When pumped with a 1.48 µm laser diode, Er-implanted Al203 ridge waveguides emit a broad spectrum consisting of several distinct peaks having wavelengths ranging from the midinfrared (1.53 µm) to the visible (520 nm). In order to explain these observations, three different upconversion mechanisms are considered: cooperative upconversion, excited state absorption, and pair-induced quenching. It is found that for samples with a high Er concentration (1.4 at. %), cooperative upconversion completely dominates the deexcitation of the Er3+ ions. For a much lower concentration (0.12 at. %), the influence of cooperative upconversion is strongly reduced, and another upconversion effect becomes apparent: excited state absorption. These conclusions are based on measurements of the luminescence emission versus pump intensity, and also on measured luminescence decay curves. The upconversion coefficient is found to be (4±1)x10-18 cm3/s; the excited state absorption cross section is (0.9±0.3)x10-21 cm2. It is shown that in spite of these upconversion effects, a high fraction of the Er3+ can be excited at low pump powers. For pump powers between 2 and 10 mW, the optimum Er concentration is calculated. The results show that for an Er concentration of 0.5 at. %, more than 2 dB/cm net optical gain is achievable at a pump power less than 10 mW.

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Journal J. Appl. Phys.
van den Hoven, G.N, Snoeks, E, Polman, A, van Dam, C, van Uffelen, J.W.M, & Smit, M.K. (1996). Upconversion in Er-implanted Al2O3 waveguides. J. Appl. Phys., 79, 1258–1266.