THz Field Induced Second Harmonic Generation in Epsilon Near Zero Indium Tin Oxide Thin Films

Epsilon near zero (ENZ) thin films have attracted considerable attention due to their unique optical properties in the near-infrared (NIR), which have enabled a wide range of interesting phenomena and diverse applications. In nonlinear optics, the near-zero permittivity of films in the NIR region has been shown to enhance second-order nonlinear processes by several orders of magnitude, therefore boosting both second harmonic (SH) generation and broadband THz generation. In this work, we investigate THz-field-induced second harmonic (TFISH) generation in indium tin oxide (ITO) thin films in the ENZ spectral region. A symmetry-breaking electric field is applied using a high field strength broadband THz pulse, which when temporally overlapped with an ultrashort optical pulse, results in the emission of second harmonic from the centrosymmetric film. The experimental results mirror very well the predictions of a four-wave mixing optical model, capturing the interplay between linear and nonlinear effects driving the NIR-THz-ITO interaction.