Third Harmonic Generation in Transparent Longitudinal Epsilon-Near-Zero Multilayers.

Epsilon-near-zero (ENZ) materials can dramatically enhance local optical fields, enabling nonlinear interactions at relatively low intensities. Yet, near their plasma frequency, isotropic ENZ media remain highly absorptive, limiting nonlinear operations that require good transparency. Longitudinal epsilon-near-zero (LENZ) metamaterials, characterized by a vanishing permittivity along the optical axis, provide a promising route to field enhancement while mitigating absorption losses and impedance mismatch. We experimentally show that an αSi/ITO multilayer with a LENZ resonance in the near-infrared enables broadband high pump transmission while still supporting ENZ-enhanced harmonic generation. We observe a strong third-harmonic signal under conditions of significantly higher pump transparency than comparable ITO films demonstrating how nonlinear processes can be driven in a transparent multilayer geometry, avoiding the high-loss pump conditions typically associated with isotropic ENZ operation. Measured third-harmonic efficiency is comparable to values reported for isotropic ENZ films, whereas the multilayer architecture provides a potential route to reduce absorption-induced heating at the pump wavelength. These results establish transparent LENZ (TLENZ) multilayers as a versatile platform for transparent, field-enhanced nonlinear interactions, combining strong light-matter interaction with reduced pump losses.