Amorphous to Crystalline Transition in Nanoimprinted Sol–Gel Titanium Oxide Metasurfaces

Crystalline titanium dioxide (TiO₂) (anatase and rutile) possesses a higher refractive index than amorphous TiO₂ and near-zero absorption in the visible region, making them an ideal material for visible metasurfaces. However, fabrication limitations hinder their implementation into flat optics. In this work, a wafer-scale manufacturing platform is proposed for crystalline TiO₂ metasurfaces. Sol-gel TiO₂ is developed as a printable material in which its material phase can be precisely controlled to produce amorphous, anatase, or rutile, depending on the crystallization temperature. Therefore, anatase or rutile metalenses can be fabricated on a wafer scale using thermal nanoimprint lithography and sintering process. The high refractive index of the crystalline TiO₂ contributes to the enhanced conversion efficiency of the fabricated metalenses. The fabricated metalenses exhibit diffraction-limited focusing and imaging capabilities, comparable to the theoretically ideal lenses.