Integrated electric-field (E-field) sensors are commonly used devices in E-field sensing. However, distortion in the modulated signal due to high half-voltage (Vπ) and obtaining a low-frequency response are challenging issues in low-frequency AC E-field sensors. The aim of this study is to investigate a modification by adding a Si layer beneath a segmented slot waveguide (SSW) and optimizing the hybrid SSW as the core of a sensor to determine the sensor features in terms of the frequency response and sensitivity. The results of reducing the Si-layer thickness and segment width with high periodicity revealed a high modulation efficiency for very low-frequency AC E-field sensors and simultaneously expanded the minimum limit of detection by incorporating sensors in very small AC E-fields. This study validated the feasibility and efficiency of using a hybrid SSW as the core of highly sensitive low-frequency AC E-field sensors.