arXiv subjects

Odysseas Tsilipakos

Publications and source records attributed to Odysseas Tsilipakos.

At least 19 recordsLinked to original sources

arXiv2025

Efficient Lasing in MoS$_2$/WSe$_2$-Based Metasurfaces Enabled by Quasi-Dark Magnetic Dipole Resonance

The novel combination of a strongly-resonant optical metasurface with the MoS$_2$/WSe$_2$ hetero-bilayer is proposed for efficient free-space lasing enabled by the enhanced coupling between the optical and matter (exciton) states. The metasurface comprises silicon-rich nitride meta-atoms periodically arrayed in a subdiffractive lattice and overlaid with MoS$_2$/WSe$_2$, which provides optically-pumped gain around $1130~\mathrm{nm}$. Light emission is enabled by exploiting a quasi-bound state in the continuum in the form of a perturbed vertical magnetic dipole resonance. Following a meticulous design process guided by full-wave simulations and multipole expansion analysis, an ultralow lasing threshold of $\sim 6~\mathrm{kW/cm^2}$ is achieved. Moreover, the thermal stability of the lasing structure is examined through heat transfer simulations; stable operation with pump power densities up to a few MW/cm$^2$ (three orders of magnitude above the threshold) is predicted. These results demonstrate that MoS$_2$/WSe$_2$-based metasurface lasers can exhibit robust operation, paving the way for highly-performing ultrathin light-emitting surfaces. The lasing response is rigorously assessed through a highly-efficient temporal coupled-mode theory framework, verified by time-domain FEM simulations showing excellent agreement. Thus, an efficient and accurate approach to design and study metasurface lasers with arbitrary geometries and surface or bulk gain media is introduced, exhibiting significant advantages over cumbersome full-wave simulations.

physics.optics↗

arXiv2025

Graphene-Gold THz Metasurfaces with Tailored Resonant Structure for Enhanced Nonlinear Response

Graphene's exceptional nonlinear optical properties combined with resonant photonic structures offer a promising pathway for efficient nonlinear applications at terahertz (THz) frequencies. In this work, we propose and demonstrate a fabrication-friendly hybrid nonlinear metasurface composed of gold patches integrated with uniform graphene, circumventing the need for complex graphene patterning. The structure supports strong localized resonances that enhance nonlinear interactions. By exploiting resonant enhancement at both the fundamental and third harmonic frequencies, we predict via simulations third-harmonic generation efficiencies as high as -15 dB (3.2%) under continuous-wave excitation at modest intensities (0.1 MW/cm$^2$). The metasurface is fabricated via electron-beam lithography and experimentally characterized using THz time-domain spectroscopy. Under pulsed excitation, we experimentally observe pronounced nonlinear frequency shifts up to 0.5~THz (12.5% fractional change), driven by self-phase modulation, consistent with simulation results. These findings highlight the potential of tailored graphene-based metasurfaces for efficient nonlinear THz photonic devices.

physics.optics↗

arXiv2025

Can 2D materials enable passively $Q$-switched lasers in the nanoscale?

Achieving compact on-chip pulsed lasers with attractive performance metrics and compatibility with the silicon photonics platform is an important, yet elusive, goal in contemporary nanophotonics. Here, the fundamental question of whether 2D materials can be utilized as both gain and saturable absorption media to enable compact integrated passively Q-switched nanophotonic lasers is posed and addressed by examining a broad range of 2D material families. The study is conducted by developing a temporal coupled-mode theory framework involving semi-classical rate equations that is capable of rigorously handling gain and saturable absorption by 2D materials, allowing to perform stability and bifurcation analysis covering broad parameter spaces. The range of pulse-train metrics (repetition rate, pulse width, peak power) that can be obtained via different 2D materials is thoroughly assessed. Our work illustrates that nanophotonic cavities enhanced with 2D materials can enable passive Q-switching with repetition rates ranging up to 50~GHz, short pulse duration down to few picoseconds, and peak power exceeding several milliwatts. Such attractive metrics, along with the ultrathin nature of 2D materials and the ability to electrically tune their properties, demonstrate the potential of the proposed platform for compact and flexible integrated laser sources.

physics.optics↗

arXiv2024

Cactus-like Metamaterial Structures for Electromagnetically Induced Transparency at THz frequencies

