Even thin photonic crystals are functional, UT finds

Photonic crystals don’t need to be thick and bulky to be able to manipulate photons by means of an energy gap, as was generally held. Researchers from the University of Twente (UT) and three Japanese institutions show this in Physics Review B. The new insight could lead to optoelectronic devices that control the flow of light in a similar way as semiconductors control the flow of electrical current. Possible applications include computing, telecommunication and enhancement of thin-film solar cells.

The research focused on photonic crystals with the so-called woodpile structure: stacked arrays of rods in two perpendicular directions. Spectral measurements unveiled that all crystals, even those of only a few layers, showed high reflectivity in broad peaks. This indicates the existence of a ‘forbidden zone,’ where light in a certain wavelength range isn’t allowed to enter the crystal and hence is reflected.

“The quick formation of the forbidden gap in our crystals is remarkable because earlier 3D crystals required a large thickness for a gap to emerge,” comments first author Takeyoshi Tajiri of the University of Tokyo. UT group leader Willem Vos adds: “The result that even thin structures are fully functional is great news for applications in photovoltaics. Here, scientists are looking for thin broadband back reflectors to improve the performance of thin solar cells.”