Improving Light Extraction From Heavy Inorganic Scintillators by Photonic Crystals

The impact of photonic crystal (PhC) slabs on the extraction of light from the heavy inorganic scintillators LuYAP and LYSO is evaluated by combining numerical transmission calculations of a scintillator with PhC coupling face with simulations of the light propagation inside the scintillator. The transmission of the scintillator-PhC coupling face is determined by means of a scattering-matrix algorithm. The PhC slab is assumed to consist of a bulk material with a triangular pattern of air holes that is sandwiched between the scintillator substrate and a layer of optical grease. By folding these data with the angular distribution of the scintillation photons arriving at the coupling face, the light collection efficiency ηL of the system is estimated. The results indicate that a scintillator coupling face equipped with a PhC slab can exhibit a significant gain in ηL. This gain is due to the extraction of photons that are lost in a scintillator with plain exit surface due to total internal reflection. The largest simulated gains of up to a factor of two are observed for small scintillators and for PhC coupling faces with n_bulk ≪ n_sub, n_avg ≈ √(n_sub·n_grease), and d ≤ a, where n_bulk, n_sub and n_grease are the respective refractive indexes of the PhC bulk, the scintillator substrate, and the optical grease, a the lattice constant of the PhC pattern, d the thickness of the PhC slab, and n_avg the average refractive index of the PhC slab determined by n_bulk and the filling factor f. Due to the approximations and idealizations of the model, these gains in light collection efficiency may be lower in practical applications and are expected to be achieved only with specular reflectors with reflectivit- - ies above 90%, and PhC bulk materials with absorption coefficients α_abs ≤ 10³-10⁴ cm^-1 over the whole wavelength range of emission of the scintillator.