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Reflectivity Reaches a New Low
sporkme writes "A new nanocoating material developed by a team of researchers from Rensselaer Polytechnic Institute has the lowest level of reflectivity ever seen ... or not seen in this case. The amount of light reflected by the composite of silica nanorods and aluminum nitride is almost the same amount reflected by air. From the article: 'Schubert and his coworkers have created a material with a refractive index of 1.05, which is extremely close to the refractive index of air and the lowest ever reported. Window glass, for comparison, has a refractive index of about 1.45. Using a technique called oblique angle deposition, the researchers deposited silica nanorods at an angle of precisely 45 degrees on top of a thin film of aluminum nitride, which is a semiconducting material used in advanced light-emitting diodes (LEDs). From the side, the films look much like the cross section of a piece of lawn turf with the blades slightly flattened.' Suggested applications include increased efficiency in solar cells, more energy-efficient lighting and advances in quantum mechanics."
oh crap where did it go?
Even more birds hitting windows!
...move along.
They made several layers with increasing refractive indices, so the RI of the outermost coating is close to air while that of the innermost coating is close to the material it coats. What's cool about this is that it cuts reflexion across the EM spectrum, it doesn't just trade off reflectivity in one part of the spectrum for that in another, like previous anti-reflective coatings have done. Unfortunately, it isn't transparent so it won't work as a coating for glass.
- None can love freedom heartily, but good men; the rest love not freedom, but license. -- John Milton
How does reducing reflection increase the efficiency of LEDs? This stuff absorbs light. It seems to me like it would only be able to make things darker, not lighter. I'm willing to accept I just don't get it... but what is it that I don't get?
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Refraction and reflection are closely related. Light reflects when it strikes a discontinuity in index of refraction. If the IOR is made to vary smoothly, on the other hand, light will not reflect. And that's exactly what they seem to have done here.
... isn't the illusion of water on the road a product of light reflecting off of the air?
No.
It's caused by the curvature of light refracted by the difference in refractive index between the hot air near the sun-heated surface and the cooler air above it. The light bends back up without "touching" the underlying surface.
You only get a little bend. This is why you need a very hot surface to get enough of a bend to be visible at all. It's also why you only get it at large distances, where the line of sight is nearly parallel to the ground.
It looks like water because you look at the ground and see a a region of like of the sky's color, shimmering due to convection current - generated patches of uneven refractive index in the air rather than surface ripples.
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There is reflection at a dielectric boundary. A dielectric is something that is not a metal, like glass. If you focus your eyes on a window, you can see a reflection of yourself because air and glass are dielectrics with different indices of refraction.
The amplitude of the reflected light wave for light that strikes perpendicular to the dielectric boundary is (n1 - n2)/(n1 + n2) - the "n's" are indices of refraction. For a boundary between air and this stuff, the reflection is (1.05 - 1.0) / (1.05 + 1.0) = (0.05/2.05) = 1/41. Compare with glass, with an index of 1.4: (1.4 - 1.0) / (1.4 + 1.0) = 0.4/2.4 = 1/6. (The difference in intensity is the square of this, though, which diminishes the difference.)
The equation for non-normal incident light is more complicated*, but even light that is a long way off normal incidence reflects by about the same amount. It's only when you start approaching 90 degrees off normal that a dielectric boundary starts reflecting lots of light. Try it with a large window pane: you have to get your head right up there and view something with a glancing reflection to see it clearly.
They're saying that they can coat a semiconductor, like an LED or a photovoltaic cell, with this stuff. Then about (1/6 - 1/41) more light either strikes the PV, or leaves the clear stuff that surrounds the LED.
This will not work as well as a magnesium fluoride coating for lenses, though. That kind of antireflective coating relies on destructive interference with the reflected light from the two dielectric boundaries, which is why they only work at a certain wavelength.
* Google "reflected light at a dielectric boundary" for the gory details.