Wavelength Redistribution and Color Purification Action of a Photonic Crystal

Currently, photonic crystals are attracting a lot of interest because of their ability to harvest light from a device into specific directions and wavelengths. In this work we have proven the theoretical prediction that in the case of an emission overlapping with the photonic stop band, the intensity is redistributed at different wavelengths. This prediction has two major consequences: (i) the total QY remains the same and (ii) the intensity increases just outside the band gap. In our case, Eu2+ is the responsible emitter in a hybrid material based on GaN on silica, which has a fairly broad emission with its maximum at 500 nm. The GaN and Eu2+ were placed inside an inverse opal of silica (air voids in silica matrix). The size of the holes in the different samples was varied between 300 and 600 nm, in order to tune the stop band in different positions with respect to the Eu2+ emission. The measured quantum yield was constant for the different samples at about 5%, the lifetime of the Eu2+ increased in the forbidden range, and its emission intensity was squeezed toward the side of the stop band, with a concomitant decrease of the lifetime. The enhancement of the emission intensity at a certain energy range opens new possibilities for the design of more efficient devices, providing color purification and intensification at whichever wavelength is needed.