Luminescence spectroscopy and NIR to VIS upconversion of Cs2GeF6: 2% Re4+

We present and analyse high-resolution absorption, luminescence and upconversion-luminescence spectra of Cs2GeF6: 2% Re4+. The crystals of Cs2GeF6: 2% Re4+ were grown from solution. Upon excitation around 11 000 cm−1 in the near infrared, visible upconversion luminescence around 17 000 cm−1 is observed. On increasing the temperature from 15 K to room temperature the integrated upconversion luminescence intensity decreases to 2%. This is attributed to the decreasing absorption cross-section at the monochromatic laser-excitation energy. The upconversion excitation spectra and the results obtained from time-dependent measurements indicate a dominant energy-transfer upconversion mechanism.