New Flat Lens Focuses Without Distortion 202
yahyamf writes "Applied physicists at Harvard have created an ultrathin, flat lens that focuses light without the distortions of conventional lenses. 'Our flat lens opens up a new type of technology,' says principal investigator Federico Capasso. 'We're presenting a new way of making lenses. Instead of creating phase delays as light propagates through the thickness of the material, you can create an instantaneous phase shift right at the surface of the lens. It's extremely exciting.'" And by "ultrathin," they mean it — 60 nanometers thin. The big advantage for this technology, aimed at telecommunications signals, is that "the flat lens eliminates optical aberrations such as the 'fish-eye' effect that results from conventional wide-angle lenses. Astigmatism and coma aberrations also do not occur with the flat lens, so the resulting image or signal is completely accurate and does not require any complex corrective techniques."
Re:A return to refractive telescopes? (Score:5, Informative)
Nope. It's IR and down, and it sounds like it probably only works in a fairly narrow frequency band. It also seems like it's probably going to stay that way, since the feature size determines the frequency it's tuned for. Visible light may require impractically small features.
You could probably build an IR telescope using it, but it would still be a tube, it's just the lens would be very thin (which is likely a problem, rather than an advantage for a large aperture - how do you keep it from flexing? Plus your telescope would probably only work properly in a narrow frequency band (and you'd have to filter out other frequencies).
Re:But... (Score:5, Informative)
Have the cameraman back up, which lessens perspective distortion [stepheneastwood.com]. When taking pictures of people you should always get as far back as possible and zoom in. Staying close and zooming out is bad.
interestingly... (Score:5, Informative)
according to this [business-standard.com] report it's not a lens, but a diffraction grating.
From linked article:
"Our flat lens opens up a new type of technology. We're presenting a new way of making lenses. It's extremely exciting," says principal investigator Federico Capasso, professor of applied physics at the Harvard School of Engineering and Applied Sciences (SEAS).
Sorry, matey, it ain't that new, it's just a new application of a well established physical property. I do seem to remember using diffraction gratings to magnify light-bending effects at college in 1992 - specifically to fire an EM pulse at 450nm (near blue part of the visible spectrum) through a sample and use a calibrated* diffraction grating to amplify the signal to a photographic plate. What you end up with, essentially, is a highly magnified image (on the order of millions of times) with a very low distortion, with which you can determine the structure of the sample (be it a crystal lattice, eg. graphite, or a double helix, eg. DNA; each molecule has its own unique diffraction pattern). Generally you would use X-rays as pretty much anything is at least partially transparent to this wavelength, but since we had to use visible light from a very low powered lasing LED, we had to use visible-transparent samples. We got stuck with a quartz crystal. Still interesting physics, though, and some very pretty pictures.
*calibrating a diffraction grating is very simple: all you do is make the spacing between the lines on the plate equal to the wavelength of the light you're using. For far blue, you'd use a 400nm grating, for red 700nm. These are but two of several calibrated plates available.
no (Score:5, Informative)
If we sandwich a few of these with the metasurfaces tuned right, could we build a telescope that is a slab instead of a tube?
Only in limited cases, because it's only applicable from near-infrared to terahertz frequencies. UV and visible band are pretty much all out from the sounds of it.
Also: the lens is very thin. Nothing else is - just the lens. Ie, the objective or sensor still has to be some distance behind it, and I'm sure there are limitations with respect to angles. So you still need a tube - especially if the lens is very large in diameter.
This is fascinating, because it sounds like it is operating as a phased array; they *delay* the light depending on where it strikes on the lens. Wild! Phased arrays work by delaying the signal, thus steering the electromagnetic wave, but that's when you're generating or receiving...not modifying and retransmitting!
However, they're doing it in this case by physical manipulation of the gold/silicon structures at construction time. It's not tuneable afterward.
That's fine for telecom / fiber applications, where you only have a fixed number of specific wavelengths. However, astronomers might not mind being restricted to imaging just that one wavelength or that high in the light spectrum.
Sadly, this limitation also makes it useless for semiconductor lithography, which is UV to x-ray range.
Re:A return to refractive telescopes? (Score:5, Informative)
THz is LOWER than near IR in frequency (thus, in the wrong direction). They said "up to" in the article, which I suppose is accurate if you're talking about wavelength, but gives entirely the wrong impression.
Re:A return to refractive telescopes? (Score:5, Informative)
Take a look at a spectrum. "Terahertz" is low frequency IR and below: it tops out at far infrared. So this thing is basically good for a good chunk of the IR spectrum and a maybe a little bit of the submillimetre stuff that isn't quite IR. Visible light is too high frequency.
Re:A return to refractive telescopes? (Score:4, Informative)
IR and down in frequency. So you could make a tiny, easily concealable square to focus infrared rays at someone to irradiate them. Or use a magnifying glass, which is slightly bulkier but has much more light collection capacity.
