Polarization-insensitive bifocal metalenses by combining nanoimprint lithography and atomic layer deposition in the visible spectrum.

This article focuses on the development and large-scale fabrication of polarization-insensitive bifocal metalenses operating at a visible wavelength of 450 nm by combining nanoimprint lithography (NIL) and atomic layer deposition (ALD). The method uses nanoimprinted resin structures enhanced with a thin, high-index titanium dioxide (TiO₂) film deposited via ALD to achieve sufficient phase modulation and improved focusing efficiency, producing metalenses with high aspect ratios (~10:1) and small critical dimensions (~90 nm) on silica substrates. The fabricated metalenses, with diameters of 480 μm, demonstrate two diffraction-limited focal spots along the optical axis and maintain consistent performance under various polarization states. This wafer-scale, low-cost, and high-throughput fabrication approach addresses limitations of traditional electron beam lithography and supports the mass production of versatile metasurfaces for applications in microscopy, imaging, and optical trapping within the visible or shorter wavelength spectrum.