Bismuth ferrite (BiFeO3) optical waveguide memristor realized in lithium niobate (LiNbO3)

An optical memristor is developed by miniaturizing Ag/BiFeO3/ITO resistive random-access memory (ReRAM) placed over a section immediately above a lithium niobate optical waveguide. The device operation depends on the interaction of the evanescent tail with the formation/rupture of the filaments inside the ReRAM. By alternately applying positive and negative biases, the ReRAM can be switched between high-resistance and low-resistance states. Due to the filament formation attributed to the electrochemical metallization (ECM) and the bipolar switching characteristics of the device, applying a positive bias switches the ReRAM to a low resistance state, creating a silver filament in the oxide layer. Since silver metal has a high optical absorption coefficient, the optical signal passing through it will be absorbed, weakening the output signal. On the contrary, when a negative bias is applied instead, the silver filament becomes ruptured in return. As a result, the optical signal passes through without being absorbed by silver atoms and causes the output optical signal to increase accordingly, resulting in electrically controlled optical output. The electrical input and optical output are inverse regarding their respective sign and magnitudes. To improve the coupling effect between the optical waveguide and the memory device, wet etching is employed to etch into the lithium niobate waveguide to insert the miniaturized ReRAM into the etched groove; the very action facilitates the light-matter interaction between ReRAM and the optical waveguide. The buried configuration is compared regarding optical modulation efficiency with a device directly stacked over the lithium niobate waveguide acting as a control sample. As expected, the device inserted into the etched groove has an extinction ratio of about 10.9%, which is noticeably higher than that of the control sample.