In order to increase the speed with which photonic integrated circuits (PICs) control light flux, researchers are investigating new materials with high optical nonlinearity. Among the promising materials are microwaveguides based on graphene, a material in which charge carrier concentrations can be effectively controlled using optical pumping or applied bias voltage. The work of a team from the Lobachevsky State University of Nizhny Novgorod—National Research University (UNN, also known as Lobachevsky University) could provide a new perspective on the dynamics of waves in nonstationary microwaveguides that could contribute to advances in PICs. While exploring potential materials for creating PICs, the UNN team developed a theory for the conversion of light waves propagating over the surface of graphene, when the concentration of electrons in graphene changes over time. The interaction of electrons with the light field was precisely taken into account. As a result of their study, the researchers ruled out the possibility of amplifying light waves by changing the concentration of electrons. (a) Illustration of a surface plasmon propagating along a graphene sheet at t 1 to N2. Courtesy of Lobachevsky University. The researchers took a general theoretical approach to calculate the graphene plasmon transformation after rapid changes of the Fermi level and carrier density, basing their approach on solving the Maxwell equations supplemented by the microscopic current equation. They derived formulas for the amplitudes of the transmitted and reflected plasmons after a rapid carrier density drop. The relation of these amplitudes and the Fourier transformed finite-difference time-domain (FDTD) fields was established. The results of the analytical and FDTD approaches refuted claims of plasmon amplification under rapid carrier changes that have appeared in recent theoretical studies, the researchers said. The team believes that the theoretical and computational approaches it presents could form a basis of time-varying electromagnetics of graphene plasmonics. Professor Alexei Maslov said, “Our study is aimed at developing the physical principles of ultrafast photon control in integrated microchips, in other words, at improving the performance of microcircuits and microchips used in microelectronics and nanoelectronics.” The research was published in Optica, a publication of The Optical Society (https://doi.org/10.1364/OPTICA.5.001508).
