Confinement-Induced Nonlocality and Optical Nonlinearity of Transdimensional Titanium Nitride in the Epsilon-Near-Zero Region
Ultrathin plasmonic films that approach the trans-dimensional (TD) thickness limit 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 optical nonlinearity for their typical representative high quality TiN epitaxial films with thicknesses down to a few nanometers. Systematic Z_scan measurements reveal a pronounced increase in nonlinear absorption with decreasing thickness. 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. Comparison with Ti1_xAlxN highlights the necessity of low-loss ENZ response for nonlinear enhancement. These findings identify TiN and similar TD plasmonic systems as a robust refractory platform for exploiting ENZ mediated nonlinear processes in ultrathin photonic material structures.