I'm a grad student in a lab that specializes in using optical tweezers for single-molecule biophysics research. I'm out sick today with nothing better to do, so feel free to ask me anything.
For example, what would happen if the tip of the beam made contact with a piece of paper, steel, etc.?
Also, what precautions, if any, need to be taken to get air out of the system you use these in? If vacuum, how much?
The beam will instantly ignite paper, and will cause damage to the most common type of laboratory laser beam blocks, which are made out of stainless steel.
We don't take the air out the system. Air is basically completely transparent in the visible and infrared. We sometimes flood a portion of the instrument with helium, since its index of refraction is ten-fold closer to 1 than air's. This causes the beam to fluctuate less due to air movement.
> specializes in using optical tweezers for single-molecule biophysics research.Curious, it appears that you guys use a highly focused beam on small particles, like 1 W focused into a diffraction limited spot, right?
How come the bio-molecules that you're manipulating with this light don't just "burn up"? Is it because they're mostly transparent at that wavelength? Or you're only exciting a marker molecule that is stuck to them?
That's correct.
> How come the bio-molecules that you're manipulating with this light don't just "burn up"? Is it because they're mostly transparent at that wavelength? Or you're only exciting a marker molecule that is stuck to them?
We trap and manipulate a micron-sized polystyrene sphere that the proteins are attached to, not the proteins themselves. The microspheres don't burn up because they don't strongly absorb (i.e., they're mostly transparent) at the laser's frequency. Paper, on the other hand, absorbs strongly and ignites.
Optical traps rely on the momentum of light and the fact that a microsphere displaced from the diffraction-limited spot refracts light in direction of the displacement. Since momentum is conserved, the light directed away from the center of the trap creates a force that pushes the sphere toward the trap. If the light were mostly absorbed or scattered, this force would be along the axis of the beam.
Back in my postdoc, we got a sample of purified melanosomes to work with. Melanosomes are small vesicles that hold pigment in your skin. Well, when we tried to trap them, we found that they absorbed the trap light, and turned it into heat. Enough to boil the sample.
We couldn’t do the experiment, obviously, but we did have a fun day playing Death Star, shooting every melanosome we could find.
If you're going to attempt to build one, please get a pair of laser glasses that are rated for the wavelength you are using.
I want to emphasize how incredibly weak these traps are. In order to apply a maximum force of 200 pN orthogonally to the path of the laser to a 1 micron polystyrene sphere requires approximately 1 W of laser power. Further, it takes about five times more laser power to apply the same amount of force axially (i.e., along the path of the beam) compared to transversely (i.e., in the plane orthogonal to the beam).
He does use optical trapping forces, but is able to create exotic traps such as vortexes with the help of spatial light modulators. My understanding of the physics is not very good but here are some links to his papers (the second one has a cool visualization of a solenoidal tractor beam)