Artificial nonreciprocal photonic materials at GHz-to-THz frequencies
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Introduction Fundamentally rooted in the time-reversibility of microscopic processes,1 Lorentz reciprocity requires that, in most circumstances, signals travel from point A to point B, and back from B to A, with the same transmission properties. The implications of this symmetry relation are ubiquitous in our day-today life, and the needs for technologies that can break it have been steadily growing in the past decades. The radio frequency (RF)/millimeter-wave/THz frequency range is especially important in this context, since continuous-wave radar systems, from military to automotive and civil applications, operate by discriminating outgoing and incoming signals at the same frequency in a single antenna, an operation that can be performed only by breaking reciprocity. Similarly, there is strong interest in full-duplex cellular and WiFi communication systems that can transmit and receive at the same time over the same frequency channel;2,3 again, an operation that inherently requires isolation and nonreciprocal responses. The most common approach to build a nonreciprocal device involves breaking time-reversal symmetry by relying on magneto-optical phenomena:4 a DC magnetic bias in gyrotropic materials, such as ferrites, forces left- and right-handed polarizations to propagate at different speeds, a symmetry breaking that depends on the direction of the magnetic bias, not on the direction of propagation, therefore enabling nonreciprocity. This basic phenomenon can be
exploited to realize isolators, circulators, nonreciprocal phase shifters, and other nonreciprocal systems. However, strong magneto-optical effects require rare, lossy, and expensive materials, such as iron garnets or ferrites, which cannot be straightforwardly integrated in complementary metal oxide semiconductor (CMOS) technology, and therefore lead to bulky, expensive, and often impractical components. For this reason, there has long been interest in developing magnetless approaches to nonreciprocity, especially focused on the low-frequency end of the spectrum, for which some alternatives to magnetic approaches have been devised in the past decades. A basic example is provided by the use of nonlinear materials, as highlighted in the article by Chen et al.5 in this issue. Nonlinear phenomena, when combined with geometrical asymmetries, can break reciprocity, since the signal itself, as it travels through a nonlinear material, can bias it and break time reversibility. However, this approach works only for signals of sufficiently high intensities, inherently leads to nonlinear signal distortions, and is hampered by interference with other signals, and therefore, it generally works only for incident signals from one port at a time.6,7 Another alternative, as discussed in the following section, is offered by a DC current bias, which can also break time-reversal symmetry. At MHz, GHz, and even millimeter-wave frequencies, transistor-based devices8 and artificial materials using transistors and other active devices as their building blocks
Andrea Alù, Advan
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