The review exposes basic concepts and manifestations of the singular and structured light fields. The presentation is based on deep intrinsic relations between the singularities and the rotational phenomena in light; it involves essentially the dynamical aspects of light fields and their interactions with matter. Due to their topological nature, the singularities of each separate parameter (phase, polarization, energy flow, etc.) form coherent interrelated systems (singular networks), and the meaningful interconnections between the different singular networks are analysed. The main features of singular-light structures are introduced via generic examples of the optical vortex and circular vortex beams. The review describes approaches for generation and diagnostics of different singular networks and underlines the role of singularities in formation of optical field structures. The mechanical action of structured light fields on material objects is discussed on the base of the spin-orbital (canonical) decomposition of electromagnetic momentum, expressing the special roles of the spin (polarization) and spatial degrees of freedom. Experimental demonstrations spectacularly characterize the topological nature and the immanent rotational features of the light-field singularities. The review is based on the results obtained by its authors with a special attention to relevant works of other researchers.

External light outcoupling structures provide a cost-effective and highly efficient solution for light extraction in organic light-emitting diodes. Among them, different microtextures, mainly optimized for devices with isotopically oriented emission dipoles, have been proposed as an efficient light extraction solution. In the paper, the outcoupling for a preferential orientation of emission dipoles is studied for the case of a red bottom-emitting organic light-emitting diode. Optical simulations are used to analyse the preferential orientation of dipoles in combination with three different textures, namely hexagonal array of sine-textures, three-sided pyramids, and random pyramids. It is shown that while there are minimal differences between the optimized textures, the highest external quantum efficiency of 51% is predicted by using the three-sided pyramid texture. Further improvements, by employing highly oriented dipole sources, are examined. In this case, the results show that the top outcoupling efficiencies can be achieved with the same texture shape and size, regardless of the preferred orientation of the emission dipoles. Using an optimized three-sided pyramid in combination with ideally parallel oriented dipoles, an efficiency of 62% is achievable. A detailed analysis of the optical situation inside the glass substrate, dominating external light outcoupling, is presented. Depicted results and their analysis offer a simplified further research and development of external light extraction for organic light-emitting devices with highly oriented dipole emission sources.

This work summarises investigations focused on the photoanode impact on the photovoltaic response of dye-sensitized solar cells. This is a comparison of the results obtained by the authors’ research team with literature data. The studies concern the effect of the chemical structure of the applied dye, TiO2 nanostructure, co-adsorbents addition, and experimental conditions of the anode preparation. The oxide substrates were examined using a scanning electron microscope to determine the thickness and structure of the material. The TiO2 substrates with anchored dye molecules were also tested for absorption properties in the UV-Vis light range, largely translating into current density values. Photovoltaic parameters of the fabricated devices with sandwich structure were obtained from current-voltage measurements. During tests conducted with the N719 dye, it was found that devices containing an 8.4 µm thick oxide semiconductor layer had the highest efficiency (5.99%). At the same time, studies were carried out to determine the effect of the solvent and it was found that the best results were obtained using an ACN : tert-butanol mixture (5.46%). Next, phenothiazine derivatives (PTZ-1–PTZ-6) were used to prepare the devices; among the prepared solar cells, the devices containing PTZ-2 and PTZ-3 had the highest performance (6.21 and 6.22%, respectively). Two compounds designated as Th-1 and M-1 were used to prepare devices containing a dye mixture with N719.

The article presents a study of a hybrid structure based on the combination of a tapered optical fibre and gold nanoparticles dispersed in a liquid crystal material. Sensitivity to changes of the refractive index of the environment in which the structure is located, as well as the possibility of changing the refractive index of liquid crystals by external factors, such as temperature and electric field, were investigated. Electro- and thermally-induced changes of the refractive index of a liquid crystal through the rotation of a molecule director, which cause changes in the light propagated in a tapered optical fibre, were described. The most important issue in the article is to determine the influence of doping a liquid crystal with gold nanoparticles the concentration of which varies between 0.1 and 0.3 wt.%. The paper presents transmission measurements in a wide optical range depending on voltage, temperature, and frequency changes. Additionally, time courses of the obtained signal were measured. The study shows that the appropriate selection of nanoparticle concentration has a huge impact on the optical wave propagation. The experimental results show that the optical changes obtained for the investigated hybrid structure prefer it for use as an electro-optical switcher, filter, or sensor.

In the last decade several papers have announced usefulness of two-dimensional materials for high operating temperature photodetectors covering long wavelength infrared spectral region. Transition metal dichalcogenide photodetectors, such as PdSe ₂/MoS ₂ and WS ₂/HfS ₂ and WS ₂/HfS ₂ heterojunctions, have been shown to achieve record detectivities at room temperature (higher than HgCdTe photodiodes). Under these circumstances, it is reasonable to consider the advantages and disadvantages of two-dimensional materials for infrared detection. This review attempts to answer the question thus posed.

