Silicon nanoclusters (NCs) strongly sensitize the luminescence of Er3+ ions. Attempts to calculate the interaction distance have assumed that the Förster [Ann. Phys. 437, 55 (1948)] and Dexter [J. Chem. Phys. 21, 836 (1953)] relationships for point-to-point energy transfer can be applied to experiments based on multilayered thin-film specimens. Here, the effective finite plane-to-plane relationships are derived for both interaction mechanisms. These relationships show that energy transfer can result from the Förster interaction despite the fact that the measured luminescence intensity varies much more weakly with NC-Er3+ separation than predicted by theory for point dipoles. An effective energy transfer distance is found for the NC-Er3+ system.
The luminescent infrared transitions in Nd3+ can be activated via a transfer mechanism from amorphous silicon nanoclusters. The Nd photoluminescence (PL) has some unusual characteristics, including a weak temperature dependence of the PL intensity. The data are explained using a simple rate equation model which enables an effective nanocluster-to-neodymium transfer time of ∼0.15μs to be extracted. This is short enough to dominate the intrinsic nanocluster decay rates at low temperatures but long enough to imply that the coupling between the nanoclusters and the Nd ions is, in fact, weaker than for Nd-doped bulk silicon or other semiconductors.
The temperature-induced quenching of the Er3+ luminescence is a significant problem in silicon-based materials systems ultimately designed for room-temperature applications. Here, we show that amorphous silicon-rich oxide, moderately annealed in order to avoid growth of Si nanocrystals, exhibits a reversed temperature dependence in which the integrated Er3+ luminescence increases in intensity upon heating from 77 up to 300 K. This behavior is attributed to a unique spectrum of interacting defects that efficiently sensitize the Er3+ levels, even in the absence of nanocrystals. The effect could have ramifications in fiber-optic emitters or amplifiers to be operated at noncryogenic temperatures.
Use of near infrared instead of visible light would markedly improve tissue penetration, making larger tumors candidates for PDT. Since common photosensitizers exhibit virtually no absorption in this wavelength region, conditions are required where the simultaneous action of two photons is possible. Healthy tissue (rat ears), sensitized by hematoporphyrin derivative, sulfonated chloroaluminium phthalocyanine (CIBA-GEIGY) or pheophorbide a (PORPHYRIN PRODUCTS), was irradiated (1064 nm, 10 nsec) with power densities up to 200 MWcm -2 and energy densities up to 200 k J~m -~. No reproducible photodynamic lesions were observed, but there was a strong sensitizer fluorescence which depended quadratically on the excitation intensity. This may be useful to determine the tissue sensitizer concentration to an otherwise inaccessible depth.
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