arXiv subjects

Alexander Mittelstädt

Publications and source records attributed to Alexander Mittelstädt.

5 recordsLinked to original sources

arXiv2026

Infrared photonics for healthcare: A roadmap for proactive and predictive health management

The field of infrared (IR) photonics is currently undergoing remarkable progress, moving rapidly towards practical sensing applications demanded by medical therapy and diagnostics (theranostics). The Developments can be divided into three main categories: (i) novel devices and measurement concepts including advanced updates of classical approaches that push medical sensing into the spotlight; (ii) new demonstrations of photonic integrated circuit (PIC-)based IR devices enabling highly miniaturized sensors for point-of-care application as well as medical and wellness wearables; and (iii) technologically-mature IR demonstrators that enable first medical sensing and treatment applications. This roadmap paper provides a consolidated overview of this highly dynamic and interdisciplinary research field with a focus on the major roadblocks that limit the widespread adoption of IR photonics in large-scale medical diagnostics. Special attention is given to the ambivalence between the molecular-level spectroscopic interpretation and a broader health-state assessment, highlighting the need for a common framework. Additionally, the paper discusses the critical importance of unified measurement standards, calibration protocols, and medical certification processes to ensure the validity of experimental results, reproducibility, and clinical trust, particularly when novel experimental techniques and AI algorithms are involved. Perspectives from major past and current contributors to application-oriented IR photonics will be provided.

physics.app-ph↗

arXiv2021

The Emission Directionality of Electronic Intraband Transitions in Stacked Quantum Dots

We investigate the emission directionality of electronic intraband (intersubband) transitions in stacked coupled quantum dots. Using a well-established eight-band $k \cdot p$ method, we demonstrate that the minor contributions from the valence band mixing into the conduction band govern the polarization and emission directionality of electronic $p$-to-$s$-type intraband transitions. Despite that the contribution from the central-cell part to the momentum matrix element is dominant, we find, the contribution from the matrix elements among the envelope functions cannot be neglected. With the help of an artificial cuboidal quantum dot, we show that the vertical emission from intraband transitions can be tuned via the emitter's vertical aspect ratio. Subsequently, we show that these results can be transferred to more realistic geometries of quantum dots and quantum dot stacks and demonstrate that the vertical emission can be enhanced from $23$ % to $46$ % by increasing the emitter's vertical aspect ratio to the isotropic case with a vertical aspect ratio of 1.0. Therefore, a stacking of a few quantum dots ($\sim$4 for the investigated structures) is already sufficient to improve the vertical radiation significantly. Additionally, we discuss the impact of the number of stacked QDs as well as the effect of the interdot coupling strength on the radiation properties.

cond-mat.mes-hall↗

arXiv2021

Incorporating microlearning videos, online exercises and assessments into introductory physics

We report on the evolution of the largest physics course at the Technical University Berlin during the COVID-19 pandemic, which boosted the development of online learning content, electronic assessments, and, not at least, the reconceptualization of our teaching methods. By shifting to distance education, we created features that can be easily implemented and combined with any physics-type course at a university level. Thereby, we incidentally made introductory physics sustainably accessible to students having difficulties visiting the university. Besides our experiences gained on blended learning methods, we provide a guide to online exercises, microlearning videos, and e-assessments, including exams realized in a virtual setting via the university's Moodle. Since the course is attended annually by more than 800 students enrolled in at least twelve bachelor degree engineering and all STEM (science, technology, engineering, and mathematics) fields, our reformations greatly impact the student's learning style. The teaching concepts presented here are supported by results from the Force Concept Inventory, student feedback, and exam results. The Force Concept Inventory shows that our online teaching did not negatively affect the student's learning process compared with a traditional face-to-face lecture. Student's feedback shows that the new format and material being available online are received very well. Finally, the exam results show that virtual exams conducted in a remote setting can be designed to minimize cheating possibilities. In addition, the test was able to yield a similar distribution of points as in the previous traditional ones.

physics.ed-ph↗

arXiv2021

Terahertz Lasing at Roomtemperature: Numerical Study of a Vertical-Emitting Quantum Cascade Laser Based on a Quantum Dot Superlattice

We investigate room temperature lasing of terahertz quantum cascade lasers using quantum dot chains as active material suitable for wireless communication and imaging technologies. Bandstructure calculations for such extended systems of coupled quantum dots are made possible by a novel 'linear combination of quantum dot orbitals'--method, based on single quantum dot wavefunctions. Our results demonstrate strong vertical-emission of coupled quantum dots, reduced phonon coupling and in-plane scattering, enabling room-temperature lasing with significantly reduced threshold current densities.

cond-mat.mes-hall↗

arXiv2019

Efficient electronic structure calculations for extended systems of coupled quantum dots using a linear combination of quantum dot orbitals method

We present a novel "linear combination of atomic orbitals"-type of approximation, enabling accurate electronic structure calculations for systems of up to 20 or more electronically coupled quantum dots. Using realistic single quantum dot wavefunctions as basis to expand the eigenstates of the heterostructure, our method shows excellent agreement with full 8-band $\boldsymbol{k}\cdot\boldsymbol{p}$ calculations, exemplarily chosen for our benchmarking comparison, with an orders of magnitude reduction in computational time. We show that, in order to correctly predict the electronic properties of such stacks of coupled quantum dots, it is necessary to consider the strain distribution in the whole heterostructure. Edge effects determine the electronic structure for stacks of $\lesssim$ 10 QDs, after which a homogeneous confinement region develops in the center. The overarching goal of our investigations is to design a stack of vertically coupled quantum dots with an intra-band staircase potential suitable as active material for a quantum-dot-based quantum cascade laser. Following a parameter study in the In$_{x}$Ga$_{1-x}$As/GaAs material system, varying quantum dot size, material composition and inter-dot coupling strength, we show that an intra-band staircase potential of identical transitions can in principle be realized. A species library we generated for over 800 unique quantum dots provides easy access to the basis functions required for different realizations of heterostructures. In the associated manuscript entitled "Room Temperature Lasing of Terahertz Quantum Cascade Lasers Based on a Quantum Dot Superlattice", we investigate room temperature lasing of a terahertz quantum cascade laser based on a two-quantum-dot unit cell superlattice.

cond-mat.mes-hall↗