Selectively strong green up-conversion luminescence in Nd3+-Ho3+ co-doped ZrF4-based fluoride glasses under 800 nm excitation

Blue, green and red up-conversion luminescences at around 490, 545 and 650 nm, which result from the Ho3+:F-5(3) --> I-5(8), (F-5(4), S-5(2)) --> I-5(8) and F-5(5) --> I-5(8) transitions, respectively, were observed in Nd3+-Ho3+ co-doped ZrF4-based fluoride glasses under 800 nm excitation. Among these up-conversion luminescences, the green emission was extremely strong and the blue and red emission intensities were very weak. Selectively strong green up-conversion luminescences of these glasses indicate a high possibility for realizing a green up-conversion laser. Up-conversion processes for the blue, green and red emissions are two-photon processes assisted by Nd3+ -->Ho3+ energy transfer. It is proposed that the up-conversion mechanism for the blue and green emissions is different from that for the red emission. The respective mechanisms are discussed.