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Optical Tweezers (wikipedia.org)
104 points by keshab on Oct 2, 2018 | hide | past | favorite | 29 comments

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.

Reading that these tweezer beams are focused with a microscope objective raises the question of how much energy is in one of these beams.

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?

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.

I also work with tweezers.

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.

Would it be possible to build your own, e.g. something like the one shown here: https://www.youtube.com/watch?time_continue=104&v=Sq7GaO8iqu...?

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.

And you’ll hear the popping noise when the tissue is vaporized.

Has someone you personally know ever suffered a laser injury?

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.

this is tractor beam yes?

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)



It is worth noting for those who don't know yet, the findings of Optical Tweezers has made Arthur Ashkin a laureate of the 2018 Physics Nobel prize, on par with Donna Strickland and Gérard Mourou for their work on generating high-intensity, ultra-short optical pulses.

As an undergrad I worked in a lab where they built an iPad app to control optical tweezers [1, 2]. It's a cool example of the possibilities of this technique - allowing you to manipulate nanoscopic particles suspended in fluid.

[1] https://www.youtube.com/watch?v=qgDMcP9e5G0

[2] http://iopscience.iop.org/article/10.1088/2040-8978/13/4/044...

Pardon my language yet that is so fucking cool. Do you know how the app interfaced with the microscope and optical tweezers? I just love the idea of a simple api that controls this crazy complicated lab equipment.

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

Perhaps you're thinking of Snake, built by Mark Setrakian? One of the most iconic battlebots ever. I could only find one video of him using the skeuomorphic controller [1], and a picture of him holding it [2]

[1] https://www.youtube.com/watch?v=NVyMzo-D97A

[2] http://www.teamdelta.com/ax/markwwac.jpg

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.

I was surprised by this explanation describing how the beam is focused:

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?

Note that usually you are also using the microscope to observe what you are trapping, so it' super convenient (but not easy), to use the same objective that the one you observe to send a laser through (in the opposite direction) and trap your sample. If you send your laser-beam quasi parallel it will focus and trap particula quasi-where your are observing.

(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...)

Usually optical traps use 1064 nm NdYAG lasers, which is in the infrared. Most objectives aren't designed for infrared light, so the objectives used for optical trapping are usually customized.

This isn't the first Nobel for optical tweezers. Steve Chu used them in atom trapping experiments at Bell Labs, although the award is for the atom trap results, not for the tweezers themselves.

Chu's prize was for laser cooling, which uses a magneto-optical trap. Magneto-optical traps confine atoms by exploiting the Zeeman effect, a spatially-varying magnetic field, and a laser with a precisely-controlled wavelength tuned to be slightly red of an electronic transition. Optical traps, on the other hand, rely on the refraction of a much larger, but still microscopic, object.

Some of Chu's work uses an optical trap, but that work did not earn his prize.

Thanks for that clarification.

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