Recent observations of periodic anomalies in conductance of two-dimensional arrays of densely packed metal or semiconductor nanowires give some indication on the importance of collective charge excitations in such systems. These structures can be viewed as parallel arrays of double-tunnel-junction systems with nanowires in the middle which are electrostatically coupled to each other. To assess possible effects of the interwire coupling on the electron transport in such arrays, we investigate the electrical behavior of a simpler system of two coupled double-junction systems under the condition of Coulomb-controlled tunneling. Using Monte Carlo simulations of the electron transport through the system and the master equation analysis, we find that a system of two coupled nanowires exhibits a spontaneous polarization of charge where the accumulation of excessive electrons on one wire is accompanied by the hole accumulation on the neighboring wire. This yields considerable net charge polarization in the transverse direction, which stochastically oscillates in time but depends periodically on the applied voltage, with both the average polarization and the polarization noise decreasing with increasing temperature. The effect gives rise to a number of changes to the well-known Coulomb-Blockade features of a double-junction system, and may lead to the appearance of periodic polarization structures or polarization waves in nanowire arrays which might be detected externally.
͓S0163-1829͑97͒09007-3͔FIG. 1. Two-wire double-junction system. Arrows show the prevailing directions of tunneling for VϾ0.
We propose a new concept of binary superimposed gratings (BSG's) as multiwavelength grating reflectors and show it as an effective structural improvement over the existing designs, to both the fabrication process and device performance. We also present a study of key design issues for widely tunable lasers based on grating mirrors with a comb-like reflection spectrum and summarize simple design rules for the grating part of the laser, based on analytical and numerical analysis. The binary supergrating consists of elements of equal size whose refractive index is allowed to be one of two possible values, which are sequenced according to a binary optics formalism to effect a spatial superposition of multiple sets of single-frequency gratings, and its implementation is well within the standard ebeam lithography limits. The calculations of lasing frequency, mode, and side-mode suppression ratio of the tunable laser are formulated and presented along with numerical examples.
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