Talk:Optical tweezers

Regarding To Do List

• Discuss: As the expression "optical tweezers" specifically refers to the classical "single beam gradient trap" type setups whose working principle is different from many of the here listed techniques (dual beam traps, optical cell sorting, optical binding, fiber based traps and indirect approaches), renaming the article to "Optical trapping" seems to be appropriate. Standard optical tweezers themselves should then be listed as the fundamental idea to use a single focussed laser beam to trap dielectric objects in 3D. THis seems in particularly required as optical traps employing more than one laser beam turned out to be the weapon of choice for the handling of biological cells, the probably most important application of optical traps in life sciences.

Furthermore it might make sense to shorten the article by no going too much into technical details. A biologist for instance (as most physicists) having come along the expression 'optical trapping' is not looking for a electrodynamical calculus in the first place. The article should much more serve the goal to communicate the following essence: 0. Beams of light do have mechanical impact on matter. 1. They can be used to hold, move, rotate and stretch microscopic objects. 2. Investigating the movement of these objects with respect to the center of the trap, allows for the measurement of extremely small forces.

In order to archive this, it is probably most convent to see optical traps in action in small .gif animations. Everything going beyond the point of a 5 to 10 minute article should be available by following links to daughter articles. At the moment, however, I do have the feeling that one needs to be an expert already to enjoy the article.

Please post your opinion on these suggestions!

• Discuss: should history section be moved to the end or remain on top?

A progress of the field on optical tweezing has very much increase the cross-fertilisation of ideas of the field in biology and optical physics.

• Example of a landmark single-molecule experiment (e.g. kinesin, RNAP)?

Landmark in both advancement of tweezing technqies and biology related experiments should all be included. A time-line of the experiments would be an idea format.

• Add section on experimental design, construction and operation of optical tweezer setups.

Methods of force and position detection. Calibration of the trap stiffness.

See Justin Molloy online resource [1]

• Discuss details of different kinds of optical tweezer setups (non-basic setups) in an organized fashion.

- single beam gradient force trap - dual beam gradient force trap - counterpropagating trap - novel beam traps - Time shared traps - Holographic optical traps - Calibration of optical trapping

 - QPD
 - Position Detectors
 - Differential detection. 
 - Effects of angular aperture
 - Lateral and axial trap stiffness measurments 

- Optical trapping in Biology

 - Single molecule
 - DNA, RNA
 - Cellular mechanical

- Optical trapping in Optical physics

 - Optical angular momentum

• Add short description of research group expertise for labs indicated at the end of the article. Possibly one-word links to relevant wiki pages?

- Great idea

leenewt

To Do List

I'd like to start working on improving the optical tweezers entry over the following days/weeks/months. In order to help with that, I've decided to create a To Do List. Others are encouraged to participate by adding/deleting/discussing entries on the list in addition to editing the article.

-RockyRaccoon 22:54, 3 September 2006 (UTC)

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Equations

I finished the equations section with some explanation of the steps throughout. Any comments of the layout? I erred on the side of including too many equations so that every step is exlpicitly shown. I figured this would allow a wider range of audiences to follow through the derivation themselves if they desire. I feel it's better that way than to include only the punchline. At first the layout looked messy, but I cleaned it up quite a lot and I'm fairly happy with it now. I think a bit more physical explanation may be needed either before or after the equations, but I'm not sure yet what to put there, if anything. Any ideas are greatly appreciated! -RockyRaccoon 07:41, 9 September 2006 (UTC)

Comments Equations

Hi RR,Really nice job on the optical tweezers site..as i did not have time to do any updates or corrections lately. Anyway, the equation is mainly on Fgradient there should also be Fscattering as well.

-leenewt

Steven Chu Quote in History Section

I was thinking about removing the quote from the Steven Chu interview. It's rather long and I feel it distracts from the main content of the article. There is already a reference to the interview and readers who are interested in this will likely follow the reference to it. But I think its a poor assumption that all readers will be interested. Any comments? -RockyRaccoon 07:37, 3 September 2006 (UTC)

• Okay, I actually removed it. Take a look and if anybody disagrees, let's discuss. I think it's much better without it.

Addition of some Quote in History Section

It looks great as well...However, i think that a quotation by either Ashkin or steven chu should be added to give the article some personal touch instead of being too technical. -leenewt

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Units...

The first sentence in the "Optical tweezers in brief" section doesn't make sense. Specifically, "...from less than one piconewton to greater than a femtonewton." A femtonewton is smaller than a piconewton. I don't know if the two words need to be switched or if the original author had the incorrect units...

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The Next Step in Optical Tweezers.....where are we heading to?

