As a young PhD student at Harvard University in the mid-1980s, Sajeev John published an audacious paper in the prestigious journal Physical Review Letters. It contained a radical notion: light could be trapped.
The single-author work caught the attention of Nobel Prize-winning physicist Philip W. Anderson of Princeton University, renowned for the discovery of "Anderson localization" - a phenomenon in which electron waves existing in solids with an irregular internal structure are prevented from conducting current.
Anderson's interest was apparently piqued by John's argument that a different kind of irregular arrangement of matter could be used to trap photons - the fundamental particles of light. A year later, Anderson, who John says had never imagined such an idea much less put forward such a notion, wrote a paper that largely agreed with John's original take - "except for one specific point."
"I set out to resolve this little misunderstanding and travelled to Princeton to lay out my theory in a chalkboard presentation," says John, now a University Professor of physics in the University of Toronto's Faculty of Arts & Science. "He listened attentively in silence before asking me to erase the board and redo my equations from scratch.
"I thought, 'OK, he's sharpening his knives here.'"
Instead, Anderson offered him a job at Princeton as an assistant professor. John was speechless.
Within his first year at Princeton, John published a second seminal paper confirming his idea of light localization through the invention of a new class of structured matter known as photonic band gap materials.
In the decades that followed - initially at Princeton, but mainly at U of T - John went on to develop the science and applications of photonic crystals: structures that can confine and manipulate light at a microscopic scale.
Much as semiconductors allowed scientists to control the flow of electrons and set the stage for modern electronics, photonic crystals are ushering in a new era for harnessing the power of light. Applications include: processing huge amounts of information at massive speeds; building next-generation semiconductor lasers with unprecedented brightness and power; diagnosing disease instantaneously using a hair-size volume of body fluid; and more efficiently converting energy from the sun into electricity or storable fuel.
For his work in launching and leading the field, John has received several of the most prestigious honours in science including the Gerhard Herzberg Canada Gold Medal for Science and Engineering , the King Faisal International Prize in Science , the Institute of Electrical and Electronics Engineers (IEEE) Quantum Electronics Award and fellowship in the U.K.'s Royal Society .
Yet, he says his greatest reward is seeing photonic crystals move from equations and computer simulations into the real world, where their potential is rapidly being realized across a wide swath of fields.
"There are so many exciting potential applications, of vital importance to society, that we're discovering left and right."
Trapping light
The defining feature of photonic crystals is the photonic band gap: a range of light wavelengths that are prevented from passing through and even existing in a material by an ingenious design. This effect is the result of the crystal's geometrical arrangement of dielectric matter that periodically repeats itself with a repetition length less than half the wavelength of light, causing unprecedented interference of light waves.
"If you arrange the matter in just the right way, you can get the light that is attempting to propagate to interfere with itself very severely," says John. "In a wave of light, you have crests and troughs - when a crest and a trough meet, they cancel each other out."
By introducing deliberate defects into a photonic crystal, researchers can create singular pathways that channel chosen light in desired circuit paths with no possibility of unwanted escape, while preventing light of undesired wavelengths from entering the crystal.
The achievement initially paved the way for an unprecedented method to guide light using wave-interference effects. Whereas traditional optical fibres use total internal reflection to herd pulses of light through a solid fibre core, photonic crystal fibres can guide light through a hollow core, allowing a vast new variety of wavelengths to be conveyed and facilitating much higher light intensities.
The ability to safely guide high-intensity light through a flexible photonic band gap fibre led to the first life-saving application in a Boston area hospital. In 2004, a patient with a recurring tracheal tumour was successfully treated using laser light delivered through a hollow-core fibre - with thousands of similar procedures performed since then.
"One point of guiding light in an air - or gaseous - core is that if you had the traditional solid core fibre, the intensity of laser light needed for certain surgeries would probably destroy the fibre itself," says John.
"Rather than line-of-sight laser surgery involving incisions and trauma to the patient, it could now be done through a flexible endoscope to deliver intense light to a region previously inaccessible without a knife."
Now, John and his team are working on another health-care application: lab-in-a-photonic-crystal chips that can identify multiple disease marker proteins from blood samples. By using photonic crystals with chosen interior surfaces coated with various DNA aptamers (synthetic, molecular receptors) that bind telltale proteins, the devices can instantaneously diagnose early-stage disease using a drop of blood.
"When those proteins attach to DNA-laced surfaces, it changes the frequencies of light that can pass through the photonic crystal - and you get an instant spectral fingerprint of the disease," says John.
Yet another promising application is solar energy capture, where photonic crystals are enabling ultra-thin solar cells that can be coated on the surfaces of buildings and automobiles.
