Lithuanian Journal of Physics

Title

2026-02-11T10:55:21+02:00

2026-02-11T11:07:18+02:00

Academician Juras Požela: A tribute on his 100th anniversary

2026-02-11T12:58:28+02:00

Bridging the terahertz gap using solid-state devices

2026-02-11T13:03:04+02:00

Terahertz (THz) frequencies nestled between the microwave and infrared ranges in the electromagnetic spectrum radiation remain one of the most attractive research topics. A particular attention is given to the issues related to the development of solid-state-based room-temperature high-power, stable and portable terahertz emitters and detectors as well as user-friendly THz imaging and spectroscopy. At the dawn of this research, four decades ago,
academician Juras Požela [J. Požela and V. Jucienė, Physics of High-Speed Transistors (Vilnius, Mokslas, 1985)] considered possible physical mechanisms – hot electrons, plasma effects, Josephson junctions, masers, etc. – that can successfully be employed to cover the THz frequencies using solid-state physics approaches. In this work, we briefly overview the recent achievements and advances illustrating an incredibly high precision of the scientific predictions given by Acad. Juras Požela based on his wide erudition, deeply sensitive intuition and great insights, gifted feeling of scientific trends and evolution. The paper presents a structured snapshot of the modern devices with highlights in their physics behind the operation and main parameters and includes contemporary topics in THz science and technology related to electrically pumped GaN-based sources and quantum semiconductor structures such as resonant tunnelling diodes, quantum cascade lasers, and quantum semiconductor superlattices. Possible challenges in further development of the described approaches and devices are illuminated.

Classification of concealed objects using terahertz imaging and artificial neural networks

2026-02-12T09:54:01+02:00

Imaging in the terahertz frequency band is applied in a number of fields, such as security, medical or quality control. However, a low resolution or distortions of the images hinder the identification or recognition of the objects. To cope with the processing of visual information, artificial neural networks are broadly employed. In this work, the monochromatic radiation of 253 GHz was used to collect the image set of the investigated objects either in the air or covered with a packing material. Such a set was later used to train convolutional and generative adversarial neural networks poised for three tasks: (i) the classification of objects; (ii) the enhancement of image resolution; (iii) the identification of cover material. The obtained results demonstrated that the packaging materials were identified with an accuracy of 83.33%, while the investigated objects were classified with an accuracy of 89.42%. The PSNR metric of images with improved resolution reached up to 22.44 dB. The optical properties such as refractive indices and absorption coefficients of the packaging materials were also defined using terahertz time-domain spectroscopy, and it was found that the accuracy of object and material classification in general does not depend on the physical properties and type of a package.

GaAsBi structures for ultrafast optoelectronics grown at different Bi fluxes

2026-02-11T13:02:33+02:00

GaAsBi is an attractive semiconductor material for the development of infrared optoelectronics devices due to possibilities of band engineering when, varying the Bi content, one can induce a rapid rising of the valence band edge. Although this property makes GaAsBi a promising material for terahertz (THz) emitters, telecommunication lasers, and low noise photodetectors, the yield of the developed GaAsBi-based devices is still low indicating a requirement for the better quality of the material. In this work, we extend previous studies focusing on the investigation of the influence of Bi flux during the molecular beam epitaxy growth. The structures were characterized using high-resolution X-ray diffraction, photoluminescence and optical pump–THz probe technique. It is shown that multiple growth runs targeting at the ~6% Bi content and near-infrared operation wavelength of around 1.2µm yielded consistent structural and optical properties, indicating that the optimal and repeatable growth protocol has been successfully established. The observed red-shifts in photoluminescence spectra and the bi-exponential decay in carrier relaxation can be associated with the existence of band-tail states and random potential due to fluctuations in the distribution of Bi content.

Impact of structural design and bismuth segregation on GaAsBi/GaAs quantum wells for near-infrared emitters: A numerical study

2026-02-11T13:10:00+02:00

In this work, we perform 8-band k·p simulations using the nextnano software to evaluate how Bi composition, quantum well width, and barrier thickness influence the interband transition energy and electron–hole envelope function overlap in GaAsBi/GaAs single- and multiple-quantum-well structures.
The results show that the optimization of Bi content, well width, or barrier thickness lead to the improved electron–hole overlap of up to approximately 10%, indicating enhanced radiative recombination efficiency. We additionally model Bi surface segregation using experimentally reported segregation probabilities and observe substantial modifications of the confinement potential, redshifts of 17–26 meV in the conduction band heavy-hole transition energy, and reductions of 5–7% in the electron–hole overlap. These effects arise from electron delocalization into Bi-enriched barriers. The study highlights that Bi segregation must be explicitly considered in the design of GaAsBi-based emitters and provides quantitative guidelines for achieving efficient and 1 µm wavelength-stable devices.

