Anomalous power dependence of upconversion emissions in Gd2O3: Er3+ nanocrystals under diode laser excitation of 970 nm

Visible green and red upconversion (UC) emissions with anomalous power dependence were observed in Gd2O3: Er3+ nanocrystals at room temperature under diode laser excitation of 970 nm. The green and red UC radiations both yield an "s"-shape power dependence, in marked contrast to the quadratic ones of the bulk counterparts. A closed positive looping UC mechanism that differs from conventional PA mechanism is proposed to explain the observed five- or six-photon processes in the "s"-shape power dependence. Power dependence analysis of the 1.55 mu m emissions from the I-4(13/2) state experimentally demonstrates our proposed model. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3078814]