Mid-infrared nonlinear optical modulation of CdO nanogratings enabled by geometric quantum interference enhancement

Epsilon-near-zero (ENZ) materials, notably cadmium oxide (CdO), exhibit strong nonlinear optical responses in the mid-infrared (MIR) region, but are limited by insufficient tunability and nonlinear strength. To overcome these constraints, we designed and fabricated a CdO-based nanograting (CdO-NG). In this structure, the geometrically induced quantum interference effect effectively tailors the Fermi level (E_F), thereby controlling the MIR nonlinear optical properties. Open-aperture Z-scan measurements at 2 µm reveal that the nonlinear optical absorption coefficient (β) increases with both etching depth and angle of incidence, exhibiting clear polarization anisotropy. This is notably demonstrated in CdO-NG10, where the β under TM polarization is -6.9 cm MW^-1, higher than the -6.35 cm MW^-1 observed under TE polarization. Further nonlinear absorption measurements conducted at 3 µm (outside the ENZ region) confirmed that this anisotropy originates from the material's intrinsic geometric structure. Furthermore, carrier-dynamics measurements revealed excellent ultrafast characteristics, including a transient response time below 220 fs and a modulation bandwidth exceeding 1.6 THz. This work highlights the potential of this geometrically engineered platform for next-generation ultrafast all-optical switches and high-power MIR photonic devices.