THz metamaterials present unique opportunities for next generation technologies and applications, as they can fill the ``THz gap'' originating from the weak response of natural materials in this regime, providing a variety of novel or advanced electromagnetic wave control components and systems. Here, we propose a novel metamaterial design, made of three-dimensional, metallic, "cactus like" meta-atoms, showing electromagnetically induced transparency (EIT) and enhanced refractive index sensing performance at low THz frequencies. Following a detailed theoretical analysis, the structure is realized experimentally using multi-photon polymerization and electroless silver plating. The experimental characterization results obtained through THz time domain spectroscopy validate the corresponding numerical data, verifying the high potential of the proposed structure in slow light and sensing applications.

physics.optics↗

arXiv2024

Temporal coupled-mode theory in nonlinear resonant photonics: From basic principles to contemporary systems with 2D materials, dispersion, loss, and gain

Temporal coupled-mode theory (CMT) is an acclaimed and widely used theoretical framework for modeling the continuous wave (CW) response and temporal dynamics of any integrated or free-space photonic resonant structure. It was initially employed to understand how energy is coupled into and out of a cavity and how it is exchanged between different resonant modes. In the 30 years that followed its establishment, CMT has been expanded to describe a broad range of nonlinear interactions as well (self- and cross-phase modulation, saturable absorption, frequency generation, gain, etc.). In this tutorial, we thoroughly present the basic principles and the evolution of CMT throughout the years, showcasing its immense capabilities for the analysis and design of linear and nonlinear resonant photonic systems. Importantly, we focus on examples of modern, open nanophotonic resonators incorporating contemporary bulk or sheet (2D) materials that may be lossy and dispersive. For each linear/nonlinear effect under study we follow a meticulous, step-by-step approach, starting from an accurate model of the physical phenomenon and proceeding to its introduction in the CMT framework all the way to the efficient solution of the resulting system of equations. Our work highlights the merits of CMT as an efficient, accurate, and versatile theoretical tool. We envision that it can serve both as an introductory reference for any reader, as well as a comprehensive handbook on how to incorporate a broad range of linear and nonlinear effects in the CMT framework.

physics.optics↗

arXiv2023

Multiresonant metasurfaces for arbitrarily-broadband pulse chirping and dispersion compensation

We show that ultrathin metasurfaces with a specific multiresonant response can enable simultaneously arbitrarily-strong and arbitrarily-broadband dispersion compensation, pulse (de-)chirping and compression or broadening. This breakthrough overcomes the fundamental limitations of both conventional non-resonant approaches (bulky) and modern singly-resonant metasurfaces (narrowband) for quadratic phase manipulations of electromagnetic signals. The required non-uniform trains of resonances in the electric and magnetic sheet conductivities that completely control phase delay, group delay, and chirp, are rigorously derived and the limitations imposed by fundamental physical constraints are thoroughly discussed. Subsequently, a practical, truncated approximation by finite sequences of physically-realizable linear resonances is constructed and the associated error is quantified. By appropriate spectral ordering of the resonances, operation can be achieved either in transmission or reflection mode, enabling full space coverage. The proposed concept is not limited to dispersion compensation, but introduces a generic and powerful ultrathin platform for the spatio-temporal control of broadband real-world signals with a myriad of applications in modern optics, microwave photonics, radar and communication systems.

physics.optics↗

arXiv2023

Impact of plasmonic modes on the formation of self-organised nano-patterns in thin films

Formation of nanoscale laser-induced periodic surface structures on thin metal films (of the size of the optical penetration depth) is a yet unexplored area that is expected to open new routes for laser patterning and a wealth of exciting applications in optics, photonics, and sensing. In contrast to the common belief that excitation of Surface Plasmon Polaritons (SPPs) on the air/metal interface plays the dominant role in the features of the induced topographies, in this work, we demonstrate that the excitation of coupled SPPs in both air/metal and metal/substrate interfaces, along with other parameters such as the thickness of the material, the photon energy, and the substrate refractive index, dictate the spatial modulation of the absorbed energy. A detailed theoretical analysis of the excited plasmonic waves and a multiscale modelling of laser-induced physical phenomena manifests that depending on the laser conditions and thickness of the irradiated solid, topographies with periodic features of diverse sizes (ranging from $λ_L/3$ to $λ_L$, where $λ_L$ stands for the laser wavelength) and different orientation can be realized. The capability to control and tune the characteristics of the produced structures on thin films is expected to enable novel surface patterning approaches.

