Lithium niobate (LN) has been the material of choice for electro-optic modulators owing to itsexcellent physical properties. While conventional LN electro-optic modulators continue to bethe workhorse of the modern optoelectronics, they are becoming progressively too bulky,expensive, and power-hungry to fully serve the needs of this industry. Here, we demonstrateplasmonic electro-optic directional coupler switches consisting of two closely spaced nm-thingold nanostripes on LN substrates that guide both coupled electromagnetic modes andelectrical signals that control their coupling, thereby enabling ultra-compact switching andmodulation functionalities. Extreme confinement and good spatial overlap of both slowplasmonmodes and electrostatic fields created by the nanostripes allow us to achieve a 90%modulation depth with 20-μm-long switches characterized by a broadband electro-opticmodulation efficiency of 0.3 V cm. Our monolithic LN plasmonic platform enables a widerange of cost-effective optical communication applications that demand μm-scale footprints,ultrafast operation and high environmental stability.
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