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Comment on "Nonreciprocal cavities and the time-bandwidth limit"

In their paper in Optica 6, 104 (2019), Mann et al. claim that linear, time-invariant nonreciprocal structures cannot overcome the time-bandwidth limit, and do not exhibit an advantage over their reciprocal counterparts, specifically with regard to their time-bandwidth performance. In this Comment [Optica 7(9), 1097-1101 (2020)], we argue that these conclusions are unfounded. On the basis of, both, rigorous full-wave simulations and insightful physical justifications, we explain that the temporal coupled-mode theory, on which Mann et al. base their main conclusions, is not suited for the study of nonreciprocal trapped states, and instead direct numerical solutions of Maxwell's equations are required. Based on such an analysis, we show that a nonreciprocal terminated waveguide, resulting in a trapped state, clearly outperforms its reciprocal counterpart, i.e. both the extraordinary time-bandwidth performance and the large field enhancements observed in such modes are a direct consequence of nonreciprocity. Additionally, herein, on the arXiv, we provide further results and explanations on the key points made in the main Comment, as well as further elucidating comments on the Reply to the Comment.