Prof. Aidinis holds a B.Sc. (Honors) Degree in Electrical Engineering from Newcastle University, U.K. and Master’s degrees from University College London in Microwaves and Modern Optics and the University of Michigan, Ann Arbor, in Semiconductor Devices. He was awarded the PhD degree in Electrical Engineering for his research work in device nano-fabrication from Imperial College London . He has been involved in research in the area of Microelectronics, Materials and Devices and Optical Communications, from positions in the Department of Electrical Engineering, Imperial College, Hirst Research Centre, General Electric Company, the Institute of Microelectronics, National Center for Scientific Research, University of Athens and Ajman University. He has participated in several European and national research and development projects in the fields of Microelectronics, Materials and Optical Communications. His publications comprise one European patent, over 95 articles in international, Scopus indexed, refereed journals and over 30, peer reviewed, conference presentations. He is a member of the Hellenic Technical Chamber, the Institute of Electrical and Electronic Engineers and a founding member of the National Scientific Organization of Microelectronics and Nanotechnology.
Recent developments in both optical wireless communication (OWC) systems and implanted medical devices (IMDs) have introduced transdermal optical wireless (TOW) technology as a viable candidate for extremely high-speed in-body to out-of-body wireless data transmissions, which are growing in demand for many vital biomedical applications, including telemetry with medical implants, health monitoring, neural recording and prostheses. Nevertheless, this emerging communication modality is primarily hindered by skin-induced attenuation of the propagating signal bit carrier along with its stochastic misalignment-induced fading. Thus, by considering a typical modulated retroreflective (MRR) TOW system with spatial diversity and optimal combining (OC) for signal reception in this work, we focus, for the first time in the MRR TOW literature, on the stochastic nature of generalized pointing errors with non-zero boresight (NZB). Specifically, under these circumstances, novel analytical mathematical expressions were derived for the total average bit error rate (BER) of various system configurations. Their results revealed significant outage performance enhancements when spatial diversity was utilized. Moreover, taking into consideration the total transdermal pathloss along with the effects of stochastic NZB pointing errors, the critical average signal-to-noise ratio (SNR) metric was evaluated for typical power spectral-density values.
By employing the time-dependent power flow equation (TD PFE), we examine the bandwidth in multimode W-type (double clad) microstructured plastic optical fibers (mPOFs) with a PMMA (polymethyl methacrylate) solid core for parametrically varying depth and width of the intermediate layer (IL) (inner cladding). The investigated W-type mPOF's bandwidth was calculated for various configurations of the air-holes in the inner cladding and varied launch excitations. We obtained that results for smaller inner cladding air-holes at longer fiber lengths exhibit greater bandwidth. On the other hand, for shorter fibers, the launch beam which only excites guided modes, has no effect on the bandwidth due to the air-hole size. The W-type mPOF with a narrower inner cladding has a greater bandwidth. The bandwidth is also larger for a narrow launch that only excites guided modes as opposed to a wider launch that excites both guided and leaky modes. Therefore, the bandwidth increases as the width of the IL is reduced, or by decreasing the diameter of air-holes in the IL, or by exciting only guided modes. W-type mPOFs can be more easily tailored for a specific use in optical fiber sensors and communications because to their programmable properties.
In recent years, the THz frequency band (0.3 THz–10 THz) has attracted an increasing research interest for the realization of emerging high-speed wireless communication links. Nevertheless, the propagation of THz signals through the atmospheric channel is primarily subjected to signal attenuation due to free space path loss (FSPL), water vapor, adverse weather conditions along with atmospheric turbulence-induced and misalignment-induced scintillations. Therefore, in this work, a multi-hop line-of-sight THz system that utilizes serially connected decode-and-forward relays is proposed to extend the total THz coverage distance under the presence of fog, rain or clear weather conditions, as well as water vapor, atmospheric turbulence, non-zero boresight pointing errors and FSPL. Under these circumstances, an average bit error rate (ABER) analysis is performed. In this context, novel closed-form ABER expressions are derived. Their analytical results demonstrate the influence of each of the above limiting factors as well as their joint impact on the ABER performance. Finally, the feasibility of extending the total THz link distance through multi-hop relaying configurations is also evaluated.
