Two-Photon Excited Luminescence in Mechanoluminescent ZnS Microparticles

Mechanoluminescent particles, including ZnS, are promising intravenous light sources for biomedical imaging. Here, we report a serendipitous finding that during femtosecond two-photon microscopy, a microsecond-scale prolonged luminescence ("comet tail") is observed, whereas it is absent or greatly reduced under confocal continuous-wave excitation. We found that the decay time constant of the comet tail is typically 2-5 μs, remains temperature-independent from 0-37 °C, increases with laser power, and decreases with the decay time constant of the particle's temperature when varying the embedding medium. We further measured the absorption cross section for ZnS microparticles, and the photothermal modeling suggests that while femtosecond pulsed excitation results in instantaneous heating of electrons, it does not lead to a greater increase in bulk temperature in ZnS compared with continuous-wave excitation. Our findings bring new understanding and perspectives to mechanoluminescent particles-based imaging technologies and applications.