Engineering Epsilon-Near-Zero Response and Polarization-Independent Radiative Ferrell-Berreman Modes in Nanoporous Gold-Silver Films

Ferrell and Berreman modes, arising from radiative bulk plasmon-polariton and phonon-polariton excitations, are absorption resonances in thin metal films and polar-dielectric media. These modes characterized by volume charge oscillations and epsilon-near-zero (ENZ) characteristics offer unique light-matter interaction pathways and have thus garnered extensive research interest. Here, we demonstrate nanoporous gold-silver (NPGS) films as a versatile platform for realizing Ferrell-Berreman random metasurfaces with thickness-dependent ENZ response control. Experimentally, we extract the effective optical constants of NPGS films with varying thicknesses, enabling precise tuning of ENZ points from visible (∽ 620 nm) to near-IR (∽ 900 nm) wavelengths. Further, we show, due to the bicontinuous and random anisotropic nature of the films, the excitation of Ferrell-Berreman modes near the ENZ wavelength exhibits polarization-indiscriminate behavior. To theoretically validate these findings, we employ a hybrid effective medium approximation that combines the Maxwell-Garnett and Bruggeman models. The Maxwell-Garnett model addresses optical scattering from surface roughness, while the Bruggeman model captures the effects of porosity on the material's optical properties. This hybrid model in light of the statistically anisotropic nature of NPGS films enables us to quantify the effective permittivity and corroborates well with the experimentally observed polarization indiscriminate resonances observed near the ENZ wavelength.