The paper reports on a study of the emission of GaN/AlN self-assembled quantum dots grown on a-plane 6H-SiC showing evidence of the suppression of the internal electric field. The strain in dots and barriers is determined by means of Raman scattering and the induced piezoelectric polarizations are estimated. These reveal a compensation of the spontaneous polarization and justify the lack of a quantum confined Stark effect found in the photoluminescence spectra. Strain effects and strong confinement are responsible for the partial depolarization of the emission and its energy dependence.
At the core of attosecond science lies the ability to generate laser pulses of sub-femtosecond duration. In tabletop devices the process relies on high-harmonic generation, where a major challenge is to obtain high yields and high cutoff energies required for the generation of attosecond pulses. We develop a computational method that can simultaneously resolve these issues by optimizing the driving pulses using quantum optimal control theory. Our target functional, an integral over the harmonic yield over a desired energy range, leads to a remarkable cutoff extension and yield enhancement for a one-dimensional model H-atom. The physical enhancement process is shown to be twofold: the cutoff extension is of classical origin, whereas the yield enhancement arises from increased tunneling probability. The scheme is directly applicable to more realistic models and, within straightforward refinements, also to experimental verification.PACS numbers: 32.80. Rm, 42.65.Ky, 42.65.Re, 42.79.Nv The revolution of attosecond science, i.e., monitoring and controlling the dynamics of electrons in their native time scale, relies on the generation of laser pulses with duration of a few dozens of attoseconds [1]. Such pulses can be generated by using large-scale free-electron laser facilities [2] or in tabletop devices using high harmonic generation (HHG), an ultrafast frequency conversion process [1]. Using tabletop devices, however, comes with a price: the generated attosecond pulses are often too long and they suffer from low intensity [1].A high harmonic spectrum has an energy range of nearly constant intensity (plateau), which ends in a distinctive cutoff [3]. Attosecond pulses are formed from the harmonics on the plateau [1]. Hence, the low amplitude of the pulses is due to low harmonic yield and the pulse duration is determined by the cutoff energy (the higher the energy the shorter the pulse) [1]. The objectives of increasing the yield and reducing the pulse duration can be addressed by temporal shaping of the driving pulsealready experimentally realizable either with multicolor fields or more sophisticated techniques [4]. Yet a crucial question remains unanswered: how to find the optimal shape of the driving pulse to enhance HHG?Numerous previous studies have tackled the issues of cutoff and yield; for a recent review see, e.g., Refs.[5] and [6]. The main scheme behind the cutoff extension has been using two-color laser fields [7,8] or chirped pulses [9][10][11], but also steepening of the carrier wave [12] or even using a sawtooth pulse should extend the cutoff [13]. In addition, also combined temporal and spatial * janne@solanpaa.fi † esa.rasanen@tut.fi synthesis of the driving field has been shown to extend the cutoff [14]. A previous study based on quantum optimal control theory (QOCT), for example, demonstrated some cutoff extension, albeit with a low yield, by maximizing the ground state occupation at the end of the pulse [15]. Yield increase of the plateau has been accomplished, e.g., by two-color fields [16][17]...
Articles you may be interested in semipolar nanostructures: A way to get high luminescence efficiency in the near ultraviolet range J. Appl. Phys. 110, 084318 (2011); 10.1063/1.3654053Study of the growth mechanisms of GaN/(Al, Ga)N quantum dots: Correlation between structural and optical propertiesThe capping of GaN quantum dots (QDs) with an Al 0.5 Ga 0.5 N layer is studied using transmission electron microscopy and atomic force microscopy in combination with theoretical calculations. The capping process can be divided into several well-distinguishable stages including a QD shape change and a local change of the Al 0.5 Ga 0.5 N capping layer composition. The phase separation phenomenon is investigated in relation with the capping layer thickness. Amount of the chemical composition fluctuations is determined from separate analysis of scanning transmission electron microscopy and high-resolution transmission electron microscopy images. The local distortion of atomic lattice in the QD surroundings is measured by high-resolution electron microscopy and is confronted with theoretically calculated strain distributions. Based on these data, a possible mechanism of alloy demixing in the Al 0.5 Ga 0.5 N layer is discussed. V C 2012 American Institute of Physics. [http://dx.
The strain state of stacks of GaN/AlN quantum dots (QDs) grown on (0001) and (1120) 6H-SiC has been investigated by means of Raman spectroscopy. Depending on the orientation of the wurtzite axis with respect to the growth direction it is found that the piezoelectric contribution to the electrostatic potential may either reinforce that arising from the spontaneous polarization or oppose it. The experimental results are compared with a theoretical model for the strain and polarization field in QDs of both orientations that allows the calculation of the electrostatic potential in the QDs. Both the experimental results and the theoretical model indicate that the internal electric field and electrostatic potential are strongly reduced in the QDs grown on (1120) 6H-SiC as compared to those grown along the wurtzite c-axis.
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