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Abstract:

We report on the use of interferometric autocorrelation measurements to investigate the non-linear absorption processes evident in single InGaN/GaN quantum dots. The near quadratic excitation intensity dependence of the photoluminescence signal in conjunction with the asymmetric collinear autocorrelation trace unambiguously confirms the process as being one involving two photons via an intermediate virtual state. These results highlight the inherently non-linear optical properties of these structures.

Abstract:

With the aim of providing a thorough description of the optical properties of Er³⁺-doped endohedral fullerenes, we studied their characteristics in the light of those of well-known Er³⁺-doped β-PbF2 single-crystals. Various Er³⁺-doped endohedral fullerenes were considered: Er2C2@C82, where the Er2C2 group is encapsulated inside a cage of 82 carbon atoms and the Er3-xScxN@C80 (x = 0, 1 and 2) family, where the Er3N, Er2ScN and ErSc2N clusters are trapped in a 80 carbon atom cage. In this article, we discuss the absorption and photoluminescence of trivalent erbium ions in fullerenes and in β-PbF2 crystals. The extinction coefficient of Er3N@C80 was found to be 4.8 (±0.5) × 10³ mol/l^-1 cm^-1 at 540 nm, due to the C80 cage absorbance. Even in a saturated fullerene solution, the absorption of Er³⁺ encapsulated inside a C80 cage cannot be observed at room temperature. We suggest that this is due to an insufficient number of Er³⁺ ions in the solution and their low absorption cross-section. Low temperature photoluminescence measurements show that the line width of Er³⁺ in a carbon cage, dissolved in a polycrystalline solvent, is similar to the one of Er³⁺ in β-PbF2 single-crystals. The quantum efficiency of Er³⁺ at 1.5 μm in fullerenes is four orders of magnitude lower than that for Er³⁺ in crystals, due to very efficient non-radiative decay processes. Molecular vibrations of the cage might be responsible for those rapid non-radiative de-excitations. © 2009 Elsevier B.V. All rights reserved.