This paper investigates wavelength dependence of equilibrium mode distribution and steady-state distribution in W-type (double-clad) microstructured polymer optical fibers (mPOFs) with a solid core for parametrically varied refractive index and width of the intermediate layer (IL) (inner cladding) by solving the time-independent power flow equation (TI PFE). In the case of wider IL, independent of wavelength, the lengths for establishing the equilibrium mode distribution and steady-state distribution are larger. We have demonstrated that the wavelength has no effect on these lengths for IL’s width that is larger. These lengths drop in a wavelength-dependent manner as the IL's width decreases. Equilibrium mode distribution and steady-state distribution occur at shorter optical fiber lengths as the depth of the IL diminishes, which is due to the similarly declining number of leaky modes. The smaller the depth of the IL, the shorter the fiber length is required for completion of the coupling process. These programmable characteristics allow double-clad W-type mPOFs to be easier customized for a particular use in optical fiber sensors and communications at various wavelengths.
We propose a space division multiplexing (SDM) in a newly constructed multicore polymer-clad silica fiber (PCSF) with seven cores arrayed in a hexagonal array, each carrying a centrally launched beam. This enables a higher SDM capacity at longer fiber lengths in the proposed seven-core PCSF if compared with previously proposed angular division multiplexing (ADM) in single-core (SC) PCSF. As a result, the SDM is not limited to short fiber lengths in the proposed seven-core PCSF, as it is in the case of the ADM channels due to mode coupling in the SC PCSF. In addition, the time-independent power flow equation (TI PFE) is used to analyze the effect of the width of the launch beam distribution on the equilibrium mode distribution (EMD) and steady state distribution (SSD) in each of the seven cores of the investigated PCSF. The width of the launch beam distribution has a considerable impact on the fiber length at which the EMD and SSD are attained, according to our numerical results. Thus, by decreasing the full width at half maximum (FWHM) of the launch beam distribution from 20 to 2°, the length at which EMD is established increases from Lc = 1020 to 1250 m, and the length at which SSD is attained increases from zs = 2650 to 3250 m. A narrow launch beam distribution leads to higher bandwidth at small and intermediate fiber lengths. On the other hand, at shorter fiber lengths, a wider launch beam distribution induces a bandwidth change from 1/z proportional to 1/z1/2 proportional curve, e.g., a slower bandwidth reduction. When building a multicore optical fiber transmission system for SDM, such characterization of multicore PCSFs under various launch conditions should be taken into account.
Inverted perovskite solar cells (PSCs) have attracted increasing attention in recent years owing to their low-temperature fabrication process. However, they suffer from a limited number of electron transport materials available with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) to be the most widely studied based on its appropriate energy levels and high electron mobility. The low relative permittivity and aggregation tendency upon illumination of PCBM, however, compromises the solar cell efficiency whereas its modest hydrophobicity negatively impacts on the device stability. Alternative electron transport materials with desired properties and appropriate degree of hydrophobicity are thus desirable for further developments in inverted PSCs. Herein, we synthesize a triethyleneglycol C60 mono-adduct derivative (termed as EPF03) and test it as a novel electron transport material to replace PCBM in inverted PSCs based on a quadruple cation (RbCsMAFA) perovskite. We also compare this derivative with two novel fullerenes decorated with two (EPF01) or one dodecyl (EPF02) long side chains. The latter two fail to perform efficiently in inverted PSCs whereas the former enabled a power conversion efficiency of 18.43%, which represents a 9% improvement compared to the reference device using PCBM (17.21%). The enhanced performance mainly stems from improved electron extraction and reduced recombination enabled by the insertion of the large relative permittivity amongst other properties of EPF03. Furthermore, our results indicate that triethylene glycol side chains can also passivate perovskite trap states, suppress ion migration and enhance photostability and long-term stability of EPF03 based perovskite solar cells.
