Excitation spectra of trigonal and orthorhombic FeIn pairs in silicon have been studied by Fourier transform spectroscopy. The spectra were identified as due to hole transitions from the deep ground state to shallow hole states. The orthorhombic configuration is stable but population of the metastable trigonal configuration was obtained by above-bandgap excitation. For the orthorhombic and trigonal centre, the positions of the 0/− FeIn levels were estimated to be 390 meV and 320 meV below the conduction band minima. Arguments are given that for the trigonal centre iron resides on the nearest-neighbour interstitial position.

The first analysis to our knowledge of the optical data storage density of photon-echo storage is presented. Mainly considering signal-to-noise ratio performance, we calculate the obtainable storage density for data storage and processing using photon echoes to be approximately 100 times the theoretical limit for conventional optical data storage. This limit is similar to that theoretically calculated for data storage by use of persistent spectral hole burning. For storage times longer than the upper-state lifetime the highest densities can, however, be obtained only if all the excited atoms decay, or are transferred, to a different state than that from which they were originally excited. The analysis is restricted to samples with low optical density, and it also assumes that for every data sequence, writing is performed only once. It is therefore not directly applicable to accumulated photon echoes. A significant feature of photon-echo storage and processing is its speed; e.g., addressing 1 kbyte/(spatial point) permits terahertz read and write speeds for transitions with transition probabilities as low as 1000 s 1 .

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