gradient force is a result of the intensity profile of the laser beam which acts as an attractive force drawing the bead towards the region of greater light intensity. In the case of a highly focused laser beam with a Gaussian intensity profile, the latter force operates like a restoring force that pulls the object into the center of the focal plane. Figure 2 illustrates how the gradient force restores an off-centered bead towards the center of the focal plane, effectively trapping the object in all dimensions.

How does trapping a bead work?

According to Newton’s third law, the object undergoes an equal and opposite momentum change, a reaction force, for the system to conserve the total momentum. The figure below illustrates the transfer of light momentum occurring when a light beam travels through a bead. In a typical optical tweezers configuration, the incoming light originates from a focused laser beam through a microscope objective and focuses on a spot in the sample. The spot subsequently creates a trap able to hold a small object in place.

Case study: Abducting cows

Could we create optical tweezers large enough to actualize sciencefiction scenarios? For example, would it be possible for UFOs to abduct cows with a laser beam for alien research programs? Or would we use lasers to move an entire spacecraft to dock it or prevent a space collision?

In theory; yes.
In practise; no.

We use tweezers to trap and move small objects, even smaller than a red blood cell. To trap and move a large enough object, such as a cow, we would need incredible amounts of laser power to muster appreciable forces.

A related problem arises from the laser light’s heating properties on an object. Cow-trapping would require so much energy that the poor animal would boil, roast, and then vaporize in the process.

The total forces on the small object (or bead) consist of a scattering force and
gradient force. The scattering force arises when a light beam is scattered by the surface of the object. This scattering produces a net momentum transfer from the light photons to the object and causes the bead to be pushed towards the beam propagation. The gradient force results from the intensity profile of the laser beam which acts as an attractive force, drawing the bead towards the region with greater light intensity. In the case of a focused laser beam with a Gaussian intensity profile (a normal distribution), the gradient force pulls the object into the center of the focal plane.

The reason the object stays in the center of the beam is because of the sum of the forces acting upon it. In the center, rays of light refract or scatter through the object the same way on both sides of the vertical plane, which cancels forces from moving the object sideways. If the object drifts to one side, it returns to the center. Think of a spring that accelerates back to the center when displaced from its equilibrium position. The figure above illustrates how the gradient force restores an off-centered bead towards the center of the focal plane, effectively trapping the object in all dimensions.