Achieving ultrasensitive temperature sensing through non-thermally coupled energy levels to overcome energy gap constraints

Highly sensitive and precise optical temperature measurements are pivotal across various fields, facilitating enhanced temperature regulation and monitoring. This study achieves an optically ultrahigh sensitivity temperature sensing system in Yb³⁺/Nd³⁺/Er³⁺ tridoped CaSc₂O₄ by leveraging non-thermal coupling energy levels, specifically Er³⁺:⁴F_9/2 and Nd³⁺:⁴F_5/2. This accomplishment circumvents the sensitivity constraint imposed by the energy gap. The maximum absolute and relative sensitivity of the optical thermometer peaks at 13.92% K⁻¹ and 4.61% K⁻¹ respectively, surpassing the values from the majority of analogous optical thermometers. Furthermore, it also exhibits exceptional temperature resolution, maintaining values below 0.03 K throughout the entire testing temperature range. Simultaneously, the temperature sensing properties reliant on the thermally coupled Er³⁺:²H_11/2/⁴S_3/2 states are explored in detail, revealing the maximum absolute and relative sensitivity for temperature measurement of 0.37% K⁻¹ and 1.53% K⁻¹, respectively. In the validation experiment, both of the optical thermometers show accurate temperature measurement capability. Additionally, the penetration depth in the biological tissues is 8 mm for the green and red light of Er³⁺ and 10 mm for the near-infrared emission of Nd³⁺. All of these studies collectively demonstrate the potential of CaSc₂O₄:Yb³⁺/Nd³⁺/Er³⁺ for achieving ultrasensitive temperature sensing and application in the deep biological tissues.