Femtofarad optoelectronic integration demonstrating energy-saving signal conversion and nonlinear functions

The introduction of photonic technologies into mature electronic circuits is in high demand to accelerate on-chip information networking and even computing. Great difficulty lies in the fact that the optoelectronic coupling at their interfaces requires a substantial charging energy determined by their capacitance. Optoelectronic devices have been too large to reduce the integrated capacitance down to the femtofarad scale. Here we use a photonic-crystal platform to demonstrate the first experimental proof of optoelectronic integration at only 2？fF. This allows us to realize a record-low attojoule-energy electro-optic modulator (an electrical-to-optical or E–O converter) and an amplifier-free photoreceiver (an optical-to-electrical, or O–E converter), which leads to ultralow-energy signal conversion. By integrating these O–E/E–O devices, we demonstrate femtofarad ‘O–E–O transistors’ with optical signal gain that show various optical nonlinear functions, including as all-optical switches, wavelength converters and cascadable optical repeaters with a femtojoule-per-bit energy consumption. These femtofarad-scale O–E/E–O/O–E–O devices promise tightly coupled photonic–electronic integration for new fields of energy-saving information processing