### Recent Submissions

• #### A Wideband Magnetic Frequency Up-Converter Energy Harvester

(Advanced Engineering Materials, Wiley, 2021-03-05) [Article]
Many sensor applications require small and noninvasive methods of powering, such as marine animal tracking and implantable healthcare monitoring. In such cases, energy harvesting is a viable solution. Vibrational energy harvesting is abundantly available in the environment. These vibrations usually are low in frequency and amplitude. Conventional vibrational harvesters convert the environmental vibrations into electrical signals; however, they suffer from low-voltage outputs and narrow bandwidths, limiting the harvesting to a small range of frequencies. Herein, a new mechanical harvester is introduced using a magnetic frequency up-converter. It is implemented using attractive-force magnetic coupling between a soft magnet and a permanent magnet to convert low-frequency vibrations into high-frequency pulses. Combined with a piezoelectric generator, the harvester generates a high output voltage for an extended bandwidth of operation. The proposed harvester shows a 50.15% increase in output voltage at the resonant frequency (12.2 Hz), resulting in 14.79 V at 1.0 g, with a maximum peak voltage of 16.28 V. The bandwidth of operation ranges from 10.77 to 22.16 Hz (11.39 Hz), which when compared with a single-beam harvester shows an increase of 3250% in the bandwidth, where the average power is greater for 92.56% of this bandwidth.
• #### Neural Coding in Spiking Neural Networks: A Comparative Study for Robust Neuromorphic Systems

(Frontiers in Neuroscience, Frontiers Media SA, 2021-03-04) [Article]
Various hypotheses of information representation in brain, referred to as neural codes, have been proposed to explain the information transmission between neurons. Neural coding plays an essential role in enabling the brain-inspired spiking neural networks (SNNs) to perform different tasks. To search for the best coding scheme, we performed an extensive comparative study on the impact and performance of four important neural coding schemes, namely, rate coding, time-to-first spike (TTFS) coding, phase coding, and burst coding. The comparative study was carried out using a biological 2-layer SNN trained with an unsupervised spike-timing-dependent plasticity (STDP) algorithm. Various aspects of network performance were considered, including classification accuracy, processing latency, synaptic operations (SOPs), hardware implementation, network compression efficacy, input and synaptic noise resilience, and synaptic fault tolerance. The classification tasks on Modified National Institute of Standards and Technology (MNIST) and Fashion-MNIST datasets were applied in our study. For hardware implementation, area and power consumption were estimated for these coding schemes, and the network compression efficacy was analyzed using pruning and quantization techniques. Different types of input noise and noise variations in the datasets were considered and applied. Furthermore, the robustness of each coding scheme to the non-ideality-induced synaptic noise and fault in analog neuromorphic systems was studied and compared. Our results show that TTFS coding is the best choice in achieving the highest computational performance with very low hardware implementation overhead. TTFS coding requires 4x/7.5x lower processing latency and 3.5x/6.5x fewer SOPs than rate coding during the training/inference process. Phase coding is the most resilient scheme to input noise. Burst coding offers the highest network compression efficacy and the best overall robustness to hardware non-idealities for both training and inference processes. The study presented in this paper reveals the design space created by the choice of each coding scheme, allowing designers to frame each scheme in terms of its strength and weakness given a designs’ constraints and considerations in neuromorphic systems.
• #### Neural Coding in Spiking Neural Networks: A Comparative Study for Robust Neuromorphic Systems

(Frontiers in Neuroscience, Frontiers Media SA, 2021-03-04) [Article]
Various hypotheses of information representation in brain, referred to as neural codes, have been proposed to explain the information transmission between neurons. Neural coding plays an essential role in enabling the brain-inspired spiking neural networks (SNNs) to perform different tasks. To search for the best coding scheme, we performed an extensive comparative study on the impact and performance of four important neural coding schemes, namely, rate coding, time-to-first spike (TTFS) coding, phase coding, and burst coding. The comparative study was carried out using a biological 2-layer SNN trained with an unsupervised spike-timing-dependent plasticity (STDP) algorithm. Various aspects of network performance were considered, including classification accuracy, processing latency, synaptic operations (SOPs), hardware implementation, network compression efficacy, input and synaptic noise resilience, and synaptic fault tolerance. The classification tasks on Modified National Institute of Standards and Technology (MNIST) and Fashion-MNIST datasets were applied in our study. For hardware implementation, area and power consumption were estimated for these coding schemes, and the network compression efficacy was analyzed using pruning and quantization techniques. Different types of input noise and noise variations in the datasets were considered and applied. Furthermore, the robustness of each coding scheme to the non-ideality-induced synaptic noise and fault in analog neuromorphic systems was studied and compared. Our results show that TTFS coding is the best choice in achieving the highest computational performance with very low hardware implementation overhead. TTFS coding requires 4x/7.5x lower processing latency and 3.5x/6.5x fewer SOPs than rate coding during the training/inference process. Phase coding is the most resilient scheme to input noise. Burst coding offers the highest network compression efficacy and the best overall robustness to hardware non-idealities for both training and inference processes. The study presented in this paper reveals the design space created by the choice of each coding scheme, allowing designers to frame each scheme in terms of its strength and weakness given a designs’ constraints and considerations in neuromorphic systems.
• #### Organic passivation of Al0.5Ga0.5N epilayers using self-assembled monolayer of Zn(II) porphyrin for improved solar-blind photodetector performance

