U of T chemistry professor Geoffrey Ozin and his research team have made a breakthrough in the assembly of novel materials called photonic crystals that may one day be used in computers based on light rather than electricity.
Photonic crystals are designed to allow light to be controlled just as basic electronic components like transistors control the flow of electricity in electronic devices.
Blueprint developed at U of T
The theory behind photonic crystals was first presented in 1987 by another U of T researcher, physics professor Sajeev John. John suggested that a material with tiny, regularly spaced holes would have the special optical properties needed to perform precise manipulation of light.
Using this theory, researchers have previously been able to create materials that manipulate the long wavelengths of light in the microwave band. Until now, however, little progress has been made towards building crystals that work in the visible and infrared spectrum — the wavelengths used in telecommunications devices.
Nudging light
The problem is that the smaller the wavelength of the light you want to manipulate, the smaller and more closely spaced the holes in the material have to be. To make crystals that work with relatively long-wavelength microwaves, one can simply drill holes in a solid using a laser. But it’s impractical to drill holes small enough to get optical-wavelength crystals.
To get around this roadblock, Ozin and his team have taken a completely different approach. Rather than starting with a solid material and removing pieces to make holes, they assemble the material particle by particle in such a way that gaps of just the right size are left in the structure.
Ozin makes this potentially painstaking task amazingly simple through a technique called self-assembly.
Self-assembly
Starting with a block of glass or silicon, the researchers then cut small and intricate patterns into it. The block is then placed in a solution of tiny silicon balls called microspheres. By carefully controlling the conditions in the solution, the microspheres can be persuaded to neatly fill up the pattern. The result is a regular array of silicon balls, the small gaps between them providing “holes” of just the right size to use as photonic crystals.
San Ming Yang, one of the co-authors of the research paper, says the technique is like packing oranges into a box. When a bunch of spheres are packed together, they typically arrange themselves in a hexagonal grid. But using Ozin’s technique, the microspheres pack themselves into a different pattern, called an FCC lattice.
A citrus solution
“Because we design the box at the beginning,” he says, “we know how the oranges will pack and what it will look like.”
“We make the box in such a way that the oranges can only form one shape ofcrystal. That’s how we can control crystal size, shape and orientation.”
Telecommunications applications are not far away. Companies like Nortel and Lucent are eager to use such crystals for routing and filtering light signals in fiber optic networks. Optical computers, on the other hand, are what Yang calls “a big challenge,” and remain on the distant horizon.
The breakthrough is just the latest in a long string of successes for the team, whose research has been published in Science and Nature.
The team’s success may be partly due to Ozin’s penchant for seeking inspiration outside chemistry.
“All of the big breakthroughs these days are occuring at the boundaries between fields.”
The self-assembly idea, for example, comes from biology. In fact, nature has been building photonic crystals for ages: butterfly wings owe their intricate patterns and colours to naturally grown photonic crystals.
Fashionable roots
Ozin connects his interest in materials chemistry to an unlikely source: his family’s London fashion business. Ozin’s sister was a top fashion designer. While the fashion business may seem worlds apart from materials chemistry, Ozin suggests that they really aren’t so different.
They share a common goal of “using novel materials to create something structurally beautiful with function and utility.”
Ozin is also quick to credit the group of students who work for him.
“My best work has emerged over the last ten years because of the superb group of students that I have been lucky enough to attract to work with me.”
Never shy of a challenge, Ozin and team plan next to tackle the holy grail of the field: so-called “panochemistry,” the ability to control the assembly of materials over all length scales. “There has never been a more exciting time to be a student of chemistry.”