Are we heading anywhere with the physics of optical tweezers? It appears that optical tweezers have a stronghold in the world of microbiologist and atom cooling, but more so as a tool. Most of the fundamental physics behind optical tweezers have been well cover by the early pioneer especially Arthur Ashkin.

One strong question that i am eager to answer is, What else has optical tweezers not been applied on? Which realms of science has optical tweezers yet to touch?....

1) Atmospheric Science where aerosol trapping is quite an immature field?

2) Ocean science where tweezers can be brought to the depth of the ocean to study the single cell micro-organism in the deep sea.

Anymore??!!

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NOTE: Link on second refrence at bottom of page is broken.--134.10.9.127 19:51, 12 January 2006 (UTC)

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I do not have time to edit this page to correct my few gripes, so I'll list things I believe to be issues, and leave it at that.

There are a number of typos, such as "reserach" instead of "research", "ataoms" not "atoms" etc. I suggest someone run this through a spellchecker. Additionally, general grammar issues, for example "...Ashkin proposed that optical micromanipulation can be analysis" (my emphasis)

The section "optical tweezers in brief" is relatively confusing and badly written. It either needs editing to be more clear, with the section of quotation edited so as to be more standardly presented, or completely re-written. Phrases like "overcomes the push of the beam" are not appropriate, in my opinion, as they are unscientific, and not terribly meaningful.

Disagree with the inclusion of certain "Key figures" whilst excluding others. I think it beyond passing strange that Arthur Ashkin, the father of optical trapping, is not included, whilst Dmitri Petrov, a relatively obscure figure is. It is my opinion that exhaustively listing all research who have played a major role in the field is not plausible, and that this section should be removed. Those wishing to read more on the subject are able to find Molloy and Padgett online, or any one of the multiple optical trapping reviews listed on the page.

Additionally, I would suggest that Padgett and Molloy's paper, "Lights, Action: Optical Tweezers" also be listed in the section entitled "Professional Paper".

To the above commenter, who states that the fundamental physics are well established, I would ask for information on trapping particles between the mie and rayleigh regimes. Certainly, Neuman and Block (Rev. sci. Instr. 75, 2787) suggest that the physics in the intermediate case are not well understood, though I will concede that my knowledge of Rohrback and Steltzer's work is not as complete as it might be. Later in the same review paper, the Jarzynski equality, and the consequence that the second law of thermodynamics appears to be subject to short time violations is discussed, with optical traps used as the tools that elucidated this discovery. I would suggest that there was both room for more work in the physics of optical trapping, and that fundamental physics can and will be advanced with the ability to apply pN forces at the nanoscale.

Alec Zorab

reply to Alec: I support the idea of adding "Lights, Action: Optical Tweezers" as profesional paper. This group is doing cutting edge research in the field for a long time. —Preceding unsigned comment added by 82.6.98.39 (talk) 17:00, 9 November 2008 (UTC)

--

Further to my comments above, I now note that according to the edit history of the main page, the links provided for the Key figurees, to which I objected, are meant to be to active research groups. Whilst I concede that this somewhat nullifies my criticism of Ashkins exclusion, I still think it strange that other researchers are not being listed, and would suggest that either someone got ot the effort of finding links to all the research groups currently actively working on optical trapping, or that this section be removed. My previous comments about the ability of the reader to decide which groups interest them, based on references from review papers still stands.

Alec Zorab

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Flawed Explanation

I believe the ray diagram figures are incorrect. The light shouldn't bend in the same direction upon entering and exiting the dielectric sphere. The light should bend upon entering and "bend back" upon exiting. From the final and initial light directions that are drawn on the right hand, the total change in momentum would be in the direction of the drawn force (opposite laser propagation) and thus the induced force on the bead would be in the direction of propagation. For the figure on the left, the induced forces would be in the propagation direction as well (reflection about x-axis of forces drawn).137.131.172.73 (talk) 21:57, 20 November 2008 (UTC)

(a) The light *does* bend in the same direction upon entering and exiting the sphere. You probably thought of a parallel plate -- with a parallel plate, the light "bends back" upon exiting. But the bead is spherical, and so it's correct that the light bends in the same direction upon entering and exiting.

(b) But the diagram was wrong, since the momentum change would cause a force on the bead in the direction of the laser, and not in the opposite direction as drawn in the diagram.

So, I replaced the wrong diagram by 2 new, correct ones (where I constructed the rays for n=1.5).

R.koebler (talk) 23:07, 2 May 2011 (UTC)

The 'Ray Optics' explanation is in my opinion conceptually flawed:
as is well known from basic school physics, the phenomenon of the refraction of light is not consistent with Newton's laws, but can only only be explained by a wave model, so it is paradoxical to combine the refraction hypothesis with Newton's laws here.

