3 ms·
No, it's not a typo. Standard "optical tweezers" pull things towards regions of higher intensity (it's the intensity gradient, not the potential gradient, that
by gjm11 16y ago
No, it's not a typo.
Standard "optical tweezers" pull things towards regions of higher intensity (it's the intensity gradient, not the potential gradient, that's relevant here), so you can't pull anything past where the intensity is maximal. But the technique described in the paper works even with no intensity gradient along the direction of beam propagation. (I think that if there were one, the gradient force would swamp their new force, but I'm not sure.)
- jessriedel 16y agoWhoops, looks like I was confused about the terminology. Thanks. In case anyone else reads this: according to the wikipedia article, the term "intensity" in "intensity gradient"--in this context--is just the strength of electric (and magnetic) field associated with the laser. The electric field, in turn, is the gradient of the electric potential [1]. Point charges will follow the potential gradient, but neutral dieletric objects (which I think is what optical tweezers are usually used on, and which usually can be modeled as dipoles) will follow the intensity gradient. Simple dipoles can be pulled with optical tweezers, but limits on laser power mean that the distance between intensity extrema (and so the maximum distance things can be pulled) is relatively short. The hypothetical tractor beam only works on objects more electrically complicated than dipoles, but doesn't rely on following an intensity gradient and can pull all the way back to the source. [1] This may not be quite applicable here because of the ambiguities in defining a potential for non-static electric fields like those of lasers.