A new plasmonic Mach-Zehnder modulator is demonstrated at a bit rate of 120 Gb/s NRZ-OOK with low peak-to-peak driving voltages of 178 mVpp below the HD-FEC limit. Such record low driving voltage requirements potentially translate into an electrical drive power consumption of 862 aJ/bit. The low drive voltages have been made possible by a new differential Mach-Zehnder modulator electrode design. The differential electrode design is optimized for the balanced driving circuitry and reduces the effectively required driving voltage by a factor of four (Vπ/4). The potential of the transmitter scheme is further demonstrated by a transmission experiment over 500 m of single mode fiber at the C-band with a BER performance below the KP4 FEC limit.