Control of proton irradiation-induced damage in low gain avalanche detectors by heat treatment techniques

2026-02-11T13:02:04+02:00

Silicon-based particle sensors are widely employed in high-energy and nuclear physics experiments conducted at the European Organization for Nuclear Research (CERN). In recent years, silicon sensors with internal gain, known as low gain avalanche detectors (LGADs), have demonstrated an excellent performance in detecting high-energy particles owing to their good spatial and timing resolution. The sensor LGAD architecture has shown a great potential for the use in the upcoming High-Luminosity Large Hadron Collider (HL-LHC) upgrade, where semiconductor sensors will be exposed to extremely high radiation fluences. In this study, the impact of high-energy proton irradiation on the electrical performance of LGADs was investigated. The variations of critical parameters such as leakage current, effective doping concentration, carrier lifetime, spectral characteristics of radiation-induced defects, and charge collection, before and after thermal annealing at different temperatures, have been analysed using the I–V, C–V, microwave-probed photoconductivity (MW-PC), photoionization spectroscopy (PIS), and transient current (TCT) methods. It was demonstrated that the 24 GeV energy proton irradiation introduces defects, such as divacancy and trivacancy complexes, boron–oxygen and carbon–oxygen complexes, as well as divacancy–oxygen and divalent bistable defects, which act as current generation and carrier recombination centres. Annealing at temperatures of up to 400°C led to the transformation or passivation of those defects, partially restoring doping profiles and improving carrier lifetimes. These results highlight the potential of defect engineering to enhance the radiation tolerance of LGADs employed in high-energy physics applications.

Surface analysis of carbon allotropes

2026-02-11T13:03:58+02:00

The spectroscopy of carbon is very important in surface analysis of solids, because its content indicates the grade of surface contamination. Adventitious carbon from air ambient is practically present on any solid material and the C 1s photoelectron spectrum is often used as a reference for the scale calibration of binding energy. Moreover, during the last two decades, new 2D carbon materials have been developed and intensively investigated: graphene, fullerenes, nanotubes and nanowalls, quantum dots, etc. Also, the growing applications of amorphous carbon (a-C), e.g. diamond-like carbon (DLC), carbon quantum dots (CQDs), etc., require the characterization of these materials.
This short overview is dedicated to the analysis of new carbon-based materials by widely used surface-sensitive techniques: X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The combination of XPS and AES techniques permits one to investigate the electron hybridization in carbon materials, i.e. to determine the ratio of sp2/sp3 configurations, which defines their main mechanical, electrical and optical properties. In addition, it was demonstrated that the same experimental approach could be successfully used for the investigation of bulk composite materials containing 2D carbon, e.g. graphene or nanotubes.

Hot carrier transport in solar cells

2026-02-11T13:03:46+02:00

The study presents the findings on photovoltage formation in solar cells subjected to pulsed laser excitation. Transient photovoltage measurements reveal that the photoresponse comprises two components with opposite polarities, expressed as U = U_f + U_ph. The fast component, which mirrors the laser pulse profile, arises from the heating of charge carriers by the incident light. In contrast, the slow component corresponds to the conventional photovoltage generated through electron–hole pair creation. The detrimental effect of hot carriers on the power conversion efficiency of perovskite solar cells can be alleviated by reducing band bending near the charge transport layers or by adopting a multijunction cell architecture. This approach enhances spectral utilization and minimizes thermalization losses.

Fabrication and magnetoresistive properties of magnetron-sputtered CoFe/Cu spin valves with antiferromagnetic IrMn pinning

2026-02-11T19:00:21+02:00

Nanolayered ferromagnetic/non-magnetic structures exhibit the giant magnetoresistance (GMR) effect and are used in a variety of applications. Spin valves are one class of devices that fall into the GMR category. In this work, the fabrication and characterization results of magnetron sputtered Ta/IrMn/CoFe/Cu/CoFe/Ta spin valve structures are presented. Two groups of samples were produced where the thickness of the Cu spacer layer or the CoFe pinned layer were varied in search of the highest magnetoresistance value. The maximum value of 4.8% magnetoresistance was obtained for a sample with the composition of Ta(5 nm) / IrMn(15 nm) / CoFe(2 nm) / Cu(2 nm) /
CoFe(5 nm) / Ta(5 nm) when the sample was shaped into a meandering channel with 2 μm width. The achieved results are promising and will be used to further develop spin valve technology for various applications.