Tuesday, June 30, 2009

My Research Explained: The Optical Tweezer

I haven't blogged this topic in a while so please forgive me, but let's refresh. A laser through a microscope gives you an optical trap. Done. How do you make tweezers? That my friends require ingenuity.

It doesn't really need that, but I wanted to put you in suspense. Are you ready to be oo-ed and ahh-ed? Good. The key component of the tweezer is movement. If you can't move the trap then it is no good for my experiments. In this first image, I have setup a basic trap using just mirrors and the microscope. The laser is a green 532nm laser so I colored the path green.

Here I have a mock-up of a steerable trap. How is it steerable? Well if you notice, there is a 1:1 telescope in there. The beam is imaged throughout this system using lenses. Each time the beam is reimaged, we call that a conjugate plane. The first lens of the 1:1 telescope is the conjugate plane of the laser (likewise the objective of the microscope is another conjugate plane). Moving this lens, amazingly, does not move the beam path on any conjugate planes after this point, but it does change the angle of the light. This change in angle allows us to move the trap.

Another key mechanism for movement is through the use of a feedback loop. We collect all the light from the trap on a quadrant photodiode (QPD). When there is something in the trap, like a bead, light gets scattered, reflected, refracted, etc. The QPD detects this and using some fancy equations we can determine how exactly whatever is in the trap is moving. A computer then sends this information to some kind of moving mechanism to move the stage of the microscope (where our sample sits) to react to the movement in the trap.

Our mechanism will be a piezo-electric stage. A piezo is a crystal that changes shape when a voltage is applied to it. The computer, in our case, sends a signal (in Volts luckily enough) to the piezo and the piezo reacts by either growing or shrinking. This growth or shrink is what pushes or pulls the stage, and is very controllable.

Now we can control the trap, but there is one more aspect to the tweezer, taking data? Luckily the QPD does this too. Because it detects how light interacts with a sample in the trap, it can tell us (with more equations) how much force is being applied to the sample. It is through this that we are able to get data about unzipping DNA.

Hopefully I have thoroughly explained how we get information from an optical trap. Next time I will begin the very long explanation of all the other parts of my research, Biology!

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