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?
> 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)
EDIT: If anyone has any idea which episode of what show this was, I would be very excited to watch it again
They used a camera connected to a LabVIEW interface to view the trapped particles, but they control it using a laser and a spatial light modulator (SLM), which is able to imprint a spatial pattern onto the laser beam, which is ultimately what creates the optical traps. The paper says "The trap coordinates [on the iPad] are synchronized over the wireless network with a desktop computer, which controls the SLM using our freely available LabVIEW software" (square brackets mine), i.e. they control the traps using a separate computer interfaced using LabVIEW to the SLM. The iPad is used to show the video stream and allow the user to set and manipulate the position of the traps. The camera is also connected via LabVIEW: "Using the JPEG compression available in the National Instruments Vision library, we can stream around ten frames per second from the control PC to the iPad over a wireless network (limited by available CPU power on the iPad). Up to 11 optical traps can be simultaneously dragged around, and they can be created and removed with a double tap on the screen. Double tapping with multiple fingers creates multiple optical traps simultaneously, which is very useful when trapping non-spherical objects."
I guess a more up-to-date iPad would manage more traps. The one they used (circa 2011) was probably 1st generation.
Optical tweezers are capable of manipulating nanometer and micron-sized dielectric particles by exerting extremely small forces via a highly focused laser beam. The beam is typically focused by sending it through a microscope objective. ...
I imagined something more complex, although what exactly I'm not sure.
Are there any other ways of doing it? Are off-the-shelf microscope lenses used routinely, or are custom lenses the norm?
(source: did my PhD with optical traps: http://matthiasphd.herokuapp.com/html/parts/part1.html#optic..., picture of trapped beed as viewd through microscope: http://matthiasphd.herokuapp.com/html/parts/part3.html#exper...)
Some of Chu's work uses an optical trap, but that work did not earn his prize.