Confinement‐Induced Nonlocality and Optical Nonlinearity of Transdimensional Titanium Nitride Films in the Epsilon‐Near‐Zero Region

ABSTRACT

Ultrathin metal films approaching the transdimensional (TD) thickness limit, an intermediate regime between 2D and 3D, provide a promising route for light–matter interaction control and manipulation, yet their nonlinear optical response near the epsilon‐near‐zero (ENZ) condition remains poorly understood. Here, we report the strongly enhanced nonlinear absorption for high‐quality refractory TiN epitaxial films grown by molecular beam epitaxy, with thicknesses down to a few nanometers. Systematic Z‐scan measurements across film thickness, wavelength, and incidence angle reveal a pronounced enhancement of nonlinear absorption in TD TiN films thinner than 10 nm. Especially in the ENZ spectral region, the TD TiN films exhibit nearly two orders of magnitude stronger nonlinear absorption over a broad range of incidence angles as compared to conventional thin films. The enhanced nonlinear absorption observed is well described by a nonlinear nonlocal electromagnetic response model that accounts for electron confinement effects unique to the TD plasmonic systems. These findings provide clear experimental evidence of ENZ‐mediated nonlinear enhancement in TD plasmonic films and establish TiN and related TD plasmonic materials as a robust platform for ultrathin photonic devices exploiting ENZ‐driven nonlinear processes.