1.3.25

Article: Single vanadium ion magnetic dopant in an individual CdTe/ZnTe quantum dot

 

The application of quantum technologies such as spintronics, solotronics, and quantum computing is highly promising when it comes to miniaturization in modern technology. In order to achieve effective devices, there is a need to investigate the spin properties of impurity interacting with the semiconductor lattice and confined carriers. Epitaxial quantum dots (QDs), representing zero-dimensional semiconductor structures, emerge as a model system offering a profound exploration of fundamental interactions in condensed matter. For instance, QDs serve as an invaluable tool for scrutinizing the spin characteristics of individual magnetic ions.

Vanadium is a transitional metal with a nuclear spin 7/2 and 3 electrons on the d shell. It exhibits spin 3/2 in V2+ configuration leading to two possible fundamental states with spin projection ±3/2 or ±1/2. Particular spin configuration is expected to depend on the strain of the crystal lattice in a QD.

In this study, we investigate self-assembled CdTe QDs doped with vanadium within a ZnTe barrier, created through molecular beam epitaxy. Our focus involves the observation of a single quantum dot containing a sole vanadium dopant, and the subsequent measurement of its magneto-optical properties. Through numerical modeling based on experimental data, we discern that the crucial phenomenon explaining the main features in the spectrum is the presence of sheer strain within the quantum dot. Ultimately, our findings lead us to the conclusion that vanadium in this context exhibits a spin of ±1/2, thereby rendering our system a realization of a qubit.

Authors: K. E. Połczyńska, T. Kazimierczuk, P. Kossacki and W. Pacuski

Physical Review B 111, 085428 (2025)

https://doi.org/10.1103/PhysRevB.111.085428

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7.1.25

Article: Carrier localization in defected areas of (Cd, Mn)Te quantum well investigated via Optically Detected Magnetic Resonance employed in the microscale

 

The object of examination is the effect of carrier localization on three properties sensitive to carrier gas density at the micrometer scale: the oscillator strength of charged excitons, local free carrier conductivity, and the Knight shift. The latter two are measured through a micrometer-scale, spatially resolved optically detected magnetic resonance (ODMR) experiment. On the surface of MBE-grown (Cd,Mn)Te quantum wells, we identify defected areas near dislocations. These regions exhibit significantly lower conductivity compared to the pristine areas, while the Knight shift values remain relatively unchanged. This behavior is attributed to carrier localization within the defected regions.

Authors: Amadeusz Dydniański, Aleksandra Łopion, Mateusz Raczyński, Tomasz Kazimierczuk, Karolina Ewa Połczyńska, Wojciech Pacuski and Piotr Kossacki

Solid State Communications, Volume 396, 2025, 115755

Published 19 November 2024

https://doi.org/10.1016/j.ssc.2024.115755

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7.1.25

Article: Spin-lattice relaxation of (Cd,Mn)Te co-doped with Co ions

 

The study investigates the effect of cobalt ion addition on the spin-lattice relaxation rate in the (Cd,Mn)Te system. Measurements using time-resolved spectroscopy of single (Cd,Mn,Co)Te/(Cd,Mg)Te quantum wells under a magnetic field reveal that even a small cobalt admixture significantly reduces the Mn relaxation time. The results indicate that cobalt ions, with their faster spin relaxation, serve as centers that enhance the spin relaxation of manganese ions, resulting in a faster and more multi-exponential relaxation process. This suggests that spin-lattice relaxation is primarily limited by the spin diffusion time to cobalt ions.

Authors: Aleksandra Łopion, Kacper Oreszczuk, Aleksander Bogucki, Karolina Ewa Połczyńska,Wojciech Pacuski, Tomasz Kazimierczuk, Andrzej Golnik and Piotr Kossacki

Solid State Communications,Volume 396, 2025, 115753,

Published 19 November 2024

https://doi.org/10.1016/j.ssc.2024.115753

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6.5.24

SpectroCalc update

SpectroCalc is a project of mine that I have been developing since 2017. It is a free Android app designed by physicist for physicists, to make our lives easier :)

SpectroCalc is a tool that seamlessly converts between different units in spectroscopy. Whether you're analyzing a spectrum and need to know the equivalent wavelength in energy (eV), or if you're in a lecture and the presenter is using wavenumbers (1/cm) but you prefer to think in nanometers, SpectroCalc has got you covered.

With SpectroCalc, you can effortlessly convert between nanometers (nm), energy (eV), wavenumbers (1/cm), and frequency (Hz) in just one click. Plus, you have the flexibility to choose whether to use constants in the air or a vacuum, ensuring accuracy in your calculations every time. Additionally, SpectroCalc offers the unique functionality of calculating the difference between two values in different units - no more problems with the estimation of the FWHM or the distance between two peaks.

It is available in the Google Play HERE.

Enjoy!

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1.5.24

Postdoc

Ladies and Gentleman, 

I am thrilled to announce that since the 1st of February, I have been a Postdoctoral Researcher at the Technical University of Denmark! The project that stole my heart is QPIC1550, which is focused on integrating quantum light sources like single-photon emitters and detectors with photonic integrated circuits (PICs). This integration is crucial for the next wave of quantum technologies, allowing precise control and manipulation of quantum light states at an unprecedented scale. By merging these components into a single chip, we create more compact, robust, and high-performance quantum systems.

