Resonant Optical Absorption and Photothermal Process in High Refractive Index Germanium Nanoparticles

Germanium has one of the highest refractive indices and extinction coefficients in the optical range, making it an attractive material to study Mie‐resonance‐induced absorption in subwavelength structures. In this work, at first mode analysis of a subwavelength sphere in a complex permittivity plane is provided to simultaneously examine the Mie and plasmon resonances at once, which are often discussed separately. Electromagnetic calculations show enhanced absorption of germanium nanospheres at Mie resonances. To experimentally confirm the predicted results, germanium nanoparticles are successfully fabricated through reactive thermal arc plasma method. The size distribution of germanium nanoparticles enables broad extinction from UV to near infrared even though the Mie resonance peak of individual nanoparticles is rather narrow. Enhanced absorption is confirmed by a solar photothermal experiment. The work proves the applicability of strong Mie resonances excited at subwavelength germanium particles.