There is in fact no consistent theory which would result in the 'radiation pressure' which is postulated to be relevant here (see my website entry regarding Radiation Pressure ( http://www.physicsmyths.org.uk/#radpress ) and my pages regarding the Wave and Particle Theory of Light applied to the Photoelectric Effect ( http://www.physicsmyths.org.uk/photons.htm ) and the Energy and Momentum Conservation Laws in Physics (http://www.physicsmyths.org.uk/conservation.htm ) (from which it is obvious that the concept of a 'momentum' (as well as 'energy') can only be applied in classical mechanics but not to light)).

Thomas

Certainly a consistent theory would be nice. But even when we don't know how they work, these optical tweezers do, in fact, work.

I see that "photons have momentum" is claimed in both the photon and the solar sail article.

If you want people to think that momentum cannot be applied to photons, perhaps you could begin with those articles.

--75.48.165.135 07:36, 1 October 2007 (UTC)

See Photoelectric effect and Matter wave for photon momentum. Photons DO have momentum, just as matter has wave-like properties (Wave-particle duality). You can refract electrons, and even make them interfere - see, the two-slit diffraction experiment with electrons instead of photons.

My problem with this diagram is that if the rays are bent further from the beam axis after they've passed the particle, then their momentum in the beam direction will have decreased in order to maintain a constant velocity magnitude at any point in the space surrounding the particle. This change in momentum (in vector form) of the light would result in a change in momentum of the particle IN THE DIRECTION OF THE BEAM, rather than towards the waist.

86.136.222.154 (talk) 14:38, 16 January 2011 (UTC)

Correct, the diagram was wrong. So, I replaced it with correct diagrams.

R.koebler (talk) 23:07, 2 May 2011 (UTC)

Research groups using optical tweezers

I've removed this section, as optical tweezers have become such a widely used tool that this is analogous to something like "research groups using electron microscopy"... such a list can never be complete. Akriasas (talk) 18:49, 7 February 2008 (UTC)

DLBT in "History and development"

The following statement appears in the "History and development" section:

In 2004 Optical tweezers made the leap from large, complicated, expensive machines to much simpler, smaller, inexpensive and ultimately portable systems with the introduction of DLBT (Diode Laser Bar Trapping).

I am proposing to remove this statement because this technique has not made a lasting, substantial impact in the "History and development of optical trapping." If I am mistaken, can somebody provide evidence of its widespread adoption? I don't wish to diminish the impact of the paper, but I do not think every change in geometry or optics needs to be mentioned in this section. —Preceding unsigned comment added by Chodges (talk • contribs) 23:38, 23 February 2008 (UTC)

I changed it to sound less dramatic. --chodges (talk) 22:38, 5 March 2008 (UTC)

Optical Gradient Force

This needs and article of its own. Someone qualified please take it on. Many new applications are missing such as NMEMs, and optical signal processing etc.. —Preceding unsigned comment added by 12.229.112.98 (talk) 16:00, 12 October 2009 (UTC)

Kapitsa–Dirac effect

the Kapitsa–Dirac effect is where a standing lightwave modifies a stream of particles wikipedia says it could use more links Is it strongly enough related to this topic to be referenced here

Doppler Effect

I'd have added this to the to-do list, but a Ctrl+F didn't even find the mention of the word Doppler, so I'm not sure if I'm hallucinating.

I recall reading a few years back that there was an optical trap mechanism for atoms whose working principle was the Doppler effect. Very briefly: you have six lasers - two along each orthogonal spatial axis, pointed at each other, all tuned to an absorption line of the atom in question. What then happens is - any random motion of the atom away from the point of intersection is compensated for by the Doppler shift. Say, the perturbation was in the +x direction... then the atom would experience a blue-shift from the laser that was lasing toward the -x direction, and pick up more momentum from this laser to go back to the centre, while concurrently, experiencing a decrease in momentum it picks up from the laser lasing toward the +x direction.

Elegantly enough, it doesn't matter if the doppler shifted photon packets are of a wavelength which doesn't correspond to a the trapped atom's quantum level transitions. The atom particle will just continue on it's perturbed course, until the incident photons are Doppler shifted enough to match. i.e. even if the absorption is at discrete intervals in the frequency domain, the shift is continuous. [Of course, this is assuming the perturbed particle is accelerating, and that its velocity is changing.]

It was a while back, and I don't remember the other specifics. — Preceding unsigned comment added by 220.224.246.97 (talk) 01:05, 3 February 2013 (UTC)

Alright.. I feel like a n00b. Here you go: Magneto-optical trap and Doppler cooling

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Nobel prize

2018 physics nobel prize went to Arthur Ashkin “for the optical tweezers and their application to biological systems”. --grin ✎ 15:24, 2 October 2018 (UTC)

References