Modeling and design of high-speed ultralow voltage GaAs electro-optic modulators enabled by transparent conducting materials

We present a comprehensive modeling study of a high-speed gallium arsenide electro-optic modulator with ultralow switching voltages and large modulation bandwidths enabled by transparent conducting (TC) electrodes. The driving voltage, optical insertion loss, and modulation bandwidth of the TC-enabled modulator are systematically analyzed. Optimized designs for both a top-down and a side conduction geometry using Ta₂O₅ as both buffer and side cladding layers are presented. The results predict half-wave voltages from 0.5 down to 0.2 V, optical insertion losses of 6-10 dB, and optical 3 dB modulation bandwidths from 25-50 GHz for a top-down conduction geometry and 15-30 GHz for a side conduction geometry, assuming that proper impedance transforming parts and terminations are used. The use of benzocyclobutane as side cladding layers in the top-down conduction geometry to realize direct impedance matching was also explored. The corresponding modulation bandwidths are 13 GHz for 0.5 V case and 6 GHz for 0.2 V case, mainly limited by RF-optical wave velocity mismatch.