Exploring ultrafast photo-thermal and photo-acoustic dynamics of material employing localized surface plasmon-mediated fiber optic sensor

• A U-bent fiber optic sensor was functionalized with AuNPs and AgNPs to study ultrafast photothermal and photoacoustic dynamics using evanescent-wave and LSPR absorption in a dual pump–probe setup. • Pump overlap on the nanoparticle-coated probe caused non-radiative LSPR heating and refractive-index shifts, which were separated from material-driven effects using resonant and off-resonant dual pump–probe excitation. • AuNP probes gave fast plasmonic-heating signals that masked slower effects, while AgNP probes, with minimal heating, sensitively captured the slower thermal dynamics. • AgNP-functionalized probes also detected shock waves and acoustic signatures generated from the absorbing material under nanosecond excitation, enabling pressure-dynamics sensing. • Overall, the U-bent fiber probe provides a compact, tunable platform for real-time ultrafast thermal and mechanical monitoring in photothermal spectroscopy, thermal-wave sensing, and advanced material diagnostics. This study explores ultrafast photothermal and photoacoustic dynamics of materials using an evanescent wave absorption-based U-bent optical fiber (U-Fb) sensor, functionalized with gold (Au) and silver (Ag) nanoparticles. Leveraging localized surface plasmon resonance (LSPR), the sensor enables rapid detection of refractive index (RI) changes due to non-radiative heat dissipation under optical excitation. The nanoparticle-functionalized probes were first calibrated under controlled temperature and RI conditions. In order to get the insight of the photothermal processes, pump–probe experiments were conducted using a 514 nm chopped continuous-wave (CW) laser and a 532 nm nanosecond pulsed laser as pump sources, while a 405 nm probe beam monitored LSPR responses under both resonant and off-resonant conditions. It was observed that when the excitation beam overlaps with the probe, nonradiative decay of LSPR absorption contributes to the thermal change of the surrounding medium. To further explore heat propagation dynamics, the distance between the fiber probe and the photothermal source was varied. This allowed for temporal analysis of RI changes and differentiation between thermal contributions from the immobilized nanoparticles and the absorbing material. The results revealed that, under chopped CW excitation, AgNP-functionalized probes exhibited higher sensing sensitivity than AuNP-functionalized probes, despite having approximately similar thermal response times. Since the AgNp LSPR is off resonant with pulse excitation and resonant with the probe beam, this configuration provides better sensitivity and the clarity in monitoring the time profile of the material thermal response than AuNp- functionalized probe. Notably, AgNP- functionalized probes also detect shock waves and acoustic signatures generating from the absorbing material under nanosecond pulse excitation, underscoring their potential in measuring the fast pressure dynamics. Overall, the U-Fb probe sensor offers a compact, tunable platform for real-time monitoring of ultrafast thermal and mechanical events, with applications in photothermal spectroscopy, thermal wave sensing, and advanced material diagnostics.