What we have seen so far is that rare earth doped nitrides can be excited to obtain emissions from the f-f inner shell transitions of rare earth ions. However a few questions remain: (i) why is that GaN is such a good host?, and (ii) how is that the energy absorbed by GaN matrix is transferred to the RE ion? Ways to probe mechanisms of this kind include pressure and temperature dependent photoluminescence. Photoluminescence simply means that light is used to excite material suitably, so that it emits well. The emission spectrum is collected and analyzed using an appropriate spectrometer. Pressure and temperature dependent PL essentially means is that you "squeeze" or heat the material, while doing the photolumiescence experiment. Variations from the room temperature, atmospheric pressure photoluminescence spectra of the material is helpful in elucidating luminescence mechanisms.

Typically heating the sample reduces the emission intensity of the material. This phenomena is called thermal quenching. However we noticed that the thermal quenching can be completely suppressed by application of pressure! This observation strongly supports a certain model of RE excitation, wherein incident light results in the formation of excitons (essentially bound electron-hole pairs), which then get "pinned" to the neighbourhood of the RE ions. This pinning is responsible for energy transfer between the exciton and the RE ion. The application of compressive hydrostatic pressure (~6.8 GPa) results in stronger localization of bound exciton on Eu3+ ion trap; which in turn enhances the efficiency of energy transfer between the exciton and the RE ion, while reducing possibility of energy back transfer between Eu3+ ion 4f-shell electrons and GaN host. A combination of these factors is responsible for observed suppression of luminescence thermal quenching by the applied hydrostatic pressure.

Further more, it was found that the Eu ions create different electronic states within the forbidden gap of GaN (think of them as electronic energy levels that should'nt exist, but are created because we have added Eu into the lattice). We also notice a few less efficient Eu3+ ions excitation pathways (other that the exciton mediated path described above) due to shallow energy levels in the forbidden gap. In simpler terms What this means is: addition of Eu within the GaN matrix, results in addition of levels within the forbidden energy gap, which provides alternate means by which conduction electrons can excite the RE ions.

An interesting outcome of our collaborative studies with Prof. Jadwicienzak's group was the discovery of the high radiation hardness in our powders. We discovered that optical properties of our Eu:GaN powders remain unaffected when 2 MeV oxygen ions (visualize them has Oxygen ions moving at about one-hundredth the speed of light!) are bombarded on to our powders with a fluence of 1.7×1012 to 5×1013 cm-2 (fluence = number of particles per unit area). For these experiments, the powder was embdedded in a matrix of another material called potassium bromide (KBr). Our studies suggest that Eu-doped GaN powder phosphor can be considered for devices meant for use in high radiation environments. To put our results in perspective, it is helpful to know that the alternative technology (Eu:GaAs) results in a system that has radiation tolerance that roughly similar or less than our system. But our system has an additional advantage of being free of arsenic, which is a toxic element.