High-aspect-ratio photoresist nanopillar arrays with broadband near-perfect optical absorption performance using PDMS-assisted colloidal lithography†

Abstract

Colloidal lithography offers a cost-effective and straightforward method for fabricating periodic arrays, utilizing colloidal spheres as microlenses to create patterns in a photoresist. However, the exposure depth remains a challenge. By utilizing PDMS to fill colloidal films, high-aspect-ratio photoresist nanopillars were employed as the structural basis for the successful fabrication of both metallic resonant type and non-metallic anti-reflective type broadband near-perfect optical absorbers. According to simulated light beams and the resulting photoresist patterns, the introduction of PDMS not only made the colloidal mask flexible and reusable, but also significantly increased the beam convergence depth, enabling the formation of high-aspect-ratio photoresist patterns. In simulations, the effective focused beam depth was sensitive to the film's refractive index, colloidal diameters, and ratios of the sphere diameter to periodicity of colloidal arrays, resulting in a depth of 2828 nm under optimal parameters. In experiments, photoresist arrays with pillar heights reaching up to 3374 nm and the corresponding depth-to-width ratio of 5.04 were achieved. Additional petal-shaped or octopus-shaped pillars were also created during PDMS-assisted lithography. A metallic absorber, based on the conformal Pt coating, achieved an average absorbance of up to 98.3% over the range from 400 nm to 1100 nm, with a minimum absorptivity of 96%. An all-dielectric optical absorber, employing photoresist nanopillars for impedance matching, exhibited an average absorptivity of 92.4% within the same wavelength range.