Acoustic doorway states in layered porous silicon structures

We report the experimental observation of doorway states phenomenon for acoustic waves in porous silicon structures. We designed and fabricated a structure capable to exhibit the acoustic doorway states phenomenon around 1 GHZ. The structure consists of two porous silicon layers, one with low porosity and another with higher porosity and smaller thickness, followed by a much more thicker layer of crystalline silicon. We study the doorway state phenomenon in the frequency and in the time domain. To do this, we measured the acoustic transmission spectra in the frequency domain, and by using the Fourier transform, we obtain the transmission in the time domain. The transmission spectrum of the fabricated sample shows acoustic doorway states in frequencies around 1 GHz. The experimental results are in good agreement with the transfer matrix calculations.