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Up-conversion (UC) generally refers to any nonlinear optical process that facilitates the conversion of low energy
radiation into higher energy emission. Typically achieved in materials incorporating rare-earth ions, exploiting their rich
density of available electronic state transitions, non-parametric UC systems are often placed in categories including
excited state absorption, photon avalanche and energy transfer. The latter, energy transfer up-conversion achieves
nonlinear excitation of chromophores as a consequence of non-radiative resonance energy transfer (RET) events, through
coupling with neighboring sensitizer ions. Being susceptible to the local electromagnetic environment, the mechanism
of RET is known to be influenced by surrounding matter, underscoring the importance of similar channels for media
control within UC materials. By developing the principles of two-center UC, a fully quantized representation of local
electronic structure and electrodynamics is extended through the introduction of a mediator species – a vicinal, nonabsorbing
chromophore. This theory underpins a new application of the medium-modified energy transfer theory to
three-center UC. The present report then considers an alternative up-conversion mechanism in which pairs of identical
donors transfer energy to the acceptor species, promoting two-photon excitation and shorter wavelength emission. The
mechanism for this three-center system proves to be significantly influenced by a fourth, essentially passive
chromophore. Investigations of the influence of this mediator, in improving or inhibiting RET, determine parameters
that can be modified to improve the UC efficiency. The results provide insight into factors that might assist the
optimization of laser active media, and the improvement of optical characteristics in designer materials.
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