Medium-dependent resonance energy transfer: A controlling role for three-center upconversion

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.