arXiv subjects

Sotiris Droulias

Publications and source records attributed to Sotiris Droulias.

At least 19 recordsLinked to original sources

arXiv2026

Unified Evaluation Methodology for AI-Native Integrated Sensing and Communication

Integrated Sensing and Communication (ISAC) couples radio sensing, data transmission, and control actions within a single closed-loop system. When Artificial Intelligence (AI)-driven policies adapt sensing and communication online across a variety of sensing tasks and objectives, end-to-end performance is shaped not only by waveform and channel conditions but also by inference latency, uncertainty, environmental dynamics, and hardware non-idealities, leading to fundamental trade-offs between sensing accuracy, communication reliability, and resource overhead. This manuscript presents a unified system architecture and evaluation methodology for AI-native ISAC, defined as ISAC in which learning-based agents adapt sensing, communication, and actuation policies online under uncertainty. We formalize the design space of closed-loop ISAC, propose a three-stage validation pipeline from bounds and feasibility analysis, through high-fidelity digital-twin simulation, to preliminary over-the-air validation, and provide a minimal reporting checklist that links technical Key Performance Indicators (KPIs) (e.g., data rate, SINR, target detection, parameter estimation, track quality, localization error, outage, latency, overhead, and energy per decision) to application-level Key Value Indicators (KVIs) (e.g., availability and mission effectiveness). Two representative instantiations, specifically Unmanned Aerial Vehicle (UAV)-based outdoor and Reconfigurable Intelligent Surface (RIS)-enabled indoor coverage extensions, are used to illustrate how to structure reproducible baselines and comparable evidence across heterogeneous deployments, helping bridge the gap between theoretical ISAC gains and deployment-ready performance claims.

cs.NI↗

arXiv2026

Bending beams behind corners: mechanisms, challenges and capabilities for wireless connectivity

Curved beams, that is, beams that are able to propagate on nonlinear trajectories, are often envisioned as ideal candidates for blockage avoidance in future wireless connectivity. Owing to this unique feature, they are considered as ideal beams for bending around and behind corners to reach users beyond the line-of-sight (LoS), thus offering unprecedented connectivity. In this work, we explain the various mechanisms of beam propagation beyond the LoS, and we demonstrate that beam bending behind corners results from an interplay between wavefront engineering and edge diffraction, with distinct characteristics that depend on the extent of blockage and the beam formation efficiency. We identify three distinct regimes of operation, namely the unblocked, the partially blocked, and the fully blocked regime, and we show that beam bending through wavefront engineering dominates in the unblocked and partially blocked regimes, while edge diffraction dominates in the fully blocked regime; as a result, curved beams cannot really bend behind the corner, unless there is some LoS between the user and the transmitter. Based on our findings, we compare curved beams with focused beams, and we demonstrate that they perform similarly in the partially blocked regime, while focused beams outperform curved beams in the unblocked and fully blocked regimes.

eess.SP↗

arXiv2026

Enhancing the physical layer security with bending beams

Wavefront engineering for applications in near-field wireless connectivity is gradually becoming common ground. In this landscape, beams that propagate on bent paths are ideal candidates for dynamic blockage avoidance and suppression of potential eavesdropping. In this work we study the physical layer security offered by bending beams, and we demonstrate their capabilities for line-of-sight and non-line-of-sight eavesdropping. We analyze the dependencies between the possible locations of an eavesdropper and the design parameters of such beams, and we introduce metrics to assess their physical layer security performance. Our results demonstrate their superiority with respect to beams generated with conventional beam-forming.

eess.SP↗

arXiv2025

Ultrafast All-optical control of Multiple Light Degrees of Freedom through Mode-mixing in a Graphene Nanoribbon Metamaterial

