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Science Forum

  1. 18 Sep '12 08:18 / 8 edits
    We could have very thin flat lenses in the near future and with no monochromatic aberrations:

    http://physicsworld.com/cws/article/news/2012/sep/17/first-flat-lens-focuses-light-without-distortion
    “...
    First flat lens focuses light without distortion
    Sep 17, 2012
    Physicists in the US have made the first ultrathin flat lens. Thanks to its flatness, the device eliminates optical aberrations that occur in conventional lenses with spherical surfaces. As a result, the focusing power of the lens also approaches the ultimate physical limit set by the laws of diffraction.
    "Imagine if you were to replace the lens in a mobile phone with a flat and ultrathin one – you could then squeeze your smartphone down to a thickness approaching that of a credit card," says team leader Federico Capasso of the Harvard School of Engineering and Applications. "Most optical components found in devices today are quite bulky because the light-beam shaping is done by changing the optical path of incident light rays, which requires changes in lens thickness. In our lens, all the beam shaping is done on its flat surface, which is just 60 nm thick."

    In an ordinary lens, light rays travel more slowly in the thicker, central regions than in the thinner, peripheral ones thanks to the smaller phase velocity of light in glass compared with air, he explains. This distribution of phase delays in the lens leads to light refraction and focusing.

    Nanostructured metasurface
    The new flat ultrathin lens is different in that it is a nanostructured "metasurface" made of optically thin beam-shaping elements called optical antennas, which are separated by distances shorter than the wavelength of the light they are designed to focus. These antennas are wavelength-scale metallic elements that introduce a slight phase delay in a light ray that scatters off them. The metasurface can be tuned for specific wavelengths of light by simply changing the size, angle and spacing between the nanoantennas.
    "The antenna is nothing more than a resonator that stores light and then releases it after a short time delay," Capasso says. "This delay changes the direction of the light in the same way that a thick glass lens would."
    The lens surface is patterned with antennas of different shapes and sizes that are oriented in different directions. This causes the phase delays to be radially distributed around the lens so that light rays are increasingly refracted further away from the centre, something that has the effect of focusing the incident light to a precise point.
    No monochromatic aberrations
    The new lens does not suffer from the image-distorting features, known as monochromatic aberrations, that are typical of lenses with spherical surfaces, adds Capasso. "Spherical aberration, coma and stigmatism are all eliminated and one gets a well-defined diffraction-limited, accurate focal spot. This is true even when light rays hit the lens away from the centre or at a large angle, so no complex corrective techniques are required."

    Flat lens keeps light on target
    The Harvard team made its lens by first depositing a nanometre-thin layer of gold. The researchers then stripped away parts of the gold to leave behind an array of V-shaped structures (the nanoantennas) that were evenly spaced in rows across the surface of a silicon wafer.
    The most obvious applications for the lens include photography and microscopy, says Capasso. "For example, compact objectives with very large numerical apertures can be envisaged, but we can also imagine optical fibres with patterned facets for new imaging and medical applications, and anywhere in general where a conventional lens could be replaced with a flat one," he says.

    Towards broadband focusing
    Although the lens is only at the proof-of-concept stage, Capasso and colleagues have already been inundated with requests from photographers and astronomers from around the world. The focusing efficiency of the lens is still quite small at present but, according to the team, could easily be increased by increasing the packing density of the optical antenna and by using different flat-lens designs. "So far, the lens only focuses specific wavelengths of light but by arranging different antenna patterns onto the metasurface it could be made broadband," says Capasso.
    The researchers fabricated their lens using electron-beam lithography, which is not the most practical technique because it is time-consuming. "Fortunately, there are many emerging nanolithography technologies that could be suitable for mass production, such as nanoimprinting and soft lithography, which might be extremely useful for patterning our lens on flexible substrates," adds Capasso. "This in itself would open up a host of exciting application areas."

    ….”





    But what about the transparency of such a lens? Wouldn't having a thin perforated metal ( of gold in this case ) layer there mean much of the light will be reflected? -I mean, metal surfaces do normally reflect light!
    Does merely that metal layer being so thin ( just one nanometre in this case which is much less than half the wavelength of visible light ) automatically mean that it is transparent?

