In this installment, we will discuss how we can use a laser to trap beads in a microscope slide and move them around. First things first, if we want to know how to move something around, we need to know the forces on it. You might be wondering how we can use light to create a force and that is a perfectly legitimate thing to wonder. If you shine a flashlight on something, it isn’t going to be pushed around. We are not wrong to think of light as exerting no force on objects in our day to day life because all of the things we deal with are relatively big and heavy. That means they are hard to push around. However, light does exert very very small forces on objects. They are just so small that they do not effect us in any noticeable way. However, when we are dealing with tiny things under a microscope, that is a different story.
Optical trapping is easiest to understand in two dimensions. For the sake of this discussion, we will restrict the trapping to the plane of the microscope slide. If I just lost you with what I said, basically we are going to move things up/down and left/right in the image that we see through the microscope; we will not worry about moving things into or out of focus. Trapping does work in three dimensions, and maybe I will make a special post about it later if there is interest, but it is more complicated to understand.
So, let’s get started! First, we start out with a bead that is suspended in a fluid in a microscope slide. That means it is undergoing Brownian Motion.
The sample above is very densely packed, for our purposes, we would want the beads dilute enough so that there were only a few in the image at any one time. Also, we use beads that are all the same size, whereas the beads above vary in size greatly. Now that we have a bead, let’s hit it with a laser and look at what happens.
So, to give the above diagram some context, the laser beam is coming down from the top of the image. When the light beams hit the bead, they are bent due to refraction. This is the same phenomena that makes a prism split light into different colors, when light hits a different material, it bends slightly. Then, as the light leaves the bead it is refracted once more, sending it off at a more extreme angle. What causes light to have force is this shifting the direction it is traveling in. Let’s just look at the beam on the right for example. When the beam is shifted to point slightly to the left, that is a change in the light’s momentum. You can equivalently think about it as a ball hitting a wall:
There is going to be a force on that wall. If we say that the wall can only move in the left/right direction, then the force will be to the right, and the wall will move that way due to the ball hitting it. This is how a fan or a propeller works except the slanted wall is moving rather than the smaller air or water molecules. So, going back to our bead, when it shifts the light beam, it feels a force to the right. And, when the light ray leaves the bead, it feels another force to the right. However, when the bead is centered in the laser beam, there is just as much force to the left from the other half of the laser. That is why the total force is zero in the picture with our bead above. For every force to the right, there is an equal force to the left; add them all up and there is no net force.
But, lets say that the bead is not centered in the beam. What would our diagram look like then? Let’s say that the beam on the right is much stronger than the beam on the left. This is equivalent to moving the bead to the left from the center of the laser beam.
The thicker lines in the above image means that there is more light in that part of the laser beam. So, looking at the picture, it is clear to see that more light is shifted to the left than to the right. This means that there is a force on the bead pushing it to the right. But remember, to get more light on the right side of the bead, we had to move it to the left. So, to summarize, move a bead to the left, it gets pushed to the right. The opposite is true as well, move a bead to the right and it gets pushed to the left. So, the bead is always pushed back to the center of the laser. Thus, a trap is born.
Once we have trapped a bead in the laser beam, when we move the laser, the bead wants to follow. In this way, we can manipulate things under the microscope to suit our needs. Not only that, by quickly switching the laser position between multiple beads, we can trap more than one quite precisely. This is best shown through a video rather than a picture though, so here is a pretty cool demonstration of trapping 14 beads that are 1 micron in diameter.
Because this system is so good at manipulating small things, it is called an optical tweezer. Just like we use tweezers to pick up small things and move them, this system can do it on a microscopic scale.
There is a lot more to optical trapping and I would love to go into more detail about the setup with the microscope or with the three dimensional trapping if there is interest. Fair warning though, it can get complicated. So, stay tuned for my next research post. It will probably be about using optical tweezers on flagella and some of the interesting things that happen.
I couldn’t figure out a way to bring this up in my post, but I have to include it. Here is a video of the game Tetris being played with an optical tweezer setup.