The evolution of optical technologies necessitates advanced solutions for selective and dynamic manipulation of light's degrees of freedom, including amplitude, phase, polarization, wavelength, and angular momentum. Metamaterials can offer such control through the interplay between the intrinsic material and geometrical properties of nanostructures or extrinsically through excitation and detection symmetry breaking, leading to customizable performance. However, achieving dynamic control over multiple light degrees of freedom remains a challenge. To address existing limitations, we present a novel dual-stack metamaterial design capable of broadband ultrafast control over amplitude, phase, polarization, spin angular momentum, and handedness of light mediated by two independently controlled nanoribbon layers that enable flexible and selective mode-mixing in both reflection and transmission. Through a combination of a thermal response model and Finite-Difference Time-Domain simulations, we investigate graphene as a suitable material for the metamaterial design, leveraging the intrinsic optical properties of graphene and its tunable conductivity through electrostatic gating and ultrafast optical excitation, achieving selective control over multiple light degrees of freedom at ultrafast timescales. This selective ultrafast mode-mixing significantly advances the capabilities of high-speed photonic systems, paving the way for compact, high data-rate optical technologies essential for future applications.

physics.optics↗

arXiv2024

Bending beams for 6G near-field communications

Future wireless connectivity is envisioned to accommodate functionalities far beyond broadband data transmission over point-to-point direct links, enabling novel scenarios, such as communication behind blockers and around corners, and innovative concepts, such as situational awareness, localization and joint communications and sensing. In this landscape, beams that are able to propagate on bent paths are ideal candidates for dynamic blockage avoidance, interference management in selected regions, and user connectivity on curved trajectories. In this work, we study beam shaping for applications in near-field wireless connectivity. We explain the underlying mechanism of beam bending and we present the design principles for tailoring the curvature of the propagation trajectory. We discuss design aspects for generation of such beams with large arrays and analyze the impact of several parameters on their performance, including the beam's footprint shape, the aperture size, the inter-element spacing, the sub-array selection of active elements, the available phase levels of the array elements and the operating frequency. We introduce the concept of near-field virtual routing (NFVR) and we demonstrate that such beams are able to address challenges of high frequency communications, such as dynamic routing, blockage avoidance and energy-efficiency, more efficiently than conventional beamforming.

eess.SP↗

arXiv2024

Hierarchical Localization for Integrated Sensing and Communication

Localization is expected to play a significant role in future wireless networks as positioning and situational awareness, navigation and tracking, are integral parts of 6G usage scenarios. Nevertheless, in many cases localization requires extra equipment, which interferes with communications systems, while also requiring additional resources. On the other hand, high frequency and highly directional communications offer a new framework of improved resolution capabilities in the angular and range domains. The implementation of integrated sensing and communications is being explored to unify the sensing and communications systems and promote a communicate-to-sense approach. To this end, a localization algorithm is presented that utilizes beam-forming and the emerging beam-focusing technique, to estimate the location of the receiver. The algorithm can be implemented with large antenna arrays, and large intelligent surfaces. The performance of the algorithm for static and mobile users is evaluated through Monte-Carlo simulations. The results are presented with the empirical CDF for both static and mobile users, and the probability of successful estimation for static users.

eess.SP↗

arXiv2024

Reconfigurable Intelligent Surfaces as Spatial Filters

The design of Reconfigurable Intelligent Surfaces (RISs) is typically based on treating the RIS as an infinitely large surface that steers incident plane waves toward the desired direction. In practical implementations, however, the RIS has finite size and the incident wave is a beam of finite $k$-content, rather than a plane wave of $δ$-like $k$-content. To understand the implications of the finite extent of both the RIS and the incident beam, here we treat the RIS as a spatial filter, the transfer function of which is determined by both the prescribed RIS operation and the shape of the RIS boundary. Following this approach, we study how the RIS transforms the incident $k$-content and we demonstrate how, by engineering the RIS shape, size, and response, it is possible to shape beams with nontrivial $k$-content to suppress unwanted interference, while concentrating the reflected power to desired directions. We also demonstrate how our framework, when applied in the context of near-field communications, provides the necessary insights into how the wavefront of the beam is tailored to enable focusing, propagation with invariant profile, and bending, beyond conventional beamforming.

