Abstracts

Anti-Stokes laser-induced white emission (LIWE), characterised by broadband spectra in the visible region and a non-linear dependency of emission intensity on the excitation power, has been observed in several optically active materials, including YAG, organometallic compounds, or organic dye solutions. This process has been interpreted in terms of different models like multiphoton ionisation, and intervalence charge transfer, among others. The quantum efficiency of LIWE can reach 16% in inorganic phosphors, fostering interest in research about the mechanisms and for the development of more efficient materials, especially regarding solar energy conversion technologies in which perovskites yield the highest power conversion efficiency combined with commercialisation viability. In this work, rare-earth perovskites manganite (REMnO₃) were studied for its structural and photophysical properties. REMnO₃ formed a single-phase compound in an orthorhombic structure to (Nd, Eu)MnO₃ and hexagonal to (Y, Er, Yb)MO₃. The samples were synthesised by self-combustion method using a mixture of the RE(NO₃)₃, Mn(NO₃)₃, and urea (ratio 1:2.5), subjected to a furnace at 650 °C for 5 minutes followed by the calcination at 900 °C for 6hours. In XRD can be identified reflection planes to the orthorhombic EuMnO₃ phase (JCPDS Card No. 96-153-1804), and no raw material or impurities were determined. The absorbance analysis does not show the typical narrow peaks to 4f-transitions, possibly, due to the presence of the Mn²⁺ in the matrix. The spectra consist of two absorption bands centred around 337 nm. The origin of these bands could be associated with the overlap of the charge transfer between the trivalent lanthanide ion (RE³⁺→O^2-) and Mn²⁺ →O^2-. LIWE was analysed under λ_exc=980 nm in function of power density (P_D), pressure and sample-compaction (power x pellet). In studied cases, a broadband emission from 400 nm to 650 nm and from 1175 to 2200 nm is observed. It was observed the blue shift to the emission of 13 nm, 22 nm and 29 nm to powder, pellet and pellet at low pressure, respectively. The supralinear dependence between the intensity of the emission and the excitation power() cannot describe the number of photons involved in the up-conversion emission and indicate the P_D threshold to promote the LIWE (2168 W cm^-2 to powder, 4508 W cm^-2 pellet and 2947 W cm^-2 pellet at low pressure). We discuss the influence of sample compaction, pressure, the mechanism that induces this anti-Stokes process, as well as the generation of current under photoexcitation.