A linear waveguide along the [11] direction of a triangular sonic crystal, composed of aluminum cylinders in air is shown both experimentally and numerically to facilitate slow sound propagation. Supercell-based calculations through the finite element method reveal a band centered at approximately 16.0 kHz with 255 Hz span, exhibiting linear variation away from band edges, for the lattice constant and cylinder radii of 21.7 mm and 10.0 mm, respectively. The experimental setup is based on monitoring the propagation of a Gaussian-enveloped sinusoidal pulse at 16.0 kHz inside the waveguide. Numerical behavior of the Gaussian pulse is investigated by time-dependent finite-element computations. The experimental and numerical group velocities are found to be 26.7 m/s and 22.6 m/s, respectively. Being congruous with the experimental findings, numerical transient study of the system reveals significant longitudinal compression commensurate with the calculated group index.
We present a comprehensive study of spectral photoluminescence (PL), photoconductivity and Hall mobility in undoped, n and p-type modulation-doped quantum wells of Ga 1−x In x N y As 1−y /GaAs with varying nitrogen concentration. We show that the increasing nitrogen composition red shifts the energy gap and this red shift is accompanied with a reduction of the 2D electron mobility in the quantum wells. True temperature dependence of the band gap, free from errors associated with nitrogen induced exciton trapping effects, is observed because in the modulation doped QW samples PL emission is dominated by bandto-band recombination and the S-shape temperature dependence is eliminated. Excellent fit to semi-experimental Varshni equation is obtained and the temperature dependence of the band gap in the linear regime (dE/dT) is tabulated as a function of nitrogen concentration and the type of dopant.
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