The presented research shows that commercially available graphene on quartz modified with rhenium oxide meets the requirements for its use as a conductive and transparent anode in optoelectronic devices. The cluster growth of rhenium oxide enables an increase in the work function of graphene by 1.3 eV up to 5.2 eV, which guarantees an appropriate adjustment to the energy levels of organic semiconductors used in organic light-emitting diode devices.

An imaging polarimeter based on the principles of high-accuracy polarimetry well known in crystal optics is proposed. The application of scientific digital cameras for performance light measurements leads to precise data on polarizers quality, i.e., maps of extinction ratio and transmission axis. Processing of numerous images, acquired at various settings in the polarizer-sample-analyser system, allows to determine the two-dimensional distribution of the phase retardation of birefringent plates. Several results of imaging polarimetry experiments on birefringent plates demonstrate the impact of multiple light reflections on the measured phase retardation values. Experimental data for LiNbO ₃ and SiO ₂ crystal plates have been presented, demonstrating the capabilities of the proposed type of imaging polarimeter in the crystal optics studies. This technique also allows the measurement of the eigen wave ellipticities, associated with the optical activity of crystals.

Various intelligent transportation systems are proposed in different forms of wireless communication technologies. Recently, the importance of visible light communication and free-space optics has been demonstrated in accomplishing vehicle-to-vehicle and infrastructure-to-vehicle communication systems, due to power efficiency, free licenses, and safety for human health. In this paper, a new hybrid relay system supported by free-space optics/visible light communication with two scenarios is proposed. The first one is that the data are transferred from the source to the relay through a free-space optics communication link and are then directed to the destination through a visible light communication link. The second scenario is that the data are transmitted from the source to the destination passing through two different relays to ensure larger coverage. A 10−6 bit error rate is achieved at a distance of 900 m for the first scenario with a remarkable signal-to-noise ratio of ~25.5 dB, while the largest distance that can be covered by the second scenario is 1200 m with a signal-to-noise ratio of ~30 dB.

Effects of temperature variation on the performance of silicon heterojunction solar cells are studied using opto-electrical simulations. It is shown that the low-temperature cell efficiency is determined by the fill factor, while at high temperatures it depends on the open-circuit voltage. Simulations revealed that the low-temperature drop in the fill factor is caused by poor tunnelling, in particular at the ITO/p-a-Si:H heterojunction. The authors link this drop in fill factor to a low maximum-power-point voltage and show how poor tunnelling is reflected in the charge redistribution determining the device voltage. The effect of the contact work function on temperature behaviour of efficiency by varying the electron affinity of ITO layers has been demonstrated. It was also demonstrated that increasing the electron affinity of ITO on the p-side minimises the work function mismatch, leading to significant improvements in efficiency, especially at low temperatures, while optimisation on the n-side results in marginal improvements over the entire temperature range. In addition to the cumulative effects of the temperature-dependent parameters, their individual contributions to the efficiency were also investigated. Moreover, it was presented that the thermal energy (kT) determines the efficiency temperature behaviour, while other parameters play only a minor role. This paper shows how temperature variations affect device performance parameters.

We developed a three-stage, amplifying, tunable diode laser system that comprises a master laser in a Littrow configuration, frequency-stabilized by dichroic atomic vapour laser lock, acousto-optic frequency shifter, injection-locked slave laser, and tapered amplifier. The slave amplifies the injected frequency-shifted master beam while suppressing (within 0.5  %) the strong dependence of its intensity on the acousto-optic frequency shifter carrier frequency, thus acting as a strongly saturated optical limiting amplifier with constant output power. The resulting beam is then amplified in a tapered amplifier. The system provides an output power above 700 mW at a wavelength of 780 nm, with a time-averaged linewidth of 0.6 MHz, and a frequency drift below 2 MHz/h. Dichroic atomic vapour laser lock enables frequency stabilization in the range of 400 MHz around D2 lines of rubidium. The mode-hop-free tuning range amounts to 2 GHz. Determined by the acousto-optic frequency shifter model used, the fine-tuning range (precision of few tens kHz) spans 70 MHz. A description of the system was presented and its performance was tested. The basic components have been designed in our laboratory.

