Cloaks, Absorbers and Energy Harvesters by Constantinos Valagiannopoulos
Physical Review Applied, 2026
A minimalistic setup comprising a single homogeneous material, in bulk or two-dimensional form, i... more A minimalistic setup comprising a single homogeneous material, in bulk or two-dimensional form, is examined with respect to its ability to absorb visible light. The performance is assessed in terms of the Rozanov bound expressing the bandwidth-thickness balance, the average absorption indicating the broadband efficiency, and the minimal absorbance characterizing the self-camouflage ability. The materials used include ordinary lossy dielectric media, substances with ideal dispersion yielding perfect absorption across the whole visible spectrum, as well as their causal counterparts. The reported results show the optimal operating conditions of such a simple design, which may serve as a key component in absorptionbased applications ranging from energy harvesting and medical imaging to photodetection and radar target concealment.
ACS Photonics, 2026
The development of silicon-compatible, high-performance infrared photodetectors is crucial for ad... more The development of silicon-compatible, high-performance infrared photodetectors is crucial for advancing thermal imaging, security, and communication systems. While germanium is a promising near-infrared material, its behavior in nanostructured forms with silicon heterojunctions reveals complex photophysics. This work demonstrates a germanium nanowire photodetector grown on a silicon-oninsulator (SOI) platform that exhibits a striking, tunable coexistence of both positive photoconductivity (PPC) and negative photoconductivity (NPC). We show that the dominant photoresponse can be switched by the wavelength of incident light: NPC dominates at visible wavelengths (e.g., 532 nm), while PPC prevails in the near-infrared (e.g., 1310 nm). Through systematic experiments and FDTD and TCAD simulations, we elucidate that this phenomenon arises from the interplay of light absorption in the different layers of the heterostructure. At short wavelengths, strong absorption in the underlying Si layer forward-biases the heterojunction, injecting carriers that quench the Ge channel conductance (NPC). At long wavelengths, absorption is confined to the Ge layer, resulting in conventional PPC. Negative photoconductivity was consistently observed over the temperature range from 78 to 298 K. Notably, the maximum responsivity of the nanowire increased from -56.7 A/W at room temperature to -1421.5 A/W at 78 K. This is attributed to the suppression of surface recombination velocity, increasing the minority carrier lifetime by 2 orders of magnitude. The -3 dB bandwidth is 2.9 kHz under 532 nm light and 3.9 kHz under 1310 nm light. The minimum noise equivalent power is determined to be 5.3 × 10 -14 W/Hz 0.5 , corresponding to a specific detectivity of 4.0 × 10 9 Jones at room temperature. Furthermore, we demonstrate that the crossover wavelength is intensitydependent and that the photocurrent follows an established logarithmic model for nanowire photoconductors. This work provides a controllable model system for studying NPC and presents a novel device architecture with tunable, multifunctional photoresponse for advanced optoelectronic applications.
IEEE Access, 2025
Wireless power transfer between an external electromagnetic (EM) source and implanted dipoles int... more Wireless power transfer between an external electromagnetic (EM) source and implanted dipoles into human body can be greatly assisted by employing a matching part atop of the skin. Two different approaches are proposed: one incorporating lossless layers that minimize the reflections and another dealing with active metasurfaces that boost the transmissivity. Obviously, the performance is higher in the latter scenario since gain media are involved; however, even with passive layers, significant enhancement in the coupling between the two radiators is achieved. These coupling scores exhibit substantial robustness under the condition of misaligned dipoles, a feature that renders them suitable to operate under experimental imperfections. The proposed concept has been experimentally demonstrated at bluetooth frequencies leading to strong enhancement of power transmission, even when the radiation patterns of the two antennas are heavily misaligned. Accordingly, the reported designs can be utilized in biomedical setups that call for fast and reliable wireless communication: from healthcare monitoring and biosensing to real-time drug delivery and bioimaging. INDEX TERMS Biomedical communication, Electromagnetic coupling, Implants, Metasurfaces, Wireless power transfer.
Physical Review B, 2024
It has long been argued that the performance of organic bulk-heterojunction solar cells criticall... more It has long been argued that the performance of organic bulk-heterojunction solar cells critically depends on the morphology of the active layer, a mixture of donor and acceptor materials in which the charge generation from sunlight occurs. In this work, optical homogenization principles are utilized to model the structure of the common active layer PM6:Y6. By systematically modifying the size and shape of Y6 acceptor inclusions within the morphology, we explore how these changes influence the refractive index and extinction coefficient of the resulting effective medium. A synergy of transfer matrix optical simulations with the Hecht equation is used to evaluate the effect of the inclusions features on the external quantum efficiency spectrum. The reported findings can be particularly helpful in the fabrication of similar classes of organic solar cells by indicating which aspects of active layer mixture may undermine the expected performance of the photovoltaic cells.
IEEE Transactions on Antennas and Propagation, 2023
Photonic nanorod heterodimers, comprising different materials, are numerically optimized to suppr... more Photonic nanorod heterodimers, comprising different materials, are numerically optimized to suppress the radiation from a sandwiched visible-light source. The employed media are taken from a long list that includes most common dielectrics, semiconductors and metals; accordingly, the reported shielding performances constitute limits for the considered simple setup when working at the regarded wavelength bands. Therefore, the reported optimal dimers, operated at various colors of the visible spectrum, provide the design process with additional degrees of freedom; they can be critical in satisfying additional constraints when fabricating photonic devices such as optical switches, sensors and nanolasers.
