Investigation of Defect Structure to Determine the Primary Photorefractive Centers Responsible for Enhanced Beam Coupling in KNbO3:Fe

A series of experiments are used to determine the location of the energy levels of various Fe centers and associated defect centers in KNbO$_{\mathrm{3}}$:Fe, as well as conclude which centers are the primary photorefractive centers. In particular, the methods include electron paramagnetic resonance, optical absorption spectroscopy, electric conductivity, and beam coupling. A correlated study using data acquired with these techniques has been performed on as-grown (unreduced) and reduced KNbO$_{\mathrm{3}}$:Fe crystals, which identified which Fe centers were reduced and which ones were not. Conditions for an increased electron source population (improved beam coupling performance) was determined and compared to beam coupling results for pure electron, pure hole, and electron-hole competition processes.