Comparative Study of Second-Harmonic Generation from Epsilon-Near-Zero Indium Tin Oxide and Titanium Nitride Nanolayers Excited in the Near-Infrared Spectral Range

We perform a comparative study of second-harmonic generation (SHG) from indium tin oxide (ITO) and from titanium nitride (TiN) nanolayers excited in the near-infrared spectrum. Both materials are compatible with Si technology and are candidate platforms for integrated nonlinear optics. In this work, we fabricate ITO samples with an e-near-zero (ENZ) condition, which can be continuously tailored in the 1150-1670 nm spectral range, and TiN samples with a metallic behavior in the same spectral range. For the ITO nanolayers, we observe tunability and enhancement of the SHG intensity when the samples are excited at their respective ENZ condition, in agreement with the electromagnetic modeling and analogous to its third-harmonic generation studied earlier. On the other hand, we show that the SHG efficiency of TiN nanolayers is lower by a factor of 50. We determine experimentally that the dominant component of the second-order susceptibility for our best ITO nanolayer is chi((2 omega))(zzz)) = 0.18 pm V-1, and we theoretically predict that the SHG process is enhanced up to 4 orders of magnitude when resonantly pumping the nanolayer at the ENZ wavelength with respect to a wavelength at 2000 nm. Remarkably, the resulting SHG efficiency is comparable with a crystalline quartz plate with thickness 0.5 mm used as a reference in our experiments in reflection configuration. Our study clearly indicates that ITO nanolayers with engineered ENZ conditions are a promising material platform for surface nonlinearities, with possible applications to nonlinear metasurfaces, Si-based flat optics, and sensing.