Design of off-axis and co-axial achromatic bifocal metalens

Metamaterials are widely used as an artificial composite material in optical systems, however, almost all imaging systems suffer from chromatic aberrations due to their inherent dispersion. In this paper, we demonstrate the achromatic bifocal metalens based on phase change materials by tuning the crystalline fraction of Ge₂Sb₂Te₅ in a linear-phasegradient metasurface, which can realize off-axis and co-axial achromatic bifocal metalens in the continuous waveband from 9.5 to12μm. Simulation results show that average focusing efficiency of the off-axis metalens in the working band is about 28.96%, average focusing efficiency of the co-axial metalens is 50.2% with the full width at half-maximum is close to the diffraction limit. This design will play an important role in the field of fluorescence microscopy, which can achieve achromatic objective and eyepiece metalens with different focal lengths over a wide waveband.