Antennas are everywhere: on radios, televisions, cell phones, computers, and wireless internet routers. Each is optimized for a specific frequency range, often in the megahertz or gigahertz. Normally, the time-reversal symmetry of electromagnetic radiation dictates that if an antenna can transmit efficiently at a particular frequency, it must be an equally good receiver at the same frequency. That reciprocity becomes a problem—and slows down communications—when antennas inevitably listen to the reflections of their own transmitted signals.
Now Andrea Alù and his colleagues at the University of Texas at Austin have designed and built an antenna that breaks time-reversal symmetry. Their device, shown in the figure, consists of two transmission lines (specialized cables that carry RF alternating currents) on a flat copper sheet. The transmission lines are interrupted by apertures spaced 2.6 cm apart, and under each aperture is a capacitor (not visible) whose capacitance changes as a function of the voltage applied to it. The static antenna can transmit and receive signals—with equal efficiency—at around 4 GHz. But passing a weak 600 MHz current along the device caused the capacitances to oscillate, thereby breaking the symmetry between transmission and reception. Under certain conditions, the antenna transmitted 50 times more strongly than it received.
Alù and company hope to optimize their proof-of-concept antenna to achieve even stronger asymmetry. They also plan to explore whether similar devices could be designed for other regions of the electromagnetic spectrum. Photovoltaic and thermophotovoltaic cells, which harvest energy in the visible and IR parts of the spectrum, suffer the same symmetry that antennas do: A good absorber must also be a good emitter, so much of the energy that’s harvested is immediately lost. Nonreciprocal cells could yield higher-efficiency solar-energy conversion. (Y. Hadad, J. Soric, A. Alù, Proc. Natl. Acad. Sci. USA 113, 3471, 2016.)