Optical fibre thermometry using ratiometric green emission of an upconverting nanoparticle-polydimethylsiloxane composite

Abstract

The thermally coupled green band emission from excited Er3+ ions has been used in the past to create optical thermometers, by doping the material in various types of media, particularly glasses. Glasses are known to be excellent hosts for Er3+ ions: however, high temperatures (>900 K) are usually required for doping these ions into glasses and a non-linear temperature response is often produced. In this work, the frequently encountered drawbacks of glass-based temperature sensors have been addressed by developing a temperature sensor created at a lower temperature (543 K), by dip-coating chemically synthesized upconverting nanoparticles (UCNP – NaYF4:(18%) Yb3+, (2%) Er3+) embedded in polydimethylsiloxane (PDMS) onto the tip of a 1000 μm optical fibre, to create the actual fibre probe. The sensor shows an excellent linear response (R2 = 0.991) over a very useful temperature range of 295 K – 473 K, with a sensitivity of 2.9 × 10-3 K-1, a temperature resolution of ± 2.7 K and response time of ∼ 5 seconds. Additionally, a probe was investigated where a pure upconverting nanoparticle powder was coated on the tip of optical fibre and its spectral and temperature response was obtained (and cross compared with that of UCNP-PDMS composite). The results obtained from the probe development work show that the UCNP-PDMS-coated optical fibre temperature sensor developed offers a better alternative to more conventional Er3+ doped glass-based temperature sensors, in terms of the thermal budget, the synthesis process and the ease of coating, creating as a result, a very linear device response.