Photonic crystals can be thought of as nanostructured materials in which periodic variations of some property (usually, the material's electric permittivity) produce a photonic "band gap", which affects how photons propagate through the material. The effect is similar to how a periodic potential in semiconductors affects the flow of electrons by defining allowed and forbidden energy bands. In the case of photonic crystals, photons with wavelengths or energies in the photonic band gap cannot travel through the crystal. This allows scientists to control and manipulate the flow of light by introducing carefully selected defects into the material.

Until now, researchers have only been able to manipulate photons inside such crystals by introducing defects into the bulk, but Susumu Noda and Kenji Ishizaki of Kyoto University have found that they could manipulate photons at the surface of 3D photonic crystals too. The effect opens up a new, easily accessible, route for manipulating photons and might one day be important for using photonic crystals to control light in optical circuits, say the scientists.

Noda and Ishizaki achieved their result by first showing that 3D photonic crystals possess surface states, and that photons can be confined and propagate through these states. Next, the researchers demonstrated that photons can be localized at desired surface points by forming a surface mode gap and introducing surface-defect structures. To their surprise, they obtained quality, or Q, factors of up to 9000, the highest ever value reported for 3D photonic crystal nanocavities. Q factors indicate how strongly, or for how long, photons can be confined in a nanocavity, and the higher the value, the better.

"Our work represents an important step in realizing a new route to manipulating photons by 3D photonic crystals," Noda told nanotechweb.org. "Accessing light from outside these crystals would be much more straightforward compared to that from manipulating it inside the materials."

Because the surface of the 3D photonic crystal does not absorb light, it might be used as a new type of sensor that would work by detecting the presence of a chemical or biomaterial as a change in the effective refractive index of the nanocavity system. Other applications include advanced photonic circuits and novel nano-photonic devices, such as improved LEDs and solar cells.

The number of stacking layers in the photonic crystals made by Noda and Ishizaki currently stands at eight and the researchers would now like to increase this because it would allow even better confinement of light and reduce light leakage via the bottom layers. They would also like to try their hand at realizing some of the applications mentioned above.

The work was published in Nature.