The efficient manipulation of graphene's saturable absorption in resonant structures potentially allows for the demonstration of advanced all-optical components for modulation, switching and routing applications. The key idea towards this objective is that a strong pump wave reduces the losses experienced by a weak probe signal as it interacts with graphene. This is a recently demonstrated phenomenon in graphene, referred to also as cross saturable absorption (X-SA), in analogy to cross-phase modulation in multichannel systems dominated by Kerr nonlinearity. X-SA is an almost instantaneous nonlinear effect limited by the electron recombination time in graphene, in most cases in the sub-picosecond time scale. Thus, X-SA in graphene is an important candidate for the realization of advanced functionalities with low-power requirements and ultra-fast response.

A possible implementation of such an all-optical nonlinear element is shown in Fig. 1. A graphene monolayer uniformly covers a silica-cladded silicon disk traveling-wave resonator in an add-drop filter configuration. The operating principle of such a device lies in the selective fulfillment of the critical-coupling condition to efficiently route a low-intensity input signal either to the through or to the drop port. This is achieved by manipulating graphene losses using a second stimulus (the strong pump wave), introduced on demand (modulation/switching actions), or following a predefined pattern (routing). This functionality is demonstrated in Fig. 2, where an input pulse is efficiently routed between the through and the drop ports in the absence (presence) of a strong control pulse.

References

[J.1] Christopoulos T., Ataloglou V. G., and Kriezis Em. E., "All-optical nanophotonic resonant element for switching and routing applications exploiting graphene saturable absorption", Journal of Applied Physics, 127 (22), 223102, (2020). [pdf]

[J.8] Nousios G., Christopoulos T., Tsilipakos O., and Kriezis Em. E., "Dynamic Routing through Saturable Absorption in Graphene Photonic Resonators: Impact of Carrier Diffusion and Finite Relaxation Time", Journal of Applied Physics, 131 (5), 053104, (2022). [pdf]

Fig. 1: Silicon-based add-drop filter for modulation, switching, and routing applications, induced by cross saturable absorption on the overlaying graphene monolayer.
(Click on image to enlarge)

Fig. 2: Typical routing scheme: An input pulse is routed between the through and the drop ports in the absence/presence of a strong control pulse.
(Click on image to enlarge)