Impact Ionization Mediated Terahertz Nonlinearity of Toroidal Resonance

ABSTRACT Nonlinear and efficient control of terahertz (THz) waves is vital for advancing compact photonic technologies, including high‐speed wireless communication, nonlinear optics, and tunable THz sources. However, realizing strong nonlinear responses even at moderate field strengths remains a significant challenge. In this work, we introduce a nonlinear, self‐consistent THz modulation scheme based on a dual toroidal metasurface fabricated on high‐resistivity silicon. When excited by sub‐picosecond THz pulses with peak electric fields ranging from 90 to 450 kV cm ⁻¹ , the metasurface induced strong field‐confinement facilitates impact ionization, resulting in a carrier density increase from ≈10 ¹⁰ cm ^{− 3} to 2.9 × 10 ^{1 8} cm ^{− 3} and a corresponding conductivity enhancement from ≈1× 10 ⁻¹² to 5806 S m ⁻¹ . These THz‐driven changes, as revealed by a theoretical analysis, lead to a complete suppression of both toroidal and LC resonances, and a strong nonlinear behavior. Further the scattering power drops by more than two orders of magnitude near THz probe strength of 300 kV cm ⁻¹ ; close to the avalanche breakdown threshold of silicon, hence confirming impact ionization driven self‐consistently tunable toroidal resonances. These findings underscore the effectiveness of nonlinear THz modulation for integration into next‐generation compact silicon photonic platforms while opening up new avenues in THz research. More importantly, we demonstrate that mode engineering provides an additional and powerful pathway to enhance device performance beyond conventional resonance‐based designs.