Research and Publications - Department of Electrical & Computer Engineering (ECE)

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Cross-frequency training with adversarial learning for radar micro-Doppler signature classification
Kurtoglu, Emre; Rahman, Mahbubur (Mahbub); Macks, Trevor; Gurbuz, Sevgi Zubeyde; Fioranelli, Francesco
Impact of Substrate and Bright Resonances on Group Velocity in Metamaterial without Dark Resonator
(Nature Portfolio, 2015) Hokmabadi, Mohammad Parvinnezhad; Kim, Jy-Hyung; Rivera, Elmer; Kung, Patrick; Kim, Seongsin M.; University of Alabama Tuscaloosa
Manipulating the speed of light has never been more exciting since electromagnetic induced transparency and its classical analogs led to slow light. Here, we report the manipulation of light group velocity in a terahertz metamaterial without needing a dark resonator, but utilizing instead two concentric split-ring bright resonators (meta-atoms) exhibiting a bright Fano resonance in close vicinity of a bright Lorentzian resonance to create a narrowband transmittance. Unlike earlier reports, the bright Fano resonance does not stem from an asymmetry of meta-atoms or an interaction between them. Additionally, we develop a method to determine the metamaterial "effective thickness", which quantifies the influence of the substrate on the metamaterial response and has remained challenging to estimate so far. By doing so, very good agreement between simulated and measured group delays and velocities is accomplished. The proposed structure and method will be useful in designing optical buffers, delay lines, and ultra-sensitive sensors.
Reevaluation of Performance of Electric Double-layer Capacitors from Constant-current Charge/Discharge and Cyclic Voltammetry
(Nature Portfolio, 2016) Allagui, Anis; Freeborn, Todd J.; Elwakil, Ahmed S.; Maundy, Brent J.; University of Sharjah; University of Alabama Tuscaloosa; Egyptian Knowledge Bank (EKB); Nile University; University of Calgary
The electric characteristics of electric-double layer capacitors (EDLCs) are determined by their capacitance which is usually measured in the time domain from constant-current charging/discharging and cyclic voltammetry tests, and from the frequency domain using nonlinear least-squares fitting of spectral impedance. The time-voltage and current-voltage profiles from the first two techniques are commonly treated by assuming ideal SsC behavior in spite of the nonlinear response of the device, which in turn provides inaccurate values for its characteristic metrics. In this paper we revisit the calculation of capacitance, power and energy of EDLCs from the time domain constant-current step response and linear voltage waveform, under the assumption that the device behaves as an equivalent fractional-order circuit consisting of a resistance R-s in series with a constant phase element (CPE(Q, alpha), with Q being a pseudocapacitance and a a dispersion coefficient). In particular, we show with the derived (Rs, Q, alpha)-based expressions, that the corresponding nonlinear effects in voltage-time and current-voltage can be encompassed through nonlinear terms function of the coefficient alpha, which is not possible with the classical RsC model. We validate our formulae with the experimental measurements of different EDLCs.
Spin-orbit torque-assisted switching in magnetic insulator thin films with perpendicular magnetic anisotropy
(Nature Portfolio, 2016) Li, Peng; Liu, Tao; Chang, Houchen; Kalitsov, Alan; Zhang, Wei; Csaba, Gyorgy; Li, Wei; Richardson, Daniel; DeMann, August; Rimal, Gaurab; Dey, Himadri; Jiang, J. S.; Porod, Wolfgang; Field, Stuart B.; Tang, Jinke; Marconi, Mario C.; Hoffmann, Axel; Mryasov, Oleg; Wu, Mingzhong; Colorado State University; University of Alabama Tuscaloosa; United States Department of Energy (DOE); Argonne National Laboratory; University of Notre Dame; University of Wyoming
As an in-plane charge current flows in a heavy metal film with spin-orbit coupling, it produces a torque on and thereby switches the magnetization in a neighbouring ferromagnetic metal film. Such spin-orbit torque (SOT)-induced switching has been studied extensively in recent years and has shown higher efficiency than switching using conventional spin-transfer torque. Here we report the SOT-assisted switching in heavy metal/magnetic insulator systems. The experiments used a Pt/BaFe12O19 bilayer where the BaFe12O19 layer exhibits perpendicular magnetic anisotropy. As a charge current is passed through the Pt film, it produces a SOT that can control the up and down states of the remnant magnetization in the BaFe12O19 film when the film is magnetized by an in-plane magnetic field. It can reduce or increase the switching field of the BaFe12O19 film by as much as about 500 Oe when the film is switched with an out-of-plane field.
