Up-conversion luminescence of a violet color from sol-gel derived Nd3+:SiO2+Al2O3 glasses

Abstract

Solid-state up-conversion lasers are found to be practical alternatives to frequency doubling techniques for the conversion of infrared radiation into visible light. Several applications, such as under-water optical communications and biomedical sensors, await the development of compact, efficient room-temperature devices that can operate at wavelengths not commonly obtainable by conventional laser sources. Such up-conversion lasers are considered as possible solutions to immediate technological problems. Trivalent rare-earth ions doped in different glasses and in certain crystals demonstrate up-conversion emissions in the visible wavelength range. The 4fn electronic level structures of these ions provide many long-lived intermediate levels, which could be populated using infrared radiation. These levels, together with some meta-stable higher lying levels, give rise to strong visible emissions [1]. Over the past few years, up-conversion emission (violet color) from Nd3+ ions has been reported in a variety of glasses based on certain chlorides, fluoroindate, fluorogallate, fluorozirconate, multi-component oxides and heavy metals [2–7]. In all these neodymium glasses, the up-conversion emissions are due to their low vibration frequencies and small phonon energy, and they are all synthesized by conventional melting methods. It is commonly believed that the Nd3+ in silica glasses cannot emit the up-conversion emission. In the literature, some researchers have stated that they did not observe violet up-conversion emission from the Nd3+doped silica glasses [6]. In addition, conventional silica fibers are not efficient in generating up-conversion signals from Nd3+ because of their large phonon frequencies. However, in our present work, we have indeed observed a relatively strong up-conversion emission in violet (399 nm) color upon excitation with a yellow light (580 nm) from Nd3+ : SiO2+Al2O3 glasses prepared by the sol-gel process. This short paper reports, for the first time, up-conversion emission in Nd3+ doped SiO2 based sol-gel glasses. Following the conventional sol-gel process [8], tetraethylorthosilicate (TEOS) was diluted in ethanol (EtOH) and water, with HCl added as a catalyst. The mole ratio of TEOS to EtOH to H2O was 1 : 8 : 8. The solution was allowed to a hydrolyze at 60 ◦C for an hour. Al(NO3)3·9H2O and Nd(NO3)3·6H2O were used as the precursors of Al2O3 and Nd2O3 respectively. They were dissolved in EtOH in separate beakers and then added drop-by-drop into the pre-hydrolyzed TEOS solution. The end solutions were stirred at room temperature in a sealed bottle for a period of one week. Dried gels were thus obtained by removing the cover of the bottle and leaving the gel in the open air for several days. A high temperature treatment was carried out on all these dried gels in an electric furnace at 1000 ◦C for about 12.5 h, and were kept there for about 5 h, and then cooled down to room temperature gradually. Based on our previous results on the laser transition (F3/2→ I11/2) properties at 1064 nm of Nd3+ doped Al3+ co-doped silica [9], we have analyzed three samples for upconversion emissions at various Nd3+ concentrations. The mole ratio of each of the chemicals used was as follows: 100SiO2 : 10AlO1.5 : xNdO1.5 (x = 0.1, 0.5 and 1.0). The up-conversion emission spectra were measured by using a Spex Fluorolog-3 spectrofluorometer, attached with a 1934D3 phosphorimeter. This system employs the Datamax software package for acquiring the spectrum and the decay curve data. The source of excitation was a xenon flash lamp. The measured data pertaining to the luminescence were corrected for the detector. Fig. 1 shows the up-conversion emission spectra of the three samples studied under excitation at 580 nm. An intense violet emission peak at 399 nm has been observed in all these samples. Three additional weaker fluorescent transitions at 342 nm, 372 nm and 452 nm have also been found. Referring to the energy diagram of Fig. 2, the emissions correspond to transitions: D3/2 → I9/2 (∼342 nm), D3/2 → I11/2 and