eess.SP↗

arXiv2024

Near-field orthogonality and cosine beams for near-field space division multiple access in 6G communications and beyond

Spatial division multiple access (SDMA), a powerful method routinely applied in multi-user multiple-input multiple-output (MIMO) communications, relies on the angular orthogonality of beams in the far field, to distinguish multiple users at different angles. Yet, with the gradual shift of wireless connectivity to the near-field of large radiating apertures, the applicability of classical SDMA becomes questionable. Therefore, to enable near-field multiple access, it is necessary to design beams that have the desired orthogonality in the near-field. In this work, we propose the concept of near-field space division multiple access (NF-SDMA), to enable SDMA in the near-field. We demonstrate analytically that the orthogonality of beams is preserved at any location of the receiver, from the near-field to the far-field of the transmitter. By judicious design, we select the family of cosine beams and we prove that they satisfy the orthogonality condition, offering a multitude of communication modes in the near-field. We demonstrate how the correlation of beams generated with uniform linear arrays (ULAs) is extended to uniform planar arrays (UPAs) in a straightforward and insightful manner. To test our analytical findings, we propagate the designed beams numerically, and we measure their orthogonality both at the transmitter and the receiver. We verify that the orthogonality of the proposed beams is successfully retrieved at a receiver that resides in the near-field of the transmitter, and is also robust to displacements of the receiver. Based on our findings, we propose codebook designs for NF-SDMA that are applicable for receivers with many elements and even with single antennas.

eess.SP↗

arXiv2024

Vortex inverted pin beams: Mitigation of scintillations in strong atmospheric turbulence

We recently introduced a new class of optical beams with a Bessel-like transverse profile and increasing beam width during propagation, akin to an "inverted pin". Owing to their specially engineered distribution, these beams have shown remarkable performance in atmospheric turbulence. Specifically, inverted pin beams were found to have reduced scintillation index as compared to collimated or focused Gaussian beams as well as other types of pin beams especially in moderate to strong turbulence. In this work, we demonstrate that inverted pin beams carrying orbital angular momentum can further suppress intensity scintillations in moderate to strong irradiance fluctuation conditions. Our results can be useful in improving the performance and link availability of free-space optical communication systems.

physics.optics↗

arXiv2024

Perceptive, Resilient, and Efficient Networks assisted by Reconfigurable Intelligent Surfaces

Wireless communications are nowadays shifting to higher operation frequencies with the aim to meet the ever-increasing demand for bandwidth. While reconfigurable intelligent surfaces (RISs) are usually envisioned to restore the line-of-sight of blocked links and to efficiently counteract the increased pathloss, their functionalities can extend far beyond these basic operations. Owing to their large surface and the multitude of scatterers, RISs can be exploited to perform advanced wavefront engineering, essentially transforming the incident beam into a non-trivial reflected beam that is able to address the challenges of high frequencies more efficiently than conventional beam-forming. In this paper it is demonstrated how advanced wavefront engineering with RISs enables beam profiles that are able to focus, bend and self-heal, thus offering functionalities beyond the current state-of-the-art. Their potential as enablers of perceptive, resilient, and efficient networks is discussed, and a localization technique based on a hybrid beam-forming/beam-focusing scheme is demonstrated.

eess.SP↗

arXiv2023

Inverted pin beams for robust long-range propagation through atmospheric turbulence

We introduce a new class of optical beams, which feature a spatial profile akin to an ``inverted pin''. In particular, we asymptotically find that close to the axis the transverse amplitude profile of such beams takes the form of a Bessel function with width that gradually increases during propagation. We examine numerically the behavior of such inverted pin beams in turbulent environments as measured via the scintillation index, and show that they outperform Gaussian beams (collimated and focused) as well as Bessel beams and regular pin beams, which are all optimized, especially in the moderate and strong fluctuation regimes.

physics.optics↗

arXiv2023

Reconfigurable Intelligent Surface: MIMO or radiating sheet?

