Abstract
Organic erbium complexes have long been of interest due to their potential for using the strong absorption into the organic to sensitise the erbium emission. Despite this interest there is remarkably little quantitative information on how effective the approach is and the discussion of the energy transfer mechanism is generally vague. Here we accurately quantify the sensitisation as a function of excitation pump density and model it using a rate equation approach. As a result, we can calculate the degree of population inversion for the erbium ions as a function of the pump intensity. We demonstrate that even when we increase the erbium concentration in the films from ~10 to ~80% we find a relatively small decrease in the sensitisation which we attribute to the large (>20 Å) Förster radius for the sensitisation process. We show that we can obtain population inversion in our films at very low pump powers ~600 mW/cm2. The calculated Förster radius for the organic erbium complexes suggests design rules for energy transfer between antennas and erbium ions in molecular systems and hybrid organic-inorganic nanoparticles.
Highlights
Erbium has long been of interest as a near infrared (NIR) emitter as its emission wavelength (1.5 μm) matches perfectly to the low loss transmission wavelengths for silica optical fibres[1,2,3,4]
For the organic erbium complex we used erbium(III) tetrakis(pentafluorophenyl) imidodiphosphinate[24], (Er(F-TPIP)3), which we have demonstrated can have a quantum efficiency of ~7% when deposited by vacuum sublimation in a vacuum of ~10−7 mbar
We have demonstrated that we can achieve population inversion for the erbium ions at excitation levels as low as 500 mW/cm[2] and that with a modest increase in the quantum efficiency of the erbium to 50% this power requirement can be reduced to only 40 mW/cm[2]
Summary
For the 44% film at low excitation powers the long component accounts for >80% of the emission but at an excitation power density of ~1000 mW/cm[2] this reduces to ~50% and remains at that level as the pump power is increased This change in the erbium decay time corresponds to the point where the experimental data start to deviate from the model (~790 mW/cm[2]). This correlation between the deviation of the 407 nm excited data from the model and the observed reduction in the lifetime of the erbium ions in the film suggests that with the higher erbium concentration films we are starting to observe ion-ion quenching with increasing pump power This effect is taken into account in the measured decay time as a function of excitation intensity the model does not account for the fact that ion-ion interactions can effectively quench more than one ion in a single process (One example of this could be cooperative up-conversion between two excited erbium ions followed by energy transfer back to the chromophore) and the number of excited erbium ions in the sample falls below the model. We have demonstrated that both the singlet and triplet states of the chromophore can effectively couple into the Er3+ ions but due to their longer lifetimes it would be best to use chromophores with higher ISC rates in order to provide a reservoir of excited states to replace Er3+ ions that are de-excited to their ground state
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