Doubly Enhanced Second Harmonic Generation through Structural and Epsilon-near-Zero Resonances in TiN Nanostructures

Enhancing the nonlinear frequency conversion efficiency at the nanoscale is important for on-chip communication, information processing and sensing. Plasmonic nanostructures can significantly enhance the nonlinear signal due to localized surface plasmon (LSP) resonance, that is, localized electric field enhancements. Ideally, a double resonance occurs, in which both the excitation and the harmonic wavelengths are enhanced, but this is restricted by the available modes. It has been recently shown that thin films of epsilon-near-zero (ENZ) materials can also enhance optical nonlinear effects if excited at the ENZ wavelength. Here, we report the first demonstration of a new mechanism to enhance the second harmonic generation (SHG), combining a structural LSP resonance at the fundamental frequency, and the material ENZ resonance at the second harmonic frequency. We show that when both resonances are present, the SHG is substantially enhanced. With its refractory nature and CMOS compatibility, our results show considerable promise for TiN micro and nanostructures in integrated on-chip nonlinear optical devices.