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

TL;DR

This paper challenges previous claims by demonstrating through simulations and physical analysis that nonreciprocal structures can surpass the time-bandwidth limit, showing advantages over reciprocal systems in trapping states and field enhancement.

Contribution

It provides evidence that nonreciprocal structures can outperform reciprocal ones in time-bandwidth performance, countering prior conclusions based on limited theoretical models.

Findings

Nonreciprocal terminated waveguides exhibit superior time-bandwidth performance.

Full-wave simulations reveal large field enhancements in nonreciprocal trapped states.

Temporal coupled-mode theory is inadequate for analyzing nonreciprocal trapped states.

Abstract

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.