QPIC1550 tackles key challenges in quantum photonics, including efficient light coupling between different components and minimizing signal processing losses. We leverage Silicon Nitride (SiN) for ultra-low loss pathways and Indium Phosphide (InP) for active devices, ensuring compatibility with existing optical networks while enhancing quantum circuit performance.

The goal is to pioneer a new class of quantum photonic integrated circuits, enabling advanced applications in telecommunications, healthcare, finance, and defense. QPIC1550 accelerates the adoption of quantum technologies by enhancing performance, scalability, manufacturability, and functionality, making them accessible for widespread use in communication, computing, and sensing.

Working on this project, I am exploring the secrets of MOVPE, which is a new kind of epitaxy added to my portfolio as a grower. Since I am responsible for the full fabrication process of the quantum light sources, I am gaining a lot of experience in the best cleanroom I have ever seen. In case you are not lucky enough to visit the Nanolab in person, you can go there for a virtual trip.

The amazing crew of DTU Nanolab is running a YouTube channel with an enormous amount of information about fabrication, I highly recommend checking it out.

Here is a snapshot of a cheerful scientist with her pink bike outside the DTU Electro building, where her office is situated. Since makeup isn't allowed in the cleanroom, every day is a no-makeup day. However, I'm thoroughly enjoying my new job and my time in Denmark. I'm incredibly grateful for this opportunity and fully committed to achieving exceptional results. Nature, here I come!

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8.11.23

My Ph. D. Thesis

For a long time in the industry of electronics, the goal of increasing the efficiency of a device was as important as decreasing its size. The ultimate limit of miniaturization in semiconductor technology is a single atom device. However, for any potential application of a given system, it is necessary to have a fundamental understanding of its various parameters and phenomena occurring therein.

Zero-dimensional semiconductor nanostructures doped with single magnetic ions, which are the subject of this thesis, are the simplest experimental system that enables optical investigations of a single atom in the solid state. In this work I discuss three examples of magnetic dopants, that can be embedded in single quantum dots, while two of them (Vanadium and Nickel) were never explored thus far. The investigated samples were manufactured by the Molecular Beam Epitaxy method and were optically characterized in spectroscopic and magnetospectroscopic measurements.

For the case of CdTe quantum dots with single Vanadium dopants a theoretical model is presented, that explains experimentally observed phenomena. Furthermore, the determination of Vanadium dopant spin equal to 1/2 shows that this system could be a solid state realization of a qubit.

The investigation of CdTe quantum dots doped with single Nickel ions showed their tendency to temporal variations of their charge state. Solving this issue required the development of a method of manufacturing electrical contacts for the studied samples. The application of an external electric field to the quantum dots allowed not only for the stabilization of their charge state but also opened up new possibilities for experimental studies.

In particular, the electrical contacting of epitaxially grown samples was utilized in order to further investigate an already partially explored system of CdSe quantum dots doped with single Iron ions. One of the most important outcomes was the first known observation of X-- and XX- states in these quantum dots.

Link to the repository HERE.

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17.5.23

Article: Coherent imaging and dynamics of excitons in MoSe2 monolayers epitaxially grown on hexagonal boron nitride

The technology of preparing heterostructures made of semiconducting transition metal dichalcogenides (TMDs) is based on the exfoliation of thin films from van der Waals bulk crystals. While the nonscalability of the exfoliation top-down approach is not an issue in the fundamental research, for which the proof-of-principle demonstrations are essential, it is a major roadblock on the academia-industry pathway of this field. In order to merge these novel materials with the semiconductor microelectronics, strain-free monolayer samples homogeneously covering wafers of a few-inch diameter are required.

Here we present studies of MoSe2 grown by molecular beam epitaxy on the silicon substrate with exfoliated hBN flakes. We performed four-wave-mixing (FWM) imaging and spatially correlated the obtained amplitude of the nonlinear optical response of the studied flakes with the layer thickness obtained from AFM measurements. It allowed us to precisely characterize the signal originating from the epitaxially-grown monolayers with respect to the surface morphology. Furthermore, by measuring the FWM signal in the temporal domain we were able to determine the dephasing dynamics of exciton complexes and ascertained their temperature dependence.

Our results show that these epitaxial monolayers, while opening the prospect of being compatible with the semiconductor optoelectronics industry, display excellent optical response, providing they crystalize on atomically flat surfaces, here provided by hBN flakes. The quality and intensity of the produced signal are comparable with their non-encapsulated counterparts obtained via exfoliation. As such, our findings fortify the viability of the MBE growth method for the production of high-quality TMD monolayers. Furthermore, its inherent versatility opens up alluring new research venues for this particular family of 2D materials. 

Authors: Karolina Ewa Połczyńska, Simon Le Denmat, Takashi Taniguchi, Kenji Watanabe, Marek Potemski, Piotr Kossacki, Wojciech Pacuski and  Jacek Kasprzak

Nanoscale, 2023,15, 6941-6946

Published 21 March 2023

https://doi.org/10.1039/D2NR04844B

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