Trapping the light fantastic

An illustration shows a brilliant circle of white light with the outline of a blue square surrounding it. Light beams extend in all directions, represented by thick, textured red lines.

Trapping the light fantastic

The University of Toronto’s Sajeev John set out to answer the fundamental question of how to trap and control light. Once he did, it opened the door for boundless commercial applications, from medicine to telecommunications.
August 17, 2015

Thirty years ago, many scientists said the University of Toronto’s Sajeev John was chasing the impossible — that he was asking questions for which the answer was simply “no.” But that didn’t deter the young physicist from setting out on an expedition to trap one of the most elusive beasts known to humanity: the photon.

Photons, which are elementary particles of light, have no mass and no electrical charge. In a vacuum, they move at nearly 300,000 kilometres per second. Most challenging of all, common, impure materials absorb photons the way a sponge absorbs water.

“You can use mirrors to trap photons for a short period of time, but then they’ll escape or get absorbed,” says John. “You have to use materials that just don’t absorb light. That was a big part of the challenge.”

While John was working out how to trap light, he also found himself having to justify why. He began with a typical scientist’s take that very fundamental questions are worth answering for their own sake.

“Sometimes small questions of science, when investigated thoroughly, lead to extraordinary answers,” he says.

But once he succeeded, entrepreneurs quickly took notice. John’s light traps — known as photonic crystals — offered an entirely new way to control light. Computer chips operating with laser light. Medical biosensors. Less expensive, more efficient solar panels. Next-generation fibre optics.

“During the telecom bubble, I was getting lots of calls from venture capitalists, but I decided not to take their money at that time,” he said. “Today, there are certainly big companies and labs that are pursuing applications.”

One application that currently holds John’s attention is “Lab on Chip” medical biosensors that can supplant lengthy diagnosis processes.

“You use a thin photonic crystal chip with channels through which a blood sample can flow,” he says. “You place certain antibodies along the interior surfaces of the crystal that act as chemically selective ‘mousetraps’ for biological markers of certain diseases. When the ‘mice’ attach to the surface, it changes the chip’s optical characteristics. You shine a laser beam through that chip and create what we call a ‘spectral fingerprint’” of the disease.

Instantaneous diagnosis.

The major challenge: even after more than a decade of research, precise light traps are costly to make.

A photonic crystal is made up of delicately-sculpted, diamond-like clusters of atoms, which don’t absorb light. Light wavelengths are large compared to single atoms, which means scientists must create lattices with billions of atoms in a precise repeating pattern, attuned to the wavelength of the photons they’re trying to trap and hold. Every time a photon tries to move in any direction, it instantaneously bounces back on itself. The photon never stops moving, but never gets anywhere.

“It is quite doable for well-funded research labs and companies, but it’s still expensive and requires specialized equipment to make high-quality crystals,” John says.

Since John first started exploring them in the 1980s, photonic crystals have continued a slow but steady march toward commercial applications. Meanwhile, the academic world has been solidly behind John’s “small question with extraordinary results.”

This story originally appeared in the fall 2014 issue of EDGE, the University of Toronto’s research magazine.