Md. Mahbub Hasan

Crystal orientation-dependent optical properties of mid-infrared GaInSb/GaInAlSb quantum well laser

The optical properties of compressively strained GaInSb quantum well (QW) laser fabricated in (001), (110), (111), (112) and (113) directions are theoretically investigated by solving one dimensional conduction and valence bands Schrödinger equation using finite difference method. The valence band mixing effect is found minimum in (113) direction and maximum in (111) orientation. The results obtained from the present study demonstrate that there is a strong correlation between the optical gain and its emission wavelength with crystal orientation of the QW material. The maximum and minimum optical gains are evaluated in the (113) and (111) crystal orientations, respectively. The peak emission wavelength can be tuned from 2.415 μm to 2.27μm by changing the crystal orientation from (110) to (111). It is also found that the threshold current and transparency carrier density are strongly dependent on the crystal orientation of the QW material.

Strain-dependent optical properties of mid-infrared GaInSb/GaInAlSb quantum well laser

The optical properties of compressively strained GaInSb/GaInAlSb mid-infrared quantum well light sources are numerically studied solving one-dimensional Schrödinger equation using finite difference method. The simulation results demonstrate that band-mixing effects and effective mass of hole are reduced when the well is highly compressively strained. The strain-dependent optical and differential gains are evaluated for different values of strain and found maximum for 1.52% compressively strained quantum well. The emission wavelength for the proposed laser can be tuned from 2.39μm to 2.27μm due to change in compressive strain from 0.60% to 1.52%. The results obtained from PSPICE simulation, indicate that the optical output power and threshold current are strongly depend on the number of well and found to be almost constant for the number of well is three and above.

Raman scattering study of vibrational modes and hole concentration in GaxMn1?xSb

The Raman scattering study of vibrational modes and hole concentration in a ferromagnetic semiconductor Ga1−xMnxSb grown by Mn ion implantation, deposition and post-annealing has been presented. The experiments are performed both in implanted and unimplanted regions before and after etching the samples. The Raman spectra measured from the unimplanted region show only GaSb-like phonon modes. On the other hand, the spectra measured from the implanted region show additional phonon modes approximately at 115, 152, 269, 437 and 659 cm−1. The experimental results demonstrate that the extra modes are associated with surface defects, crystal disorder and blackish layer that is formed due to Mn ion implantation, deposition and annealing processes. Furthermore, we have determined the hole concentration as a function of laser probing position by modeling the Raman spectra using coupled mode theory. The contributions of GaSb-like phonon modes and coupled LO-phonon plasmon mode are taken into consideration in the model. The hole-concentration-dependent CLOPM is resolved in the spectra measured from the implanted and nearby implanted regions. The hole concentrations determined by Raman scattering are found to be in good agreement with those measured by the electrochemical capacitance–voltage technique.

Speaker invariant principal component determination for Bangla vowel perceptual space

In this article, speaker invariant principal components have been determined from the multi-dimensional perceptual space for Bangla vowels. Linguistic content base nine-dimensional perceptual space has been formed from the formant frequency related properties of ten Bangla vowels (/a/, /ã/, /i/, /ĩ/,/u/, /ũ/ /e/, /ai/,/õ/, and /au/). Average normalized eigen-value profile is calculated using Bangla vowel perceptual spaces of fifteen speakers. Speaker invariance is numerically evaluated using the correlation of first and second principal components of the fifteen speakers. The contributions of perceptual vector members in first and second principal components are also investigated.

Crystallographic orientation-dependent performance of 445nm InGaN Blue Laser

A numerical approach is presented to study the crystallographic orientation-dependent performance of 445nm InGaN/GaN quantum well (QW) Blue Laser by solving a six-band k.p Hamiltonian for wurtzite crystal using finite difference method. Eulers rotation technique is used to modify the wave vector and Hamiltonian matrix in conventional (0001) crystal orientation. It is found that there is a substantial correlation of the energy band dispersion profile, peak gain and peak emission wavelength with change of crystal orientation. The regular optical gains are inspected as 3845, 4460, 4178, 4880 and 4750 cm−1 corresponding to peak emission wavelength of 440, 445, 450, 454 and 458nm at (0001), (101̄0), (112̄0), (112̄2) and (1011̅) orientations which shows that highest optical gain and lowest effective mass are attained for (112̄2)) orientation.

Canonical correlation based impersonation quality determination algorithm for natural morphed speech

In this article, impersonation experiments were conducted utilizing natural morphed speeches between (/a/-/b/-/a/) and (/a/-/g/-/a/) as stimuli. The sound stimuli are produced utilizing the natural glottal source and morphed linear predictive coding (LPC) filtering coefficients, which represent the vocal tract states. An algorithm has been proposed for determination of impersonation quality based on the redundancies of the canonical variates for the morphed sounds. The redundancy is defined by canonical variate loading factor and corresponding canonical correlation coefficient. The quality of imitated speech for different impersonators is evaluated by applying the proposed algorithm.

Analysis of the local frequency spectrum for the chirp signal

In this article, the statistical moments of the local spectrum obtained from Gabor transform technique for the chirp signal are presented both analytically and numerically. The chirp signal estimation considering only ridge frequency component and electrically band limited frequency components is also discussed.

A mathematical modelling of dislocations reduction in In x Ga 1−x N/GaN heteroepitaxy using step-graded interlayers

In this work, a mathematical approach has been developed to calculate dislocations reduction in step-graded InGaN heteroepitaxy. The generation of different types of misfit dislocations (MDs) in expected slips and their reduction with increasing interlayer have been calculated. The significant improvements of MDs confirm the prohibition of their generation with increasing the interlayer up to 4 where 8% In composition difference used for each step-graded interlayers. The system of differential equations for threading dislocations (TDs) according to all possible burger vectors have been developed considering annihilation, fusion reaction as well as TDs blocking by MDs and solved them numerically. The results show the decreasing nature of TDs with the average densities of 8.68×1010, 7.4×1010 and 1.32×1010 cm-2 for edge, screw and mixed character respectively at the top surface of the epilayer. The results have good agreement with the published experimental values.