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Connecting Theory and Practice in Optoelectronics

Category Archives: nanostructure

Fundamental limit of quantum device simulations

InGaN_QWThis pictures provides an atomistic view of an InGaN/GaN quantum well [1]. Inside this quantum well (QW), 17% of the Gallium atoms are replaced by Indium atoms which are here randomly distributed (red dots). Such QWs are employed in many modern light-emitting devices, from full-color displays to LED lamps. The emission wavelength is controlled by the Indium concentration.

QW models and device simulations typically ignore this atomistic structure and assume a uniform QW alloy layer with uniform material properties. Such continuum models still deliver reasonable results in many cases. But some phenomena are hard to explain this way and require an atomistic approach. One example is the much discussed efficiency droop that seems to be mainly caused by strong Auger recombination. Recent studies link this effect to the non-uniformity of InGaN quantum wells  (details). Read more of this post

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NUSOD 2017 Preview: Dynamic simulation of quantum cascade laser structures with optical nonlinearities

QuantuQCL_combm cascade lasers (QCLs) utilize quantized electron states as laser levels, which can be custom-tailored to the specific application by adequately designing the multi-quantum-well active region. With dephasing times between the upper and the lower laser level of about a picosecond, coherent light-matter interaction, along with other effects such as dispersion and spatial hole burning, governs the laser dynamics [1]. Besides leading to multimode instabilities [1], the ultrafast dynamics in QCLs is increasingly exploited to implement innovative functionalities, such as the generation of frequency combs [2,3] and picosecond optical pulses [4] in the mid-infrared and terahertz regime.

For a targeted design of such structures and a deeper understanding of the complex QCL dynamics, a detailed theoretical model is required. We have developed a multi-domain simulation approach, which couples a Maxwell-Bloch type description of the light-matter interaction with advanced ensemble Monte Carlo (EMC) carrier transport simulations to eliminate empirical electron lifetimes [5]. In the figure, simulation results for a QCL-based terahertz frequency comb source [3] are presented. The laser dynamics gives rise to a two-lobed power spectrum consisting of equidistant discrete comb lines [Fig. (a)]. Furthermore, a periodic temporal shifting of the power between the two spectral lobes is observed [Fig. (b)]. The two-lobed comb spectrum and the associated temporal switching dynamics have also been observed in experiment [3,5], validating our simulation model.

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NUSOD 2017 Preview: Simulation of Nonlinear Polariton Dynamics in Microcavity wires for Polaritonic Integrated Circuits

Microcavity_wireIntegrated optical technologies for quantum computing based on linear optics have been under active development in the recent decade. However, the nonlinear optical interactions ‘on a chip’ offer new unexplored functionalities and may play a key role. Recently a new class of nonlinear system has emerged in which light and matter play equally important roles. The basic building blocks of these systems are quantum states of matter coupled to enhanced optical fields found in microstructures. One example is the microcavity exciton-polariton: a mixed light-matter quasiparticle, resulting from the strong coupling of quantum well (QW) excitons to cavity photons.

Microcavity exciton-polaritons have numerous advantages over bare photons and excitons. For instance, due to the excitonic component, they exhibit weaker diffraction and tighter localisation, and the strong interparticle interactions result in lower operational powers ~fJ/mm2 and faster switching speeds ~a few ps. Polariton waves can be confined in structures with sub-micron size, which opens up possibilities for fabrication of polaritonic integrated circuits based on structured semiconductor microcavities on a chip. Laterally etched microcavity wires [1] (Fig. 1a) enhance further the polaritonic nonlinearities and thus are a particularly promising integration platform due to broad transparency window, mature fabrication technology and the possibility of monolithic integration with semiconductor diode lasers and VCSELs. In this respect, new theories and numerical methods are needed to model the nonlinear polariton dynamics in non-planar microcavity wires.

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Call for Papers: Special Journal Issue on Multiscale Modeling of Solar Cells

In the last two decades, there has been an increasing interest in multiscale modeling applied to electronic devices. Several factors are driving this trend. On the one hand, device dimensions of “classical” devices like MOSFETs have continuously been scaled down in order to increase device performance. On the other hand, specific properties of quantum structures are systematically utilized in modern devices. The embedding of the active device region in its environment including access regions and contacts, and the mutual interaction between different aspects like optics, thermal heating, strain and carrier transport requires an involved multiscale/multiphysics simulation approach which can handle different physical models and different length or time scales.

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NUSOD 2016 Preview: Electronic properties of polytype GaAs superlattices

enBandsGaAs can be considered as the prototype compound semiconductor and is used for a wide range of applications including infrared light emission, acoustic sensing, transistor technology and photovoltaics. Nanowires (NWs) made from GaAs commonly exhibit polytypism, i.e., some segments of the NW crystallize in the zincblende (ZB) and others in the metastable wurtzite (WZ) crystal structure, thus turning the NW into a crystal-phase nanostructure. While the electron states of the ZB phase are principally derived from a single conduction band (CB), two energetically close bands exist in the WZ modification. Read more of this post