physics.optics↗

arXiv2022

Multimode non-Hermitian framework for third harmonic generation in nonlinear photonic systems comprising 2D materials

Resonant structures in modern nanophotonics are non-Hermitian (leaky and lossy), and support quasinormal modes. Moreover, contemporary cavities frequently include 2D materials to exploit and resonantly enhance their nonlinear properties or provide tunability. Such materials add further modeling complexity due to their infinitesimally thin nature and strong dispersion. Here, a formalism for efficiently analyzing third harmonic generation (THG) in nanoparticles and metasurfaces incorporating 2D materials is proposed. It is based on numerically calculating the quasinormal modes in the nanostructure, it is general, and does not make any prior assumptions regarding the number of resonances involved in the conversion process, in contrast to conventional coupled-mode theory approaches in the literature. The capabilities of the framework are showcased via two selected examples: a single scatterer and a periodic metasurface incorporating graphene for its high third-order nonlinearity. In both cases, excellent agreement with full-wave nonlinear simulations is obtained. The proposed framework may constitute an invaluable tool for gaining physical insight into the frequency generation process in nano-optic structures and providing guidelines for achieving drastically enhanced THG efficiency.

physics.comp-ph↗

arXiv2022

Multi-Wideband Terahertz Communications via Tunable Graphene-based Metasurfaces in 6G Networks

The next generation of wireless networks is expected to tap into the terahertz (0.1--10 THz) band to satisfy the extreme latency and bandwidth density requirements of future applications. However, the development of systems in this band is challenging as THz waves confront severe spreading and penetration losses, as well as molecular absorption, which leads to strong line-of-sight requirements through highly directive antennas. Recently, reconfigurable intelligent surfaces (RISs) have been proposed to address issues derived from non-line-of-sight propagation, among other impairments, by redirecting the incident wave toward the receiver and implementing virtual-line-of-sight communications. However, the benefits provided by a RIS may be lost if the network operates at multiple bands. In this position paper, the suitability of the RIS paradigm in indoor THz scenarios for 6G is assessed grounded on the analysis of a tunable graphene-based RIS that can operate in multiple wideband transparency windows. A possible implementation of such a RIS is provided and numerically evaluated at 0.65/0.85/1.05 THz separately, demonstrating that beam steering and other relevant functionalities are realizable with excellent performance. Finally, the challenges associated with the design and fabrication of multi-wideband graphene-based RISs are discussed, paving the way to the concurrent control of multiple THz bands in the context of 6G networks.

physics.app-ph↗

arXiv2021

Experimental demonstration of ultrathin broken-symmetry metasurfaces with controllably sharp resonant response

Symmetry-protected resonances can be made to couple with free space by introducing a small degree of geometric asymmetry, leading to controllably-sharp spectral response. Here, we experimentally demonstrate a broken-symmetry metasurface for the technologically important low millimeter wave spectrum. The proposed metasurface is fabricated on an ultrathin polyimide substrate, resulting in a low loss and flexible structure. Measurements inside an anechoic chamber experimentally verify the theoretically predicted sharp spectral features corresponding to quality factors of several hundreds. The demonstrated sharp response is also observed with the complementary structure which responds to the orthogonal linear polarization (Babinet's principle). The designed metasurfaces can be exploited in diverse applications favoured by a controllably-sharp spectral response, e.g., filtering, sensing, switching, nonlinear applications, in either reflection or transmission mode operation. More generally, the demonstrated fabrication process provides a generic platform for low-cost, large-scale engineering of metasurfaces with minimal substrate-induced effects.

physics.app-ph↗

arXiv2020

A multi-functional reconfigurable metasurface: Electromagnetic design accounting for fabrication aspects

