We present ultrawide flat-band reflectors enabled with multilevel resonant leaky-mode structures. The reflectors are designed using particle swarm optimization. We show that three-level silicon-on-insulator structures provide bandwidths of approx840 nm for TE polarization and approx835 nm for TM polarization, across which reflectance is greater than 99percent and 97.5percent, respectively. For the germanium-on-insulator system, the 99percent TE and TM bandwidths are approx1100 nm and approx1050 nm. The results indicate the potential of multilevel resonant leaky-mode elements in electromagnetics and photonics.
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