Jessica Vermeer
30 January 2020

Researchers from University of Twente’s Mesa+ Institute have developed a fully experimental method for determining the bandgap in crystals with a ‘3D photonic bandgap’. These crystals are used to control light and can be applied in new types of solar cells, sensors and miniature lasers.

Photonic crystals have a special structure, which forbids a range of wavelengths passing through. This adds control of light in silicon and opens up the possibility of connecting electronics and photonics. The crystals are created with nanoscale fabrication, leading to a perfectly periodic pattern of pores.

UT photonic crystals
Credit: University of Twente

Until now, there was no practical way to evaluate the quality of a photonic crystal and to tell, for example, how the pore size and forbidden range match. Instead, theoretical models were used to determine the characteristic wavelength region. However, these idealized models always start with some assumptions, eg about fabrication disorders – not all of which can be included.

Using the new experimental technique, it’s possible to rapidly evaluate the quality of a photonic crystal, making it easier to tune the fabrication process for new applications in optoelectronics and quantum photonics. By shining a light on the crystal, of broad bandwidth and over many angles of incidence, the Mesa+ researchers can measure reflectivity, identifying the exact range that’s forbidden. They do this for two perpendicular polarizations of the input light. High-quality crystals show over 90 percent of reflectivity in the forbidden band.