Extracting epsilon-near-zero wavelength of ultrathin plasmonic film

Strong optical nonlinearities of plasmonic thin films exist at their epsilon-near-zero (ENZ) wavelengths, which are essential to be acquired first for the design and fabrication of ENZ photonic devices. However, it has been challenging to obtain the ENZ wavelength precisely when the film thickness is reduced to tens of nanometers or less. By enhancing both electric field intensity and light-matter interaction distance in the film, we propose that the ENZ wavelength and the medium model of ultrathin films can be extracted accurately from the transmittance and reflectance spectra under oblique light excitation. A characteristic valley in the transmittance spectrum, which originates from the increased light absorption caused by the ENZ electric field enhancement, can be used to determine the ENZ wavelength with significantly improved fitting accuracy of the Drude parameters. The work in this paper provides an accurate and effective method for the acquisition of ENZ wavelength and will contribute to the research of nonlinear plasmonic devices.