Optical Tweezers

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)

http://physics.nyu.edu/grierlab/conveyor7c/

http://physics.nyu.edu/grierlab/photobessel14c/

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.

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.

I remember as a child watching an episode of some version of battlebots, where a competitor had an articulated grabbing arm on top of his robot. The arm had, I believe, two articulating joints and then an articulating grabber/jaw at the end. To control the arm he had made a small 2d model of it and could simply move that around on a clipboard like backing, and it would map those movements to the servos in the arm. I was blown away by how intuitive and simple of an interface he had made for this contraption that I imagined would work with a complicated series of levers similar to those used by backhoe operators. I credit this in no small way with inspiring my ever-persistent need to automate as much as possible in my adult life.

EDIT: If anyone has any idea which episode of what show this was, I would be very excited to watch it again

The paper I linked describes it a bit more, but it's unfortunately behind a paywall. I'll summarise it here.

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.

Thanks for that clarification.