Wednesday, May 6, 2009

My Research Explained: The Optical Trap

So if you recall, last time I spoke about some properties of light and electricity. This will become crucial to the understanding of how an optical trap works. Good thing, that is what I am going to explain this time.

In order to have a trap, one needs a laser and something to focus the laser. You also need something to trap... like a dielectric particle. Let's call that a microsphere (micro- because it is small on the order of micrometers and -sphere because it is a sphere). Ok so lets put all that together, and we have a trap. But how does it work?

Remember when I said light is a particle? The photons will collide with our particle. Because each photon has momentum, it will transfer its momentum to our microsphere. Think of it like a ball bearing hitting a hockey puck. The ball bearing is really small, so there won't be much transfer of momentum, but if there is a lot of them there will be some pushing. That is what happens here. There are a lot of light particles that collide with the microsphere and they slowly push the sphere. In direct laser light nothing will happen, but if you focus the laser light that is where the interesting stuff happens.

Remember how I discussed there is a part of light that is electrical? Well, this is important because we are trying to trap a dielectric particle (the microsphere). Because our laser is focused, we are creating an electrical gradient. Basically the electric field is increasing as you get closer the center of the focus. The microsphere is attracted toward the center because of this electrical gradient. So now we have a second force acting on our particle. The first is due to the photons pushing the sphere, and the second is due to the sphere wanting to be at the focus. It just so happens that the forces balance very near the center, but a little beyond it.

Here's another perspective. The trap works just like a spring. When you pull on a spring it wants to return to it's original shape. The sphere is the same. If we pull on the sphere and move it from the center it wants to go back to where it is perfectly balanced. Take a look at the image below:

I've modeled the energy of a spring. Basically what it is saying is that when you displace (x) the spring the energy vastly increases. Things in nature always like to be at low energy states (we see light because atoms get excited and then they emit a photon returning them to their lowest energy level). So when you pull on a spring you are removing it from it's lowest energy state and it will return there as soon as you let go (the bottom of the parabola in the figure).

You can do all sorts of cool things with an optical trap. We use it to manipulate and apply forces to microscopic objects. Other people can trap single atoms in it (instead of a dielectric sphere they use atoms) and measure energy properties or whatever they do (it's obvious I don't know anything about that stuff). Some people use traps to play Tetris:

So now we have a particle sitting in a trap. How do we go from that to having tweezers? Well that answer will come next time when I discuss the Optical Tweezer Setup.

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