Plasmon-Induced Transparency by Hybridizing Concentric-Twisted Double Split Ring Resonators
(Nature Portfolio, 2015) Hokmabadi, Mohammad Parvinnezhad; Philip, Elizabath; Rivera, Elmer; Kung, Patrick; Kim, Seongsin M.; University of Alabama Tuscaloosa
As a classical analogue of electromagnetically induced transparency, plasmon induced transparency (PIT) has attracted great attention by mitigating otherwise cumbersome experimental implementation constraints. Here, through theoretical design, simulation and experimental validation, we present a novel approach to achieve and control PIT by hybridizing two double split ring resonators (DSRRs) on flexible polyimide substrates. In the design, the large rings in the DSRRs are stationary and mirror images of each other, while the small SRRs rotate about their center axes. Counter-directional rotation (twisting) of the small SRRs is shown to lead to resonance shifts, while co-directional rotation results in splitting of the lower frequency resonance and emergence of a PIT window. We develop an equivalent circuit model and introduce a mutual inductance parameter M whose sign is shown to characterize the existence or absence of PIT response from the structure. This model attempts to provide a quantitative measure of the physical mechanisms underlying the observed PIT phenomenon. As such, our findings can support the design of several applications such as optical buffers, delay lines, and ultra-sensitive sensors.
Modelling of segmented high-performance thermoelectric generators with effects of thermal radiation, electrical and thermal contact resistances
(Nature Portfolio, 2016) Ouyang, Zhongliang; Li, Dawen; University of Alabama Tuscaloosa
In this study, segmented thermoelectric generators (TEGs) have been simulated with various state-of-the-art TE materials spanning a wide temperature range, from 300 K up to 1000 K. The results reveal that by combining the current best p-type TE materials, BiSbTe, MgAgSb, K-doped PbTeS and SnSe with the strongest n-type TE materials, Cu-Doped BiTeSe, AgPbSbTe and SiGe to build segmented legs, TE modules could achieve efficiencies of up to 17.0% and 20.9% at Delta T = 500 K and Delta T = 700 K, respectively, and a high output power densities of over 2.1 Watt cm(-2) at the temperature difference of 700 K. Moreover, we demonstrate that successful segmentation requires a smooth change of compatibility factor s from one end of the TEG leg to the other, even if s values of two ends differ by more than a factor of 2. The influence of the thermal radiation, electrical and thermal contact effects have also been studied. Although considered potentially detrimental to the TEG performance, these effects, if well-regulated, do not prevent segmentation of the current best TE materials from being a prospective way to construct high performance TEGs with greatly enhanced efficiency and output power density.
Biomechanical Modeling of Human Skin Tissue Surrogates
(MDPI, 2018) Chanda, Arnab; University of Pittsburgh; University of Alabama Tuscaloosa
Surrogates, which precisely simulate nonlinear mechanical properties of the human skin at different body sites, would be indispensable for biomechanical testing applications, such as estimating the accurate load response of skin implants and prosthetics to study the biomechanics of static and dynamic loading conditions on the skin, dermatological and sports injuries, and estimating the dynamic load response of lethal and nonlethal ballistics. To date, human skin surrogates have been developed mainly with materials, such as gelatin and polydimethylsiloxane (PDMS), based on assumption of simplified mechanical properties, such as an average elastic modulus (estimated through indentation tests), and Poisson's ratio. In addition, pigskin and cowhides, which have widely varying mechanical properties, have been used to simulate human skin. In the current work, a novel elastomer-based material system is developed, which precisely mimics the nonlinear stress-stretch behavior, elastic modulus at high and low strains, and fracture strengths of the natural human skin at different body sites. The manufacturing and fabrication process of these skin surrogates are discussed, and mechanical testing results are presented.
