Modeling The Nonlinear Optical Response of ENZ Films

The epsilon-near-zero (ENZ) regime of optical materials represents the transition point between metallic (negative permittivity) and dielectric (positive permittivity) response, similarly characterized by strong alterations in reflection, transmission, and absorption. This intriguing region of optical response has given rise to range of unique effects in both the linear and nonlinear optics fields, including wavelength expansion, slow light, and field confinement. In particular, the ability of the ENZ regime to enhance various nonlinear optical effects has garnered significant attention in recent years. Among them, a key is the ability to modulate the refractive index of the ENZ film on the order of unity on a picosecond timescale via nonlinear optical exciation. This has given rise to a significant interest in utilizing ENZ films for a number of optical devices including saturable absorbers, nonlinear activation functions, optical switches, and more. However, key to designing these devices is understanding the nonlinearity and modeling the change in complex index (permittivity) that will be experienced for a given excitation. In this talk we will highlight our recent efforts to model the intensity dependent index of Drude-based ENZ thin films, highlighting the Nonlinear Epsilon Near Zero (NLENZ) Calculator application available online via nanohub.org. We will introduce the basic theory of the non-parabolic conduction band and how it gives rise to nonlinear response of ENZ materials. We will then provide a high level outline of the models for interband and intraband nonlinearities, culminating in the use of the NLENZ app to predict the nonlinear properties of the material, using only experimental parameters and the material band structure, which can then be extracted and used in combination with other FEM or FDTD solvers for complex device design. We expcet this tutorial style talk to be of interest to researchers and materials scientists who are interested in working with ENZ materials in new devices or developing new materials with an ENZ response, and hope to support the further development and study of these unique effects by furthering the ability of the community to simulate mateiral and device responses incorprating ENZ.