Dual-Mode MOSCAP Silicon Microring Resonator for Energy-Efficient Analog Optical Computing

Silicon microring resonators have established themselves as key building blocks for optical neural networks (ONNs); however, achieving low-voltage operation remains a critical challenge for scalable photonic hardware. Here, we demonstrate a subvolt silicon microring resonator using a metal-oxide-semiconductor capacitor (MOSCAP) gated by a high-mobility transparent conductive oxide (HMTCO). The suppressed free carrier absorption enabled by the high mobility preserves a high Q-factor while achieving a large electro-optic efficiency of 175 pm/V. This results in an ultracompact V_πL of 0.09 V·cm, an extinction ratio (ER) > 6 dB with only 0.5 V driving voltage, and an energy efficiency of 13.5 fJ/bit. In addition, the MOSCAP configuration enables dual-mode operation in accumulation and depletion modes depending on the gate bias, achieving both analog and nonlinear photonic responses within a single structure. In the accumulation regime, the device provides continuous, monotonic resonance tuning, enabling analog optical weight encoding with a subvolt electrical swing, demonstrated at 5-bit and 6-bit resolution. In contrast, in the depletion regime, the device produces an intrinsically nonlinear transfer curve that approximates sigmoid and reverse soft-ReLU activation functions, selectable via probe-wavelength detuning, without requiring any additional nonlinear component. By realizing analog weight encoding and nonlinear activation within a single compact structure, this dual-mode MOSCAP MRR establishes a versatile building block for scalable, energy-efficient integrated ONNs.