IEEE Transactions on Microwave Theory and Techniques, 2023
Thin tubes can trap the electromagnetic (EM) energy, emitted wirelessly from a near-field source.... more Thin tubes can trap the electromagnetic (EM) energy, emitted wirelessly from a near-field source. The optimal dimensions of these hollow cylinders are determined for an extensive range of complex permittivities characterizing their material and the reported power enhancement is found practically independent of the antenna location. The spatial distribution of the signal reveals the reshaping in the paths of incoming rays and unveils the nature of the developed resonances in the vicinity of the photonic tubes, for both wave polarizations. The concept is experimentally demonstrated at the UHF band with the use of dense dielectric claddings; enhancement up to two orders of magnitude is recorded. The presented results constitute limits in terms of the EM energy accumulation for a simple configuration and, thus, can be utilized in various wireless power transfer (WPT) applications.
Physical Review Applied, 2023
Application of various electron-transport layers (ETLs) with desired morphology and dimensions is... more Application of various electron-transport layers (ETLs) with desired morphology and dimensions is a key factor to be considered for the structural designs of perovskite solar cells (PSCs) in order to obtain enhanced optical and electrical properties. The metal-oxide ETLs composed of one-dimensional nanorod arrays are one of the most frequently used nanostructures for electron transporting in PSCs. In this work, computer-simulation methods are employed to investigate the optoelectrical properties of PSCs with planar and nanorod-based SnO 2 ETLs. A two-dimensional optical model is used to determine the optical responses of devices and the standard drift-diffusion model is utilized to simulate their behavior and performance. The aspect ratio and the density of SnO 2 nanorods are varied to examine their effect on the optical and the electrical properties of resulting devices and the findings are contrasted with those of planar devices. It is found that under optimum conditions, PSCs with thin planar SnO 2 ETLs, which are referred to as reference planar devices, outperform PSCs with nanorod-based SnO 2 ETLs even if the light-harvesting properties of the latter are improved with the implementation of structured ETLs. The underlying reasons for this phenomenon are analyzed and rationalized through a detailed analysis and comparison of electric field, current density, and carrier recombination distributions in PSCs. The findings of this work can be used as a theoretical guide for designing and fabrication of high-performance planar and structured PSCs using SnO 2 ETLs.
Journal of Applied Physics, 2022
Collecting energy from the time-dependent electrodynamic fields into a vacuum volume can be subst... more Collecting energy from the time-dependent electrodynamic fields into a vacuum volume can be substantially assisted by spherical metasurfaces wrapped around the respective domains. The combinations of sizes and surface admittances that lead to maximal concentration of power into the cells are identified and enhancement by several orders of magnitude has been recorded. The spatial distribution of the signals in the vicinity of these meta-bubbles unveils the nature of the sustained resonances and demonstrates their ability to wirelessly couple with other equipment nearby. The reported optimal regimes of operation can be employed in integrated photonic systems involving an extensive range of applications from energy storage and optical memory to electromagnetic filtering and power accumulation.
IEEE Vehicular Technology Magazine, 2021
T he 6G era comes with the challenge of offering highly energy-efficient and autonomous communica... more T he 6G era comes with the challenge of offering highly energy-efficient and autonomous communications securely. In this direction, we report energy efficiency (EE), energy harvesting (EH), and secure performance by employing power-collecting metasurface-coated devices capable of supporting ultralowpower (ULP) transmissions. In contrast to what are called reconfigurable intelligent surfaces (RISs), where the reflected signal can be combined at the receiver by being treated as transmitted from a relay, the proposed metasurface claddings can be deployed at either the transmitter or receiver or at both. The passive metasurface-coated devices can achieve ultrahigh EE and EH in addition to signal detection, combined with an enhanced secrecy rate at the legitimate user or, if used from the other side of the link, spying capabilities of eavesdroppers under ULP transmission. To quantify their efficiency, we provide a holistic model for the utilization of the metasurface shells. Building upon this model, we present preliminary results that reveal the unprecedented superiority of the proposed concept compared to the RIS paradigm. Additionally , we enumerate the main advantages of the new concept and define its role in the 6G era. Finally, possible research directions are discussed.
Physical Review Research, 2020
Simple plasmonic multilayers are found to perfectly trap electromagnetic waves around specific in... more Simple plasmonic multilayers are found to perfectly trap electromagnetic waves around specific incidence directions, regardless of the sample thickness. Such a counterintuitive feature is acquired via increasing unboundedly the effective path length of the ray into the film with proper tilt of the optical axis. The obtained optimal setups support wide-band operation while being substantially robust with respect to fabrication defects. Accordingly, the reported designs may exceptionally serve a range of objectives, from photovoltaic conversion and sensing to light modulation and detection, as parts of photonic devices requiring high efficiency combined with compact packaging.
IEEE Transactions on Antennas and Propagation, 2018
Conventional wireless power transfer systems consist of a microwave power generator and a microwa... more Conventional wireless power transfer systems consist of a microwave power generator and a microwave power receiver separated by some distance. To realize efficient power transfer, the system is typically brought to resonance, and the coupled-antenna mode is optimized to reduce radiation into the surrounding space. In this scheme, any modification of the receiver position or its electromagnetic properties results in the necessity of dynamically tuning the whole system to restore the resonant matching condition. It implies poor robustness to the receiver location and load impedance, as well as additional energy consumption in the control network. In this paper, we introduce a new paradigm for wireless power delivery based on which the whole system, including transmitter and receiver and the space in between, forms a unified microwave power generator. In our proposed scenario, the load itself becomes a part of the generator. Microwave oscillations are created directly at the receiver location, eliminating the need for dynamical tuning of the system within the range of the self-oscillation regime. As a proof-of-concept demonstration, we experimentally show that, such self-oscillating wireless power delivery systems can provide robust operation against changes in the environment or in the location of the load. The proposed concept has relevant connections with the recent interest in parity-time symmetric systems, in which balanced loss and gain distributions enable unusual electromagnetic responses.
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Cloaks, Absorbers and Energy Harvesters by Constantinos Valagiannopoulos