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Abstract Using electrodynamical description of the average power absorbed by a conducting film, we present an expression for the electric-field intensity enhancement (FIE) due to epsilon-near-zero (ENZ) polariton modes. We show that FIE reaches a limit in ultrathin ENZ films inverse of second power of ENZ losses. This is illustrated in an exemplary series of aluminum-doped zinc oxide nanolayers grown by atomic layer deposition. Only in a case of unrealistic lossless ENZ films, FIE follows the inverse second power of film thickness predicted by S. Campione, et al. [ Phys. Rev. B , vol. 91, no. 12, art. 121408, 2015]. We also predict that FIE could reach values of 100,000 in ultrathin polar semiconductor films. This work is important for establishing the limits of plasmonic field enhancement and the development of near zero refractive index photonics, nonlinear optics, thermal, and quantum optics in the ENZ regime. 

Using basic considerations on the average power absorbed in ultra-thin conducting films, we derive a closed-form expression for the average electric- field intensity enhancement (FIE) due to epsilon-near-zero (ENZ) polariton modes. We show that FIE in ENZ media with realistic losses reaches a maximum value in the limit of ultra-small film thickness. The maximum value is reciprocal to the second power of ENZ losses. This is illustrated in an exemplary series of aluminum-doped zinc oxide nanolayers of varying thickness grown by atomic layer deposition technique. The limiting behavior of FIE is shown in exact cases of the perfect absorption, normal incidence, and in a case of ultra- thin lossless ENZ films. Only in the case of lossless ENZ films FIE is inversely proportional to the second power of film thickness as it was predicted by S. Campione, et al. [Phys. Rev. B 91, 121408(R) (2015)]. We also show that FIE could achieve values as high as 100,000 in ultra-thin polar semiconductor films, which have losses as small as 0.02 close to the longitudinal optic (LO) phonon frequency. 

Enhanced and controlled light absorption as well as field confinement in an optically thin material are pivotal for energy-efficient optoelectronics and nonlinear optical devices. Highly doped transparent conducting oxide (TCO) thin films with near-zero permittivity can support ENZ modes in the so-called epsilon near zero (ENZ) frequency region, which can lead to perfect light absorption and ultra-strong electric field intensity enhancement (FIE) within the films. To achieve full control over absorption and FIE, one must be able to tune the ENZ material properties as well as the film thickness. Here, we experimentally demonstrate engineered absorption and FIE in aluminum doped zinc oxide (AZO) thin films via control of their ENZ wavelengths, optical losses, and film thicknesses, tuned by adjusting the atomic layer deposition (ALD) parameters such as dopant ratio, deposition temperature, and number of macro-cycles. We also demonstrate that under ENZ mode excitation, though the absorption and FIE are inherently related, the film thickness required for observing maximum absorption differs significantly from that for maximum FIE. This study on engineering ENZ material properties by optimizing the ALD process will be beneficial for the design and development of next- generation tunable photonic devices based on flat, zero-index optics. 

A gate-tunable plasmonic optical filter incorporating a sub- wavelength patterned metal–insulator–metal metasurface heterostructure is proposed. An additional thin transparent conducting oxide (TCO) layer is embedded in the insulator layer to form a double metal–oxide-semiconductor configu- ration. Heavily n-doped indium tin oxide (ITO) is em- ployed as the TCO material, whose optical property can be electrically tuned by the formation of a thin active ep- silon-near-zero layer at the ITO–oxide interfaces. Full-wave electromagnetic simulations show that amplitude modula- tion and shift of transmission peak are achievable with 3–5 V applied bias, depending on the application. Moreover, the modulation strength and transmission peak shift increase with a thinner ITO layer. This work is an essential step toward a realization of next-generation compact photonic/ plasmonic integrated devices.