Graphene phase modulators operating in the transparency regime

Next-generation data networks need to support Tb/s rates. In-phase and quadrature (IQ) modulation combine phase and intensity information to increase the density of encoded data, reduce overall power consumption by minimising the number of channels, and increase noise tolerance. To reduce errors when decoding the received signal, intersymbol interference must be minimised. This is achieved with pure phase modulation, where the phase of the optical signal is controlled without changing its intensity. Phase modulators are characterised by the voltage required to achieve a π phase shift V_π, the device length L, and their product V_πL. To reduce power consumption, IQ modulators are needed with<1V drive voltages and compact (sub-cm) dimensions, which translate in V_πL<1Vcm. Si and LiNbO_3 (LN) IQ modulators do not currently meet these requirements, because V_πL>1Vcm. Here, we report a double single-layer graphene (SLG) Mach-Zehnder modulator (MZM) with pure phase modulation in the transparent regime, where optical losses are minimised and remain constant with increasing voltage. Our device has V_πL∼0.3Vcm, matching state-of-the-art SLG-based MZMs and plasmonic LN MZMs, but with pure phase modulation and low insertion loss (∼5dB), essential for IQ modulation. Our V_π L is∼5 times lower than the lowest thin-film LN MZMs, and∼3 times lower than the lowest Si MZMs. This enables devices with complementary metal-oxide semiconductor compatible V_πL (<1Vcm) and smaller footprint than LN or Si MZMs, improving circuit density and reducing power consumption by one order of magnitude.