We investigated different Er3+ defect sites found in Er-doped GaN layers by site-selective combined excitation-emission spectroscopy and studied the role of these sites in different direct and multistep excitation schemes. The layers were grown by molecular beam epitaxy and were 200 nm thick. Two majority sites were found along with several minority sites. The sites strongly differ in excitation and energy transfer efficiencies as well as branching ratios during relaxation. For this reason, relative emission intensities from these sites depend strongly on emission and excitation. The sites were identified for several transitions and a comprehensive list of energy levels has been compiled. One of the minority sites appears strongly under ultraviolet excitation above the GaN band gap suggesting that this site is an excellent trap for excitation energy of electron-hole pairs.

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In order to study the role of defects within the domain inversion process in ferroelectric LiNbO3 crystals, we investigated the optical properties of intentionally introduced Er3+ defect complexes across a 180° domain wall produced at room temperature by electric-field poling. Using site-selective excitation-emission spectroscopy for well chosen excitation energies, we found drastic differences in the Er3+ emission, which are due to a rearrangement of the defect complexes. We used these changes in a confocal luminescence microscope to image ferroelectric-domain structures. This powerful imaging method with a 700 nm, 50 ms spatial and temporal resolution can be used to study real-time dynamics of domain walls.