Light propagation in conventional photonic systems is ruled by the Lorentz reciprocity law. According to the reciprocity principle, if the positions of the source and receiver are swapped the transmission level is unchanged. Thus, reciprocal systems are inherently bidirectional, independent of any spatial asymmetry.
“One-way” light flows can be obtained with ferrites or other iron garnets biased with a static magnetic field. Indeed, nonreciprocal devices, such as isolators and circulators, are essential building blocks of communication systems. However, at optical frequencies the standard solutions are bulky and/or challenging to integrate on-chip. Thereby, there is a great interest in novel approaches that can enable the “one-way” functionality in future integrated photonic platforms.
In project One-Way, a research team from IT, coordinated by Mário Silveirinha, studied several unconventional mechanisms to break the Lorentz reciprocity. The main research direction was inspired by the electrodynamics of systems with moving components. Results have shown that in some conditions the relative motion of two uncharged material bodies, such as a metal sphere moving very fast and parallel to a metal sheet, may trigger the emission of coherent light. The emitted energy is due to the spontaneous conversion of the kinetic energy of the moving body into radiation, some sort of a laser pumped by the physical “motion”. However, the effect is significant only when the distance between the two objects is on the order of 10nm and the relative velocity is about one tenth of the speed of light in vacuum.
To overcome this restriction, researchers suggested imitating the translational motion of a uncharged body in vacuum with a drift electric current in a solid state material. Indeed, the electromagnetic response of a material with drifting electrons is to some extent equivalent to that of the corresponding moving structure and with no drifting electrons. Building on this idea, it was showned that a “cavity” formed by a drift-biased graphene sheet and a second graphene sheet with no drift current may spontaneously emit mid-IR radiation due to the conversion of the drifting electrons kinetic energy into light.
Furthermore, the drifting electrons can effectively drag the plasmon waves along the direction of motion, in the same manner as a boat near a water fall is dragged by the stream when the stream velocity exceeds some threshold value. Thus, the drift-current bias can enable a “one-way” propagation. The gain provided by the drift-current biasing may allow for the amplification of the signal, and thereby to boost the propagation length of the plasmons.
These findings may open interesting perspectives in nonreciprocal electromagnetics and offer new opportunities to control the flow of light with one-atom thick nonreciprocal devices and design robust and practical light sources at the nanoscale.