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.

The unique properties of the emerging photonic materials, conducting nitrides and oxides, especially their tailorability, large damage thresholds, and, importantly, the so‐called epsilon‐near‐zero (ENZ) behavior, have enabled novel photonic phenomena spanning optical circuitry, tunable metasurfaces, and nonlinear optical devices. This work explores direct control of the optical properties of polycrystalline titanium nitride (TiN) and aluminum‐doped zinc oxide (AZO) by tailoring the film thickness, and their potential for ENZ‐enhanced photonic applications. This study demonstrates that TiN–AZO bilayers support Ferrell–Berreman modes using the thickness‐dependent ENZ resonances in the AZO films operating in the telecom wavelengths spanning from 1470 to 1750 nm. The bilayer stacks also act as strong light absorbers in the ultraviolet regime using the radiative ENZ modes and the Fabry–Perot modes in the constituent TiN films. The studied Berreman resonators exhibit optically induced reflectance modulation of 15% with picosecond response time. Together with the optical response tailorability of conducting oxides and nitrides, using the field enhancement near the tunable ENZ regime can enable a wide range of nonlinear optical phenomena, including all‐optical switching, time refraction, and high‐harmonic generation.

All-optical switches offer advanced control over the amplitude, phase, and/or polarization of light at ultrafast timescales using optical pulses as both the carrying signal and the control. Limited only by material response times, these switches can operate at terahertz speeds which is essential for technology-driven applications, such as all-optical signal processing and ultrafast imaging, as well as for fundamental studies, such as frequency translation and novel optical media concepts such as photonic time crystals. In conventional systems, the switching time is determined by the relaxation response of a single active material, which is generally challenging to adjust dynamically. This work demonstrates that the zero-to-zero response time of an all-optical switch can instead be varied through the combination of so-called “fast” and “slow” materials in a single device. When probed in the epsilon-near-zero (ENZ) operational regime of a material with a slow response time, namely, plasmonic titanium nitride, the proposed hybrid switch exhibits a relatively slow, nanosecond response time. The response time then decreases as the probe wavelength increases reaching the picosecond time scale when the hybrid device is probed in the ENZ regime of the faster material, namely, aluminum-doped zinc oxide. Overall, the response time of the switch is shown to vary by two orders of magnitude in a single device and can be selectively controlled through the interaction of the probe signal with the constituent materials. The ability to adjust the switching speed by controlling the light-matter interactions in a multi-material structure provides an additional degree of freedom in the design of all-optical switches. Moreover, the proposed approach utilizes “slower” materials that are very robust and allow to enhance the field intensities while “faster” materials ensure an ultrafast dynamic response. The proposed control of the switching time could lead to new functionalities and performance metrics within key applications in multiband transmission, optical computing, and nonlinear optics.

We demonstrate actively tunable third harmonic generation (THG) in zinc oxide, increasing THG by 600%. This is done using an interband pump, generating free carriers to increase Kerr nonlinearities, and enhancing fields through permittivity reduction.