A Reconfigurable Intelligent Surface (RIS) redirects and possibly modifies the properties of incident waves, with the aim to restore non-line-of-sight communication links. Composed of elementary scatterers, the RIS has been so far treated as a collection of point scatterers with properties similar to antennas in an equivalent massive MIMO communication link. Despite the discrete nature of the RIS, current design approaches often treat the RIS as a continuous radiating surface, which is subsequently discretized. Here, we investigate the connection between the two approaches in an attempt to bridge the two different perspectives. We analytically find the factor that renders the two approaches equivalent and we demonstrate our findings with examples of RIS elements modeled as antennas with commonly used radiation patterns and properties consistent with antenna theory. We analyze the equivalence between the two theoretical approaches with respect to design aspects of the RIS elements, such as gain, directivity and coupling between elements, with the aim to provide insight into the observed discrepancies, the understanding of which is crucial for assessing the RIS efficiency.

physics.app-ph↗

arXiv2022

Analytical characterization of RIS-aided terahertz links in the presence of beam misalignment

The terahertz (THz) frequency band has recently attracted considerable attention in wireless communications as potential candidate for providing the necessary high bandwidth for demanding applications. With increasing frequency, however, the communication link becomes more vulnerable to blockage and pathloss increases. While both effects can be mitigated with the judicious utilization of directional beams and Reconfigurable Intelligent Surfaces (RISs), high directivity could potentially increase the probability of undesired misalignment between the beam that is steered by the RIS, and the user. It is therefore crucial to characterize and understand the stochastic behavior of misalignment in RIS-aided THz links. In this work, beam misalignment in RIS-aided links is studied theoretically and analytical models are derived, the validity of which is verified through numerical calculations. It is demonstrated that there is a distinction in the stochastic behavior of misalignment between pointing errors that occur on the steering plane or normally to the steering plane, with direct consequences on the link robustness on misalignment. The analytical models capture the impact of misalignment under these qualitatively different conditions and provide the necessary tools for assessing the stochastic RIS performance with respect to crucial link parameters, such as the transmitter's beam width, the transmitter-RIS distance, the RIS-receiver distance, and the steering angle of the RIS.

eess.SP↗

arXiv2022

Reconfigurable chirality with achiral excitonic materials in the strong-coupling regime

We introduce and theoretically analyze the concept of manipulating optical chirality via strong coupling of the optical modes of chiral nanostructures with excitonic transitions in molecular layers or semiconductors. With chirality being omnipresent in chemistry and biomedicine, and highly desirable for technological applications related to efficient light manipulation, the design of nanophotonic architectures that sense the handedness of molecules or generate the desired light polarization in an externally controllable manner is of major interdisciplinary importance. Here we propose that such capabilities can be provided by the mode splitting resulting from polaritonic hybridization. Starting with an object with well-known chiroptical response -- here, for a proof of concept, a chiral sphere -- we show that strong coupling with a nearby excitonic material generates two distinct frequency regions that retain the object's chirality density and handedness, which manifest most clearly through anticrossings in circular-dichroism or differential-scattering dispersion diagrams. These windows can be controlled by the intrinsic properties of the excitonic layer and the strength of the interaction, enabling thus the post-fabrication manipulation of optical chirality. Our findings are further verified via simulations of the circular dichroism of a realistic chiral architecture, namely a helical assembly of plasmonic nanospheres embedded in a resonant matrix.

physics.optics↗

arXiv2022

Chirality sensing employing PT-symmetric and other resonant gain-loss optical systems

