  Modern communication networks require high performance and scalable
electro-optic modulators that convert electrical signals to optical signals at
high speed. Existing lithium niobate modulators have excellent performance but
are bulky and prohibitively expensive to scale up. Here we demonstrate scalable
and high-performance nanophotonic electro-optic modulators made of
single-crystalline lithium niobate microring resonators and micro-Mach-Zehnder
interferometers. We show a half-wave electro-optic modulation efficiency of
1.8V$\cdot$cm and data rates up to 40 Gbps.
