Phonon-Engineered Indium Fluoride Glass Enabling Upconversion Fiber-Probe Infrared Sensing

Conventional IR sensors are hindered in tortuous spaces by rigid form factors and alignment sensitivity. Here, an alignment-tolerant alternative is realized by converting IR to visible light within a low-phonon upconversion (UC) fiber and electrically reading out the emission via a fiber-integrated photodetector.

Bi3+-modified InF3-based fluoride glasses exhibit a reduced maximum phonon energy (~512 to ~468 cm-1). Structural analysis and simulations reveal sixfold-coordinated, distorted Bi–F motifs with bond lengths of 2.16 and 2.50 Å, accompanied by rearrangement of the fluoride network. With 3 mol% Er3+, a maximum UC quantum yield (UCQY) of 3.2% is achieved under 980 nm excitation.