Strong Solar Radiation Forces from Anomalously Reflecting Metasurfaces for Solar Sail Attitude Control
(Nature Portfolio, 2018) Ullery, Dylan C.; Soleymani, Sina; Heaton, Andrew; Orphee, Juan; Johnson, Les; Sood, Rohan; Kung, Patrick; Kim, Seongsin M.; University of Alabama Tuscaloosa; National Aeronautics & Space Administration (NASA)
We examine the theoretical implications of incorporating metasurfaces on solar sails, and the effect they can have on the forces applied to the sail. This would enable a significant enhancement over state-of-the-art attitude control by demonstrating a novel, propellant-free and low-mass approach to induce a roll torque on the sail, which is a current limitation in present state-of-the-art technology. We do so by utilizing anomalous optical reflections from the metasurfaces to generate a net in-plane lateral force, which can lead to a net torque along the roll axis of the sail, in addition to the other spatial movements exhibited by the sail from solar radiation pressure. We characterize this net lateral force as a function of incidence angle. In addition, the influence of the phase gradients and anomalous conversion efficiencies characteristics of the metasurfaces are independently considered. The optimum incidence angle that corresponded with the maximum net lateral-to-normal force ratio was found to be -30 degrees for a metasurface exhibiting 75% anomalous conversion efficiency with a phase gradient of 0:71k(0).
A Simple Analytical Model for Magnetization and Coercivity of Hard/Soft Nanocomposite Magnets
(Nature Portfolio, 2017) Park, Jihoon; Hong, Yang-Ki; Lee, Woncheol; Kim, Seong-Gon; Rong, Chuangbing; Poudyal, Narayan; Liu, J. Ping; Choi, Chul-Jin; University of Alabama Tuscaloosa; Korea Institute of Materials Science (KIMS); Mississippi State University; University of Texas Arlington
We present a simple analytical model to estimate the magnetization (sigma(s)) and intrinsic coercivity (H-ci) of a hard/soft nanocomposite magnet using the mass fraction. Previously proposed models are based on the volume fraction of the hard phase of the composite. However, it is difficult to measure the volume of the hard or soft phase material of a composite. We synthesized Sm2Co7/Fe-Co-i MnAl/Fe-Co-i MnBi/Fe-Co-i and BaFe12O19/Fe-Co composites for characterization of their sigma(s) and H-ci. The experimental results are in good agreement with the present model. Therefore, this analytical model can be extended to predict the maximum energy product (BH)(max) of hard/soft composite.
Effect of Donor-Acceptor Vertical Composition Profile on Performance of Organic Bulk Heterojunction Solar Cells
(Nature Portfolio, 2018) Bi, Sheng; Ouyang, Zhongliang; Shaik, Shoieb; Li, Dawen; Dalian University of Technology; University of Alabama Tuscaloosa
In organic bulk heterojunction solar cells (OSCs) donor-acceptor vertical composition profile is one of the crucial factors that affect power-conversion efficiency (PCE). In this simulation study, five different kinds of donor-acceptor vertical configurations, including sandwich type I and type II, charge transport favorable, charge transport unfavorable, and uniform vertical distribution, have been investigated for both regular and inverted OSC structures. OSCs with uniform and charge transport favorable vertical composition profiles demonstrate the highest efficiencies. High PCE from charge transport favorable configuration can be attributed to low recombination because of facilitated charge transport in active layer and collection at electrodes, while high PCE from uniform structure is due to sufficient interfaces for efficient exciton dissociation. OSCs with sandwich and charge transport unfavorable structures show much lower efficiencies. The physical mechanisms behind simulation results are explained based on energy band diagrams, dark current-voltage characteristics, and comparison of external quantum efficiency. In conclusion, experimental optimization of vertical composition profile should be directed to either uniform or charge transport favorable vertical configurations in order to achieve high-performance OSCs.
