Double-Mode Thermometer Based on Photoluminescence of YbGd2Al2Ga3O12: Cr3+ Operating in the Biological Windows

A Near-Infrared (NIR) ratiometric luminescence thermometer with the composition of Yb1Gd2Al1.98Cr0.02Ga3O12 was prepared and studied. When excited by 660 nm in the first biological transparent window (BTW), the sample shows a peak structure of around 1000 nm due to the 2F5/2 → 2F7/2 transitions of Yb3+ via the energy transfer process from Cr3+. Due to the Boltzmann distribution, the Yb3+ PL intensities in the shorter wavelength side (i.e., 1st BTW) and longer wavelength side (i.e., 2nd BTW) exhibit opposite temperature dependencies. The luminescence intensity ratio (LIR) of Yb3+ in shorter and longer wavelength ranges works as a luminescence thermometer with a relative sensitivity of 0.55% K−1 at 310 K. In addition, YbGd2Al1.98Cr0.02Ga3O12 can also be employed for temperature sensing based on the LIR of Cr3+ (2E → 4A2) at around 700 nm and Yb3+ (2F5/2 → 2F7/2) at around 1000 nm, achieving a remarkable relative sensitivity of 2.69% at 100 K. This study confirms that the YbGd2Al1.98Cr0.02Ga3O12 thermometer fulfills the requirements for biological temperature measurements.