
Optical Tweezers - keshab
https://en.wikipedia.org/wiki/Optical_tweezers
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arctux
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.

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dmead
this is tractor beam yes?

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arctux
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).

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MartianSquirrel
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.

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sleavey
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](https://www.youtube.com/watch?v=qgDMcP9e5G0)

[2]
[http://iopscience.iop.org/article/10.1088/2040-8978/13/4/044...](http://iopscience.iop.org/article/10.1088/2040-8978/13/4/044002/meta)

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matthberg
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.

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scrumbledober
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

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chimere
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](https://www.youtube.com/watch?v=NVyMzo-D97A)

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

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apo
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?

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carreau
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...](http://matthiasphd.herokuapp.com/html/parts/part1.html#optical-
tweezer), picture of trapped beed as viewd through microscope:
[http://matthiasphd.herokuapp.com/html/parts/part3.html#exper...](http://matthiasphd.herokuapp.com/html/parts/part3.html#experimental-
observations))

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dekhn
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.

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arctux
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.

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dekhn
Thanks for that clarification.

