Off-axis focusing by using nanoimprinted dielectric metasurface with free-form phase distribution

This study presents the design and fabrication of a dielectric metasurface with freeform phase distribution and with a large period unit cell. The dielectric metasurface is fabricated using i-line stepper, dry etching, and nanoimprint technology. The phase distribution of the metadevice is the combination of a blazed grating for deflection and an aspherical lens for eliminating the off-axis aberration. The optical measurement result shows the off-axis focusing spot is with loss aberration and the corresponding Strehl ratio is 0.34. The diffraction efficiency is around 2%. The low efficiency is mainly attributed to the rounding of the rectangular nanostructures during the pattern transfer and relatively thin thickness. Moreover, the polarization-dependency of this large period metasurface is also discussed.