Biomechanical Modeling of Prosthetic Mesh and Human Tissue Surrogate Interaction
(MDPI, 2018) Chanda, Arnab; Ruchti, Tysum; Upchurch, Weston; University of Pittsburgh; University of Alabama Tuscaloosa; Brigham Young University; University of Minnesota Twin Cities
Surgical repair of hernia and prolapse with prosthetic meshes are well-known to cause pain, infection, hernia recurrence, and mesh contraction and failures. In literature, mesh failure mechanics have been studied with uniaxial, biaxial, and cyclic load testing of dry and wet meshes. Also, extensive experimental studies have been conducted on surrogates, such as non-human primates and rodents, to understand the effect of mesh stiffness, pore size, and knitting patterns on mesh biocompatibility. However, the mechanical properties of such animal tissue surrogates are widely different from human tissues. Therefore, to date, mechanics of the interaction between mesh and human tissues is poorly understood. This work addresses this gap in literature by experimentally and computationally modeling the biomechanical behavior of mesh, sutured to human tissue phantom under tension. A commercially available mesh (Prolene((R))) was sutured to vaginal tissue phantom material and tested at different uniaxial strains and strain rates. Global and local stresses at the tissue phantom, suture, and mesh were analyzed. The results of this study provide important insights into the mechanics of prosthetic mesh failure and will be indispensable for better mesh design in the future.
Statistical models for meal-level estimation of mass and energy intake using features derived from video observation and a chewing sensor
(Nature Portfolio, 2019) Yang, Xin; Doulah, Abul; Farooq, Muhammad; Parton, Jason; McCrory, Megan A.; Higgins, Janine A.; Sazonov, Edward; University of Alabama Tuscaloosa; Boston University; University of Colorado Anschutz Medical Campus; University of Colorado Denver
Accurate and objective assessment of energy intake remains an ongoing problem. We used features derived from annotated video observation and a chewing sensor to predict mass and energy intake during a meal without participant self-report. 30 participants each consumed 4 different meals in a laboratory setting and wore a chewing sensor while being videotaped. Subject-independent models were derived from bite, chew, and swallow features obtained from either video observation or information extracted from the chewing sensor. With multiple regression analysis, a forward selection procedure was used to choose the best model. The best estimates of meal mass and energy intake had (mean +/- standard deviation) absolute percentage errors of 25.2% +/- 18.9% and 30.1% +/- 33.8%, respectively, and mean +/- standard deviation estimation errors of -17.7 +/- 226.9 g and -6.1 +/- 273.8 kcal using features derived from both video observations and sensor data. Both video annotation and sensor-derived features may be utilized to objectively quantify energy intake.
Towards health monitoring using remote heart rate measurement using digital camera: A feasibility study
(Elsevier, 2020) Hassan, M. A.; Malik, A. S.; Fofi, D.; Karasfi, B.; Meriaudeau, F.; University of Alabama Tuscaloosa; Universiti Teknologi Petronas; Centre National de la Recherche Scientifique (CNRS); Universite de Bourgogne
The paper presents a feasibility study for heart rate measurement using a digital camera to perform health monitoring. The feasibility study investigates the reliability of the state of the art heart rate measuring methods in realistic situations. Therefore, an experiment was designed and carried out on 45 subjects to investigate the effects caused by illumination, motion, skin tone, and distance variance. The experiment was conducted for two main scenarios; human-computer interaction scenario and health monitoring scenario. The human-computer scenario investigated the effects caused by illumination variance, motion variance, and skin tone variance. The health monitoring scenario investigates the feasibility of health monitoring at public spaces (i.e. airports, subways, malls). Five state of the art heart rate measuring methods were re-implemented and tested with the feasibility study database. The results were compared with ground truth to estimate the heart rate measurement error. The heart rate measurement error was analyzed using mean error, standard deviation; root means square error and Pearson correlation coefficient. The findings of this experiment inferred promising results for health monitoring of subjects standing at a distance of 500 cm. (C) 2019 Elsevier Ltd. All rights reserved.
Ultra-High Efficiency and Broad Band Operation of Infrared Metasurface Anomalous Reflector based on Graphene Plasmonics
(Nature Portfolio, 2019) Soleymani, Sina; Gungordu, M. Zeki; Kung, Patrick; Kim, Seongsin M.; University of Alabama Tuscaloosa
Infrared metasurface anomalous reflector with ultra-high efficiency and broad band operation is designed via multi-sheet graphene layer with triangular holes. The anomalous reflection angle covers the range of 10 degrees to 90 degrees with the efficiency higher than 80%, over a broad spectral range from 7 mu m-40 mu m of infrared spectrum. It reaches above 92% at the center wavelength in the spectral response. By increasing the periodicity of phase gradient, we can expand this frequency band even further without losing efficiency. The compact design of metasurface affords the adjustability of the electrochemical potential level of graphene by means of gating. Additionally, the impact of the number of graphene sheets for the optimum efficiency of the proposed structure is investigated. By adding the secondary graphene metasurface with opposite direction of phase gradient, we demonstrated the tunability of the reflection angle from 0(r) to - 0(r) with bias voltage.
