Understanding all-optical switching at the epsilon-near-zero point: a tutorial review

Epsilon-near-zero (ENZ) materials that operate in the spectral region where the real part of the permittivity crosses zero have recently emerged as a promising platform for all-optical switching because of the large, optically induced reflectance and transmittance modulation they offer at ultrafast speeds. To gain insights into the ENZ modulation, this study focuses on the reflectance and transmittance modulation of commonly used ENZ switching schemes and applies an analytical framework both for intraband and interband pumping. We consider the effects of the wavelength, the angle, and the probe polarization on the modulation amplitude for different configurations, specifically highlighting the locations of the maximum reflectance/transmittance modulation and the maximum refractive index modulation, which often occur at different wavelengths around the ENZ point. We find that the maximum modulation, while proximal to the ENZ point, can occur away from the ENZ point and even slight deviations can result in seemingly anomalous modulation behavior. The occurrence of resonances at the ENZ region for ultrathin films further increases the modulation strength. This work paves the path for practical and effective all-optical modulation approaches employing ENZ materials, and will help design the best experimental configurations for future material studies and nonlinear optical experiments employing ENZ materials.