The Building Blocks of Upconversion Absorption and Energy Transfer

The Building Blocks of Upconversion Absorption and Energy Transfer 

A wide variety of upconversion mechanisms have been proposed and observed. The majority of these involve some combination of absorption and nonradiative energy-transfer (ET) steps. Absorption may come in two basic forms. Ground-state absorption (GSA, Fig. 2 a) results in promotion of an ion from its ground state to an excited state. Excited-state absorption (ESA, Fig. 2 b) involves absorption of a photon by an excited ion, and results in promotion of that ion to a higher excited state. 

The simplest upconversion mechanism possible is therefore that of GSA followed by ESA (GSA/ESA). GSA/ESA upconversion is inherently a single-chro-mophore effect, and as a consequence this mechanism is relatively insensitive to 

The basic building blocks of upconversion, and related processes. Solid arrows indicate radiative processes, and dashed arrows indicate nonradiative processes 

When a single excitation beam is used, photons of the same energy must be capable of inducing both excitation steps, and this places a restriction on the wavelengths at which this mechanism is effective (see Sect. 6). The expressions at Eq. (6a-c) are valid in the limit of low pump powers. Eor discussion of the analogous properties under high pump power conditions, see Sect. 5. 

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