All-Inorganic TiO₂ Nanoparticle-Based Metalenses Manufactured by Direct Nanoimprint Lithography for High Energy Applications: Femtosecond Laser-Induced Damage Threshold Testing

Abstract

Femtosecond laser-induced damage threshold (LIDT) testing is carried out at 515 nm on 4-mm-sized metalens arrays that are manufactured by direct nanoimprinting of a TiO₂ nanoparticle (NP)-based ink containing either polymeric or inorganic binders. The all-inorganic TiO₂ metalenses exhibit ≈80% absolute focusing efficiency and demonstrate an LIDT of ≈90 mJ cm⁻² based on a single-shot determination using Liu's method, while the metalenses with the polymeric binder achieve ≈137 mJ cm⁻² and an efficiency of ≈76%. Despite the higher LIDT of the TiO₂-polymer composite metalenses in the single-shot experiment, these lenses exhibit significant damage at fluences as low as ≈8 mJ cm⁻² when subjected to ≈6 × 10⁸ pulses at 60 kHz. On the other hand, the all-inorganic metalenses remain intact under identical conditions at ≈64 mJ cm⁻². That is, the inorganic binder provides superior long-term stability relative to the polymeric binder and is a more viable solution for high-energy applications. Structural damages observed in nanostructures result in a reduced deflection efficiency and increase light scattering at the focal plane of the metalens. The LIDT testing is also performed in the nanosecond regime at 532 and 1064 nm with the all-inorganic metalenses, yielding thresholds of ≈0.5 and ≈5 J cm⁻², respectively.