Catalyst-free, vertical array of InAs nanowires (NWs) are grown on Si (111) substrate using MOCVD technique. The as-grown InAs NWs show a zinc-blende crystal structure along a < 111 > direction. It is found that both the density and length of InAs NWs decrease with increasing growth temperatures, while the diameter increases with increasing growth temperature, suggesting that the catalyst-free growth of InAs NWs is governed by the nucleation kinetics. The longitudinal optical and transverse optical (TO) mode of InAs NWs present a phonon frequency slightly lower than those of InAs bulk materials, which are speculated to be caused by the defects in the NWs. A surface optical mode is also observed for the InAs NWs, which shifts to lower wave-numbers when the diameter of NWs is decreased, in agreement with the theory prediction. The carrier concentration is extracted to be 2.25 × 1017 cm-3 from the Raman line shape analysis. A splitting of TO modes is also observed.PACS: 62.23.Hj; 81.07.Gf; 63.22.Gh; 61.46.Km
Previous studies on modulation instabilities (MIs) in birefringent optical fibers focus on the ordinary linearly and circularly ones. This paper reports an analysis of MIs in the general elliptically birefringent fibers with the emphasis on investigating the effects of ellipticity angle (0° ≤ θ ≤ 90°). Both symmetric and antisymmetric CW states are considered. In the anomalous dispersion regime, for the symmetric (antisymmetric) CW states, we show that MI gain increases dramatically (reduces first and then enhances greatly) as the increment of θ. In the normal dispersion regime, for the both CW states, the distinctive feature is that the gain of the MI bands reduces first, vanishes at θ = 45°, reappears across this ellipticity angle, and quickly increases after then.
The folded acoustic phonon and confined optical phonon modes in chirped InGaAs-InAlAs superlattices have been studied by micro-Raman spectroscopy. The folded and confined phonons were probed at the (1 10) and (001) faces. Raman scattering measurements for the chirped superlattices and an InGaAs film were performed at different temperatures.The temperature dependence of the longitudinal optical phonon frequencies indicates that a compressive strain existed in the InGaAs layers of the chirped superlattices, which is significant for analyzing the device failure of quantum cascade lasers operating at higher active region temperatures.
The confined longitudinal optical, transverse optical and interface phonon modes in chirped GaAs-AlGaAs superlattices grown on the (001)-oriented GaAs substrate are studied by the micro-Raman spectroscopy. The phonon modes are probed at the (001) and (1 10) faces. The temperature dependence of the longitudinal optical, transverse optical and interface phonon modes are achieved. The temperature dependence of the longitudinal optical phonon frequencies demonstrates that a tensile strain exists in the GaAs layers of the chirped superlattices, which is significant for analyzing the device failure of a terahertz quantum cascade laser.
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