Research

Similar to almost all types of devices, the fabrication of an electromagnetic wave device is expensive and time-consuming. Consequently, relying only on experimental characterization in developing an electromagnetic system is prohibitive and inefficient both timely and financial. To overcome this difficulty, accurate electromagnetic simulation is needed. 

Such simulation allows for optimization and compromises to be made using a computer rather than through expensive experimental cycles. The experimental characterization will be then needed only at the final development stage for fine-tuning and verification. For the electromagnetic simulation to satisfy this requirement, it should take into account all physical phenomena that occur in the natural system. All full-wave electromagnetic simulators solve Maxwell’s equations subjected to certain boundary conditions. These equations are coupled partial differential equations that can be solved directly in the differential form either in the frequency domain using Finite Element Method (FEM) or in the time domain using Finite-Difference Time-Domain (FDTD) technique.

Another approach to solving these equations is to rewrite them in the form of integral equations in terms of unknown surface/volume conduction/polarization current along the localized conductor/dielectric via the concept of Green's functions. These functions are the response of the unit sources embedded within laterally unbounded layered media. This integral equation can be solved using the Method of Moments (MoM). The computational electromagnetics research of the Department of Physics is devoted the most to the latter technique. Over several years, a quasi-3D solver for microwave/plasmonic circuits and microwave/plasmonic antennas, as well as a mode solver for microwave/plasmonic transmission lines, have been developed. These solvers are based on the integral equations formulation solved using the MoM.

The computational physics research of the Department of Physics is interested in different multiphysics modeling techniques to model the electrical, optical, and thermal characteristics of nanostructures and devices. Numerical modeling of Maxwell equations, electronic transport equations, Schrodinger equations, diffusion equations, wave, and heat equations are proposed using both finite element and finite difference methods. These modeling techniques allow for full integration with electronic CAD.

An efficient sensitivity analysis methodology for extracting the sensitivity information with respect to all the design parameters is proposed, which allows for efficient tolerance and yield analysis. This sensitivity information will also help in accelerating the optimization process. Various optimization techniques have also been proposed, including gradient-based techniques, convex optimizations, and topology optimization. The interest is also devoted to smart analytical and semi-analytical methods for modeling the nanophysics of different structures.

Different simulation and computational tools are used to help in understanding the structural, optical and electrical properties of the nanostructured materials developed in our laboratory at AUC. In fact, this step helps the design of optimized nanostructures for better functioning in the applications of interest. It saves too much time to focus only on the potential materials instead of the trial and error protocols followed by many researchers to identify the proper materials for their applications.

To this end, the Density Functional Theory (DFT) calculations are used to help in understanding the structural and electronic properties of the materials. Recently, the DFT has been coupled with the Finite Difference Time Domain (FDTD) codes to develop a way that enables the calculation of the optical, structural and electronic properties of the materials. Starting with screening the materials of interest to determine the most promising candidates, trials of synthesizing those materials are made in the laboratory in order to use them in devices. The next step involves the modeling and simulation of the fabricated devices to understand the physics involved as well as to help design more efficient materials. So far, COMSOL, FDTD and T-CAD have been used for device simulation.

Dynamical system methods are used to explore the general behavior of f(T) cosmology. The great advantage of this technique is that it reduces the system to a one-dimension autonomous system, which is easy to analyze using the phase space portraits. Phase space portraits are utilized to show that f(T) cosmology can describe the universe's evolution in agreement with observations. Nevertheless, f(T) cosmology can present a rich class of more exotic behaviors, such as the cosmological bounce and turnaround, the phantom-divide crossing, the Big Brake and the Big Crunch. Moreover, a new model of f(T) gravity is presented that can lead to a universe in agreement with observations, free of perturbative instabilities, and applying the Om(z) diagnostic test, we confirm that it is in agreement with the combination of SNIa, BAO and CMB data at 1σ confidence level.

