  Densely integrated active photonics is key for next generation on-chip
networks for addressing both footprint and energy budget concerns. However, the
weak light-matter interaction in traditional active Silicon optoelectronics
mandates rather sizable device lengths. The ideal active material choice should
avail high index modulation while being easily integrated into Silicon
photonics platforms. Indium tin oxide (ITO) offers such functionalities and has
shown promising modulation capacity recently. Interestingly, the nanometer-thin
unity-strong index modulation of ITO synergistically combines the high
group-index in hybrid plasmonic with nanoscale optical modes. Following this
design paradigm, here, we demonstrate a spectrally broadband, GHz-fast
Mach-Zehnder interferometric modulator, exhibiting a high efficiency signified
by a miniscule VpL of 95 Vum, deploying an one-micrometer compact
electrostatically tunable plasmonic phase-shifter, based on heterogeneously
integrated ITO thin films into silicon photonics. Furthermore we show, that
this device paradigm enables spectrally broadband operation across the entire
telecommunication near infrared C-band. Such sub-wavelength short efficient and
fast modulators monolithically integrated into Silicon platform open up new
possibilities for high-density photonic circuitry, which is critical for high
interconnect density of photonic neural networks or applications in GHz-fast
optical phased-arrays, for example.
