  In modern communication system, modulation is a key function that embeds the
baseband signal (information) into a carrier wave so that it can be
successfully broadcasted through a medium such as air or cables. A flexible
signal modulation scheme is hence essential to wide range of applications based
on flexible electronics. Here we report a fully bendable all-graphene modulator
circuit with the capability to encode a carrier signal with quaternary digital
information for the first time. By exploiting the ambipolarity and the
nonlinearity in a graphene transistor, we demonstrated two types of quaternary
modulation schemes: 4-ary amplitude-shift keying (4-ASK) and quadrature
phase-shift keying (QPSK). Remarkably, 4-ASK and QPSK can be realized with just
1 and 2 all-graphene transistors, respectively, representing a drastic
reduction in circuit complexity when compared with conventional digital
modulators. In addition, the circuit is not only flexible but also highly
transparent (~95% transmittance) owing to their all-graphene design with every
component (channel, interconnects, load resistor, and source/drain/gate
electrodes) fabricated from graphene films. Taken together, these results
represent a significant step toward achieving a high speed communication system
that can be monolithically integrated on a flexible and transparent platform.