The implications of the constant-roll condition on the inflationary era of a scalar field coupled to a teleparallel f(T) gravity are investigated in detail. The resulting cosmological equations constitute a reconstruction technique that enables us to find either the f(T) gravity, which corresponds to a given cosmological evolution or the Hubble rate of the cosmological evolution generated by a fixed f(T) gravity. The phase space of the constant-roll teleparallel gravity is analyzed in some detail the physical significance of the resulting fixed points and trajectories is discussed. The observational indices of a theory with given f(T) gravity are calculated, and all the implications of the constant-roll condition on these are discussed. The resulting theory is compatible with the current observational data for a wide range of values of the free parameters of the theory.

A comprehensive study of the electronic structure of the materials is crucial for understanding a wide variety of their physical properties, such as optical and electronic ones. Most properties of solids are directly related to the manner in which electrons occupy the states. It also helps in further progress in the development and potential use of materials for several applications. Angle-resolved photoemission spectroscopy (ARPES) has become one of the most powerful tools for the experimental study of the electronic structure of solids and solid-state surfaces. This can also be enhanced by the application of synchrotron radiation as a tunable photon source for photoemission as well as high angle and energy resolutions.

The family of transition metal dichalcogenides (TMDCs) is a class of important quasi-2-D layered materials. It has received much attention due to its interesting anisotropy properties originating from its remarkable 2D structure. These materials possess unique morphology as thin, flexible, high-quality dangling bond-free surfaces. ZrS_xSe_2-x are semiconductor compounds of TMDCs. Most recently, the ZrSe₂ has been demonstrated to replace the mainstream semiconductor for electronics and their applications, i.e., the silicon. This is motivated by their moderate band gaps and their possibility of forming native HfO₂ and ZrO₂ with desirable higher relative dielectric constant values “high-k”.

The comprehensive experimental study of the electronic band structure of TMDCs like ZrS_xSe_2-x single crystals was determined by means of ARPES. The obtained results from the ARPES can be compared to the band structure calculations performed using MBJ potential with spin-orbit coupling based on density functional theory (DFT).

The ultimate goal of experimental high-energy nuclear physics research is to understand the phase diagram of nuclear matter. Understanding the phase diagram at the atomic level has indeed led to almost all of the innovations and technology that currently exist. The underlying physics theory, which controls the atomic stability, and, therefore, all the baryonic matter around us, stems from the electromagnetic force, which is very well established and almost fully understood at the atomic scale. The nuclear force in its strong and weak versions are the forces that govern the stability of the nuclear matter. Such forces are not fully understood at the level of the bulk of nuclear matter. Due to the huge difference in the energy scales of atomic and nuclear physics (six orders of magnitude), it turns out the temperature at which the phase transitions took place for the nuclear matter is hotter than the core of the solar system’s sun. Such a type of experiment to explore the nuclear matter phase diagram, cannot be performed on a bench on a laboratory beaker, and hence the collider experiment.

The Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory is constructed to explore the nuclear matter phase diagram, to understand the physics of the first few microseconds of the early universe “Big Bang”, and to search for the origin of the proton’s spin from its constituents’ spin and their orbital angular momenta. The latter might lead to a novel technology of what so-called the spin current, similar to the normal electrical “charge” current. The Department of Physics is going to be involved in the field of experimental high-energy nuclear and particle physics research, the STAR experiment, at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory. At the STAR experiment, an extremely large data set has to be computationally analyzed with supercomputers in order to reveal its patterns and to model it, searching for the Quark Gluon Plasma phase-QGP (early Universe phase) and investigating the origin of the proton’s spin.

Such type of research explores the most fundamental aspects of nature and forms, along with cosmology and astrophysics, our current knowledge about the past, present, and future of the Universe. It also offers an excellent opportunity for the students to practice how science and technology go hand-in-hand through many different settings, e.g., accelerator, detector, simulation, and data mining at high-performance computing facilities. 

