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.
Our strongest trap is powered by 1.5 W 1064 nm laser beam. (For comparison, laser pointers are usually under 0.005 W.) Unlike most other light sources, laser power measures the output power rather than electrical input power.
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.
...
> The beam will instantly ignite paper, and will cause damage to...
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?
> Curious, it appears that you guys use a highly focused beam on small particles, like 1 W focused into a diffraction limited spot, right?
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.
Yes, but high-power lasers need to be treated with respect. Reflections are capable of instantly and irreversibly blinding. There's an overused joke in labs that work with them: "do not look into laser with remaining eye".
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.
Spent alot of time working with low to medium power lasers in the entertainment industry, and I'm happy to say that was also a joke in that industry as well.
Powerful lasers can be scary, especially if they're in the infrared. The first warning you receive that it's reflecting into your eye is blood seeping into the vitreous humor from your retina's capillaries.
I met one person who took a hit from a pulsed laser (the -other- Nobel from today). Pulsed lasers can be even worse. Fortunately, it was in his peripheral vision. But he described hearing and feeling the pop.
My undergraduate advisor had a small hole in his retina due to a high-power laser. The injury would have been far worse if it weren't a visible laser, since his blink reflex greatly reduced the damage.
Kind of? It's the only way that I know of for a laser to move a particle towards the laser's source. But it has some severe limitations: the force is extremely weak, on the order of 0.1 pN/(nm W); the particles have to refract, rather than absorb or scatter, the light; particles larger than 10 microns in diameter tend to be melted or vaporized since they can't dissipate the energy quickly enough.
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).
Depends who you ask... I saw David Grier (http://physics.nyu.edu/grierlab/) give a talk in which he said that this kind of optical tweezers doesn't really count as a a tractor beam. He went on to define the characteristics of a tractor beam and how he realized such a device in a laboratory setting.
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)