The paper presents noise measurements in low-resistance photodetectors using a cross-correlation-based transimpedance amplifier. Such measurements usually apply a transimpedance amplifier design to provide a current fluctuation amplification. In the case of low-resistance sources, the measurement system causes additional relevant system noise which can be higher than noise generated in a tested detector. It mainly comes from the equivalent input voltage noise of the transimpedance amplifier. In this work, the unique circuit and a three-step procedure were used to reduce the floor noise, covering the measured infrared detector noise, mainly when operating with no-bias or low-bias voltage. The modified circuit and procedure to measure the noise of unbiased and biased detectors characterized by resistances much lower than 100 Ω were presented. Under low biases, the reference low-resistance resistors tested the measurement system operation and techniques. After the system verification, noise characteristics in low-resistance InAs and InAsSb infrared detectors were also measured.

Orthogonal frequency division multiplexing has been widely used in many radio frequency wireless communication standards as a preferable multicarrier modulation scheme. The modulated signals of a conventional orthogonal frequency division multiplexing system are complex and bipolar. In intensity-modulated direct detection optical wireless communications, transmitted signals should be real and unipolar due to non-coherent emissions of an optical light emitting diode. In this paper, different hybrid optical systems have been proposed to satisfy real and unipolar signals. Peak-to-average power ratio is one of the biggest challenges for orthogonal frequency division multiplexing-based visible light communications. They are based on a combination of non-linear companding techniques with spreading or precoding techniques. Simulation evaluation is performed under direct current-biased optical orthogonal frequency division multiplexing, asymmetrically clipped optical orthogonal frequency division multiplexing, and Flip-orthogonal frequency division multiplexing systems in terms of peak-to-average power ratio, bit error rate, and spectral efficiency. The proposed schemes are investigated to determine a scheme with a low peak-to-average power ratio and an acceptable bit error rate. MATLABTM software has been successfully used to show the validity of the proposed schemes.

An adaptive and precise peak wavelength detection algorithm for fibre Bragg grating using generative adversarial network is proposed. The algorithm consists of generative model and discriminative model. The generative model generates a synthetic signal and is sampled for training using a deep neural network. The discriminative model predicts the real fibre Bragg grating signal by the calculation of the loss functions. The maxima of loss function of the discriminative signal and the minima of loss function of the generative signal are matched and the desired peak wavelength of fibre Bragg grating is determined. The proposed algorithm is verified theoretically and experimentally for a single fibre Bragg grating peak. The accuracy has been obtained as ±0.2 pm. The proposed algorithm is adaptive in the sense that any random fibre Bragg grating peak can be identified within a short wavelength range.

In this work the influence of the cavity parameters on optical losses of a simple intensity-based in-line refractive index sensor utilizing a micromachined side-hole fibre was studied by means of numerical simulations. To perform these simulations, the Authors used the finite-difference time-domain method. The proposed sensor setup consists of light source, micromachined optical fibre as a sensor head, and a detector which makes it low-cost and easy to build. The changes of the external refractive index can be, therefore, recovered by direct measurements of the transmitted intensity from which insertion loss values can be calculated. By changing geometry of the cavity micromachined into the side-hole optical fibre, it was possible to determine its influence on the final sensor sensitivity and measurements range. Based on the provided analysis of simulations results, a simple fibre optic sensor can be fabricated mainly for sensing external liquids refractive index for application in biochemistry or healthcare.

The solid dielectrics used in the capacitors exhibit rather high-frequency relaxations. This means that in the radio-frequency range, the capacitors exhibit a constant capacity. When a liquid crystal is put into the capacitors, it is observed that in the radio-frequency range the capacity changes (decreases with frequency). This is due to the fact that liquid crystals exhibit relaxation in the radio-frequency range. In this paper, the formulas for the electric response of a low-frequency RC filter with liquid crystal characterized by complex electric permittivity are derived. One Debye-type relaxation is assumed in the calculations. The influence of strengths and relaxation time (frequency) of relaxation mode in liquid crystal on the electric response of low-frequency filters is discussed.

In order to minimize the receiver complexity and improve the performance of the spectral amplitude coding - optical code division multiple access system, a novel one-dimensional zero cross-correlation code using Pascal’s triangle matrix has been suggested. This research article shows that the position of chip “1” in the code sequences is one of the important factors affecting system performance. In fact, mathematical results show that, for the all-wavelength direct detection, it is possible to reduce the number of filters without sacrificing system performance. In addition, compared to one-wavelength direct detection, the signal-to-noise ratio value is increased with an increasing weight by using wide-bandwidth filters as decoders. Performance of the proposed system in terms of the minimum bit error rate is validated using the OptiSystem software. Compared with the previous systems at 622 Mbps, the suggested system gave the best values of bit error rate of around 10−43, 10−35, and 10−26 for higher, medium, and lower service demand, respectively.

