Materials Today

Volume 15, Issue 10, October 2012, Pages 425


Pinning down the butterfly effect: Optical Materials

The iridescent wing of the butterfly remains at once beautiful and intriguing, but for materials scientists it also serves as a natural model for photonic crystals. Now, a European collaboration has mapped how light behaves in complex photonic materials by breaking through the limit of light resolution. The work could lead to the development of new bio-sensors or perhaps more efficient solar panels.

Researchers at King's College London and scientists at the European research institutes ICFO in Barcelona, Spain and AMOLF in Amsterdam, The Netherlands, have sidestepped the optical microscopy limit by combining electronic excitation with optical detection to get an insider view of a photonic crystal and so examine how light is confined in such structures down to a spatial resolution of just 30 nanometres, ten times lower than the optical resolution limit. King's scientist Riccardo Sapienza explains that this work reveals how it is now possible to test optical theories in photonic materials to a new level of accuracy [Sapienzaet al., Nature Mater (2012) 11, 781–787; doi: 10.1038/nmat3402].

The team made a two-dimensional photonic crystal by etching a hexagonal pattern of holes into a thin film of silicon nitride. Such a structure is inspired by nature's photonic crystals found in butterfly, bird feather “coloration” and the shells of beetles as well as the iridescence of opal gemstones all of which are renowned for their shimmering of light. When a photonic crystal “catches the light”, some colors are inhibited as others are reflected. The tiny platelets of a butterfly's wings give rise to this effect whereas in the artificial photonic crystal the tiny holes allow some wavelengths of light to be confined within so-called crystal defect cavities.

The team uses an electron gun in the technique of cathodoluminescence to access the photonic properties of the crystal on the nanoscale. “Each time a single electron from the electron gun reaches the sample surface it generates a burst of light as if we had placed a fluorescent molecule at the impact location,” explains Sapienza. “Scanning the electron beam we can visualize the optical response of the nanostructure revealing features ten times smaller than ever done before.”

As nanofabrication techniques advance so the possibility of constructing more complex artificial photonic crystals with specific optical properties will become possible. Such crystals could then be used as optical waveguides and cavities for telecommunications and sensing devices. “Our research provides a fundamental insight into light at the nanoscale and, in particular, helps in understanding how light and matter interact,” says Sapienza. “This is the key to advances in nanophotonic science.”

“The next step is to use our technique to unveil new physical phenomena. We will start investigating how light can be localized and trapped in materials which show no iridescence, like random networks, for which the present theoretical models struggle to give a definitive answer,” Sapienza told Materials Today.