An article written by M. Kitaura, H. Zen, K. Kamada, S. Kurosawa, S. Watanabe, A. Ohnishi, K. Hara was published in Applied Physics Letters
Title; Visualizing Hidden Electron Trap Levels in Gd3Al2Ga3O12:Ce Crystals Using a Mid-Infrared Free Electron Laser
DOI: 10.1063/1.5008632
Advanced Energy Generation Division　Quantum Radiation Energy Research Section

Stimulation of Ce3+ 5d-4f luminescence by mid-infrared (MIR) light pulses from a free-electron laser (FEL) is pronounced above 0.31 eV. It is concluded that the electron trap levels are located 0.31 eV below the bottom of the conduction band. This study demonstrates that MIR-FEL is applicable for the exploration of hidden electron trap levels.