Showing papers by "Odysseas Tsilipakos published in 2022"

Multiwideband Terahertz Communications Via Tunable Graphene-Based Metasurfaces in 6G Networks: Graphene Enables Ultimate Multiwideband THz Wavefront Control

Abstract: The next generation of wireless networks is expected to tap into the terahertz (THz) band (0.1–10 THz) 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 (non-LoS) 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 article, 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 multiwideband graphene-based RISs are discussed, paving the way to the concurrent control of multiple THz bands in the context of 6G networks.

Recent advances in strongly-resonant and gradient all-dielectric metasurfaces

Abstract: All-dielectric metasurfaces have been intensively researched as a low-loss, flat-optics platform for the advanced manipulation of electromagnetic wave propagation. Among the numerous metasurface-enabled functionalities, particular focus has been recently placed...

XR-RF Imaging Enabled by Software-Defined Metasurfaces and Machine Learning: Foundational Vision, Technologies and Challenges

Abstract: In this work, we present a new approach to Extended Reality (XR), denoted as iCOPYWAVES, which seeks to offer naturally low-latency operation and cost effectiveness, overcoming the critical scalability issues faced by existing solutions. Specifically, iCOPYWAVES is enabled by emerging PWEs, a recently proposed technology in wireless communications. Empowered by intelligent metasurfaces, PWEs transform the wave propagation phenomenon into a software-defined process. To this end, we leverage PWEs to: i) create, and then ii) selectively copy the scattered RF wavefront of an object from one location in space to another, where a machine learning module, accelerated by FPGAs, translates it to visual input for an XR headset using PWE-driven, RF imaging principles (XR-RF). This makes an XR system whose operation is bounded in the physical-layer and, hence, has the prospects for minimal end-to-end latency. For the case of large distances, RF-to-fiber/fiber-to-RF is employed to provide intermediate connectivity. The paper provides a tutorial on the iCOPYWAVES system architecture and workflow. Finally, a proof-of-concept implementation via simulations is provided, demonstrating the reconstruction of challenging objects in iCOPYWAVES-produced computer graphics

Dynamic routing through saturable absorption in graphene photonic resonators: Impact of carrier diffusion and finite relaxation time

Abstract: We assess the continuous wave and dynamic routing performance of a compact silicon-on-insulator disk resonator overlaid with a graphene monolayer at telecommunication wavelengths. Switching action is enabled by saturable absorption in graphene, controlled by a pump wave of only a few milliwatts. Graphene saturable absorption is modeled through a carrier rate equation that incorporates both the finite relaxation time and diffusion of photo-generated carriers, providing a realistic account of carrier dynamics. The overall nonlinear response of the resonator is evaluated with a rigorous mathematical framework based on perturbation theory and temporal coupled-mode theory. We thoroughly investigate the effects of carrier diffusion and finite relaxation time, both separately and together. We also take into account nonlinear refraction via a Kerr effect term and quantify its impact on the overall response. In order to suppress the Kerr effect, we replace silicon with silicon-rich nitride, allowing for the individual contributions of the resonator core and graphene (of opposite sign) to exactly compensate each other. Our results contribute to the understanding of carrier dynamics and their impact on the performance of practical graphene-based switching components.

Multi-functional metasurface architecture for amplitude, polarization and wavefront control

Abstract: Metasurfaces (MSs) have been utilized to manipulate different properties of electromagnetic waves. By combining local control over the wave amplitude, phase, and polarization into a single tunable structure, a multi-functional and reconfigurable metasurface can be realized, capable of full control over incident radiation. Here, we experimentally validate a multi-functional metasurface architecture for the microwave regime, where in principle variable loads are connected behind the backplane to reconfigurably shape the complex surface impedance. As a proof-of-concept step, we fabricate several metasurface instances with static loads in different configura-tions (surface mount capacitors and resistors of different values in different connection topologies) to validate the approach and showcase the different achievable functionalities. Specifically, we show perfect absorption for oblique incidence (both polarizations), broadband linear polarization conversion, and beam splitting, demon-strating control over the amplitude, polarization state, and wavefront, respectively. Measurements are performed in the 4-18 GHz range inside an anechoic chamber and show good agreement with theoretically-anticipated results. Our results clearly demonstrate the practical potential of the proposed architecture for reconfigurable electromagnetic wave manipulation.

Multimode non-Hermitian framework for third harmonic generation in nonlinear photonic systems comprising two-dimensional materials

Abstract: 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.

Enhanced Refractive Index Sensing with Direction-Selective Three-Dimensional Infrared Metamaterials

Abstract: Three-dimensional (3D) geometries ensure large surface area interaction of photonic devices with the surrounding medium. Here, we demonstrate that metamaterials composed of 3D metallic Split Cube Resonator (SCR) elements assembled in various arrangements enable resonantly enhanced refractive index sensing. The proper arrangement of the SCR elements results in almost perfect narrow-band direction-selective absorption, which is highly sensitive to the refractive index of the surrounding environment. The experimental sensitivity achieved exceeds 5.5 μm per Refractive Index Unit (RIU) with theoretical predictions showing that this can reach 11 μm/RIU. The structures allow easy fabrication via direct laser writing and highly selective electroless metal plating. Thus, the proposed metadevices are ideal candidates for assisting cost-effective infrared polarization-resolved sensing and direction-selective spectral filtering in and out of the infrared atmospheric transparency window.

Light resonators imprinted onto optical fibers using multi-photon lithography

Abstract: The optical fiber sensing field is in continuous seek of new processing methods, light localization structures and transduction mechanisms for developing devices with novel functionalities and/or improved performance, while targeting existing or emerging application fields. Herein, we are reviewing work performed on the imprinting of optical resonators, onto optical fibers using multi-photon, three-dimensional lithography. Two major resonating cavity designs are presented: hollow Fabry-Perot resonators imprinted onto the endface of optical fibers, and micro-ring resonators attached onto micronic diameter optical fiber tapers; both types of devices operate at the 1.5μm spectral band. Results are presented on the design, spectral characterization and simulation of those hybrid type of photonic devices, while their sensing capabilities are exemplified in the tracing of organic solvent vapors, which upon case can reach sub-ppm detectivity levels.

Whispering Gallery Mode Resonances in Thermally Poled Borosilicate Glass Hetero-Fibers

Abstract: Whispering gallery mode (WGM) resonances in radially, thermally poled glass hetero-fibers, are investigated for the first time. Upon radially oriented thermal poling, both TE and TM polarized responses undergoing a spectral “cleaning” process, namely higher-order radial modes are being suppressed, or even fully annihilated, compared to the spectral behavior of the pristine fibers. Second-harmonic generation (SHG) microscopy, Energy Dispersive X-Ray Spectroscopy (EDX) and micro-Raman (μ-Raman) measurements were conducted in order to reveal the spatial profile of the structural and optical changes introduced to the thermally poled hetero-fibers, and correlate them with the WGM spectral measurements. The specific selective, mode cleaning behavior is attributed to refractive index changes generated in proximity to the fiber's circumference, which are associated with radially-symmetric ionic re-distribution. The thermally poled hetero-fiber WGM cavities have been rigorously simulated using the finite element method, the calculated modal eigenstates and transmission spectra, further confirm the specific mode selection mechanism, introduced by the azimuthally symmetric thermal, poling process.