Low-Field Terahertz Quantum Tunneling in Metal-TiO₂-Metal Nanogaps via Schottky Barrier Engineering

Light-driven quantum tunneling offers an attractive platform for coherent electron control at ultrafast time scales; however, sustainable operation remains challenging due to the high electric fields required for quantum tunneling, often leading to thermal damage caused by Joule heating. Here, we experimentally investigate how quantum barrier engineering influences the nonlinear terahertz (THz) quantum tunneling response across tailored nanometer-scale junctions. By integrating lower-band-gap oxide materials such as titanium dioxide (TiO₂) into metal-insulator-metal junctions using atomic layer lithography, we substantially reduce the onset field of Fowler-Nordheim tunneling, 13 kV cm^-1, which is 4 times lower than that in existing higher-band-gap aluminum oxide junctions. At an incident field amplitude of 34 kV cm^-1, we achieved the enhanced field of 14 MV cm^-1 (1.4 V nm^-1) at the TiO₂ nanogap, allowing up to 60% modulation of THz transmission. Beyond the expected barrier-height dependence, we identify the thermal conductivity of the tunnel barrier as a critical factor for reproducibility. TiO₂ combines a lower barrier with higher thermal conductivity, enabling stable operation over 1000 reversible cycles and revealing thermal transport in THz tunneling stability. We offer a comprehensive theoretical analysis that confirms the experimental findings, emphasizing the importance of both barrier material selection and thermal management. We expect our findings to advance the development of robust, low-field THz quantum plasmonic devices for ultrafast, energy-efficient optoelectronic applications.