Molecular chirality detection and enantiomer discrimination are very important issues for many areas of science and technology, prompting intensive investigations via optical methods. However, these methods are hindered by the intrinsically weak nature of chiro-optical signals. Here, we investigate and demonstrate the potential of gain materials and of combined gain-loss media to enhance these signals. Specifically, we show that the proper combination of a thin chiral layer with a gain-loss bilayer can lead to large enhancements of both the circular dichroism (CD) response and the dissymmetry factor, g, compared to the chiral layer alone. The most pronounced enhancements are obtained in the case of a Parity-Time (PT) symmetric gain-loss bilayer, while deviations from the exact PT symmetry lead to only moderate deterioration of the CD and g response, demonstrating also the possibility of tuning the system response by tuning the gain layer properties. In the case of PT-symmetric gain-loss bilayers we found that the largest CD enhancement is obtained at the system lasing threshold, while the g-enhancements at the anisotropic transmission resonances of the systems. Our results clearly demonstrate the potential of gain materials in chirality detection. Moreover, our gain-involving approach can be applied in conjunction with most of the nanophotonics/nanostructures-based approaches that have been already proposed for chirality sensing, further enhancing the performance/output of both approaches.

physics.optics↗

arXiv2021

Chiral sensing with achiral anisotropic metasurfaces

Recently, we proposed a metasurface design for chiral sensing that (i) results in enhanced chiroptical signals by more than two orders of magnitude for ultrathin, subwavelength, chiral samples over a uniform and accessible area, (ii) allows for complete measurements of the total chirality (magnitude and sign of both its real and imaginary part), and (iii) offers the possibility for a crucial signal reversal (excitation with reversed polarization) that enables chirality measurements in an absolute manner, i.e., without the need for sample removal. Our design is based on the anisotropic response of the metasurface, rather than the superchirality of the generated near-fields, as in most contemporary nanophotonic-based chiral sensing approaches. Here, we derive analytically, and verify numerically, simple formulas that provide insight to the sensing mechanism and explain how anisotropic metasurfaces, in general, offer additional degrees of freedom with respect to their isotropic counterparts. We provide a detailed discussion of the key functionalities and benefits of our proposed design and we demonstrate practical measurement schemes for the unambiguous determination of an unknown chirality. Last, we provide the design principles towards broadband operation - from near-infrared to near-ultraviolet frequencies - opening the way for highly sensitive nanoscale chiroptical spectroscopy.

physics.optics↗

arXiv2020

Chiral sensing with achiral isotropic metasurfaces

Metasurfaces, the two-dimensional analogues of metamaterials, are ideal platforms for sensing molecular chirality at the nanoscale, e.g. of inclusions of natural optically active molecules, as they offer large accessible areas (they are essentially surfaces) and can accommodate the necessary strong resonances for coupling the probing radiation with the chiral inclusions. Here, we examine theoretically achiral isotropic metasurfaces, and we treat them as polarizable surfaces that support resonant electric and magnetic currents, which are coupled via the chiral inclusions. We derive analytically, and verify numerically, expressions that provide insight to the enhancement mechanism of the magneto-electric coupling and explain why circular dichroism signals (difference in absorption between left- and right- circularly polarized waves) can arise from both the real and the imaginary part of the chirality parameter $κ$. Our analysis demonstrates distinct chiroptical signals where the contributions from both the real and imaginary part of $κ$ can be independently observed and, based on such measurements, we propose a scheme for the unambiguous determination of an unknown chirality.

physics.app-ph↗

arXiv2020

Scattering properties of PT-symmetric chiral metamaterials

The combination of gain and loss in optical systems that respect parity-time (PT)-symmetry has pointed recently to a variety of novel optical phenomena and possibilities. Many of them can be realized by combining the PT-symmetry concepts with metamaterials. Here we investigate the case of chiral metamaterials, showing that combination of chiral metamaterials with PT-symmetric gain-loss enables a very rich variety of phenomena and functionalities. Examining a simple one-dimensional chiral PT-symmetric system, we show that with normally incident waves the PT-symmetric and the chirality-related characteristics can be tuned independently and superimposed almost at will. On the other hand, under oblique incidence, chirality affects all the PT-related characteristics, leading also to novel and uncommon wave propagation features, such as asymmetric transmission and asymmetric optical activity and ellipticity. All these features are highly controllable both by chirality and by the angle of incidence, making PT-symmetric chiral metamaterials valuable in a large range of polarization-control-seeking applications.

physics.optics↗