Misleading article title and blurb! (Score:4, Informative)
The lens is tuned to a single wavelength of light and was demonstrated with a laser.
It's not apochromatic and not instantly useful to most lensing applications.
The authors say that it could potentially be, in the future. But that often means "give us more funding."
Re:What about chromatic aberrations? (Score:2, Informative)
That's really all that matters these days. Everything else can be easily corrected in software. Chromatic aberrations are more difficult to deal with nicely.
-Matt
Nope Matt...
Every optical aberration suppresses the system's Modulation Transfer Function (MTF) fundamentally resulting in a loss of information. Despite what CSI has taught you, you can never fully "correct it in software" after the fact. How well you can do depends primarily on signal to noise ratio (SNR), your detector's linearity calibration, and your prior knowledge of the objects power spectral density (usually just a rough semi-educated guess). Therefore, it's still important to have low-aberration systems for good imaging.
Now to answer your question... The principle of operation for this "lens" is similar to a fresnel lens / zone plate. This means it really only works at the single wavelength it was designed for (i.e. if you shove broadband light into it, you'll get a metric fuckton of chromatic aberration)
Re:A return to refractive telescopes? (Score:4, Informative)
The very same. It's possible these lenses might work well for things like that where regular lenses perform poorly. It's really aimed at telecom though - focusing fibre optic lasers.
Plasmonic devices=a bit far from any practical use (Score:5, Informative)
Whenever us optical engineers hear about plasmonics, we internally roll our eyes, because metal almost always absorbs far too much light to be useful. Even tens of nanometers of penetration and/or propagation can extinguish almost all of the light. This essentially relegates the entire field to the realm of theoretical curiosities and nothing more. (This work uses 60nm thick gold)
The authors of this paper admit that absorption is their biggest obstacle, as this lens only passes 10% of the incoming light. There are other issues for making this work a reality, but they pale in comparison to the classic brick wall you get when passing light through metal.
Re:A return to refractive telescopes? (Score:1, Informative)
Just to clarify: the demonstrated lens operates at 1.55 micron (near-IR). The same phase-control concept has already been demonstrated in the mid-IR by the same authors, in the terahertz (THz) by some other authors. The approach is trivially generalizable to any longer wavelength (shorter frequency) which means millimeter wave, radio waves, etc, though it is unclear if it is very useful in the radio frequency region compared to conventional receiving/transmitting phased arrays.
Re:A return to refractive telescopes? (Score:5, Informative)
Re:A return to refractive telescopes? (Score:4, Informative)
Re:A return to refractive telescopes? (Score:5, Informative)
Re:Co-author checking in. (Score:4, Informative)
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.
Re:A return to refractive telescopes? (Score:5, Informative)
No, unfortunately the concept is not generalizable to gamma ray frequencies (or xrays). It involves plasmonic components, which require metals with plasma frequencies above the operating frequency (otherwise the metals stops acting as a metal). There is no metal which would still behave "metallic" at gamma ray frequencies, I believe.
Quite right. More fundamentally, this won't work on any ionizing radiation, as you no longer achieve any cohesive refractive effect when your photons are randomly ejecting electrons via Compton or photo-electric effects, which become the dominant interactions at energies beyond UV.
Re:Its all about latitude... (Score:5, Informative)
Best explanation I have found that says it succinctly. I hope it helps.
Dynamic range = difference between highest and lowest(brightest/darkest) value that can be recorded on a medium.
Latitude = The degree of variation allowed above or below a certain setting, derived directly from dynamic range. i.e latitude a film is for a certain exposure, how many stops of headroom it has above and below before you lose details.
And just for fun...
Contrast = the difference between intermediate tonal values within a certain range. Generally, contrast is inversely related to dynamic range. A wider range allows finer graduations and hence lower contrast if desired. Contrast is directly related to the tonal response of the medium and can be visualized as a curve from light to dark. The steeper gradient of the curve, the higher the contrast at that point.
Re:But... (Score:3, Informative)
Potentially, a post could get alternately modded funny and overrated dozens of times, burning through a pile of karma. I'm a little bit surprised there haven't been coordinated attacks on an individual poster.
Re:But... (Score:4, Informative)
The problem is that lately all you see for the first half of a discussion is an endless stream of jokes ("classic" and "not so classic"). Believe it or not, most people don't come here for the jokes, they come here for technical discussion.
The problem is that slashdot has the "funny" mod, and as far as comment visibility, treats it the same as the "insightful" or "interesting" mod.
To hack around this shortcoming in the mod system, some mods choose to mod down the jokes to try to improve the S/N ratio.
If you don't want to see "Funny" posts, adjust your modifier preferences and give "Funny" a negative value.
I consider troll threads more disruptive than joke threads since they garner more responses. Half the replies in a story might be to such a thread when it is posted near the top.
Re:But... (Score:5, Informative)
Perspective distortion was used extensively in the filmimg of LOTR. It's how they made the hobbits look much smaller than the actors actually were.