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Shake that grating

Using phonon scattering and temporal degreees of freedom for superresolution at the nanoscale

(This means "we can see very small things if we shake them just right".)

Have you ever wondered why we can't see viruses under a microscope? The answer to this question is the diffraction limit. But what is the diffraction limit, exactly? In signals processing terms, it is the spatial frequency bandwidth for propagating optical waves. Using optical microscopes to look at a 130 nm flu virus is akin to trying to pick out the sounds of piccolos in a Strauss concerto, over a very bad cell phone connection. You can probably discern the piccolos if you are right in the concert hall, but the task is hopeless for someone on the other end of the line. Likewise, in optics, you can see arbitrarily small things – provided you are close enough to them so that you can pick up the signal from "the piccolos" (in physics, we call them evanescent waves). That is to say, your detector must be placed in the "near field", which is the area less than a micron from the object.

This, unfortunately, is not always practical or easy to do. But over the years researchers have come up with various tricks to get around the bandwidth problem. One conceptual approach is to take the high frequency information, encode it using low-frequency data, transmit it to the far-field detector, and decode it there. This idea has been around for several decades, but the encoding/decoding step is nontrivial to accomplish, and so this scheme hasn't seen adoption in applications.

Recently, we came up with an unusual new way to perform the high spatial frequency encoding, which may turn this around. In order to lower the spatial frequency of the light wave for transmission, it is necessary to scatter its evanescent components on some periodic subwavelength structure. We showed that this is possible to do with a dynamic phonon grating, validating Jacob Khurgin's original idea.

The phonon grating creates a periodic potential that converts evanescent waves to propagating optical signals. Because it's a dynamic structure, there is much flexibility in encoding and processing the signal. Most intriguingly, phonon scattering is associated with the temporal frequency (i.e. energy) shift of the incident photons. Because of this, it is possible to uniquely detect the scattered signal in the far field using coherent detection, greatly simplifying the decoding step.

This paper was a long time in the making, and it is finally getting some attention, perhaps due to the fact that nano-opto-mechanics with phonons is becoming a popular research area. It's about time the lattice vibration quasiparticles got some good publicity, they certainly deserve it!

A related project, in which we model a system where the scattering is performed by a chirped anisotropic nanostructure (but ultra-high frequency vibrations are still used to aid coherent detection) also got published very recently. In fact, it is the first article in the first 2012 issue of APL (mendeley link). I am currently taking the corresponding issue/article designation of {100,011101} as a divine sign that I should do more programming projects. (I believe that in the coming few months the Almighty will be pleased on that angle.)

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