  In this study, rigorous numerical calculation was utilized to characterize
the focal properties of mid-infrared silicon microlens with the size about tens
of micrometers. It is found that the focal shift phenomenon also exists in
mid-infrared regime, which behaves differently from that of visual and
near-infrared wavelength. Focal properties of silicon microlens were also
measured experimentally, showing well coherence with simulation results. Our
results provide systemic understanding of focal shift in mid-infrared regime,
at that wavelength special consideration should be paid in micro-nano optics,
especially with the integration between infrared optical system and other
devices.