Solar-blind ultraviolet cameras with image intensifier with CMOS detector typically use various count methodologies to measure the optical energy of an electrical corona. However, these count methodologies are non-radiometric without considering parameters such as distance, focus-, zoom-, and gain setting of a camera. An algorithm which considers the calibration and radiometric measurement of optical energy for the slow frame rate intensifier type cameras is presented. Furthermore, it is shown how these calibration data together with the flowcharts are used for the conversion from raw measured data to radiometric energy values.

Al ₂O ₃/TiO ₂ thin films were deposited onto monocrystalline silicon surfaces using an atomic layer deposition. Their surface morphology and optical properties were examined for their possible use in solar cells. The surface condition and chemical composition were characterized using a scanning electron microscope and the thickness was measured using a spectroscopic reflectometer. The refractive index and the reflection characteristics were determined. First, the optical properties of the Al ₂O ₃ thin film and its influence on recombination in the semiconductor were examined. In this way, it can fulfil a double role in a solar cell. Since reflection reduction was only achieved in a narrow range, it was decided to use the Al ₂O ₃/TiO ₂ system. Thanks to this solution, the light reflection was reduced in a wide range (even below 0.2%).

An indoor localization system is proposed based on visible light communications, received signal strength, and machine learning algorithms. To acquire an accurate localization system, first, a dataset is collected. The dataset is then used with various machine learning algorithms for training purpose. Several evaluation metrics are used to estimate the robustness of the proposed system. Specifically, authors’ evaluation parameters are based on training time, testing time, classification accuracy, area under curve, F1-score, precision, recall, logloss, and specificity. It turned out that the proposed system is featured with high accuracy. The authors are able to achieve 99.5% for area under curve, 99.4% for classification accuracy, precision, F1, and recall. The logloss and precision are 4% and 99.7%, respectively. Moreover, root mean square error is used as an additional performance evaluation averaged to 0.136 cm.

This paper presents the results of a thermal computational analysis of a two-dimensional laser array emitting from a surface. The array consisted of eight equispaced ridge-waveguide edge-emitting nitride diode lasers. Surface emission of light was obtained using mirrors inclined at 45°. The authors investigate how the geometrical dimensions of the array emitters and their pitch in the array affect the increase and distribution of temperature in the device. They also examine the influence on the temperature increase and distribution of the thickness of the insulating SiO2, the thickness of the gold layer forming the top contact of the laser, and the thickness of the GaN substrate, as well as the influence of the ridge-waveguide width.

A theoretical analysis of the mid-wavelength infrared range detectors based on the HgCdTe materials for high operating temperatures is presented. Numerical calculations were compared with the experimental data for HgCdTe heterostructures grown by the MOCVD on the GaAs substrates. Theoretical modelling was performed by the commercial platform SimuAPSYS (Crosslight). SimuAPSYS fully supports numerical simulations and helps understand the mechanisms occurring in the detector structures. Theoretical estimates were compared with the dark current density experimental data at the selected characteristic temperatures: 230 K and 300 K. The proper agreement between theoretical and experimental data was reached by changing Auger-1 and Auger-7 recombination rates and Shockley-Read-Hall carrier lifetime. The level of the match was confirmed by a theoretical evaluation of the current responsivity and zero-bias dynamic resistance area product (R0A) of the tested detectors.

In this study, the copper doping effect on the NiAl structural stability, strength, and electronic structure was investigated. The samples were prepared using induction melting at 2073 K. This material presents good mechanical and physical properties such as high-temperature strength, fatigue or impact, and corrosion resistance which meet technical requirements of many applications. The microstructure of the Cu-doped nickel aluminide was studied using a metallurgical microscope and its lattice parameter was also studied and characterized using an X-ray diffractometer for different concentrations of Cu. The lattice constant of the existing phases was calculated, and it was found that the lattice distortion and gamma prime phase energy have high values allowing the increase of the entropy term of the alloy and subsequently increasing its hardness. From the ab-initio calculation, it was determined that the Cu atoms have the Al sites as a preferred site and prefer to bond with Ni atoms which leads to the improvement of the material hardness. Ab-initio density functional theory was applied to study the formation energy that revealed increasing with Cu amount.

The paper presents a design and performance analysis of a photosensor device enabling the measurement of the visible light illuminance. The sensor is designed for use in the light metering matrix of a mobile measurement platform allowing the correct operation of in-pavement airport lamps. This kind of control can be required by regulations and must meet the standards defined by the European Union Aviation Safety Agency (EASA). An important assumption of the solution was to obtain the highest possible speed of a measurement acquisition so that the control process would take place in a relatively short time. The proposed module concept is dedicated to the task of testing the quality of airport lamps, due to the characteristics of the photosensitive elements matching the light beams emitted by luminaries. The device is based on a VTP1220FBH photodiode and an ATmega328P microcontroller, which, in addition to the analogue-to-digital conversion and correction, sends the results back to the master unit via the I ²C bus.