Upconversion processes

Gamelin DR, Gudel HU (2001) Upconversion Processes in Transition Metal and Rare Earth Metal Systems. 214 1-56 Ganachaud F, see Elaissari A (2003) 227 169-193 Garda R, see Tromas C (2002) 218 115-132 Geraldes CFGC, see Frullano L (2002) 221 25-60... 

Gamelin DR, Gudel HU (2001) Upconversion Processes in Transition Metal and Rare Earth Metal Systems. 214 1 - 56... 

If an ion possesses two excited states with approximately same energy separation, energy transfer can occm between two ions in the first excited state (Figme 4c). One ion returns uomadiatively to the ground state while the second is promoted to the second excited state, then decays with emission of a photon whose energy is about twice that of incident photons. There are other upconversion processes two-step absorption, cooperative sensitization, cooperative luminescence. Upconversion by energy transfer is the most efficient process. 

Upconversion Processes in Transition Metal and Rare Earth Metal Systems... 

Note that ETU, like all ET processes, is inherently a pairwise or multicenter effect, and is therefore very strongly dependent upon ion concentration. Whereas GSA/ESA mechanisms often dominate upconversion processes in low-doped samples, upconversion in highly-doped samples is very often dominated by ET processes, in particular GSA/ETU. 

The same parameters are used for each of the two simulation modes, and involve the usually encountered situation of (A 2a + k2b) > ki. The specific parameters used are hsted in the figure caption. Importantly, all simulations reflect low-excita-tion-densities and low-upconversion rates, where the upconversion processes contribute in only a perturbative way to the intermediate-level populations and decay rates (i.e., kiNi>ENi or 2wetu )- This corresponds to a power range that is experimentally easily accessible. As shall be described in Sect. 5, the use of higher pumping powers may in some cases lead to significant and interesting deviations from the behavior described in this section. 

The energy- and time-dependence of an upconversion process are extremely useful for identification of the upconversion mechanism. A third variable parameter that has received relatively little attention is the excitation power. It is often stated in the literature that two-photon excitation processes such as upconversion must follow a quadratic power dependence, or more generally. 

Two-color excitation can thus provide orders of magnitude intensity gains for GSA/ESA upconversion processes through the optimization of both Oq and o. Eor practical applications such as lasing or imaging, this can lead to superior performance [6]. Two-color upconversion excitation methods have been applied in other cases to generate efficient upconversion where little or none was observed under one-color conditions, or to advance mechanistic studies. 

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