Now that I have some time to read the paper, on the second page there's this sentence: "The Fourier decomposition of a PIB consists of plane wave components whose k-vectors form a cone that make an angle Theta_0 with the z-axis." The way it must get momentum is having the outgoing light be more collimated than the incoming light.
My new question is, How is this different from an Optical Trap? I'm imagining the point of the cone at the particle, but that may not actually be the case, and my skimming of the article is not enough to make sense of figure 3.
Not only is the point of the cone not at the particle; it's not even in the same space as the particle! The cone is in the Fourier transform of the beam.
A standard optical trap, applied to very small objects, works like this: the electric field causes polarization in the thing being trapped, turning it into a little dipole; it then exerts a force on the induced charges, basically pulling on each end of the dipole; that force is bigger in regions of greater field intensity; the net effect is a small force pulling the object towards where the field is bigger. (In an electromagnetic wave there is also a magnetic field, which also acts on the objects, but the effects of those average out to zero over each cycle of the wave.)
Now, really the effect of an electric field on a particle is more complicated than just turning it into a dipole. There will be higher-order stuff as well. The authors of this paper claim that if you look at the quadrupole term and consider interactions between the different terms (warning: I am not at all sure I understand this bit, and they've left the details to a supplementary section that appears not to be included in the paper on the arXiv), then with a suitably shaped beam and the right sort of particle you can get a backward pull against the direction of propagation of the wave. (So, yes, the light will have to emerge travelling more-directly-forward than it went in, so to speak. Which is ... counterintuitive.)
So it's different from an optical trap in that it relies on higher-order effects, and a more sophisticated beam shape, and it can only work on very small objects (the higher-order stuff gets averaged away for bigger ones; so I don't think you could use this technique for things like biological specimens). But it uses much the same underlying physics.
[EDITED to add: their trick also doesn't work for the very smallest objects, substantially below one wavelength. It works only in the "Mie regime", meaning with particles whose size is comparable to the wavelength.]
Now that I have some time to read the paper, on the second page there's this sentence: "The Fourier decomposition of a PIB consists of plane wave components whose k-vectors form a cone that make an angle Theta_0 with the z-axis." The way it must get momentum is having the outgoing light be more collimated than the incoming light.
My new question is, How is this different from an Optical Trap? I'm imagining the point of the cone at the particle, but that may not actually be the case, and my skimming of the article is not enough to make sense of figure 3.