Enhancement of the third harmonic generation efficiency of ITO nanolayers coupled to Tamm plasmon polaritons

We study the enhancement of third harmonic generation in indium tin oxide nanolayers coupled to Tamm plasmon polaritons (TPPs). The TPPs are excited at the interface between a thin gold mirror and a silicon dioxide/silicon nitride (SiO2/Si3N4) distributed Bragg reflector with a 30 nm thick indium tin oxide (ITO) nanolayer embedded inside the topmost dielectric layer under the metal mirror. This ITO nanolayer exhibits epsilon-near-zero (ENZ) behavior at near-infrared wavelengths. By tuning the angle of incidence and the TPP resonance conditions, we achieve sub-wavelength confinement of the electromagnetic field, resulting in an 8× enhancement of the nonlinear optical response of the structure compared with the isolated ITO nanolayer at its optimal ENZ condition. We further investigate the dependence of the THG signal on the incident angle and sample orientation, confirming that the enhancement is driven by the excitation of the TPP mode with a characteristic asymmetric behavior. Numerical simulations of local field factors based on the transfer matrix method fully support our findings. Our study demonstrates that the TPP-ENZ platform offers a versatile and highly efficient approach to enhancing nonlinear optical processes, with potential applications in frequency conversion, optical signal processing, and the development of more efficient nonlinear photonic devices.