Stability bounds on superluminal propagation in active structures

Robert Duggan, Hady Moussa, Younes Ra’di, Dimitrios L. Sounas and Andrea Alù ()
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Robert Duggan: The University of Texas at Austin
Hady Moussa: CUNY Advanced Science Research Center
Younes Ra’di: CUNY Advanced Science Research Center
Dimitrios L. Sounas: Wayne State University
Andrea Alù: The University of Texas at Austin

Nature Communications, 2022, vol. 13, issue 1, 1-10

Abstract: Abstract Active materials have been explored in recent years to demonstrate superluminal group velocities over relatively broad bandwidths, implying a potential path towards bold claims such as information transport beyond the speed of light, as well as antennas and metamaterial cloaks operating over very broad bandwidths. However, causality requires that no portion of an impinging pulse can pass its precursor, implying a fundamental trade-off between bandwidth, velocity and propagation distance. Here, we clarify the general nature of superluminal propagation in active structures and derive a bound on these quantities fundamentally rooted into stability considerations. By applying filter theory, we show that this bound is generally applicable to causal structures of arbitrary complexity, as it applies to each zero-pole pair describing their response. As the system complexity grows, we find that only minor improvements in superluminal bandwidth can be practically achieved. Our results provide physical insights into the limitations of superluminal structures based on active media, implying severe constraints in several recently proposed applications.

Date: 2022
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DOI: 10.1038/s41467-022-28713-x

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