  Efficient electro-optic (EO) modulators crucially rely on advanced materials
that exhibit strong electro-optic activity and that can be integrated into
high-speed and efficient phase shifter structures. In this paper, we
demonstrate ultra-high in-device EO figures of merit of up to n3r33 = 2300 pm/V
achieved in a silicon-organic hybrid (SOH) Mach-Zehnder Modulator (MZM) using
the EO chromophore JRD1. This is the highest material-related in-device EO
figure of merit hitherto achieved in a high-speed modulator at any operating
wavelength. The {\pi}-voltage of the 1.5 mm-long device amounts to 210 mV,
leading to a voltage-length product of U{\pi}L = 320 V{\mu}m - the lowest value
reported for MZM that are based on low-loss dielectric waveguides. The
viability of the devices is demonstrated by generating high-quality
on-off-keying (OOK) signals at 40 Gbit/s with Q factors in excess of 8 at a
drive voltage as low as 140 mVpp. We expect that efficient high-speed EO
modulators will not only have major impact in the field of optical
communications, but will also open new avenues towards ultra-fast
photonic-electronic signal processing.
