Telluride temperature test, just right

Researchers in India have used laser spectroscopy and other techniques to study a novel doped telluride glass of technological interest. The material could have potential as new kind of high temperature sensor.

Rare earth metals have been used to dope glasses for use in solid state lasers and other optical devices since the early 1990s. Their ions have favourable energy levels and transitions that extend from the near-infrared (NIR) to the visible region of the electromagnetic spectrum. The earliest devices exploited down-conversion emissions but more recently researchers have been focusing on developing devices that utilise up-conversion instead. Such devices hold promise for displays, optical and thermal sensors, and on-line amplifiers.

It is important to take into account the up-conversion efficiency of a particular rare earth ion when developing such devices. This depends not only on the energy levels of the ion but also of the glassy host material too. For instance, neodymium(III), Nd³⁺, ions have densely packed energy levels and this almost precludes up-conversion emissions because the non-radiative relaxation rates are so large. However, dope a low phonon host with Nd³⁺ and up-conversion emissions become viable.

Despite this observation, there have been few attempts to observe up-conversion emission from Nd³⁺ ions using visible pump radiation. Of course, silicate, borate, phosphate oxide glasses doped with neodymium ions do not show up-conversion emission because of their high phonon frequencies. Ultraviolet up-conversion emission has been observed in the ultraviolet-blue but with heavy modifiers. UV/blue up-conversion was seen with Nd³⁺ doped fluorozirconate glass, for example, while such emissions are also possible with Nd³⁺ doped lithium oxide modified tellurite glass using 532 and 800 nanometre excitations.

Indeed, tellurite glass has a much lower phonon frequency than some other materials. It is also highly chemically resistant and thermally stable, which makes it a potentially useful material for optoelectronics.

Now, R.K. Verma, K. Kumar, and S.B. Rai of the Laser Spectroscopy Laboratory, at Banaras Hindu University, in Varanasi, India, have looked at tellurite glass as a host and used barium carbonate, barium fluoride, and barium chloride as glass modifiers. They synthesised the doped glass using the melt quench method. They point out that the modifiers help them to improve the un-conversion intensity.

In their experiments, the team has observed up-conversion emissions in the ultraviolet/blue region with a Nd³⁺ doped tellurite glass with 532 nm excitation. They observed up-conversion bands at 360, 387, 417 and 452 nm showing two-photon character. The team adds that barium chloride was the best glass modifier for boosting up-conversion intensity to a maximum compared with the carbonate and fluoride. They explain that this enhancement correlates with a reduction in average phonon frequency of the glass sample.

The team has also carried out temperature-dependent fluorescence studies on three of the emitting levels. The differences observed as temperature changes suggest that Nd³⁺ doped tellurite glass could be used as a temperature sensor.