We demonstrate up-conversion of noncoherent sunlight realized by ultralow excitation intensity. The bimolecular up-conversion process in our systems relies on the presence of a metastable triplet excited state, and thus has dramatically different photophysical characteristics relative to the other known methods for photon up-conversion (two-photon absorption, parametric processes, second harmonic generation, sequential multiphoton absorption, etc.).

We report on the experimental demonstration of phase-dependent electromagnetically induced transparency (EIT). This effect is based on a quantum interference in closed-loop schemes of interaction of atoms with the radiation. It allows us to control optical properties of the medium by the phases of the laser fields. We discuss some of the possible applications of phase-dependent EIT.

We demonstrate up-conversion with an external quantum yield of 3.2% realized at ultra-low excitation intensities (∼0.1 W cm-2). The bimolecular up-conversion process in our systems relies on the presence of a metastable triplet excited state, and thus has substantially different photophysical characteristics when compared with the other known methods for photon up-conversion (two-photon absorption, parametric processes, second harmonic generation, sequential multiphoton absorption, etc). Thus, we state the general requirements for the materials' parameters of the couple sensitizer/emitter for obtaining triplet-triplet annihilation up-conversion. One of the applications we suggest for active materials compositions is the achievement of energetically stepwise up-conversion of the terrestrial solar spectrum towards blue wavelengths: as such they could be used for improving the efficiency of the solar cells. Up-conversion with non-coherent excitation by sunlight with wavelengths between 550 and 700 nm is demonstrated.

Different approaches towards protection of triplet excited states against deactivation by molecular oxygen are summarized and reviewed.