Site-selective upconversion excitation of Er3+: KYF4

A spectroscopic study of the green (S-4(3/2) --> I-4(15/2)) and blue (P-2(3 /2) --> I-4(11/2)) emissions from Er3+:KYF4 induced by a red tunable laser excitation is realized at liquid nitrogen temperature, for two Er3+ concent rations: 0.1 and 5 at.%. The emission spectrum can only be accurately inter preted by considering that it is due to two sites, whose contributions are successfully isolated using a two-step absorption process via I-4(13/2) for the green emission and a three-step absorption process via I-4(13/2) and S -4(3/2), for the blue emission. The excited-state energy transfer prevails over the excited-state absorption at high concentration (5 at.%).