For the short-term goals, research in experimental high-energy nuclear physics at RHIC is pursued to study nuclear matter under extreme conditions. In the long-term future, the plan is to join the Electron-Ion Collider (EIC) in order to study the gluon role in the nucleus and nucleon structure functions and search for the possible precursor state to the QGP, the proposed Color Glass Condensate (CGC).

New D-dimensional charged Anti-de-Sitter black holes in f(T) gravity and D≥4 is introduced. These solutions are characterized by flat or cylindrical horizons. The interesting feature of these solutions is the existence of inseparable electric monopole and quadrupole terms in the potential, which share related momenta, in contrast with most of the known charged solutions in General Relativity and its extensions. Furthermore, these solutions have curvature singularities that are milder than those of the known charged solutions in General Relativity and Teleparallel Gravity. This feature can be shown by calculating some invariants of curvature and torsion tensors. Furthermore, the total energy of these solutions using the energy-momentum tensor is calculated. Finally, this research shows that these charged solutions violate the first law of thermodynamics in agreement with previous results.

The antenna is the interface between the guided and radiated electromagnetic waves. In its transmitting mode, it converts the information signal generated by an electronic chip into corresponding radiation, which enables wireless communication. As for the receiving mode, the wireless radiation is captured by the antenna and converted into a signal that feeds another electronic circuit. Aside from being used in communication systems, the antenna is responsible for manipulating inspection signals in radar, sensing and imaging systems. The departmental research in this area covers a wide range of operating frequencies, such as planar antennas operating in the RF and microwave ranges, micromachined antennas operating in the millimeter-wave and sub-millimeter-wave ranges, as well as nanoantennas operating in the infrared and visible light ranges of frequencies. The research in integrated antennas has developed several new antennas for different applications, such as wireless local area network (WLAN), radio frequency identification (RFID), ultra-wideband (UWB) communication, wireless personal area networks (WPAN), biomedical imaging, radar systems, THz imaging systems, solar and thermal energy harvesting, optical communications, and biomedical sensing.

Quantitatively measuring the mechanical properties of soft matter over a wide range of length and time scales, especially if a sample is as complex as typical biological materials, remains challenging. Polymer physics traditionally deals with systems in thermodynamic equilibrium. Biological Cells use the cytoskeleton, an active, dissipative polymer structure, in combination with motor proteins to organize their complex and heterogeneous interior structure, and to drive essential dynamic processes such as motility, growth or cell division. These processes are based on highly complex regulatory circuits that integrate the mechanics via mechano-sensory pathways with the biochemistry of the cell, which in turn feeds back on the physical material properties of the cell in its environment, forming a non-equilibrium system. A tool of choice to study the dynamical properties of polymer networks down to microscopic scales has been microrheology (MR). MR techniques track the motions of micron-sized probes embedded in the system to be studied. One can either follow single probes (1-particle MR) or evaluate the correlated fluctuations of pairs of probes (2-particle MR).

In active MR, probe particles are manipulated with laser light (optical tweezers). In passive MR, thermal fluctuations of the probes are recorded, and the fluctuation-dissipation theorem is used to extract viscoelastic parameters. In passive MR, no strain is applied to the material, which is particularly useful in soft biological materials where small strains cause nonlinear responses (e.g., strain hardening or shear thinning). Displacements are measured with high bandwidths of up to 100 kHz using laser trapping techniques combined with interferometry. Proper calibration is crucial and must take into account probe-particle polydispersity and inhomogeneities in the materials.

The nanophotonics research is interested in the different nanophysical properties of nanostructures and nanodevices, especially the electric and optical properties of these structures and systems. The NRL has extensive experience in the design and fabrication of micro and nano-scale photonic structures and devices at the chip -scale for telecommunication, interconnects and biomedical applications. Different novel materials have been exploited in order to achieve superior performance for these applications, including, for example, black silicon, plasmonics, doped semiconductors, transparent oxides, and graphene. The  Silicon photonic represents a major field of interest to the NRL, where various functional devices have been designed, fabricated and tested, such as optical filters, multiplexers, optical modulators, resonators and interferometers.

