
Metamaterial based flat lens promises possible revolution in optics - mdf
http://www.bbc.com/news/science-environment-36438686
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wlesieutre
My understanding of these flat metamaterial lenses is that they're fine-tuned
for a single wavelength of light; you can't put one in a camera and expect it
to work like a glass lens did. I'm sure there are applications where control
of monochromatic light is important, but I couldn't tell you what they are.

It's an awfully important detail to completely omit from an article.

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alcubierredrive
For smartphones, where thinness is very important but area is less so, it
would not be unprecedented to make an array of 4 monochromatic cameras and
rectify and combine the images computationally.

    
    
      R G
      G B

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BurningFrog
Unfortunately, that is not how light and perception works.

There are thousands/millions¹ of separate visible light frequencies. Our eyes
and brains takes that all in and does an enormously lossy mapping of that to 3
perceived colors.

If you only record 3 of those thousands/millions of frequencies, you will lose
99.9% of the light, and mostly make black photos.

¹ depending on how wide the frequency interval considered monochromatic is.

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niketear
This is incorrect.

Cone cells have fairly peaked frequency responses, and due to random
projections caused by the cascading, this is sufficient to fully reconstruct
the signal, i.e. the converse of the statement is true, you can retain 99.9%
of the perceptual information using 3 sensors, all you need is to perceive in
time and some randomness in the sensor placement.

~~~
tgb
They're not really that peaked, the M and L cones largely overlap, even, and
are spread over at least a third of the entire visible spectrum. Pictures at:
[https://en.wikipedia.org/wiki/Cone_cell](https://en.wikipedia.org/wiki/Cone_cell)

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dkbrk
Full article: [http://science.sciencemag.org.sci-
hub.cc/content/352/6290/11...](http://science.sciencemag.org.sci-
hub.cc/content/352/6290/1190)

These lenses are designed for a specific wavelength, and if I am reading the
paper properly, only work with circularly polarised light. Essentially, for a
given design wavelength and focal length there is a desired phase shift at
each point on the lens. This phase shift is caused by the titanium dioxide
"nanofins" which rotate the circularly polarised light to produce the desired
phase shift. The phase shift is determined by the angle at which each fin is
rotated. This produces a pattern of fins rotated relative to one another,
which can be seen in the images of the BBC article.

While the lenses are designed for a target wavelength, they're not entirely
useless at other wavelengths, they just have terrible chromatic aberration. In
all other respects they seem to be excellent (especially for their size), but
this makes them useless for most commercial applications.

To manufacture the lenses, they start with a substrate of silicon dioxide; not
actually glass as said in the article, but quartz, like sand. This is coated
by a resist, which is patterned by electron-beam lithography. The resist is
"positive", meaning that the exposed part is removed when developed. A thin
layer of titanium dioxide is deposited using atomic layer deposition. This is
a type of thin film deposition technique that allows the deposition of a
single atomic layer at a time. This is accomplished by introducing two
different precursors one at a time alternately in sequence, the number of
cycles determines the number of layers. With this they can essentially deposit
just enough TiO2 to fill the holes left in the resist, though it also
deposited on top of the unexposed resist.

The TiO2 remaining on top of the undeveloped resist is etched off and the
undeveloped resist is removed, leaving just the nanofins. The nanofins have a
high "aspect ratio", meaning height-to-width, which makes them challenging to
produce using most semiconductor fabrication techniques. They are however
quite large compared to modern semiconductors, on the order of hundreds of
nanometers, which makes most things easier. Semiconductor fabrication uses
photolithography, this used electron-beam lithography. While electron-beam
lithography can in principle produce smaller feature sizes than
photolithography (due to the smaller wavelength of electrons), that was not
needed for this application; rather electron-beam lithography does not require
the creation of a photomask and is consequently much more useful for small
scale prototyping.

Commercially producing these lenses at scale could potentially be done with
photolithography, though there would be a large upfront cost due to the need
to fabricate photomasks. Monocrystalline silicon substrates are standard and
silicon-dioxide-on-silicon is extremely common; I suspect the lenses could be
fabricated on such a SiO2-Si substrate and the silicon on the back face
removed, leaving optically transparent lenses.

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jwatte
"Shapes on the surface of this lens are smaller than the wavelength of light
involved: a thousandth of a millimetre."

A micron is 1000 nm and visible light is about 900 nm and down. Close but no
cigar.

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gradi3nt
You have to love pop sci headlines with phrases like "...promises possible..."

I promise you, BBC's Roland Pease, that it's possible the sun won't rise
tomorrow and Linus Torvalds with announce that he will be Microsoft's next
CEO.

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iamleppert
How is this different than what can be achieved using holographic optical
elements, which can routinely make optical lenses and materials using the
principle of holography and can be diffraction limited, producing feature
sizes that are 1/n the wavelength of light (depending on the mastering
process)?

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tener
Looks exciting on paper, but I wonder how much work is needed to bring this to
mass production.

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infogulch
It looks like they had that in mind while they were designing it:

"But our lenses, being planar, can be fabricated in the same foundries that
make computer chips. So all of a sudden the factories that make integrated
circuits can make our lenses."

If this is true, I imagine old foundries could produce these since they
probably don't need anything near the precision or consistency that current-
gen chips require.

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Aelinsaar
I'm not sure if this is where and when metamaterials break into the mass
market, but it's bound to happen sometime in the next 5-10 years, why not now?

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swframe
Can it see proteins, cell walls or viruses?

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styrophone
Is this much different from Diffractive Optical Elements in use today?

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erikj
I wonder if it can be used to improve VR optics and make HMDs cheaper.

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bobsil1
Better for transparent AR lenses than the current holographic and waveguide
approaches.

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justinclift
Gah. Maybe we should add a "[flash]" warning for links that seems to want
Adobe Flash installed for important parts of the content. :(

