Nonlinear refractive index compensation enables accurate spectral shifting in varactor-driven plasmonic waveguides

Abstract

Precise control of spectral shifting in time-varying media is essential for reconfigurable microwave-photonic systems. However, varactor-based waveguides inherently exhibit strong capacitance–voltage nonlinearity, which is directly imprinted onto the refractive-index modulation and leads to severe distortion of the expected sinusoidal frequency trajectories. Here, we experimentally establish the complete nonlinear response chain, from applied voltage to refractive index to output spectrum in a varactor-loaded spoof surface plasmon polariton waveguide. By combining full wave simulations of the varactor-loaded waveguide with time-delay measurements, we obtain an accurate refractive-index–voltage mapping that reveals highly asymmetric refractive-index modulation under sinusoidal driving. This distortion explains the experimentally observed frequency-shift asymmetry and the deviation of 0.51 MHz from the ideal sinusoidal trajectory. Inspired by predistortion, we design an inverse-function-based compensation voltage waveform using the experimentally calibrated n–V mapping. This compensated waveform restores a nearly ideal sinusoidal refractive-index profile inside the waveguide. The symmetric frequency shift exhibited by the compensated device agrees excellently with analytical theory and COMSOL simulations. These results provide a broadly applicable workflow for calibration and control, mitigating intrinsic varactor nonlinearity in reconfigurable waveguides and enabling accurate dynamic spectral control.