Our lab uses optical tweezers to unzip DNA and in the process of unzipping, you must turn off the laser beam in order to position it over a bead that you intend to pull on. Yes, you can manipulate the position of things with lasers. And yes, it's like a tractor beam albeit a tractor beam that's only useful in the microscopic scale. To turn "off" the laser beam, you need to block the beam such that it does not reach the sample plane in the microscope.
In this Instructable, I will discuss my version of a laser shutter. This design came about because a fellow lab mate was trying to make a laser shutter with a speaker. I was intrigued by this idea (which is very clever) and decided to make my own shutter using a hard drive as the actuator and not a speaker.
After making the shutter, I researched to see if other people had tried this before. Much to my elation, others had. They even made the circuits from scratch and if you can get the paper, it's a nice short read.
Maguire, L. P., S. Szilagyi, and R. E. Scholten. 2004. High performance laser shutter using a hard disk drive voice-coil actuator . Review of Scientific Instruments 75, no. 9: 3077. doi:10.1063/1.1786331.
I had the power of the Arduino platform when I designed this shutter so, I didn't have to build a circuit from scratch. With this design, anyone can make one with very little electronics know how and very cheaply.