All-optical Mach-Zehnder Switch on a Graphene-on-silicon Nitride Chip

All-optical switches play an important role in optical cross-connects and optical communication networks. With the advantage of light-controlling-light, all-optical switches could be used for achieving an efficient light signal processing, which avoids the conversion between optical signals and electronic signals. Silicon nitride (Si
₃
N
₄
) is an ideal candidate for all-optical devices with a low optical loss, since Si
₃
N
₄
has a large bandgap of ~ 5eV and doesn't have two- photon absorption (TPA) effect as well as the free-carrier absorption (FCA) effect in the C-band. However, Si
₃
N
₄
has a weak thermooptic effect, and thus it is still hard to implement an all-optical high-efficient Si
₃
N
₄
switch based on the thermo-optic effect. In this paper, an all-optical Mach- Zehnder switch on graphene-on-Si
₃
N
₄
chip is proposed and experimentally demonstrated. In our device, the Mach-Zehnder switch is based on the Si
₃
N
₄
waveguide. And the graphene overlies one of the two arms of the switch. Then, the graphene can absorb the pump light and convert the light power into heat, which in turn changes the refractive index of the Si
₃
N
₄
underneath based on the thermo-optic effect. In this case, for the probe signal light, the phase delay of the arm is tuned and optical switching is then implemented. Note that, in this process, the weakness of the thermo-optic effect and thermal conductivity of Si
₃
N
₄
is compensated by graphene due to its high thermal conductivity. In the experimental demonstration, all optical switching with an extinction ratio of 15 dB is demonstrated while the device footprint is ~ 0.24 mm
²
. Furthermore, a tuning efficiency of 0.0112 π/mW is implemented with a switching power of 89.1 mW, which is much higher than many other thermo-optic switches. Note that, the proposed device configuration may provide a simple and effective way to realize all-optical high-efficient light control.