The recent increasing requirements for high speed and robust wireless communication links have given rise to the development of THz wireless communication systems (0.3-10 THz), as a very promising alternative to their MMW and FSO counterparts. However, the performance of THz wireless systems is subject to free space path loss, signal attenuation due to humid air, atmospheric turbulence and pointing errors between transmitter and receiver terminals. Thus, in this work we first introduce the stochastic impact of generalized PEs with nonzero boresight in the THz area, while atmospheric turbulence is modeled through the suitable gamma gamma distribution. Considering also the deterministic impact of free space path loss and signal attenuation, an outage probability analysis is performed for a typical line of sight THz link. In this context, novel closed form expressions are derived. Their analytical results demonstrate the joint influence of these effects on THz signal transmissions, while simulation results validate the accuracy of our analysis.
Over the last years the rapidly growing demands for higher wireless data transfer rates have recently motivated the research community to focus on the exploitation of higher frequency bands, such as the infrared (IR) frequency band and even more recently the terahertz (THz) frequency band (0.3-10 THz) which bridges the gap between millimeter wave (MMW) and IR frequency ranges. Nevertheless, the development of both free space optical (FSO) and THz communication links depends strongly on the randomly varying characteristics of their atmospheric channels along with the stochastic misalignment between transmitter and receiver terminals. Thus, in this work we first introduce Gamma distribution atmospheric turbulence (AT) model in the THz area. In this context, an outage performance comparison between a line of sight (LOS) THz link and a FSO link in terms of outage probability (OP) metric is provided for different AT and stochastic pointing error (PE) conditions. Additionally, the OP for the THz link due to free space path loss (FSPL) and atmospheric attenuation along with stochastic PEs is evaluated. Novel closed-form OP expressions are derived, while proper analytical results reveal and quantify the impact of the above factors. Simulation results are further included to validate our analytical results.
FSO is one of the most widespread, low-cost, wireless, optical communicational technologies with sufficiently high throughput, transmission reliability, and high-level security. Nevertheless, many fading effects act on the optical pulses used, during their propagation, causing performance degradation. In this work, group velocity dispersion and time jitter, modeled by the truncated normal distribution, are jointly investigated analytically and numerically. The availability of the studied model is expressed in terms of outage probability, while its reliability is given in terms of its average bit error rate, through the derived novel mathematical expressions. To the best of authors’ knowledge, this is the first time that the outage and the BER performance are estimated analytically, through specific approximations, taking into account the abovementioned physical effects. Furthermore, using the obtained mathematical forms, the corresponding numerical results are presented by assuming typical parameter values for realistic FSO links.
A series of titanium dioxide (TiO2) modified with 3-aminopropyltriethoxysilane (APTES) was prepared by high-temperature calcination in an argon atmosphere in the temperature range from 800 to 1,000°C. The properties of the obtained samples were compared with those of pure TiO2 annealed under the same conditions. Examining electron paramagnetic resonance (EPR) parameters at room temperature for APTES–TiO2 showed an intense resonance line from defects related to conducting electrons with geff from 2.0028 to 2.0026 and 1.9052 for temperatures 800, 900, and 1,000°C, respectively, while for pure calcined TiO2, these ERP lines were not observed. With the increase in the calcination temperature to 900°C for APTES–TiO2 samples, the EPR increases linearly. This has been combined with a relatively high anatase content and small crystallites. The EPR line intensity at RT calculated for 1 g of sample showed an almost linear relationship with the photoactivity in removing ORANGE II dyes from water.
