Lithuanian Journal of Physics

Title

Lietuvos mokslų akademija — 2024-12-13

Gintautas Jurgis Babonas (1941–2024)

Vytautas Karpus — 2024-12-13

Reciprocal space mapping of ZnMgY face-centred icosahedral quasicrystals

Vytautas Karpus — 2024-12-13

The reciprocal lattice of face-centred icosahedral ZnMgY quasicrystals was investigated using X-ray diffractometry. A large-scale planar cut of the reciprocal ZnMgY lattice was recorded making use of a PIXcel area detector. The θ-2θ scans along the C5, C3 and C2 symmetry axes revealed numerous diffraction peaks with a large dynamical range of about 106. Low structure-factor diffraction peaks, corresponding to large complementary reciprocal lattice vectors g┴ up to g┴a = 24 (where a is the quasilattice constant), were observed, indicating a high structural quality of ZnMgY quasicrystals. A static linear phason strain was detected in one of the five investigated ZnMgY samples.

Volumetric carrier injection in InGaN quantum well light emitting diodes

Saulius Marcinkevičius — 2024-12-13

InGaN/GaN quantum well (QW) light emitting diodes (LEDs) are essential components of solid-state lighting and displays. However, the efficiency of long wavelength (green to red) devices is inferior to that of blue LEDs. To a large degree, this occurs because the equilibration of injected holes between multiple QWs of the active region is hindered by GaN quantum confinement and polarization barriers. This drawback could be overcome by volumetric hole injection into all QWs through semipolar QWs present on the facets of V-defects that form at threading dislocations in polar GaN-based structures. In this work, we have tested the viability of this injection mechanism and studied its properties by time-resolved and near-field spectroscopy techniques. We have found that indeed the hole injection via the V-defects does take place, the mechanism is fast, and the hole spread from the V-defect is substantial, making this type of injection feasible for efficient long wavelength GaN LEDs.

Emission induced by strong coupling between hybrid Tamm-surface plasmon polariton mode and rhodamine 6G dye exciton

Ernesta Bužavaitė-Vertelienė — 2024-12-13

otal internal reflection ellipsometry (TIRE) and leakage microscopy were applied for the study of photonic-plasmonic nanostructures supporting hybrid Tamm–surface plasmon modes and their strong coupling with Rhodamine 6G organic dye excitons. The optical response of TIRE has shown that Tamm and surface plasmon polaritons interact strongly and the formed hybrid plasmonic mode alters resonances in the energy spectra. Moreover, both TPP (Tamm plasmon polaritons) and SPP (surface plasmon polaritons) components in the hybrid mode are strongly coupled with R6G-PMMA (poly(methyl methacrylate)) layers at the inner and outer interfaces of the 50 nm gold layer, respectively. Leakage microscopy in the back focal plane optical configuration proves the energy transfer of excited emitters through the 50 nm gold layer in the strong coupling regime. Polaritonic emission in the strong coupling has better coherence properties than conventional spontaneous fluorescence emission from pure Rhodamine 6G organic dye molecules.

Tellurium/|GaAs heterojunctions fabricated by thermal evaporation in vacuum

Vaidas Pačebutas — 2024-12-13

Heterostructures containing thin tellurium layers thermally evaporated on differently doped GaAs substrates were systematically investigated by using THz pulse excitation spectroscopy. The observed differences of the THz excitation spectra were explained by the details of the energy band lineups in the heterostructures. Comparison of the simulation results of the heterojunction between tellurium and semi-insulating GaAs with the measured THz pulse emission spectrum allowed one to estimate the electron affinity in the tellurium layer. In addition, a near-infrared photodetector based on the Te heterojunction with n-type GaAs was demonstrated.

Optimization of AlGaAs barrier for InGaAs quantum wells emitting in the near infrared

Andrea Zelioli — 2024-12-13

The results of a study aimed at optimizing the optical properties of InGaAs quantum well structures by employing different barrier designs are presented. Single rectangular InGaAs quantum wells with approximately 21% indium and AlGaAs barriers, with different Al content 12, 20 and 30%, were theoretically modelled using the nextnano3 software to calculate band edges and levels in the quantum wells. A series of samples were grown using molecular beam epitaxy to clarify the influence of the Al fraction in the AlGaAs barriers on the optical properties of the quantum structures. Atomic force microscopy measurements were used to evaluate the surface roughness of the grown structures, while photoluminescence investigations provided insight into the optical quality and carrier confinement effects. The investigations revealed that the introduction of AlGaAs barriers resulted in an increased carrier confinement inside of the quantum well, but consequently resulted in the degradation of the InGaAs quantum well quality with the increase of aluminium content in the barrier. It was determined that a barrier with 12% of Al can be used to balance these effects, by providing a sufficient confinement, while retaining a satisfactory crystalline quality of the quantum structures.

Dependence of terahertz photoconductive switch performance on metal contact geometry

Ignas Nevinskas — 2024-12-13

This study investigates the emission and spectral characteristics of photoconductive THz switches employing coplanar stripline contact geometries fabricated on a GaAs substrate. The experimental results reveal how the power outputs as well as the spectral shape are significantly influenced by the strip width dimension. Utilizing the Drude–Lorentz conductivity model, photocarrier dynamics were analyzed through an RLC circuit framework, offering insights into how the contact design influences the spectral response. Our findings suggest that matching the photocurrent impedance to that of the metallic contacts is critical to improving the eﬀiciency of these devices.

Single Pixel Reconstruction Imaging: taking confocal imaging to the extreme

Simona Streckaite — 2024-12-13

Light nanoscopy is attracting widespread interest for the visualization of fluorescent structures at the nanometre scale. Recently, a variety of methods have overcome the diffraction limit, yet in practice they are often constrained by the requirement of special fluorophores, nontrivial data processing, or a high price and complex implementation. Therefore, confocal microscopy, yielding a relatively low resolution, is still the dominant method in biosciences. It was shown that image scanning microscopy (ISM) with an array detector could improve the resolution of confocal microscopy. Here, we review the principles of the confocal microscopy and present a simple method based on ISM with a different image reconstruction approach, which can be easily implemented in any camera-based laser-scanning set-up to experimentally obtain the theoretical resolution limit of confocal microscopy. Our method, single pixel reconstruction imaging (SPiRI), enables high-resolution 3D imaging utilizing image formation only from a single pixel of each of the recorded frames. We achieve the experimental axial resolution of 330 nm, which was not shown before by basic confocal or ISM-based systems. The SPiRI method exhibits a low lateral-to-axial FWHM aspect ratio, which means a considerable improvement in 3D fluorescence imaging. As a demonstration of SPiRI, we present the 3D-structure of a bacterial chromosome with an excellent precision.