AAWSAP DIRD, Aerospace Applications of Programmable Matter, December 2009

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(permittivity, permeability, index of refraction, and coefficients of reflection,
transmission, and absorption) do not necessarily match those of any natural material.
Photonic crystals exploit this principle by varying the density or refractive index of a
material in a regular, periodic way. Just as light is affected by the spacing of atoms in a
natural crystal, it can be affected by the (much larger) spacing of sub-wavelength
features in a photonic crystal. Thus, photonic crystals can efficiently reflect some
wavelengths of light while absorbing, transmitting, bending, or scattering others. This
can be useful, for example, in telecommunications, where a single optical fiber may
carry thousands of different signals. Because similar effects occur naturally in many
gemstones (opal, for example), photonic crystals can also serve as artificial gems.
Whereas photonic crystals are generally insulators, another class of materials-called
superlattices-is made from semiconductors, metals, and other substances stacked in
very thin layers. Such materials "look" like crystals to the electrons and photons moving
through them but can have properties that do not occur-or occur only weakly-in
nature. Two examples are "magnetoresistive" materials made from alternating layers of
iron and a nonmagnetic material such as chromium, Even in very tiny quantities, such
materials can be used to sense magnetic fields with much greater sensitivity than can
any natural material and are widely used in hard disk drives and digital compasses.
In a "metamaterial," the sub-wavelength features are conductive metals surrounded by
a transparent dielectric material such as glass, air, or empty space. In much the same
way a metal rod interacts with radio waves and can thus serve as an antenna, the
"atoms" of a meta material create strong resonances at particular wavelengths that can
have more profound effects on light than can any of the material systems described
above. The best known of these is negative index of refraction, a property that allows
materials to bend light "the wrong way" and thus defy the laws of classical optics.
LIQUID CRYSTALS
Although they were discovered in the late 19th century and have been used in video
displays since the 1960s, liquid crystals can be thought of as an advanced 21st century
technology that somehow fell backward in time. They are essentially a fourth state of
matter, possessing some properties of a liquid and some of a crystalline solid. In
addition, they are "birefringent," meaning they have a different index of refraction,
depending on the angle or direction of light passing through them. They are capable of
serving as photonic crystals, and under the influence of light, heat, electric and
magnetic fields, and mechanical or chemical alignment layers, they can be rearranged
at a moment's notice so that their optical properties are transformed.
Today, people are accustomed to thinking of liquid crystals as pixels in a video display,
However, their capabilities extend far beyond this. With proper substrate design and
the application of electric fields and other alignment or disalignment mechanisms1 liquid
crystals can be used to reversibly block, bend, focus, scatter, reflect, transmit, twist,
polarize, diffuse, and absorb light in limitless combinations.
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