Nanoimprinting of Mesoporous Titania: Direct Patterning and Refractive Index Control in the Visible

Abstract

Porous materials and nanocomposites have emerged as promising media for constructing high performance, tunable, responsive, and versatile optical platforms, with applications spanning diffractive and waveguide optics, metasurfaces, photonic sensors, and structural coatings. Despite their potential, achieving precise control over the structural and optical properties of these materials has remained a significant challenge, often requiring complex fabrication processes and post-lithography processing steps that limit scalability, sustainability, and practical implementation. In this work, upon the use of nanoimprinting of refractive index (NIRI) is reported to directly pattern mesoporous titania (pTiO₂) and achieve localized, compression-based modulation of refractive index. Two formulations of sol-gel derived pTiO₂ thin films are interrogated, revealing tunable refractive indices ranging from n ≈ 1.5 to n ≈ 2.2 at visible wavelengths in response to nanoimprinting-induced compression. Using a variety of micro-patterned stamps, the direct fabrication of planar diffractive and gradient index optics is demonstrated without any curing, etching, or post-lithography processing steps. These findings demonstrate that direct imprinting of pTiO₂ is a sustainable and effective method for controlling refractive index and fabricating optical devices, paving the way for future advancements in the development of compression-tuned optical materials and their diverse applications.