  Electro-optic modulators for high-speed on-off keying (OOK) are key
components of short- and mediumreach interconnects in data-center networks.
Besides small footprint and cost-efficient large-scale production, small drive
voltages and ultra-low power consumption are of paramount importance for such
devices. Here we demonstrate that the concept of silicon-organic hybrid (SOH)
integration is perfectly suited for meeting these challenges. The approach
combines the unique processing advantages of large-scale silicon photonics with
unrivalled electro-optic (EO) coefficients obtained by molecular engineering of
organic materials. In our proof-of-concept experiments, we demonstrate
generation and transmission of OOK signals with line rates of up to 100 Gbit/s
using a 1.1 mm-long SOH Mach-Zehnder modulator (MZM) which features a
{\pi}-voltage of only 0.9 V. This experiment represents not only the first
demonstration of 100 Gbit/s OOK on the silicon photonic platform, but also
leads to the lowest drive voltage and energy consumption ever demonstrated at
this data rate for a semiconductor-based device. We support our experimental
results by a theoretical analysis and show that the nonlinear transfer
characteristic of the MZM can be exploited to overcome bandwidth limitations of
the modulator and of the electric driver circuitry. The devices are fabricated
in a commercial silicon photonics line and can hence be combined with the full
portfolio of standard silicon photonic devices. We expect that high-speed
power-efficient SOH modulators may have transformative impact on short-reach
optical networks, enabling compact transceivers with unprecedented energy
efficiency that will be at the heart of future Ethernet interfaces at Tbit/s
data rates.
