Omar Siddiqui

This paper presents the design, implementation, and performance evaluation of a Ring Dipolar Array (RDA) structure optimized for efficient, frequency-selective RF energy harvesting. The proposed RDA utilizes engineered electromagnetic interactions within its array arrangement to enhance energy capture at targeted frequency bands while preserving a compact design. The resonant characteristics of the ring dipolar elements of the system are utilized to achieve enhanced frequency selectivity and improved RF-to-DC power conversion efficiency. The energy harvesting performance is systematically investigated under varying load conditions and different network configurations, including series and parallel connections, to determine the optimal operating parameters of the system. Both simulation and experimental validations confirm the effectiveness of the proposed structure. The RDA successfully harvests ambient RF energy at 2.6 GHz (S band), to achieve a practical microwave-to-DC conversion under different load conditions. Under series and parallel topologies, the conversion efficiency reaches 31% and 22%, respectively for input power of -4 dBm. The reconfiguration between series and parallel arrangements is entirely passive, involving no switches or control power, and relies on the intrinsic electrical connections of the array. The proposed design provides reconfigurable operation adaptable to varying load conditions, allowing flexible series–parallel arrangements to deliver high voltage or high current. These findings underscore the potential of the RDA as an efficient and practical approach to frequency-selective ambient RF energy harvesting in modern wireless power transfer applications.

We use dual spiral transmission line stubs to generate the highly sensitive Electromagnetically Induced Transparency (EIT) at radio frequencies. The phenomenon is the radio analog of the quantum interference that cancels photon absorption at a particular frequency, thereby creating a transparency window in an absorbing medium. The single stub configuration produces a Lorentz resonance which we exploit to characterize dielectric constant of concrete samples. The dual stub configuration introduces the interference at a nearby frequency (which can be tuned by stub length) creating a sharp EIT resonance. With the spiral stub shape, the resonance phenomenon can be observed at VHF frequencies allowing deep energy penetration and also increasing the effective surface area for wave-matter interaction without increasing its physical cross sectional . We practically demonstrate non-destructive concrete analysis at 120 MHz frequency by characterizing pure, carbonized and acidified concrete samples based on their dielectric constant.We apply EIT-based differential sensing, facilitating precise comparison between intact and affected concrete samples, which enables the detection of subtle variations associated with common concrete degradation factors.

An innovative method is introduced for energy harvesting by combining a bowtie frequency selective rectenna surface with a traditional photovoltaic solar panel. The design of the rectenna antenna surface aims to optimize the geometry of the bowtie antennas for enhanced ambient RF energy absorption and rectification efficiency. Several integration techniques are explored to efficiently merge the RF and solar energy harvesting components into a unified system, considering factors such as spatial arrangement and DC electrical connections in both series and parallel. Tested efficiency analysis under various loads demonstrates the feasibility and effectiveness of the hybrid system as it captures the RF and solar energy and converts it into usable electrical energy. The RF-solar hybrid energy harvesting system can be employed in environment settings where energy sources exist intermittently.

We propose a quad-stub microwave differential sensor which exhibits a single electromagnetic transparency window and Fano resonance when operated under symmetrical resonant conditions. When the resonance balance is slightly disturbed by introducing small dielectric variations on one of the stub pairs, a second transparency window appears in the stop-band whose amplitude corresponds to the extent of the differential imbalance. The narrow sensing window falls within the frequency range of 2.25 – 2.5 GHz. The differential sensing can be exploited in highly sensitive chemical sensing problems such as to differentiate quality of biofuel samples. Experimentally, the concept has been demonstrated by detecting ethanol concentration in aqueous solutions with varying molality. The high sensitivity is shown by resolving a 2.5 % v/v ethanol-aqueous concentration. The compact design facilitates measurements for sample volumes as small as approximately 20 $\mu$ L. A good agreement is demonstrated between simulation and experimental results. Overall, the proposed sensor possesses a simple and compact geometry that can also be scaled to operate at optical frequencies. Furthermore, a metal slot waveguide structure is also designed to demonstrate the differential sensing mechanism adaptability to THz frequencies. The ease of operation and its capability to detect minor differences samples make it a competent candidate for high-sensitivity sensing applications in biofuel detection and biomedical sensing.

