Preparation and analysis of porous silicon multilayers for spectral encoding applications

This paper compares two methods used to encode information into the optical spectrum of a porous Si film using a modulated current-time waveform. In the first approach, a waveform consisting of a sequence of current steps is used to produce a porous silicon multilayer structure. Each layer in the multilayer acts as a Fabry-Perot interference film, and the superposition of their contributions results in a complicated reflectivity spectrum. The spectrum can be deconvoluted by means of a Fast Fourier Transformation (FFT), which yields the optical thickness of each of the individual layers and their combinations. The second type of waveform consists of several superimposed cosine waves of differing frequencies. The resulting reflectivity spectrum consists of a series of peaks, each of whose position in the spectrum (frequency at which a peak is at maximum amplitude) corresponds to the frequency of a cosine wave component in the current-time waveform. The two methods of encoding information are compared in terms of repeatability, tunability, and information capacity. The superimposed cosinusoidal waves are found to provide greater information density and a simpler means of decoding. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim