Science Literature

Transparent materials like glass are extremely useful, but the proportion of light reflected from the surface can, in some applications, be a problem. For many years, spectacles have been supplied with antireflection coatings, and there is increasing interest in improving "the light coupling into solar cells, [and enhancing] the extraction of light from light-emitting diodes." The critical parameters are the refractive index of the medium (such as air) and the refractive index of the substrate (such as glass or plastic). The bigger the differences, the more significant are the reflection problems. It has been recognised for some time that a coating with a gradual change in the refractive index will yield substantial benefits. Graded index coatings are designed so that the "refractive index is gradually increased from that of the surrounding material to the refractive index of the substrate, and the reflection is decreased by optical impedance matching at the interfaces." (For more technical details, go here). The main problem is that fabricating these graded index coatings is "very challenging".

Moth eyes have a graded index coating to help them see in the dark. (Credit: Rick Cowen, source here, for larger image, click here)

It was as long ago as 1973 that Clapham and Hutley confirmed that moths had a graded transition of refractive index between their eyes and the air. Since then, several research groups have been active, and commercial products are already in existence. New research by a Dutch group has been reported recently.

"To maximize the amount of light entering their eyes, to help them see at night, the insects' eyes are covered in tapered nanostructures. This creates an "effective medium" where the refractive index gradually increases as light travels from air through to the insects' optical nerve. The resulting effective index is graded from close to one at the top to close to 3.4 at the bottom, which means that very little light is reflected out of the eye. Inspired by these biostructures, Gomez Rivas and colleagues have mimicked the effect by growing nanowires of different lengths - creating a metamaterial with optical properties that change gradually as a function of distance."

This group has experimented with cylindrical and conically-shaped nanorods. "The team used gallium phosphide (GaP) nanorods on top of a GaP substrate, then measured reflection and transmission simultaneously". This allowed them to check whether the observed effects are actually due to the graded refractive index or whether absorption was significant.

"We have shown that the enhanced transmission and corresponding reduced reflection of these layers is related to a graded refractive index in the nanorod layer. The enhanced direct transmission and the total transmission and reflection presented in this work show that scattering and absorption are weak in these layers. The antireflection behavior over a broad wavelength and angular range, and the low amount of light scattered, renders these nanorod antireflection coatings a very promising material for enhancing light coupling in solar cells or for more efficient light extraction from LEDs."

The goal is, of course, to develop commercial products. But more development work is needed to fine-tune the manufacturing process and to determine the optimum conditions to reduce both reflectance and cost.

"Rivas told physicsworld.com that his team's long term goal is to turn this into a product but, for the next few years, they will continue to seek even lower reflection. "In theory we could get 99 per cent transmission; the difficulty is that our nanowires will need to get longer and thicker and then scattering come into play," he said."

In the meantime, spare a thought for the moths, whose eyes have set the standard for antireflection coatings. Clapham and Hutley said that "nature may have anticipated the problems", but what they really meant is that the Blind Watchmaker has been at work: beneficial mutations preserved by the process of natural selection. When we consider that teams of researchers over many years are just beginning to emulate what we find in the natural world, we must conclude either that Darwinian processes are highly effective (and should be easily demonstrated by field observations), or that something other than Darwinian processes must have been involved. Can we allow students the liberty to apply their critical minds to evaluate these options? As a group of senior academics wrote recently to The Daily Telegraph:

"Is it surprising that there is a degree of incredulity that random mutation and natural selection alone can account for the vast complexity of life? A much more open and honest debate is needed. If we lose a spirit of critical inquiry, even about Darwinism, we dishonour the pursuit of science."

Broad-band and Omnidirectional Antireflection Coatings Based on Semiconductor Nanorods
Silke L. Diedenhofen, Gabriele Vecchi, Rienk E. Algra, Alex Hartsuiker, Otto L. Muskens, George Immink, Erik P. A. M. Bakkers, Willem L. Vos, Jaime Gomez Rivas.
Advanced Materials, 2009, 21, 1-6 | doi 10.1002/adma.200802767

Excerpt from 2nd para: The fabrication of graded index coatings is very challenging when the substrate is surrounded by air, as a solid material that matches a refractive index of 1 is needed. Obviously, the refractive index of any dense solid material is too large to match the refractive index of air. To overcome this problem, more effort has been made over the last years to obtain graded index coatings, by applying nanostructuring techniques inspired by biological structures such as moth eyes.

See also:

Clapham, P.B. and Hutley, M.C. Reduction of lens reflexion by the 'Moth Eye' principle, Nature, 244, 281 (1973)

Dacey, J. Moth eyes inspire more efficient solar cell, Physics World (9 February 2009)