We analyze the effect of launch beam distribution on space-division multiplexing (SDM) performance in multimode multicore silica optical fibers (MM MC SOF) with seven cores. The time-independent power flow equation (TI PFE) is used to explore the effect of the width of the distribution of the Gaussian launch beam on power flow in each of the seven cores. We show that the optical fiber length at which the equilibrium mode distribution (EMD) and steady-state distribution (SSD) are obtained is greatly influenced by the width of the Gaussian launch beam distribution. We further show that when the width of the Gaussian launch beam distribution widens, the optical fiber length at which angular division multiplexing (ADM) in each of the seven cores can be realized with minimal crosstalk between neighboring angular optical channels decreases. We demonstrate that, for increasing the capacity of an optical fiber transmission system, an SDM system with two- and three-channel ADM and multicore optical fiber multiplexing can be implemented with the proposed seven-core MM MC SOF at optical fiber lengths up to ≈1 km (2 ADM channels × 7 cores) and ≈200 m (3 ADM channels × 7 cores), respectively. Such characterization of MM MC SOFs under various launch conditions is important for building a multicore optical fiber SDM transmission system.
We propose a space division multiplexing (SDM) in a newly constructed multicore polymer-clad silica fiber (PCSF) with seven cores arrayed in a hexagonal array, each carrying a centrally launched beam. This enables a higher SDM capacity at longer fiber lengths in the proposed seven-core PCSF if compared with previously proposed angular division multiplexing (ADM) in single-core (SC) PCSF. As a result, the SDM is not limited to short fiber lengths in the proposed seven-core PCSF, as it is in the case of the ADM channels due to mode coupling in the SC PCSF. In addition, the time-independent power flow equation (TI PFE) is used to analyze the effect of the width of the launch beam distribution on the equilibrium mode distribution (EMD) and steady state distribution (SSD) in each of the seven cores of the investigated PCSF. The width of the launch beam distribution has a considerable impact on the fiber length at which the EMD and SSD are attained, according to our numerical results. Thus, by decreasing the full width at half maximum (FWHM) of the launch beam distribution from 20 to 2°, the length at which EMD is established increases from Lc = 1020 to 1250 m, and the length at which SSD is attained increases from zs = 2650 to 3250 m. A narrow launch beam distribution leads to higher bandwidth at small and intermediate fiber lengths. On the other hand, at shorter fiber lengths, a wider launch beam distribution induces a bandwidth change from 1/z proportional to 1/z1/2 proportional curve, e.g., a slower bandwidth reduction. When building a multicore optical fiber transmission system for SDM, such characterization of multicore PCSFs under various launch conditions should be taken into account.
Nanocomposites based on nanocrystalline titania modified with graphene-related materials (reduced and oxidized form of graphene) showed the existence of magnetic agglomerates. All parameters of magnetic resonance spectra strongly depended on the materials’ modification processes. The reduction of graphene oxide significantly increased the number of magnetic moments, which caused crucial changes in the reorientation and relaxation processes. At room temperature, a wide resonance line dominated for all nanocomposites studied and in some cases, a narrow resonance line derived from the conduction electrons. Some nanocomposites (samples of titania modified with graphene oxide, prepared with the addition of water or butan-1-ol) showed a single domain magnetic (ferromagnetic) arrangement, and others (samples of titania modified with reduced graphene oxide) exhibited magnetic anisotropy. In addition, the spectra of EPR from free radicals were observed for all samples at the temperature of 4 K. The magnetic resonance imaging methods enable the capturing of even a small number of localized magnetic moments, which significantly affects the physicochemical properties of the materials.
The tremendous development of both optical wireless communications (OWC) and implantable medical devices (IMDs) has recently enabled the establishment of transdermal optical wireless (TOW) links that utilize light waves to transfer information inside the living body to the outside world and conversely. Indeed, numerous emerging medical applications such as cortical recording and telemetry with cochlear implants require extremely high data rates along with low power consumption that only this new technology could accommodate. Thus, in this paper, a typical TOW link is investigated in terms of outage capacity which is a critical performance metric that has so far not been evaluated for such wireless systems in the open technical literature. More precisely, an outage capacity analysis is performed considering both skin‐induced attenuation and stochastic spatial jitter, i.e., pointing error effects. Analytical expressions and results for the outage capacity are derived for a variety of skin channel conditions along with varying stochastic pointing errors which demonstrate the feasibility of this cross‐field cooperation. Lastly, the corresponding simulation outcomes further validate our suggestions.