  Electro-optic modulation, the imprinting of a radio-frequency (RF) waveform
on an optical carrier, is one of the most important photonics functions, being
crucial for high-bandwidth signal generation, optical switching, waveform
shaping, data communications, ultrafast measurements, sampling, timing and
ranging, and RF photonics. Although silicon (Si) photonic electro-optic
modulators (EOMs) can be fabricated using wafer-scale technology compatible
with the semiconductor industry, such devices do not exceed an electrical 3-dB
bandwidth of about 50 GHz, whereas many applications require higher RF
frequencies. Bulk Lithium Niobate (LN) and etched LN modulators can scale to
higher bandwidths, but are not integrated with the Si photonics fabrication
process adopted widely over the last decade. As an alternative, an
ultra-high-bandwidth Mach-Zehnder EOM based on Si photonics is shown, made
using conventional lithography and wafer-scale fabrication, bonded to an
unpatterned LN thin film. This hybrid LN-Si MZM achieves beyond 100 GHz 3-dB
electrical bandwidth. Our design integrates silicon photonics light
input/output and optical components, including directional couplers, low-radius
bends, and path-length difference segments, realized in a foundry Si photonics
process. The use of a simple low-temperature (200C) back-end integration
process to bond a postage-stamp-sized piece of LN where desired, and achieving
light routing into and out of LN to harness its electro-optic property without
any etching or patterning of the LN film, may be broadly-useful strategies for
advanced integrated opto-electronic microchips.
