Refractive Index Tuning of All-Inorganic TiO2 Nanocrystal-Based Films and High Aspect Ratio Nanostructures Using Atomic Layer Deposition: Implications for High-Throughput Fabrication of Metalenses

High refractive index (RI) components and nanostructures are of great interest for compact optics, waveguides, photonics, and metamaterials. Herein, we demonstrate that atomic layer deposition (ALD) can be used as a short postfabrication step to dramatically increase the RI of nanocrystal (NC)-based films and patterned nanostructures in conjunction with a single-step, direct nanoimprint lithography (NIL). The initial RI of TiO2 NC-based films was n = 1.95 at 543 nm which can then be increased to 2.00 by post-calcination and further up to 2.15 by TiO2 ALD. Fifteen cycles of ALD were sufficient to achieve this rapid increase of RI, and any intermediate RI value can be tuned by adjusting the number of ALD cycles accordingly. Nanoscale interstitial gaps between TiO2 NCs allowed a uniform diffusion of ALD precursors, resulting in a much denser structure and the RI increase. An array of 4 mm-sized metalenses were fabricated to demonstrate the effects of the RI tuning on the performance of optical devices. The focusing efficiencies of the as-imprinted metalenses (smallest dimension 80 nm, highest aspect ratio 8) were 61% on average, but the post-treatments including calcination and ALD significantly increased the average efficiency to 67% and up to 75% for the best-performing lens. This approach combines the fast, scalable, and versatile solvent-assisted NIL method to pattern optical nanostructures with a short post-patterning deposition and densification step that significantly enhances optical performance.