Enhanced Nonlinearity of Epsilon‐Near‐Zero Indium Tin Oxide Nanolayers with Tamm Plasmon‐Polariton States

Abstract Recently, materials with vanishingly small permittivity, known as epsilon‐near‐zero (ENZ) media, emerged as promising candidates to achieve nonlinear optical effects of unprecedented magnitude on a solid‐state platform. In particular, the ENZ behavior of indium tin oxide (ITO) thin films resulted in Kerr‐type nonlinearity with non‐perturbative refractive index variations that are key to developing more efficient Si‐compatible devices with sub‐wavelength dimensions. In this contribution, exceptional enhancement of the nonlinear index variation of 30 nm‐thick ITO nanolayers by silicon dioxide/silicon nitride (SiO₂/SiN) Tamm plasmon‐polariton structures fabricated by radio‐frequency magnetron sputtering on transparent substrates under different annealing conditions is proposed and demonstrated. In particular, the linear and nonlinear optical properties of ITO thin films and resonant photonic structures are investigated using broadband spectroscopic ellipsometry and intensity‐dependent Z‐scan nonlinear characterization, demonstrating enhancement of optical nonlinearity with refractive index variations as large as Δn ≈ 2 in the non‐perturbative regime. This study reveals that the efficient excitation of strongly confined plasmon‐polariton Tamm states substantially boosts the nonlinear optical response of ITO nanolayers, providing a stepping stone for the engineering of more efficient infrared devices and nanostructures for a broad range of applications, including all‐optical data processing, nonlinear spectroscopy, sensing, and novel quantum detection modalities.