    Note it says it corrects for monochromatic aberrations, but what about chromatic aberrations?
    I assume the usual method of correcting for chromatic aberration that involves having many layers of lenses sandwiched together would correct for it just fine if each lens-layer was this type of ultra-thin lenses? -but, wouldn't that even more badly effect its transparency? -My concern here is that it might waste some of the incoming light through either reflection or absorption so that the best lenses might not ever be these thin flat ones but I don't know if that is a valid concern because I don't know enough about the physics of it.
  2. 18 Sep '12 12:05
    humy, I don't know much about the science of this but you post has me thinking of all the products that can benefit from this technology. It will be interesting to see where this all leads. Thanks for the post!!
  3. Standard member mikelom
    Ajarn
    18 Sep '12 15:21
    Just use the eyes of dead humans....... end of problem........ apart from the liturgy of how to connect to ................

    -m. (Joke )
  4. Standard member sonhouse
    Fast and Curious
    19 Sep '12 16:57
    Originally posted by humy
    We could have very thin flat lenses in the near future and with no monochromatic aberrations:

    http://physicsworld.com/cws/article/news/2012/sep/17/first-flat-lens-focuses-light-without-distortion
    “...
    First flat lens focuses light without distortion
    Sep 17, 2012
    Physicists in the US have made the first ultrathin flat lens. Thanks to its flatness, the de ...[text shortened]... is a valid concern because I don't know enough about the physics of it.
    I just read that piece, the first iteration was monochromatic, they said future versions may be more broadband white light apps. I was thinking about eyeglasses, even lighter than the ultralight plastic ones now available, they have high refractive indexes which allows the same bending with thinner lenses. These new lenses should be less than a mm thick if they can get them full spectrum. Also telescopes should be able to use these lenses if they can be made big enough.
  5. 20 Sep '12 00:36
    Just glasses alone would be wonderful. I know several people that can't wear glasses because they are too heavy and cause sores on their noses.
  6. Standard member sonhouse
    Fast and Curious
    20 Sep '12 13:03
    Originally posted by joe beyser
    Just glasses alone would be wonderful. I know several people that can't wear glasses because they are too heavy and cause sores on their noses.
    Contacts are best but some people like me, can't wear them. I got in an industrial accident 20 years ago and burned my eyelids inside with sulphuric acid and have been unable to wear contacts ever since. When I put them on, within 5 minutes my eyes and lids are flaming red. Vision is ok but have to wear regular glasses. It sucks actually.
  7. 21 Sep '12 12:52
    Originally posted by mikelom
    Just use the eyes of dead humans....... end of problem........ apart from the liturgy of how to connect to ................
    And, unfortunately, how to get enough of them. We already have problems getting enough for transplants.

    Richard
  8. 21 Sep '12 16:43
    Originally posted by sonhouse
    Contacts are best but some people like me, can't wear them. I got in an industrial accident 20 years ago and burned my eyelids inside with sulphuric acid and have been unable to wear contacts ever since. When I put them on, within 5 minutes my eyes and lids are flaming red. Vision is ok but have to wear regular glasses. It sucks actually.
    My girlfriend cant wear contacts anymore either. She was told she has to use the hard contacts because of her eyes but she cant tolerate them. The flexible ones have done some damage over the years.
  9. 23 Sep '12 09:18
    Would these flat lenses be better than parabolic mirrors for telescopes?
  10. 23 Sep '12 10:17 / 5 edits
    Originally posted by twhitehead
    Would these flat lenses be better than parabolic mirrors for telescopes?
    Purely in terms of performance, I doubt it because, assuming very good transparency of this type of lens can be achieved, although, just with reflectors, monochromatic aberration will be absent, unlike with reflectors, chromatic aberration probably would not be completely absent so reflectors will probably have that significant advantage over these ultra-thin lenses. I bet that reflectors will always be the best of the two when it comes to performance at least for wavelengths longer than X-rays ( could these ultra-thin lenses be made to work for X-rays and gamma rays? -I know reflectors are not generally good for this ) .

    But it might be cheaper? -not sure about that because extra supports may be needed to prevent such thin delicate lenses from flexing or breaking as a result of vibration/movement that comes with normal use of the telescope and that could add to the cost a lot. But, on the other hand, expensive support structures are also needed to support reflectors and I don't know how the two would compare in terms of cost.
  11. Standard member sonhouse
    Fast and Curious
    23 Sep '12 14:57
    Originally posted by humy
    Purely in terms of performance, I doubt it because, assuming very good transparency of this type of lens can be achieved, although, just with reflectors, monochromatic aberration will be absent, unlike with reflectors, chromatic aberration probably would not be completely absent so reflectors will probably have that significant advantage over these ultra-thin l ...[text shortened]... also needed to support reflectors and I don't know how the two would compare in terms of cost.
    The part of the article I didn't understand was the statement there were thousands of them in an inch or cm. Does that mean they can not be made in meter sizes which you would need for a refracting telescope?