Antennas are everywhere: on radios, televisions, cell phones, computers, and wireless internet routers. Each is optimized for a specific frequency range—for example, cellular frequency bands are around 800 MHz and 1.9 GHz. Normally, the time-reversal symmetry of Maxwell’s equations dictates that if an antenna can transmit efficiently at a particular frequency and in a particular direction, it must be an equally good receiver at the same frequency and in the same direction.

That reciprocity comes in handy when measuring the reception pattern of an antenna; one need only measure the transmission pattern, which is easier. But reciprocity becomes a problem—and can slow down communications—when antennas are forced to listen to the reflections of their own signal or to other transmitted signals at the same frequency.

antenna that breaks time-reversal symmetry. 1 113, 3471 (2016). 1. Y. Hadad, J. C. Soric, A. Alù, Proc. Natl. Acad. Sci. USA, 3471 (2016). https://doi.org/10.1073/pnas.1517363113 antenna transmitted 50 times more strongly than it received. Now Andrea Alù and postdocs Yakir Hadad and Jason Soric of the University of Texas at Austin have designed and built anthat breaks time-reversal symmetry.By passing a weak alternating electric signal through the device, they altered the way it interacts with transmitted and received signals. Under certain conditions, thetransmitted 50 times more strongly than it received.

Guided waves Section: Choose Top of page ABSTRACT Guided waves << Free radiation REFERENCES CITING ARTICLES 2 et al. , Nat. Photonics 7, 579 (2013). 2. D. Jalas, Nat. Photonics, 579 (2013). https://doi.org/10.1038/nphoton.2013.185 electromagnetic signals through a magnetic material that’s been biased with an external magnetic field. Electrons in the material orbit with a particular handedness, thereby breaking time-reversal symmetry, and allow signals to pass in one direction but not the other. Such so-called magnetic isolators are deployed in some RF communications applications. But they’re expensive and bulky, so there’s active interest in developing alternatives. Reciprocity-breaking devices are not new.A common approach is to passsignals through athat’s been biased with an externalElectrons in the material orbit with a particular handedness, thereby breaking time-reversal symmetry, and allow signals to pass in one direction but not the other. Such so-called magnetic isolators are deployed in some RF communications applications. But they’re expensive and bulky, so there’s active interest in developing alternatives. acoustic waves. 3 et al. , Science 343, 516 (2014). 3. R. Fleury, Science, 516 (2014). https://doi.org/10.1126/science.1246957 waves propagating clockwise and counterclockwise around the ring are endowed with different resonant frequencies. Three ports equally spaced around the ring coupled acoustic waveguides to the cavity, and the waves passing through them behaved in a nonreciprocal way: Sound waves entering the first port exited only the second, those entering the second exited only the third, and those entering the third exited only the first. For the past several years, Alù and his group have been working on breaking reciprocity in new and different contexts. They first built a nonreciprocal device, called a circulator, for guidedIt works by circulating a continuous flow of air through a ring-shaped acoustic cavity so that soundpropagating clockwise and counterclockwise around the ring are endowed with differentfrequencies. Three ports equally spaced around the ring coupled acoustic waveguides to the cavity, and thepassing through them behaved in a nonreciprocal way: Soundentering the first port exited only the second, those entering the second exited only the third, and those entering the third exited only the first. magnetic materials. The circulation of air played the role of the circulation of electrons in a magnetic field, and the splitting of acoustic resonances was analogous to the Zeeman splitting of electronic energy levels. The next step was to translate the concept back into the electromagnetic realm while keeping the scale macroscopic. 4 et al. , Nat. Phys. 10, 923 (2014). 4. N. A. Estep, Nat. Phys., 923 (2014). https://doi.org/10.1038/nphys3134 LC circuits. In place of the air flow, the researchers circulated an electric signal that, in turn, caused the circuits’ resonant frequencies to oscillate. The acoustic device was inspired by the physics ofThe circulation of air played the role of the circulation of electrons in aand the splitting of acoustic resonances was analogous to the Zeeman splitting of electronic energy levels. The next step was to translate the concept back into therealm while keeping the scale macroscopic.The acoustic waveguides were replaced by microwave ones, and the ring-shaped cavity was replaced by a ring of coupledcircuits. In place of the air flow, thecirculated an electric signal that, in turn, caused the circuits’frequencies to oscillate. Key to the device’s operation was the variable capacitor built into each of the coupled circuits. Its capacitance varies with the applied voltage—or, put another way, the voltage across it is not linearly related to the stored charge. The capacitor is similar to a standard semiconductor diode, with a reverse bias applied to create a region depleted of charge carriers. The depletion region serves as the capacitor’s dielectric layer. The greater the reverse-bias voltage, the thicker the depletion region and the lower the capacitance. The variable capacitances resulted in variable resonant frequencies for the coupled circuits and led to the nonreciprocal behavior of electromagnetic waves, with no magnetic fields required.