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    Binding energies and oscillator strengths of impurity states in wurtzite InGaN/GaN staggered quantum wells
    (Amer Inst Physics, 2012) Yildirim, Hasan; Aslan, Bulent
    Using the variational methods, we have calculated the binding energies of the lowest donor states, 1s and 2p(+/-), in wurtzite InGaN/GaN staggered quantum wells. The binding energies in narrow wells are larger in magnitude than the values in bulk GaN due to the quantum confinement effects. However, the energies decrease sharply in wider wells because of the weakening confinement due to the strong built-in electric field inside the well. The binding energies of donors placed at the opposite edges of the well are quite different as the built-in electric field forms an asymmetric, triangular potential inside the well. The oscillator strength of the possible transitions between the donor states is then computed by modelling them as the states of a two-level atom. A magnetic field applied along the growth direction splits up the degenerate 2p(+/-) states. The amount of splitting in the quantum well is found to be small possibly due to the heavy electron effective mass inside the well. The oscillator strength of the transition between the donor states becomes greater with the increasing magnetic field. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4751438]
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    Donor-related third-order optical nonlinearites in GaAs/AlGaAs quantum wells at the THz region
    (Iop Publishing Ltd, 2011) Yildirim, Hasan; Aslan, Bulent
    GaAs/AlGaAs quantum wells doped with donor atoms are investigated for nonlinear optical applications in the THz range. The electronic properties of the quantum wells are obtained numerically by applying an iterative shooting algorithm. Donor binding energies are computed through the evaluation of variational wavefunctions. The solution of the density matrix equations of motion for non-interacting two-level atoms within the rotating wave approximation is used to formulate the third-order optical nonlinearities. Transitions between the 1s and 2p+ impurity states because of an incident light polarized perpendicularly to the growth direction are considered as the origins of optical nonlinearity. Following a set of computations for a quantum well doped at the center, it is found that the nonlinear susceptibility decreases when the well becomes wider or the Al concentration increases. Additionally, when the doping center is shifted to the well edge, the nonlinear susceptibility decreases too. A large nonlinear figure of merit is obtainable in wider wells compared to the narrower wells although the latter delivers larger nonlinear susceptibilities.
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    Effects of magnetic field on the terahertz nonlinear optical properties in donor-doped GaAs/AlGaAs quantum wells
    (Wiley-V C H Verlag Gmbh, 2012) Yildirim, Hasan; Aslan, Bulent
    Effects of the magnetic field on nonlinear optical properties at THz range in GaAs/AlGaAs quantum wells doped with donor atoms are investigated. Expressions for the third-order nonlinear optical susceptibilities are obtained through the solution of the density matrix equations of motion within the rotating wave approximation. Donor binding energies are calculated variationally by means of an iterative shooting algorithm. Magnetic field has strong effect on the nonlinear susceptibility: it removes the degeneracy in energies of 2p +/- impurity states and increases the absolute value of the nonlinearity. It is also shown that a large and tunable optical nonlinear figure of merit is possible with the magnetic field applied in the growth direction. The nonlinear optical quantities are also calculated for donor distributions with different full width at half maximum values in the absence of magnetic field and the observed features at low energy part are attributed to the increasing homogeneity in the donor distribution.
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    Intersubband transitions in InxGa1-xN/InyGa1-yN/GaN staggered quantum wells
    (Amer Inst Physics, 2014) Yildirim, Hasan; Aslan, Bulent
    Intersubband transition energies and absorption lineshape in staggered InGaN/GaN quantum wells surrounded by GaN barriers are computed as functions of structural parameters such as well width, In concentrations, and the doping level in the well. Schrodinger and Poisson equations are solved self-consistently by taking the free and bound surface charge concentrations into account. Many-body effects, namely, depolarization and excitonic shifts are also included in the calculations. Results for transition energies, oscillator strength, and the absorption lineshape up to nonlinear regime are represented as functions of the parameters mentioned. The well width (total and constituent layers separately) and In concentration dependence of the built-in electric field are exploited to tune the intersubband transition energies. (C) 2014 AIP Publishing LLC.
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    On the donor states in double InxGa1-xN/InyGa1-yN/GaN staggered quantum wells
    (Iop Publishing Ltd, 2013) Yildirim, Hasan; Aslan, Bulent
    We have calculated the binding energies of the donor states, 1s and 2p +/-, with respect to the lowest sub-band energy in a double quantum well composed of wurtzite InGaN staggered quantum wells with GaN barriers. All the energies and the wavefunctions were calculated by applying the variational methods. We have found that the binding energies of donors placed in the right quantum well are larger and independent of the middle barrier width of up to 40 angstrom. This is because of the strong built-in electric field which brings more confinement to the donor wavefunctions in the right staggered quantum well. The binding energies are found to be strong functions of the donor position in the double quantum well system which is the consequence of the large asymmetry introduced by the built-in electric field.
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    Second harmonic generation in asymmetric MgSe/CdSe/ZnCdMgSe quantum well structures
    (Wiley-V C H Verlag Gmbh, 2017) Yildirim, Hasan; Aslan, Bulent
    MgSe/CdSe/ZnCdMgSe step quantum well structures and coupled quantum well structures under an applied electric field have been investigated for the process of second harmonic generation (SHG) with the double-resonant condition. The structural parameters of the quantum wells having equally spaced three consecutive energy levels within the conduction band have been determined by solving Schrodinger equation. It is shown that the energy level separation can be continuously tuned between 214 and 472meV by changing the thickness and the material composition of the layers. The product of dipole matrix elements used in determining the second-order nonlinear susceptibility is calculated to be as much as 473 angstrom 3. This is lower than the values obtained in widely known GaAs based material systems because of the high effective mass of electron in these selenides. However, the second-order nonlinear susceptibility 2 that controls SHG is found to be of the order of 10-7 V-1 at its maximum; this is comparable to the values found in similar systems.