Numerous major advances in research and technology over the last decade or two have been made possible by the successful development of nanostructures made of metals, insulators and especially semiconductors. The practical interest in nanostructures is related to their unique properties and superior performance when compared to their bulk counterparts. The nanoscale dimension, for example, coincides with the characteristic length scale of charge diffusion in solids, rendering nanomaterials a promising candidate for controlling charges for targeted applications such as efficient photon-to-charge conversion. One significant difficulty with nanostructures is how to synthesize them in a well-ordered fashion. The aligned arrangement permits maximum photon absorption and charge separation, which is ideal for solar fuel and photovoltaics applications. The commitment is to understand the science that governs the synthesis of nanostructures and how the resulting materials’ morphology and crystal structure influence their physical properties. Through these studies, the aim is to develop methodologies that will enable the creation of nanostructures by design. These materials will act as active components in devices operating at near the theoretical limit efficiencies in harvesting solar energy.

Polymer physics research has been proven to have a wide variety of industrial applications. Such type of research can be pursued in a collaboration with the Chemistry and Engineering colleagues, and with the newly established Global Health and Human Ecology program at the AUC. The availability of the Thin Film Vacuum Deposition Chamber and the Atomic Force Microscopy might lead to a great opportunity to study surface physics at the Micro and Nano scales. Investigating the chemical, optical, electrical, and mechanical properties of polymeric films of few atomic and molecular layers, and how these properties change with the exposure to high-energy electromagnetic waves will enhance the industrial and medical applications of the polymer materials.

The Department of Physics is interested in developing different types of sensors for a variety of applications. Micro/Nano ElectroMechnical Systems (MEMS/NEMS) for different sensing applications are successfully fabricated and measured. The developed nanostructures are used to fabricate optical and bio-sensors and study how the interface between the photoactive layer and the substrate affects the performance of such sensors.

Photovoltaics have the potential to provide a low-cost solution for energy harvesting over the visible frequency band. To convert sunlight into electricity, we are exploring the use of nanostructured materials in solar cell designs. We are working on dye-sensitized solar cells as well as solid-state photovoltaics. This research work concentrates on various techniques to enhance the absorption capabilities of solar cells.

These techniques include light trapping, surface texturing, and nanostructuring of the solar cell. In general, trapping light on the surface, or inside the absorption layer is crucial in order to make thin-film solar cells viable. A few different approaches have been proposed to enhance the optical field localization and, hence, increase the optical absorption. These approaches include the utilization of the black silicon made on nanowires. Other approaches include using the localized plasmonic effect to enhance the field localization. This research work is also developing novel thermo-electric converters for mid-infrared energy harvesting at the chip-scale level to harvest energy from consumer electronics.

In the 21 century, one of the most important problems facing humanity is developing an enduring, sustainable energy economy. A central theme of our research is the interplay between structure, composition, and physical properties of nanostructured materials, such as optical absorptivity and electrical conductivity.

The aim is to control the morphology and composition in nanoscale materials with two main thrusts: solar energy conversion to chemical fuels in light-harvesting materials and electricity generation in solar cells. Energy storage is one of the most critical issues for current society. This may be achieved by multiple means such as chemical, thermal, electrochemical, electrical, magnetic, kinetic, and mechanical energy storage.

The materials for energy storage applications can be metals, alloys, nonmetallic inorganic materials, organic materials, metal-organic frameworks, or various composites. The focus is on the development of durable materials for energy storage, mainly supercapacitor devices and redox flow batteries.

The development of inexpensive and scalable materials systems that directly convert sunlight into an energy-dense chemical fuel would enable the storage, and hence utilization, of solar energy on a massive scale. The simplest chemical fuel is H2, which could be derived from the photoelectrochemical splitting of water and later used in a fuel cell, burned like natural gas, or used as a feedstock to make methanol from CO2. Although nanostructuring the semiconductor materials can improve their photoelectrochemical water splitting performance, by increasing the exposed area to the electrolyte and shortening minority carrier diffusion length, nanostructure engineering cannot change their intrinsic electronic properties.

