Leaky-mode resonant reflectors with extreme bandwidths.

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 approximately 840 nm for TE polarization and approximately 835 nm for TM polarization, across which reflectance is greater than 99% and 97.5%, respectively. For the germanium-on-insulator system, the 99% TE and TM bandwidths are approximately 1100 nm and approximately 1050 nm. The results indicate the potential of multilevel resonant leaky-mode elements in electromagnetics and photonics.