Avalanche upconversion in Tm:YALO(3).

We report the sudden appearance of upconversion fluorescence and resonant increasing absorption above a threshold intensity in Tm:YALO 3 at room temperature. These effects are shown to arise from nonlinear, cooperative , excited-state dynamics. Comparison with quantum theory reveals essential roles both for pairwise cross relaxation and excitation migration among Tm impurity ions. Absorption features with thresholds in rare-earth crystals were first observed in Pr:LaCl 3 and Pr:LaBr 3 by Chivian et al.' in spectral regions devoid of ground-state transitions. Their experiments were a prelude to development of avalanche upconversion lasers 2 ' 3 operating at shorter wavelengths than their excitation and sustained by optical pumping in regions of transparency. While empirical modeling of the nonlinear absorption 4 observed under avalanche conditions has supported a picture that involves cooperative energy-pooling pair processes between excited and neighboring ground-state ions, basic premises of this model have not been directly tested experimentally. Furthermore no microscopic theory has been advanced that explains observations made to date consistently. In this Letter we present experimental results on avalanche absorption and upconversion in a new system at room temperature as well as a quantum theoretical framework for calculating system behavior. We show that when incident light is tuned away from ground-state absorption wavelengths and into resonance with an excited-state transition of Tm, nonlinear absorption and upconversion fluorescence appear above a distinct threshold intensity. We verify directly that population in the lower resonant excited state undergoes a rapid increase that may aptly be called an avalanche and cannot be explained by linear dynamics. Theoretical analysis indicates that the cooperative dynamics responsible for the avalanche involve two distinct processes, namely, near-neighbor cross relaxation and excita-tion migration between pairs. Collectively, these results furnish a consistent conceptual basis for a detailed understanding of avalanche upconversion and show that in selected media this phenomenon can occur at high enough temperatures to render applications practical. Figure 1 displays both the low-intensity absorption spectrum and the excitation spectrum of induced absorption features near 649.5 nm in the biaxial host crystal YALO 3. A continuous-wave, single-mode DCM ring dye laser polarized along a principal axis was tuned to the wavelength of the excited-state transition of Tm 3 ` at 649.5 nm (Fig. 2) and focused to a spot radius of 45.0 Am in an 8-mm-thick sample. The dopant concentration was 2%. Absorption between 640 and 660 nm was negligible at low incident intensities. However, as …