Abstract
Brillouin light scattering experiments were done on porous silicon-based optical Bragg mirrors with modulation wavelengths of ∼100 nm. By using a combination of pseudo-reflection and backscattering geometries, phonon dispersion curves along the superlattice modulation axis were mapped. Excellent agreement is obtained with the bulk acoustic mode band structure calculated using a one-dimensional elastic continuum model. In addition to zone-folding of the bulk longitudinal mode dispersion curve, the samples are marked by a surface-localized acoustic mode at the superlattice-air interface. The frequency of this mode lies near the upper edge of a phononic band gap centered at ∼16 GHz. These results, along with optical reflectance data showing visible-range photonic band gaps, reveal that these samples are one-dimensional hypersonic phononic-photonic crystals.
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