Abstract
In their paper in Optica 6, 104 (2019)OPTIC82334-253610.1364/OPTICA.6.000104, 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, 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.
Highlights
To begin, we recall that temporal coupled-mode theory [3] describes the evolution of a field inside a cavity according to the following equation: da dt
Systems cannot overcome the time-bandwidth (T-B) limit. These assertions appear to conflict with our previous work on a terminated unidirectional waveguide (Fig. 1b), which we deployed to report that large T-B violations in linear, time-invariant nonreciprocal systems could be achieved [2]
We recall that temporal coupled-mode theory [3] describes the evolution of a field inside a cavity according to the following equation: da dt where a is the field amplitude inside the cavity, ω0 the resonance frequency dictated by the cavity, γi and γr the intrinsic and radiative loss rates, respectively, |s+|2 the power incident onto the cavity from an external system, e.g. a waveguide, and κin the coupling coefficient between that external system and the cavity
Summary
We recall that temporal coupled-mode theory [3] describes the evolution of a field inside a cavity according to the following equation: da dt. These assertions appear to conflict with our previous work on a terminated unidirectional waveguide (Fig. 1b), which we deployed to report that large (by a factor of ~1000) T-B violations in linear, time-invariant nonreciprocal systems could be achieved [2].
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