In this paper we present the theoretical considerations and the design evolution of a proof-of-concept reconfigurable metasurface, primarily used as a tunable microwave absorber, but also as a wavefront manipulation and polarization conversion device in reflection. We outline the design evolution and all considerations taken into account, from the selection of patch shape, unit cell size, and substrate, to the topology of the structure that realizes the desired tunability. The presented design conforms to fabrication restrictions and is co-designed to work with an integrated circuit chip for providing tunable complex loads to the metasurface, using a commercially available semiconductor process. The proposed structure can perform multiple tunable functionalities by appropriately biasing the integrated circuit: Perfect absorption for a wide range of incidence angles of both linear polarization states, accommodating a spectral range in the vicinity of 5 GHz, with potential also for wavefront control, exemplified via anomalous reflection and polarization conversion. The end vision is for such a design to be scalable and deployable as a practical HyperSurface, i.e., an intelligent multi-functional metasurface capable of concurrent reconfigurable functionalities: absorption, beam steering, polarization conversion, wavefront shaping, holography, and sensing.

physics.app-ph↗

arXiv2020

Scalability Analysis of Programmable Metasurfaces for Beam Steering

Programmable metasurfaces have garnered significant attention as they confer unprecedented control over the electromagnetic response of any surface. Such feature has given rise to novel design paradigms such as Software-Defined Metamaterials (SDM) and Reconfigurable Intelligent Surfaces (RIS) with multiple groundbreaking applications. However, the development of programmable metasurfaces tailored to the particularities of a potentially broad application pool becomes a daunting task because the design space becomes remarkably large. This paper aims to ease the design process by proposing a methodology that, through a semi-analytical model of the metasurface response, allows to derive performance scaling trends as functions of a representative set of design variables. Although the methodology is amenable to any electromagnetic functionality, this paper explores its use for the case of beam steering at 26 GHz for 5G applications. Conventional beam steering metrics are evaluated as functions of the unit cell size, number of unit cell states, and metasurface size for different incidence and reflection angles. It is shown that metasurfaces 5$λ\times$5$λ$ or larger with unit cells of $λ/3$ and four unit cell states ensure good performance overall. Further, it is demonstrated that performance degrades significantly for angles larger than $θ> 60^o$ and that, to combat this, extra effort is needed in the development of the unit cell. These performance trends, when combined with power and cost models, will pave the way to optimal metasurface dimensioning.

physics.app-ph↗

arXiv2020

Exploration of Intercell Wireless Millimeter-Wave Communication in the Landscape of Intelligent Metasurfaces

Software-defined metasurfaces are electromagnetically ultra-thin, artificial components that can provide engineered and externally controllable functionalities. The control over these functionalities is enabled by the metasurface tunability, which is implemented by embedded electronic circuits that modify locally the surface resistance and reactance. Integrating controllers within the metasurface cells, able to intercommunicate and adaptively reconfigure it, thus imparting a desired electromagnetic operation, opens the path towards the creation of an artificially intelligent (AI) fabric where each unit cell can have its own sensing, programmable computing, and actuation facilities. In this work we take a crucial step towards bringing the AI metasurface technology to emerging applications, in particular exploring the wireless mm-wave intercell communication capabilities in a software-defined HyperSurface designed for operation is the microwave regime. We examine three different wireless communication channels within the landscape of the reflective metasurface: Firstly, in the layer where the control electronics of the HyperSurface lie, secondly inside a dedicated layer enclosed between two metallic plates, and, thirdly, inside the metasurface itself. For each case we examine the physical implementation of the mm-wave transponder nodes, we quantify communication channel metrics, and we identify complexity vs. performance trade-offs.

physics.app-ph↗

arXiv2019

The Software-Defined Metasurfaces Concept and Electromagnetic Aspects

We present the concept and electromagnetic aspects of HyperSurFaces (HSFs), artificial, ultrathin structures with software controlled electromagnetic properties. The HSFs key unit is the metasurface, a plane with designed subwavelength features whose electromagnetic response can be tuned via voltage-controlled continuously-tunable electrical elements that provide local control of the surface impedance and advanced functionalities, such as tunable perfect absorption or wavefront manipulation. A nanonetwork of controllers enables software defined HSFs control related to the emerging Internet of Things paradigm.

