Photonic Crystals Conduct Light

Photonic crystals have the advantage that their properties can be predetermined very exactly. However, a disadvantage is that they only conduct light along straight lines or divert it at a small angle. A trick finds a remedy; individual defects in crystal capture photons and emit them again vertically to their original direction. This could make possible considerably smaller optoelectronic components than so far produced.

Photonic crystals are synthetically produced materials with special properties: in these materials the electrical polarisability varies periodically. That does not yet sound particularly exciting but has momentous consequences. So-called photonic band gaps (PBG) are created. They prevent electromagnetic waves of particular frequencies being able to spread through the crystal while other frequencies are not impressed. Material scientists can construct a wave conductor for them from photonic crystal. To do so they destroy the band structure with defects so that light can no longer disperse across these areas but only where the crystal is still intact. Through the specific placing of such destructive defects light must take exactly the path prescribed by the scientists.

However, this does not leave much room for manoeuvre because the light path can only deviate slightly from a straight line. It is therefore absolutely impossible to produce as small components as desired, even for the most skilful engineers. But Susumu Noda, Alongkarn Chutinan and Masahiro Imada from the Department of Electronic Science and Engineering at Kyoto University did not want to resign themselves to this fact. In order to save unnecessary precision experiments they first set up a simulation to examine a slab from a photonic crystal with regularly arranged holes as disturbances, from which however one row was missing. The photons could only cross the slab along this straight line. Near the wave conductor the scientists drilled one single other circular hole. This revealed itself to be a resonator and thus captured photons of a particular wavelength – in this case 1.539 micrometers. The scientists' trap caught around half of all the passing light particles. However, there were no permanent captives because then the crystal emitted the photons vertically to the surface into the air (Nature, 5 October 2000). "Nobody had previously found that one single defect can serve as a coupler between the plain and the vertical direction to it," explained Noda. With normal photonic crystals this is not in fact possible because the air surrounding them has completely different refraction characteristics. This differences block the way for the photons from one medium to the other.

After the promising simulation the scientists set about the practical work and produced a real photonic crystal made of indium, gallium, arsenic and phosphorous with a thickness of 0.25 micrometers. The transferred the defect pattern from theory to the crystal through cauterisation. And also in reality the additional defects revealed themselves to be resonators, which could even be precisely modulated. "By changing the size of the defect we can very easily adjust the wavelength of the photons which are captured and then emitted again," said Noda.

"Our results also provide a new way to develop ultra-small optical devices," is how Noda and his colleagues rate their experiment. The development could bring enormous consequences for global network communications because of their flexibility, efficiency and very small scale.