Brillouin scattering studies of porous silicon-based one-dimensional hypersonic phononic-photonic crystals

Brillouin light scattering spectroscopy and optical reflectance measurements were used to characterize the acoustic phononic band structure and the photonic band structure, respectively, of binary periodic mesoporous silicon superlattices with a repeated porosity sequence of ~55% (~45%) and modulation wavelengths of the order of 100 nm. The samples exhibited visible-range forbidden photon frequency bands, or photonic band gaps, for photons propagating along the modulation axis of the superlattice. This manifested as a peak in the near-normal incidence broadband optical reflectance spectrum. Using a combination of pseudo-reflection geometry Brillouin spectroscopy and near-normal incidence 180°-backscattering geometry Brillouin spectroscopy, a portion of the bulk acoustic band structure directed approximately along the superlattice periodicity was mapped for a series of samples. The obtained agreement between the data and calculations based on a one-dimensional elastic/electromagnetic continuum model suggested that the longitudinal bulk acoustic phonon dispersion curve had underwent zone-folding into the mini-Brillouin zone corresponding to the artificial periodicity. In particular, two peaks associated with folded bands of the longitudinal band structure were observed in the Brillouin spectra. For the transverse bulk acoustic mode, on the other hand, the Brillouin spectra showed evidence of a single band. Analysis of the Brillouin peaks associated with the longitudinal modes suggested that phonon attenuation in porous silicon was several orders of magnitude larger than that observed in bulk crystalline silicon. It was, therefore, suggested that the film porosity played a significant role in the phonon attenuation. Additionally, another peak, attributed to a surface-localized mode with a frequency within a hypersonic phononic band gap in the longitudinal bulk mode band structure centered at ~16 GHz, was also observed. This surface-localized mode was a manifestation of the hypersonic phononic band gap. Taking into account the Brillouin scattering results along with those obtained through the optical reflectance measurements, it was inferred that the π-Si superlattices were one-dimensional hypersonic phononic-photonic crystals. -- Oblique acoustic phonon propagation was also characterized using oblique 180°-backscattering geometry Brillouin scattering. The frequency of the probed folded longitudinal acoustic phonon modes depended on the phonon propagation direction. No evidence of coupling between the folded longitudinal bulk acoustic bands and the transverse bulk acoustic band was obtained. In addition to the bulk acoustic modes, peaks ascribed due to a Rayleigh surface mode and a pseudo-surface mode were observed in the oblique 180°-backscattering geometry Brillouin spectra. These surface modes were dispersive, with the phonon velocity increasing with phonon wavelength. This surface mode dispersion appeared to be an artefact of the periodic structure of the probed samples.