"Faster" Could be "Slower": Uncovering the Salient Characteristics of Slow-light Guided Signals with the Finite-Difference-Time-Domain (FDTD) Method
Keywords:
Finite Difference Time Domain (FDTD) method, group index, group velocity, group velocity dispersion, left-handed materials, negative-index media, slow light, wave dispersionAbstract
Slow-light mesophotonic waveguides have gained increasing interest in the recent years because of their catalyzing potential to transform applications relying on all-optical signal manipulation or enhanced light-matter interactions. The quests in this area have been targeting waveguide platforms with a giant group velocity index as determined by modal type of analyses in frequency domain. We show here that these efforts with frequency-domain methods have entirely missed on important mode features which are nevertheless crucial in practically effecting an ultra-slow guided pulsed signal with a large time delay. We utilize first-principle electromagnetic (EM) simulations in time-domain and show that contrary to conventional wisdom, the group-index by itself is not in general a good measure of the slow-down factor for a pulsed light signal propagating within the waveguide. We present a counterexample comparing two modes which demonstrates that the “faster” mode, the one with the lower group index, is the one that leads to larger effective time delays. The time-domain analysis in this counter-example uncovers a new figure of merit for practical slow-light platforms which indicates that along with a near-zero group velocity, a relatively low group-velocity dispersion value is simultaneously required.
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