Mechanical Modeling of Healthy and Diseased Calcaneal Fat Pad Surrogates
(MDPI, 2019) Chanda, Arnab; McClain, Stephen; University of Pittsburgh; University of Alabama Tuscaloosa; Georgia Institute of Technology
The calcaneal fat pad is a major load bearing component of the human foot due to daily gait activities such as standing, walking, and running. Heel and arch pain pathologies such as plantar fasciitis, which over one third of the world population suffers from, is a consequent effect of calcaneal fat pad damage. Also, fat pad stiffening and ulceration has been observed due to diabetes mellitus. To date, the biomechanics of fat pad damage is poorly understood due to the unavailability of live human models (because of ethical and biosafety issues) or biofidelic surrogates for testing. This also precludes the study of the effectiveness of preventive custom orthotics for foot pain pathologies caused due to fat pad damage. The current work addresses this key gap in the literature with the development of novel biofidelic surrogates? which simulate the in vivo and in vitro compressive mechanical properties of a healthy calcaneal fat pad. Also, surrogates were developed to simulate the in vivo mechanical behavior of the fat pad due to plantar fasciitis and diabetes. A four-part elastomeric material system was used to fabricate the surrogates, and their mechanical properties were characterized using dynamic and cyclic load testing. Different strain (or displacement) rates were tested to understand surrogate behavior due to high impact loads. These surrogates can be integrated with a prosthetic foot model and mechanically tested to characterize the shock absorption in different simulated gait activities, and due to varying fat pad material property in foot pain pathologies (i.e., plantar fasciitis, diabetes, and injury). Additionally, such a foot surrogate model, fitted with a custom orthotic and footwear, can be used for the experimental testing of shock absorption characteristics of preventive orthoses.
Conjugated Polymer Controlled Morphology and Charge Transport of Small-Molecule Organic Semiconductors
(Nature Portfolio, 2020) He, Zhengran; Zhang, Ziyang; Bi, Sheng; Chen, Jihua; Li, Dawen; University of Alabama Tuscaloosa; Columbia University; Dalian University of Technology; United States Department of Energy (DOE); Oak Ridge National Laboratory; Center for Nanophase Materials Sciences
In this study, we report an effective approach to tune the crystallization, microstructure and charge transport of solution-processed organic semiconductors by blending with a conjugated polymer additive poly(3-hexylthiophene) (P3HT). When 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) was used as a model semiconductor material to mix with different amount of P3HT, their intermolecular interactions led to distinctive TIPS pentacene film morphologies, including randomly-oriented crystal ribbons, elongated needles with enhanced long-range order, and grasslike curved microwires with interlinkages. Each type of morphology was found to further correlate to considerably different charge transport and device performance. As compared to pristine TIPS pentacene devices, bottom-gate, top-contact OTFTs with 2% in weight P3HT additive showed a 2-fold and 5-fold improvement of average field-effect mobility and performance consistency (defined as the ratio of average mobility to the standard deviation), respectively. The improvement in transistor electrical performance can be attributed to the combined effect of enhanced crystal orientation and uniformity, as well as increased areal coverage. This work can be applied beyond the particular example demonstrated in this study and to tune the charge transport of other small-molecule organic semiconductors in general.
