We consider fundamental limits on information acquisition from localized regions of molecular-scale electronic systems. Our approach is based on a quantitative measure we call the volume accessible information, defined as the (Shannon) mutual information associated with the best possible quantum measurement that can access a system through a specified readout volume. Using results and techniques from quantum measurement theory, we obtain a general expression for an upper bound on the volume accessible information that depends only on the manner in which information is encoded in electron states and specification of the readout volume. An illustrative study of a model tight-binding system indeed reveals that the volume accessible information is sharply reduced at
small sampling volumes, where the state distinguishability required for reliable information extraction is diminished.
The current status of InAsSb/InAlAsSb quantum-well (QW) lasers emitting between 3 and 4 micrometer is described. QW lasers grown on GaSb substrates, with emission wavelengths at approximately 3.9 micrometer, have operated pulsed up to 165 K. At 80 K, cw power of 30 mW/facet has been obtained. Ridge-waveguide lasers have operated cw up to 128 K. QW lasers grown on InAs have emission wavelengths between 3.2 and 3.55 micrometer. Broad- stripe lasers have operated pulsed up to 225 K and ridge-waveguide lasers have operated cw to 175 K. Theoretical analysis of the laser gain using a 6 by 6 k (DOT) p model to calculate the valence subband structure is reported.
A novel technique for bringing the light- and heavy-hole valence bands in a quantum well, (QW), into approximate degeneracy is described and demonstrated. It utilizes pseudomorphic tensile strain in the barriers generated by lattice mismatch between the barrier and the substrate material. An important consequence of this strain is that the splitting of the light- and heavy-hole valence band energies at the Brillouin zone center, due to the quantum confinement effect, is approximately cancelled. Unlike a similar result in systems with tensily strained wells, this degeneracy is not sensitive to the exact QW width (for QW widths greater than 5 nm) or the precise strain present in the layer. It is thus more amenable to the growth and fabrication of devices which should simultaneously exhibit the polarization isotropy of bulk structures and the enhanced performance of QWs. The technique is demonstrated by an optical investigation of GaAs/GaAs1 - yPy quantum wells grown on GaAs substrates by metalorganic chemical vapor deposition.
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