In this letter, we propose metasurface absorbers for energy harvesting that work on the principles of destructive interference and phase cancellation. The metasurface is designed by arranging four L-shaped resonant unit cells in a fourfold symmetrical geometry leading to a cross configuration. At resonance frequency, such a geometrical configuration results in destructive interference of the electromagnetic waves emanating from diagonal elements. When used in a reflection mode as a metasurface absorber, the reciprocity theorem necessitates the flow of helicoidal resonant currents that vectorially cancel each other to make the overall metasurface nonradiative that, therefore, produces a null in the backscattering direction. A 90% absorption in simulation is achieved without explicit consideration for a free-space impedance matching or using dissipative substrate. The absorbed RF energy is harvested at 2.25 GHz to practically demonstrate the conversion efficiency of over 55% from microwave to dc.

Although Polarization Modulation offers a high spectral efficiency, it has been not exploited much in modern communication systems because of the involvement of high switching speeds and complexity of associated hardware. We propose a dynamically controllable graphene metasurface capable to switch the polarization state of incident THz waves in real-time. The metasurface is designed by patterning two-dimensional graphene sheets with Fano-resonant chiral unit cells. Since the reflectance spectrum is characterized with co and cross polarized fields that bear asymmetric Fano line-shapes, several combinations of linear and elliptical polarization states are observed in a narrowband spectrum. At a fixed frequency, the electrostatic tunability property of graphene allows to switch between different polarization states by varying the chemical potential between 500 and 700 meV. The polarization state modulation with the proposed chiral graphene based metasurface is applied to implement a quaternary modulated digital communication system. Full-wave simulation results show the strong possibility of polarization modulation in THz communication systems. Graphene metasurfaces on account of the high electron mobility and discrete Fermi levels offer high switching speeds and seamless integration with digital systems.

The water content of plants plays a significant role in the safety and maintenance of plants' health. Low moisture content in leaves can damage the plants, thus early detection of water content requires very high sensitivity sensors. In this paper, we propose a plant moisture sensor based on Lorentz dispersion to detect plant's moisture level of different concentrations. The high sensitivity of Lorentz sensor comes from its narrowband resonant characteristics manifested by rapid slope change of its phase spectrum, a phenomenon known as anomalous dispersion. This unique signature in its phase spectrum adds a second degree of freedom in sensing and detection, as well as removes ambiguity in the identification of the resonant frequency. We practically measured the moisture content of different samples of leaves and analyzed quick distinctive deviations in the phase and magnitude responses of the sensor based upon the water content in the leaves. The proposed plant moisture sensor is supposed to be integrated with an internet network to inform the farmers and agriculturalists about plants' health. In this way, the concept of Internet of Trees (IoTr) can be employed. Consequently, timely watering of the plants, as well as appropriate usage of water will be guaranteed.

Lorentz Dispersion regime is a narrowband spectral region characterized by a sharp resonance dip and an anomalous phase response. Due to its high dispersive nature, the Lorentz resonance is very sensitive to the changes in the physical dimensions of the resonators and the external perturbations approaching the resonant cavity. The proposed Lorentz structure consists of an open microstrip stub which contains a small gap. A metallic perturbation brought closer to the gap changes the electric field distribution leading to an effective increase in the stub’s electrical size and a resonance shift. An experimental demonstration in the frequency range 1 to 1.5 GHz show that the relative resonance spectral shift correspond to the proximity of the metallic object.

Forward Transmission Matrix Analysis (FTM) is a powerful analysis technique which employs a systematic approach to solve complicated microwave circuits. In this paper, we apply FTM to analyze a Lorentz sensor and demonstrate its application for detection of high permittivity liquid samples. The permittivity is calibrated using the resonance signature of the phase spectrum rather than the traditional use of amplitude spectrum. We anticipate that the proposed detector can be potentially applied in biomedical applications such as blood glucose estimation.