Markus Wermuth and Hans U. Güdel
Luminescence spectroscopy and NIR to VIS upconversion of Cs₂GeF₆: 2% Re⁴⁺
J. Phys.: Condens. Matter 13, 9583-9598 (2001)                 

Abstract: 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 11000 cm^–1 in the near infrared, visible upconversion luminescence around 17000 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.