Amazing Laser Application 4: Optical Tweezers!

What’s the application? Optical tweezers use focused light beams to trap small particles in the focus of the beam, and drag them around by moving the beam.

What problem(s) is it the solution to? 1) “How do we move these tiny little things around without touching them?” 2) “How do we measure the forces exerted by biological molecules?” 3) “How do we tie knots in DNA strands?”

How does it work?The basic optical tweezer scheme uses a single beam of light focused down to a very small spot. If you take some small (mostly) transparent object and place it in the beam, it will feel a force pulling it right to the center of the focus. To understand the origin of the force, look at this figure I swiped from a group at Stanford:

The curved reddish lines at top and bottom represent the outer extent of the laser beam, and the bell curve on its side represents the intensity distribution across the beam. The light is strongest toward the center of the beam, and drops off to either side. The highest intensity point is right in the center of the beam, where it is at its narrowest point, and that’s where the particle will be trapped.

the particle to be trapped is represented as a little glass sphere, mostly because physicists love to approximate things as spheres, but also because tiny glass spheres are standard test particles for tweezer systems. Any other object– a living cell, say– will affect the light in similar ways, so the sphere is a good test system.

When the sphere is in the beam, it acts like a little lens, bending the rays of light that enter the sphere, shown as arrows in the diagram. Light enters the sphere along one trajectory, and leaves at a different angle, that depends on the exact angles involved, and the index of refraction of the trapped particle.

We know from our discussion of laser cooling that light has momentum. When the light beams bend, then, they are changing the direction of the light’s momentum. A change in the direction of momentum is caused by a force, and in order for the light’s momentum to change, there must be a corresponding change in the momentum of the sphere– that is, a force in the opposite direction of the “force” exerted on the light.

If the sphere sits in a uniform light field, the upward force from light rays near the top getting bent downward is exactly balanced out by a downward force from light rays near the bottom getting bent upward, and nothing happens. If the intensity of the light varies significantly over the size of the sphere, though, the two forces won’t be equal. A sphere that is slightly off-center in the focused beam will have more light entering on one side than the other (indicated by the different weights of the arrows), leading to a larger force pulling the sphere toward that side. This pulls a particle toward the center of the tweezer beam. A similar argument shows that there is a force pulling the sphere toward the focus point (in the left-right direction in the figure).

If you can make a tight enough focus, then, you will get a force that sucks small objects toward the center of the focus. It’s not a very big force, but then, we’re talking about small objects, so it doesn’t need to be. The focused beam can then be used to move these objects around to, for example, spell the name of your institution just by moving the focus from one place to another. This is sort of similar to the use of tweezers to manipulate objects too small to move with your fingers, hence “optical tweezers.”

Why are lasers essential? You need a bright, narrow, tightly focused beam of light for this to work, and there’s no better source of that than a laser. If you choose your laser wavelength carefully, you can arrange it so that they don’t interact with biological tissues in a harmful way, and even use this technique to move around living cells, or pieces thereof.

Why is it cool? Dude, you can manipulate microscopic objects with light! This has all sorts of uses in biophysics, from studying the forces exerted by the molecules that move things around in cells, to studying the binding strengths of various biochemical receptors, to stretching and manipulating single molecules. Steve Chu’s group famously tied a knot in a DNA strand and watched it unwind itself.

This has been used to study the ways living cells interact with each other, and to push organelles around inside living cells, and to transport things through cell walls. It gives you an incredible ability to mess around with microscopic creatures.

Why isn’t it cool enough? It only works for dielectric substances, and requires extremely tight focusing, so there are serious constraints on what sorts of things you can manipulate. And biophysics is all soft and squishy, not like real physics…

(This post is the fourth of twelve highlighting amazing laser applications, in honor of the 50th anniversary of the first laser. These posts serve as a lead-up to an audience poll asking what the coolest laser application is, so if you like lasers and radio buttons, watch this space over the next week or so.)