Light the night: nanophotonics and photonic crystals

24 March 2017

Recent advances in material physics, optics and nanotechnology have enabled scientists to manipulate photons at nanoscale, creating new and exciting technological opportunities. As light propagates in microstructures less than a hundredth of the diameter of human hair, interesting new properties emerge, which can be exploited to synthesise materials to cool buildings in hot sunny days without using electricity, detect minuscule contaminations in fluids, and may enable faster computers that run on light signals.

This may sound like science fiction, but it is already science fact. And they would not have even been imaginable without nanophotonics -- a discipline of study on the behaviour of light and its manipulation at extremely small scales.

Dr Wah Tung Lau, a postdoctoral fellow at University of Toronto, conducted many of his research in nanophotonics using a class of minute structures known as photonic crystals, which his postdoctoral advisor Sajeev John invented in 1987. It is a category of artificial optical material inscribed with orderly-spaced micro-patterns of size comparable to the wavelengths of a photon, the quantum of light. At such microscales, the wave nature of photons is in full expression, and the behaviours of photons depend excessively on their frequencies.

“Due to destructive interference caused by the microstructures, certain frequency intervals of photons can be completely rejected from propagating in the crystal. Such forbidden bands of frequency, known as photonic band gaps, is the prime feature associated with photonic crystal,” explained Dr Lau. “As its band gap frequencies depend sensitively on its geometric details, we can manipulate the behaviours of photons in photonic crystal by engineering its microstructure.”

He added: “Photons can be used to carry signal, or energy. Usually, for signalling, we need to handle very narrow range of frequencies. For energy applications, we deal with thermal radiation, which is broadband of light. My research uses photonic crystal to implement these applications.”

Photonic heat insulation

Dr Lau’s most important work is on thermal insulation using photonic crystals. For his graduate thesis, his doctoral advisor had suggested him to design nanostructures that enhance the extraction of heat. This is a pressing research topic as heat accumulation is the main problem that hinders performances of everything from microprocessors to air-conditioning units. After trying various options, they realised that with its ability to block photons, photonic crystals can function as a thermal insulator to suppress heat transmission.

“We discovered a general statistical law of photonic band gaps that can apply to the entire thermal radiation spectrum. This was counterintuitive at the time, because typical research in photonic crystals concerns only narrow-band of frequencies adjacent to its lowest-order band gaps, and the crystal structure needs to be precisely designed to tune the band gap frequencies. However, for the broad thermal spectrum, all higher-order band gaps are involved,” explained Dr Lau.

“Designing structures that tune so many parameters of band gap frequencies becomes too tedious. Our statistical theory shows that the total size of all orders of band gaps, which is related to its thermal insulation capabilities, is theoretically a constant independent of the detail design of the microstructure.”

Dr Lau and his advisor also found that photonic crystals could block heat flow more effectively than in a vacuum. Despite this theoretical breakthrough, it turned out that there were other simpler and more effective approaches to achieve thermal insulation. Still, Dr Lau values the intellectual process carried out in the study.

Years after he left his group, his doctoral advisor Shanhui Fan and former colleagues come up with a new idea to enhance heat dissipation. “They invented a composite ultrathin film to be mounted at rooftops, which can passively cool buildings below the ambient temperature without applying electricity.”

Light guiding and sensors

Another important area of research for Dr Lau concerns the use of photons as conveyors of signals. “The electrical wires that connect devices or microprocessors on motherboards can potentially be replaced by photonic waveguides, for higher channel capacity and lower power dissipation,” he explained. “Over years, waveguides with much simpler structures have been adapted with great success. To these days, the phase out of electronic circuit board by photonics are less hindered by the transmission channels, but the electron-photon conversion at the terminals of such channels.”

Dr Lau also collaborated with a team to work on photonic crystal slabs, which are thin dielectric slabs of wide area inscribed with two-dimensional lattice of tiny air holes, and can function as frequency-selective mirrors.

After spending years in the field of nanophotonics, Dr Lau is now contemplating a career move. Nevertheless, he is passionate about his research work, and he thinks photonic crystal is a major scientific innovation with a bright future, even for now, its full potential in the industry has yet to be fulfilled.

“Scientific inventions can be difficult, involve trial and error, and even generations of dedicated effort. This is also a reason why it is a noble profession and valuable ideal to pursue,” he added.

Dr Wah Tung Lau obtained his MS and PhD degrees in electrical engineering from Stanford University, specialising in theoretical and computational nanophotonics. In 2010 he received Croucher Fellowship and moved to University of Toronto for his research.

To view Dr Lau’s personal Croucher profile, please click here.