NUSOD Blog

Connecting Theory and Practice in Optoelectronics

NUSOD 2018 Preview: How reliable are pulse power simulations?

error2The reliability of numerical simulations very much depends on the physical mechanisms and the material parameters included in the model. Unrealistic results may arise from ignoring relevant physical processes or from using inaccurate parameters. In fact, key parameters are often not exactly known and employed as fit parameters to find agreement with measurements.

Broad-area laser diodes can achieve close to 100W output power with very short current pulses (see picture), but the maximum power is still severely limited by various saturation mechanisms. Different models have been developed for these pulse power limitations. After some parameter fitting, most of these models show good agreement with the measurement. However, contradicting conclusions are drawn from such simulations, even for the same laser, partly because the assumed saturation processes are different (details). Thus, pulse power simulations seem to be quite unreliable. Read more of this post

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NUSOD 2018 Preview: Superinjection in single-photon emitting diamond diodes

pin_diode_with_names_v02_300dpiFig. 1. Schematic illustration of the diamond p-i-n diode with a color center in the i-region.

Electrically-pumped single-photon sources based on color centers in diamond are considered to be the most promising platform for generating single photons on demand, which is required for the emerging quantum information technologies [1]. Such devices should emit photons at a very high rate. In the case of electrically-pumped devices, this means that there should be a huge number of both electrons and holes in the vicinity of the color center [2]. The higher the non-equilibrium carrier density, the higher the photon emission rate. However, in diamond, it is hard to create more than 1011 cm-3 of free electrons at room temperature [3,4]. There are two main reasons for this. First, at least ~10% of implanted donors are compensated by accidentally created acceptor-type defects and impurities. Second, donors have a high activation energy of 0.6 eV, which is ~23 kT at room temperature. These doping problems limit the maximum single-photon electroluminescence rate. Read more of this post

NUSOD 2018 Preview: Towards Model-Based Geometry Reconstruction of Quantum Dots from TEM

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TEM images of InAs QDs (Courtesy of TU Berlin [1])

The growth of semiconductor quantum dots (QDs) with desired electronic properties would highly benefit from the assessment of QD geometry, distribution, and strain profile in a feedback loop between epitaxial growth and analysis of their properties. However, the reconstruction of geometric properties of semiconductor quantum dots (QDs) from imaging of bulk-like samples (thickness 100-300 nm) by transmission electron microscopy (TEM) is a difficult problem. A direct reconstruction by solving the tomography problem is not feasible due to the limited image resolution (0.5-1 nm), the highly nonlinear behavior of the dynamic electron scattering, nonlocal effects due to strain and strong stochastic influences due to uncertainties in the experiment. Here, we outline a novel concept for 3D model-based geometry reconstruction (MBGR) of QDs from TEM images. This will include (a) an appropriate model for the QD configuration in real space including a categorization of QD shapes (e.g., pyramidal or lens-shaped) and continuous parameters (e.g., size, height), (b) a database of simulated TEM images covering a large number of possible QD configurations and image acquisition parameters (e.g. bright field/dark field, sample tilt), as well as (c) a statistical procedure for the estimation of QD properties and classification of QD types based on acquired TEM image data. To this end we need an accurate mathematical model for the numerical simulation of the TEM images.

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NUSOD 2018 Preview: Ultrashort solitons and their control in the regime of event horizons in nonlinear dispersive optical media

We introduce several settings for the description of pulse propagation in dispersive nonlinear optical media.

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A short and intense optical soliton that propagates along a fiber with Kerr nonlinearity, creates a localized nonlinear perturbation of the refractive index. A low-intensity pump wave of similar group velocity can be reflected at the solitonic barrier, thereby undergoing a pronounced frequency change [1,2]. During the reflection process the pump wave and the soliton exchange energy, and the soliton is compressed or dispersed, and propagates with changed peak power and shifted frequency. The soliton can be compressed up to a few-cycle regime. As a result, a soliton can be manipulated by a much weaker control wave. For instance, it can be switched on and off [1], or be used to model event horizons [3] by trapping the pump wave [4]. We present an analytic theory [5] of interactions like the one shown in Fig. 1, quantify optimal pulse parameters [6], and demonstrate how a pump wave can be suitably chosen so as to compensate the Raman effect robustly [7]. Read more of this post

NUSOD 2018 Preview: Highly Accurate Discretizations for non-Boltzmann Charge Transport in Semiconductors

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Suppose one wants to predict charge carrier transport in semiconductors with very high precision, in particular for non-Boltzmann (e.g. Fermi-Dirac and Gauss-Fermi) statistics.
How could one go about this? The van Roosbroeck system models the charge carrier flow in the presence of a self-consistent electrical field. When discretizing the van Roosbroeck system using the Voronoï finite volume method [1-3], the crucial part is the approximation of the carrier fluxes between neighboring control volumes. The Scharfetter-Gummel (SG) scheme provides a thermodynamically consistent discrete carrier flux approximation for non-degenerate semiconductors. Degeneracy effects requiring non-Boltzmann statistics become relevant e.g. at cryogenic temperatures, for high doping concentrations or in organic materials. Read more of this post