"With photonic crystals, you can make a very thin piece of silicon - a factor of 50 thinner than traditional solar cells - and have it absorb and convert sunlight more efficiently than the thick ones," says John, noting that existing silicon solar cells struggle to absorb light at near-infrared wavelengths of sunlight.
Prototypes of this technology are being developed in collaboration with photovoltaics experts in Germany and microfabrication expertise from Australia.
Road to discovery
The son of a physicist father and biochemist mother, John grew up in Ottawa and London, Ont. He became interested in math prior to grade school and recalls working through texts like Calculus for the Practical Man well before his teens.
After topping his undergraduate class at the Massachusetts Institute of Technology, John attended Harvard, where his doctoral research focused on demonstrating how vibrational waves can be trapped in certain acoustic materials.
Soon afterward, John wondered if his theories about sound waves could also apply to light. He decided to test his conjecture. "We're all familiar with propagating light," says John, referring to how light waves travel freely through space and many physical materials. "It's part of our daily experience. But no one had ever considered whether light could also be trapped."
It was the pursuit of that question that led to John publishing his 1984 paper showing how light waves could be weakly confined in structurally disordered material. "It was just an innocent paper," says John, who then moved on to other research in superconductors and semiconductors - that is, until Nobel laureate Anderson praised his original conclusions.
Later, after being recruited as Princeton faculty, John began exploring a different route to trapping light - using highly ordered structures rather than randomly disordered ones - leading to his landmark second paper describing how specially designed, repeating lattice structures could trap photons over a range of frequencies. The approach called for a precisely engineered arrangement of dielectric matter that would create a forbidden range of wavelengths at which light cannot propagate through the structure.
A similar concept was independently put forward by Eli Yablonovitch, a scientist at Bell Communications Research; the two papers were published in the same journal, Physical Review Letters, a few weeks apart. The two met for lunch at the Institute for Advanced Study in Princeton to discuss their ideas and agreed on a name for their proposed structures: photonic band gap materials.
Two years later, just as the potential of photonic band gap materials was inspiring research groups around the world, John raised eyebrows when he decided to leave Princeton and accept a senior position at U of T - a homecoming of sorts.
He says there were also important research considerations behind the move.
"The interconnectedness with different departments at U of T, like engineering and chemistry, meant there were unique opportunities to think outside of your own discipline. There was also more incentive to collaborate with talented international groups with capabilities not yet established in the U.S."
The bet paid off. In U of T's department of physics - and the former Ontario Laser and Lightwave Research Centre - John spent much of the 1990s developing theoretical roadmaps for photonic crystals while working with a diverse team comprising his own students and postdocs from the U.S., Canada, and overseas.
He and his collaborators - including U of T materials chemist Geoffrey Ozin , physicist Henry van Driel and scientists in Spain - achieved a major breakthrough at the turn of the century: silicon photonic crystals that could self-assemble without needing to be manually fabricated. Crucially, these crystals featured a photonic band gap near 1.5-micrometre wavelength - the wavelength of choice in optical communications.
A focus on the future
Whereas conventional fibre optics enable long-distance guiding of light, photonic crystals make it possible to manipulate light on microscopic scales, opening the door to potentially game-changing innovations in computing and beyond.
"You can create optical microchips, analogous to electronic microchips, with minuscule circuits of light on a scale less than the thickness of a human hair," John says.
He adds that by packing light-guiding structures onto microchips, researchers could one day build computers that operate with hundreds of independent wavelength channels, carrying information at extremely high speeds - nothing travels faster than light - while being less vulnerable to heat generation than electronic computers and more stable than quantum computers. John cautions, though, that the technology currently trails theoretical predictions in this realm.
After four decades in the field he created, John remains most energized by bold scientific questions and the inquisitive, independent thinking required to pursue them.
"He has an astounding talent for breaking down complex topics in a way that is clear and easy to understand - it's incredible how he sees and understands physics so quickly," says Raymond Langer, a PhD student working with John on lab-in-a-chip biosensors for medical diagnostics.
He adds that John is an ideal mentor.
"Sajeev provides you with support, when necessary, but he also gives you the space to overcome obstacles on your own and come out a much better physicist."
For his part, John says a key part of his role at the university is to encourage the next generation of scientists to pursue questions they find genuinely compelling and important - the same approach that launched his own academic career - rather than simply joining the bandwagon of the most hyped research field of the day.
"I didn't start out with any plan to invent materials to microscopically control light on a chip," says John. "I was simply doing innocent, curiosity-driven physics."simply doing innocent, curiosity-driven physics."
