  Three-dimensional direct laser writing via two photon polymerization is used
to fabricate anti-reflective structured surfaces composed of sub-wavelength
conicoid features optimized to operate over a wide bandwidth in the
near-infrared range from 3700 cm$^{-1}$ to 6600 cm$^{-1}$ (2.7 to 1.52 $\mu$m).
Analytic Bruggemann effective medium calculations are used to predict nominal
geometric parameters such as the fill factor of the constitutive conicoid
features of the anti-reflective structured surfaces presented here. The
performance of the anti-reflective structured surfaces was investigated
experimentally using infrared transmission measurements. An enhancement of the
transmittance by 1.35\% to 2.14\% over a broadband spectral range from 3700 cm$
^{-1} $ to 6600 cm$^{-1}$ (2.7 to 1.52 $\mu$m) was achieved. We further report
on finite-element-based reflection and transmission data using
three-dimensional model geometries for comparison. A good agreement between
experimental results and the finite-element-based numerical analysis is
observed once as-fabricated deviations from the nominal conicoid forms are
included in the model. Three-dimensional direct laser writing is demonstrated
here as an efficient method for the fabrication and optimization of
anti-reflective structured surfaces designed for the infrared spectral range.
