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Nanoporous Coatings Exhibit Low Refractive Index

Photonics Spectra
Nov 2002
Benjamin D. Butkus

Researchers at Massachusetts Institute of Technology in Cambridge have developed an aqueous-based process to uniformly cover the surface of most materials with an antireflective layer. They say the method produces surfaces with antireflective properties as good as or better than those created with most techniques, while eliminating the use of harsh chemical solvents that pose environmental hazards or damage sensitive optical components. Also, unlike other techniques, it can uniformly coat large, highly contoured or textured surfaces.

A method developed at MIT creates nanoporous coatings with reflective losses of only 0.01 percent at a quarter-wavelength optical thickness. A modified ink-jet printer can use the process to make patterned reflective films, such as the one seen here.

This method coats glass or polymeric optical surfaces, resulting in reflective losses as low as 0.01 percent at a quarter-wavelength optical thickness -- compared with between 4 and 5 percent for most uncoated surfaces. The coating's success lies in the technique by which the researchers lower its refractive index to a suitable value. While most glass and plastic optical elements show a refractive index between 1.45 and 1.7, an effective antireflection coating must exhibit an index closer to 1.2 -- difficult to achieve considering that most available antireflective substances have values of about 1.35.

A solution has been to use a substance with homogeneously distributed pores smaller than the wavelength of light. This incorporates the refractive index of air, creating a "sponge" with an reduced index. Unfortunately, the methods used, such as chemical etching, evaporation techniques or dissolution of spin-coated films, require potentially damaging chemicals and temperatures or a well-exposed surface.

The MIT researchers, using a previously reported process to create the porous thin films, alternately dipped substrates into positively and negatively charged polymers, and then immersed them in acidic solution. This typically creates a multi-layer film containing pores with diameters of 50 to 500 nm, which are too big and which scatter light. They added a step, in which they dipped the substrate in solutions with an extremely low pH, ranging from 1.8 to 2.5. This resulted in a material exhibiting a refractive index as low as 1.15 because the pores were at least an order of magnitude smaller.

The coating is not at its most stable at this point, however, as subsequent washes and acidic immersions cause it to alternate between nonporous and porous states. According to lead researcher Michael F. Rubner, this may prove advantageous in nonoptical applications, but for optical use, a permanent porous state is desired. The scientists achieved such permanence with a heat-induced cross-linking step, which "locked in" the nanoporosity.

The coating shows a partially graded index of refraction, which increases the antireflective properties over a wider range of wavelengths and incidence angles. The researchers believe they can improve this by layering permanent films of varying refractive indices to produce a gradient that will resist reflection from the visible to the near-infrared.

The process will be especially useful for coating textured and contoured surfaces. "As long as we can get water in the area, we can create this antireflective coating on many relatively inaccessible areas, such as the inside of a tube," Rubner said.

An additional advantage of the process is that it can create micron-scale patterned multilayer thin films, with areas of nonreflective and reflective properties, using a modified commercial ink-jet printer. Besides the films' optical applications, the researchers believe that the low dielectric constant and good insulating properties could make them useful in microelectronic applications.

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