Breast cancer biomarker detection through the photoluminescence of epitaxial monolayer MoS2 flakes
(Nature Portfolio, 2020) Catalan-Gomez, Sergio; Briones, Maria; Cortijo-Campos, Sandra; Garcia-Mendiola, Tania; de Andres, Alicia; Garg, Sourav; Kung, Patrick; Lorenzo, Encarnacion; Luis Pau, Jose; Redondo-Cubero, Andres; Autonomous University of Madrid; Consejo Superior de Investigaciones Cientificas (CSIC); CSIC - Instituto de Ciencia de Materiales de Madrid (ICMM); University of Alabama Tuscaloosa
In this work we report on the characterization and biological functionalization of 2D MoS2 flakes, epitaxially grown on sapphire, to develop an optical biosensor for the breast cancer biomarker miRNA21. The MoS2 flakes were modified with a thiolated DNA probe complementary to the target biomarker. Based on the photoluminescence of MoS2, the hybridization events were analyzed for the target (miRNA21c) and the control non-complementary sequence (miRNA21nc). A specific redshift was observed for the hybridization with miRNA21c, but not for the control, demonstrating the biomarker recognition via PL. The homogeneity of these MoS2 platforms was verified with microscopic maps. The detailed spectroscopic analysis of the spectra reveals changes in the trion to excitation ratio, being the redshift after the hybridization ascribed to both peaks. The results demonstrate the benefits of optical biosensors based on MoS2 monolayer for future commercial devices.
Inpainting for Saturation Artifacts in Optical Coherence Tomography Using Dictionary-Based Sparse Representation
(IEEE, 2021) Liu, Hongshan; Cao, Shengting; Ling, Yuye; Gan, Yu; University of Alabama Tuscaloosa; Shanghai Jiao Tong University
Saturation artifacts in optical coherence tomography (OCT) occur when received signal exceeds the dynamic range of spectrometer. Saturation artifact shows a streaking pattern and could impact the quality of OCT images, leading to inaccurate medical diagnosis. In this paper, we automatically localize saturation artifacts and propose an artifact correction method via inpainting. We adopt a dictionary-based sparse representation scheme for inpainting. Experimental results demonstrate that, in both case of synthetic artifacts and real artifacts, our method outperforms interpolation method and Euler's elastica method in both qualitative and quantitative results. The generic dictionary offers similar image quality when applied to tissue samples which are excluded from dictionary training. This method may have the potential to be widely used in a variety of OCT images for the localization and inpainting of the saturation artifacts.
A novel method for in-situ extracting bio-impedance model parameters optimized for embedded hardware
(Nature Portfolio, 2023) Simic, Mitar; Freeborn, Todd J.; Sekara, Tomislav B.; Stavrakis, Adrian K.; Jeoti, Varun; Stojanovic, Goran M.; University of Novi Sad; University of Alabama Tuscaloosa; University of Belgrade
A novel method for embedded hardware-based parameter estimation of the Cole model of bioimpedance is developed and presented. The model parameters R-infinity, R-1 and C are estimated using the derived set of equations based on measured values of real (R) and imaginary part (X) of bioimpedance, as well as the numerical approximation of the first derivative of quotient R/X with respect to angular frequency. The optimal value for parameter alpha is estimated using a brute force method. The estimation accuracy of the proposed method is very similar with the relevant work from the existing literature. Moreover, performance evaluation was performed using the MATLAB software installed on a laptop, as well as on the three embedded-hardware platforms (Arduino Mega2560, Raspberry Pi Pico and XIAO SAMD21). Obtained results showed that the used platforms can perform reliable bioimpedance processing with the same accuracy, while Raspberry Pi Pico is the fastest solution with the smallest energy consumption.
Flexible PCB Failures From Dynamic Activity and Their Impacts on Bioimpedance Measurements: A Wearable Case Study
(IEEE, 2021) Critcher, Shelby; Freeborn, Todd J.; University of Alabama Tuscaloosa
Wearable health monitoring systems that collect data in free-living environments are becoming increasingly popular. Flexible printed circuits provide a commercially available option that can conform to the shape of a wearable system and support electronic sensing and flexible interconnect. However, repetitive dynamic activity can stress and damage the interconnect of flexible PCBs which degrades data quality. This case study evaluated the performance of flexible PCBs providing interconnect between electrodes and sensing electronics for tissue bioimpedance measurements in a wearable system. Resistance data (1 kHz to 128 kHz) was collected from localized knee tissues of 3 participants using the wearable design with flexible PCBs over 7 days of free-living. From electrical and optical inspection after use trace cracking of the flexible PCBs occurred, degrading tissue resistances reported by the wearable system. Exploration of these results advances understanding of how flexible PCBs perform in free-living conditions for wearable bioimpedance applications.

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