A light idea: Students create 'largest' quasi-crystal

In an attempt to harness the speed of light for new applications, the Princeton Materials Institute may have created the world's largest quasi-crystal.

A senior in the Mechanical and Aerospace Engineering department, Orion Crisafulli '03 has devoted his senior thesis to finding crystals useful for optical circuit technology.

In a project entitled "Photonic Band-Gap Structures of Quasi-Crystals" Crisafulli and third-year physics graduate student Weining Man, are studying the optical properties of photonic quasi-crystals.

"In a nutshell, a photonic crystal is a material that's capable of conducting light in much the same way as a semiconductor material conducts electrons," Crisafulli said.

"With these materials, you can control the flow of light with more precision than most conventional optical fibers," he said. "And they have some substantial applications to future optical communication and information technologies."

A photonic crystal conducts light in much the same way as an electrical wire conducts electrons with the advantage that light moves significantly faster than electricity.

An ideal crystal has a periodic structure — one that is projected periodically throughout space. If you look at part of the crystal and then move a certain distance in one direction, the two locations look identical.

In essence, the entire crystal is just a repetition of "unit-cells."

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Crisafulli and Man are working in the lab of physics professor Paul Steinhardt to study crystals that are quasi-periodic.

Quasi-periodic crystals do not have a structure that repeats the same unit-cell as you go through space, but the arrangement is not random either. Certain rules determine the appearance of each subsequent segment throughout the structure, although regions are not identical as would be the case in an ideal crystal.

In addition, quasi-periodic crystals differ in that they have symmetrical axes which are not permitted by the rules of periodic crystals. One such example is the quasi-crystal that has a fivefold axis of symmetry which simply could not occur in a periodic crystal.

"Very little is currently known about the optical properties of these [quasi]crystals, but we think that their unusual symmetries may make them useful as photonic crystals," Crisafulli said.

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Although quasi-crystals are theoretically infinite in the length of each unit, only finite structures can be manufactured in the lab to study the potential properties of a proposed configuration.

Steinhardt, who was the first to discover quasi-crystals and is an expert in the field of materials engineering, said he believes that Crisafulli and Man have generated the world's largest icosahedral — twenty-sided — quasi-crystal. The crystal created by the two students is the largest of its kind, not in actual size, but rather in the number of building blocks that make up the structure. The structure is designed so that each site is occupied by a rhombohedron — a six-sided figure — and the rhombohedra are, in turn, connected by rods.

The crystal manufactured by Crisafulli on the theoretical side and Man on the experimental side, is composed of over one thousand such rhombohedra and has allowed the students to perform preliminary experiments on the optical properties of this specimen.

Previously, such a structure would have to be manufactured and tested using inefficient trial and error methods, but Crisafulli has developed a computer program to help determine which structures might be the most promising to study.

A native of Hawaii, Crisafulli began programming for this project at the University of Hawaii where computers are still running the calculations that are being put into practice at the lab in PMI.

The program is an amalgamation of the junior paper of Ray Yang '04, who designed a method for generating such crystal structures by computer, and code freely distributed by the Massachusetts Institute of Technology that aids in simulating the properties of prospective crystals.

The crystal was generated by a process called "stereolithography" whereby lasers strike a resin material and harden it. The lasers are controlled by computers that receive instructions from complex calculations that try to determine a viable structure.

The machine used by Crisafulli and Man for this project is housed in PMI, and because of the high cost in owning and maintaining the instrument, it is shared among the labs.

Using this polymer resin, it is possible to create macro-sized crystals that can be seen and studied fairly easily.

For circuitry, however, the crystals will have to be made on the micro-scale and shrunk down to only a few micrometers in length. For the creation of such a minute crystal, polymer resin cannot be used.

There are however, several other viable substances. One such proposed material is gallium arsenide, a semiconductor used in the manufacture of solar cells.

Having spent the fall looking into the two dimensional properties of quasi-crystals, Crisafulli intends to pursue their three dimensional properties in the spring and will likely spend the rest of the semester investigating the optical properties of similar structures.

Understanding the optical properties of quasi-crystals "has the potential to precipitate a computing revolution," Crisafulli said.

Optical circuits would have the ability to function thousands of times faster than conventional electric circuits found in today's computers. Optical circuitry could potentially solve problems and calculations that today's computers are unable to handle.