I posted a comment below regarding the difference between a diffractive lens (such as a zone plate) this "antenna-based" lens. In short: the physical principles behind the focusing are different (diffraction vs. direct imprinting of a curved wave-front) and as a result the curved wave-front is formed at different distance ("far away" from the lens in the far-field zone vs. almost immediately after the lens, respectively).

This is a good point. But please keep in mind that this sort of lens is made up of a lot of individual elements that can be independently adjusted; therefore you may be able to correct for the effects that you are talking about (such as the detector and screen being flat surfaces). This has not been demonstrated yet, but it doesn't seem impossible.

This is absolutely correct. The individual phase elements utilized to create these lenses are resonant antennas, and so the phase is by definition a function of the frequency (and wavelength). This is true for every resonance phenomenon: http://en.wikipedia.org/wiki/File:Universal_Resonance_Curve.svg

Could you link me to the type of lens you're referring to? I only get a lot of german when I do some googling, and the first link I found sent me to the german wikipedia entry for airy disks. Do you just mean an airy black/white/grayscale pattern?

The 10% is definitely not good enough for such an application. We'll have to work on making the efficiency much higher. It's not at all clear what are the upper bound on achievable efficiency with these sort of resonators is yet.

You're correct, of course. However, sometimes "selecting the right material" is just not that simple! For example take a look at some IR transparent materials: there are certainly plenty but some are toxic (think zinc selenide), some very hard (diamond), some soluble in water (sodium chloride), etc. Just because we know many really good transparent materials in the visible, doesn't mean that this can immediately be generalized to all frequencies even though maxwell's equations are scale invariant. Furthermore, fabrication is not always available. Say you want to make a small lens (a "microlens") for some weird frequency... say 3.3um. Maybe you've made a laser at that wavelength and you need something to focus it. Taking some unknown material and machining out of it a small hemispherical lens may not be as easy as you make it sound. On the other hand scaling up the antenna design a bit and using conventional lithography is probably not very hard.

Anyway, I'm not making the claim that this approach is better than any other approach. It's just different, and these differences may sometimes be useful.

I'm not sure yet! Sometimes we make things because we can and because others may find it useful, not necessarily because we see a clear-cut application in the very near future. Maybe it can be used in interesting frequency ranges; maybe having a frequency-selective lens is useful; maybe this approach yields more to making it dynamically tunable than conventional phase-addressing approaches; we just don't know at the moment. I should also say that there are plenty of applications (in certain imaging, for example), where losing a large portion of your light is OK as long as you still have some left. You might start with a powerful laser beam and end up with just a few photons, but as long as these are detectable you might be in business.

Well, actually a Fresnel zone plate is a focusing device that can be as thin as the presently demonstrated "ultra-thin" lenses. However, the physical principle of operation is actually completely different. Zone plates operate on the principle of diffraction of light from apertures (http://en.wikipedia.org/wiki/Zone_plate), and by properly designing dark and bright zones one can use diffraction to focus light at a point.

I certainly won't compare this type of antenna-based lens to a zone plate as far as applications go because I don't want to speculate too much. I should say though that while you need to be in the "far field zone" to see the desired beam from a diffraction optics element such as a zone plate (see http://en.wikipedia.org/wiki/Fraunhofer_diffraction), our design forms the desired wave-front almost immediately after the layer of antennas. I'm not sure if it's useful to have a curved wave-front that a lens imparts immediately after the flat wave surface or it's just a curiosity, but it's at the very least interesting. For other applications (for example we've demonstrated vortex plates here with the same technique: http://www.seas.harvard.edu/capasso/wp-content/uploads/publications/Genevet_APL_100_013101_2012.pdf) this may more relevant.

Yeah, the resonators are certainly not isotropic, and the incident polarization changes their behavior significantly. In particular, this lens was actually designed to operate in *cross-polarization*. This means that you send linearly polarized light in, and then you put a crossed polarizer (with respect to the first one) in front of the lens and the light that is able to get through is focused.

Hi John,

Thank you for your comment. It's not true though that we're making small micro-lenses. A micro-lens is still a regular lens; at the very least that means that it is larger than the wavelength of light. The antennas used here are smaller than the wavelength of light, made of metal, and change the phase of the light due to a resonance behavior. You can see the particular shapes and sizes of the structures used in Fig. 2 of the paper (http://arxiv.org/abs/1207.2194)

An open version of the article can be found here: http://arxiv.org/abs/1207.2194

As an FYI, many articles that are pay walled can be found on the arxiv pre-print server for free.

Can it be adapted to work with visible light? Yeah, though it will take some re-design.

Can it be adapted to be *useful* with visible light? Unclear for a variety of reasons. The first is that shifting to the visible will increase metal losses, so more of the light will simply be absorbed instead of focused. Not that the efficiency isn't an issue already: from the article you can see that with the current design, the maximum attainable efficiency is ~10%, with the rest of the light being absorbed (not that much actually) and scattered somewhere else (this is the big one). In fact the presently demonstrated lens has an even lower efficiency, though scaling it up to the 10% figure is fairly trivial. Anyway, in the visible the 10% figure probably drops with the current design, though some design improvements could likely be made. I don't want to give you an upper bound on the efficiency because frankly I'm not sure. Anyway, do you want a lens that only focuses some percentage (say between 10% and 40% just to have some numbers) of the light and throws away the rest? We've gotten so good at making regular old lenses in the visible, that I'm not so sure. On the other hand go to a different frequency range where good lenses are less common, and all of a sudden the present approach may have some value.

Hi everyone. I'm a co-author on the article, and I'd be happy to answer any questions you may have, though probably tomorrow. I'm hoping that this goes better than the last time I tried this (see here: http://slashdot.org/comments.pl?sid=1747464&cid=33185134), where no questions were asked and most of the discussion centered around mildly funny jokes. I appreciate those as much as the next person, but if anyone likes, we can discuss science =].

I would like to piggy-back on this excellent comment.

There is another issue as well. Look at this diagram of a "beam expander" (or telescope): http://www.cvimellesgriot.com/glossary/imagesDir/BeamExpander.gif

You'll see that while it has two lenses which no doubt have some thickness, there is also some space in the middle. With the approach in the article, the lenses can be very thin. However, to make a telescope there still has to be space in the middle. Can that be overcome to some extent (for example with very high numerical aperture ultra-thin lenses)? That's yet to be determined.

As with anything, it depends on what technology is used to make it.

If you wish to make visible or near-IR lenses, you are stuck with things like electron beam lithography and focused ion beam, so it's extremely expensive. Some newer fabrication techniques such as nano-imprint lithography could maybe bring the cost down. .

If you want to scale the entire lens up to, say, terahertz frequencies, you can make the same structures with photolithography and the price goes down tremendously because photolithography is a parallel process (every part of the pattern gets written at once instead of writing each spot point by point).