FDTD modeling of traveling-wave MIM diode for ultrafast pulse detection

We investigated the performance of traveling-wave metal-insulator-metal (TW-MIM) diodes for detecting ultrafast optical pulses. The TW-MIM structure is modeled as a deeply-confined metal-insulator-metal plasmonic waveguide with a nonlinear conductive insulator layer. A quadratic model for the nonlinear conductivity was derived from the semi-classical J-V relation of the MIM junction and incorporated into a nonlinear FDTD method for simulating the TW-MIM detectors. Due to the extremely fast electron tunneling time and the short plasmonic decay length, TW-MIM diodes exhibit much broader optical and electrical bandwidths than conventional traveling wave photodiode detectors. Simulations of Al-Al₂O₃-Al TW-MIM structures show that these devices can detect ultrafast pulses with bandwidth exceeding 10 THz over a wide range of carrier frequencies from THz to the infrared.