Various methods are exploited, including elemental doping, interfacial hetero-junction design, quantum dot sensitization, plasmonic modification and inducing oxygen vacancies to enhance the photoelectrochemical water oxidation performance of nanostructured metal oxides, including TiO2, Nb2O5, Ta2O5, WO3, and α-Fe2O3. Biogas is a renewable source of energy produced by the anaerobic digestion (AD) of animal manures, agricultural residues, and organic waste from food, sewage sludge and different energy crops. Although AD, the main process for biogas production, resulted in the formation of CO2 as one of its by-products, this CO2 is consumed in a closed cycle via photosynthesis and taken again to AD as agricultural wastes and animal manures.

The main challenge in the currently used protocols to produce biogas is the selectivity towards producing the largest CH4 content. To this end, the catalytic effect of a plethora of nanomaterials on the biogas production rate and selectivity are explored.

The engine based on thermoacoustic is one of the emerging technologies in the field of energy conversion. It converts thermal energy from agnostic heat sources to mechanical energy in the form of acoustic oscillations of high amplitudes that can drive a thermoacoustic refrigerator/heat pump or can be converted to electrical energy using a linear alternator or a bi-directional turbine.

This emerging technology brings attractive opportunities, including the inherent adaptability to renewable heat sources, the ability to utilize relatively low temperatures or to upgrade low-temperature heat sources, the lack of major moving parts, as well as environmentally-friendly operation without gaseous emissions, ozone depletion or global warming hazards. However, research is needed to enhance the power density and the conversion efficiency of these devices to allow efficient and economic competition with conventional energy conversion techniques. Currently, significant research is being carried out worldwide in thermoacoustics energy conversion.

The interest in using and developing renewable energy resources and technologies is of considerable interest due to a growing demand for worldwide energy and the great damage it may cause to the environment. Solar energy should be a potential natural source of this kind of energy. Despite having several options for solar cell devices, thin film solar cell semiconducting materials are believed to be promising materials for potential use in solar cells such as CIGS, CdTe, and CZTS. For example, CdTe is one of the prime candidates with nearly ideal photovoltaic properties. This is due to its low cost and high efficiency reported for CdTe solar cells. CdTe could be doped with both n- and p-type materials using a large number of preparation methods/techniques. CdTe is an II–VI binary compound semiconductor with about 1.5 eV direct band gap, which lies in the optimum range of the solar spectrum for photovoltaic energy conversion. CdTe also has a relatively high absorption coefficient > 10⁵ cm^-1.

The research concerns the theoretical simulation of the electrical parameters and the photovoltaic parameters by applying software packages such as the Solar Cell Capacitance Simulator in one Dimension (SCAPS-1D). SCAPS is a software which has been developed at the University of Gent. The software SCAPS can simulate both DC and AC electrical characterization of thin film heterojunction solar cells. It can also provide current, voltage, and temperature in both dark and illuminations conditions. It is widely used for the simulation of different types of solar cells, e.g., CIGS and CdTe-based solar cells. The SCAPS-1D simulation results are reported to agree well with the corresponding experimental ones which give a powerful motivation to be used as a potential tool.

Clean water scarcity has become one of the biggest problems in many countries due to the increase in demand for potable water. It is also accompanied by the increase in population, climate changes and industry development. It stands to reason that if more than 98% of the available water is either sea or brackish water, modern, more effective and reasonably priced approaches to removing salts from such water streams have to be explored. Currently, commercial desalination techniques, such as reverse osmosis (RO), multistage flash distillation (MSF), and electrodialysis (ED), are widely used.

However, these technologies suffer from significant disadvantages, such as excessive energy consumption, intensive cost, high salinity residual, and ion exchange processes that produce secondary chemical wastes. To this end, new energy-efficient desalination technology based on capacitive deionization is developed. This involves the fabrication of durable nanostructured membranes with high salt adsorption capacity.