Engineering ENZ–plasmon polaritons in birefringent asymmetric-cross metasurfaces [Invited]

Birefringent metasurfaces supporting ENZ–plasmon polaritons enable tailored, polarization-dependent nanoscale light–matter interactions. We demonstrate a coupled ENZ–plasmon polariton platform using asymmetric gold cross-nanoantennas and an ultrathin ITO film near its ENZ wavelength of 1415 nm. The asymmetric cross-antennas induce structural birefringence, resulting in distinct plasmonic resonances for orthogonal polarizations that hybridize with the ENZ mode via localized near-field coupling. The strong coupling reduces the spectral separation between polarization-dependent resonances by about threefold in the presence of an ENZ layer, compared with antennas on glass. These findings establish ENZ coupling as a way to engineer birefringent polaritonic responses around telecom wavelengths.