physics.app-ph↗

arXiv2019

Toroidal eigenmodes in all-dielectric metamolecules

We present a thorough investigation of the electromagnetic resonant modes supported by systems of polaritonic rods placed at the vertices of canonical polygons. The study is conducted with rigorous finite-element eigenvalue simulations. To provide physical insight, the simulations are complemented with coupled mode theory (the analog of LCAO in molecular and solid state physics) and a lumped wire model capturing the coupling-caused reorganizations of the currents in each rod. The systems of rods, which form all-dielectric cyclic metamolecules, are found to support the unconventional toroidal dipole mode, consisting of the magnetic dipole mode in each rod. Besides the toroidal modes, the spectrally adjacent collective modes are identified. The evolution of all resonant frequencies with rod separation is examined. They are found to oscillate about the single-rod magnetic dipole resonance, a feature attributed to the leaky nature of the constituent modes. Importantly, we observe that ensembles of an odd number of rods produce larger frequency separation between the toroidal mode and its neighbor than the ones with even number of rods. This increased spectral isolation, along with the low quality factor exhibited by the toroidal mode, favors the coupling of the commonly silent toroidal dipole to the outside world, rendering the proposed structure a prime candidate for controlling the observation of toroidal excitations and their interaction with the usually present electric dipole

physics.optics↗

arXiv2019

On the Calculation of the Quality Factor in Contemporary Photonic Resonant Structures

The correct numerical calculation of the resonance characteristics and, principally, the quality factor $Q$ of contemporary photonic and plasmonic resonant systems is of utmost importance, since $Q$ defines the bandwidth and affects nonlinear and spontaneous emission processes. Here, we comparatively assess the commonly used methods for calculating $Q$ using spectral simulations with commercially available, general-purpose software. We study the applicability range of these methods through judiciously selected examples covering different material systems and frequency regimes from the far-infrared to the visible. We care in highlighting the underlying physical and numerical reasons limiting the applicability of each one. Our findings demonstrate that in contemporary systems (plasmonics, 2D materials) $Q$ calculation is not trivial, mainly due to the physical complication of strong material dispersion and light leakage. Our work can act as a reference for the mindful and accurate calculation of the quality factor and can serve as a handbook for its evaluation in guided-wave and free-space photonic and plasmonic resonant systems.

physics.app-ph↗

arXiv2019

Dynamic Anapole in Metasurfaces made of Sculptured Cylinders

We present all-dielectric polaritonic metasurfaces consisting of properly sculptured cylinders to sustain the dynamic anapole, i.e. a non-radiating alternating current distribution. One way for the anapole to emerge, is by combining modes based on the first and the fourth Mie resonance of a cylinder made of high permittivity LiTaO$_3$ for operation in the low THz. The circular cross-section of each cylinder varies periodically along its length in a binary way, from small to large, while its overall circular symmetry has to be broken in order to remove parasitic magnetic modes. Small cross-sections are the main source of the \textit{electric dipole} Mie mode, while large cross-sections sustain the fourth, \textit{mixed toroidal dipole} Mie mode. With proper adjustment, the generated electric and toroidal moments interfere destructively producing a non-radiating source, the dynamic anapole, the existence of which is attested by a sharp dip in the reflection from the metasurface, due exclusively to electric and toroidal dipoles. Moreover, we show that, by breaking the circular symmetry of each cylinder, a substantial toroidal dipole emerges from the \textit{magnetic quadrupole} Mie mode, which in combination with the electric dipole also produces the dynamic anapole. The sensitivity of the anapole states to the material dissipation losses is examined leading to the conclusion that the proposed metasurfaces offer a scheme for realistically implementing the anapole.

physics.optics↗

arXiv2019

Joint Compressed Sensing and Manipulation of Wireless Emissions with Intelligent Surfaces

Programmable, intelligent surfaces can manipulate electromagnetic waves impinging upon them, producing arbitrarily shaped reflection, refraction and diffraction, to the benefit of wireless users. Moreover, in their recent form of HyperSurfaces, they have acquired inter-networking capabilities, enabling the Internet of Material Properties with immense potential in wireless communications. However, as with any system with inputs and outputs, accurate sensing of the impinging wave attributes is imperative for programming HyperSurfaces to obtain a required response. Related solutions include field nano-sensors embedded within HyperSurfaces to perform minute measurements over the area of the HyperSurface, as well as external sensing systems. The present work proposes a sensing system that can operate without such additional hardware. The novel scheme programs the HyperSurface to perform compressed sensing of the impinging wave via simple one-antenna power measurements. The HyperSurface can jointly be programmed for both wave sensing and wave manipulation duties at the same time. Evaluation via simulations validates the concept and highlight its promising potential.

cs.ET↗