(Semiconductor Science and Technology, IOP Publishing, 2021-03-03) [Article]
We report on the passivation of surface states of Al0.5Ga0.5N epilayers by employing self-assembled monolayers (SAM) of organic molecules, which led to a significant improvement in the performance of Al0.5Ga0.5N based solar-blind photodetector. The formation of SAM of meso-(5-hydroxyphenyl)-10,15,20-tri(p-tolyl) porphyrin (ZnTPP(OH)) on the surface of Al0.5Ga0.5N was probed by contact angle measurement (CA), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The successful passivation of surface states was confirmed by Kelvin probe force microscopy (KPFM) as a significant decrease in the surface potential of Al0.5Ga0.5N by ~280 mV was observed. The inference was supported by a four-fold increase in the photoluminescence (PL) intensity of the near-band edge emission (NBE) peak upon passivation. As a result, the dark current of the as-fabricated solar-blind photodetector reduced by 2 orders of magnitude, without compromising with the magnitude of the photo current at 270 nm. The role of SAM was evident in improving the performance of the photodetector as the peak value of photo-to-dark current ratio (PDCR) enhanced by ~36 times. The peak responsivity of the photodetector increased from 1.6 to 2.2 mA/W at 10 V. The significant reduction in the dark current and enhancement in the responsivity led to an improvement in the specific detectivity by ~10 times. Additionally, the response speed of the photodetector was found to improve significantly from 4 to 0.5 s.
• #### Towards Detecting Red Palm Weevil Using Machine Learning and Fiber Optic Distributed Acoustic Sensing

(Sensors, MDPI AG, 2021-02-25) [Article]
Red palm weevil (RPW) is a detrimental pest, which has wiped out many palm tree farms worldwide. Early detection of RPW is challenging, especially in large-scale farms. Here, we introduce the combination of machine learning and fiber optic distributed acoustic sensing (DAS) techniques as a solution for the early detection of RPW in vast farms. Within the laboratory environment, we reconstructed the conditions of a farm that includes an infested tree with ∼12 day old weevil larvae and another healthy tree. Meanwhile, some noise sources are introduced, including wind and bird sounds around the trees. After training with the experimental time- and frequency-domain data provided by the fiber optic DAS system, a fully-connected artificial neural network (ANN) and a convolutional neural network (CNN) can efficiently recognize the healthy and infested trees with high classification accuracy values (99.9% by ANN with temporal data and 99.7% by CNN with spectral data, in reasonable noise conditions). This work paves the way for deploying the high efficiency and cost-effective fiber optic DAS to monitor RPW in open-air and large-scale farms containing thousands of trees.
• #### Evidence of Carrier Localization in AlGaN/GaN based Ultraviolet Multiple Quantum Wells with Opposite Polarity Domains Provided by Nanoscale Imaging

(physica status solidi (RRL) – Rapid Research Letters, Wiley, 2021-02-25) [Article]
AlGaN based multiple-quantum-wells (MQWs) incorporating opposite polarity domains was grown by MOCVD. A direct demonstration of carrier localization effect was provided by a combination analysis of space-resolved luminescence peak position and Ga/Al composition distribution. Furthermore, through Raman spectroscopy, it is found that compressive strain plays a key role in improving the optical properties of UV-MQWs despite of the inferior crystalline quality in the N-polar domains. This suggests that incorporating sub-micrometer scale polarity domains in the MQWs is a promising perspective for the development of efficient UV emitters.
• #### AniGAN: Style-Guided Generative Adversarial Networks for Unsupervised Anime Face Generation

(arXiv, 2021-02-24) [Preprint]
In this paper, we propose a novel framework to translate a portrait photo-face into an anime appearance. Our aim is to synthesize anime-faces which are style-consistent with a given reference anime-face. However, unlike typical translation tasks, such anime-face translation is challenging due to complex variations of appearances among anime-faces. Existing methods often fail to transfer the styles of reference anime-faces, or introduce noticeable artifacts/distortions in the local shapes of their generated faces. We propose Ani- GAN, a novel GAN-based translator that synthesizes highquality anime-faces. Specifically, a new generator architecture is proposed to simultaneously transfer color/texture styles and transform local facial shapes into anime-like counterparts based on the style of a reference anime-face, while preserving the global structure of the source photoface. We propose a double-branch discriminator to learn both domain-specific distributions and domain-shared distributions, helping generate visually pleasing anime-faces and effectively mitigate artifacts. Extensive experiments qualitatively and quantitatively demonstrate the superiority of our method over state-of-the-art methods.
• #### Design of 3–5 GHz tunable memristor-based OOK-UWB transmitter

(AEU - International Journal of Electronics and Communications, Elsevier BV, 2021-02-21) [Article]
This paper presents the design of a tunable memristor-based On-off keying (OOK) impulse-radio ultrawideband (IR-UWB) transmitter (TX), operating within the 3–5 GHz band. The proposed TX contains a CMOS-Memristor-based Ring Oscillator, an OOK modulator, a control signals circuit, and a pulse shape filter. The linear programming of the memristor device offers a smooth linear frequency variation of the ring oscillator. Accordingly, in the proposed circuit, two external control signals Vctrl and Vtune are used to tune the memristance value and the output pulse width, respectively, allowing adjusting the power spectral density (PSD) bandwidth and its central frequency. The IR-UWB TX is suitable for OOK modulation schema. It is designed and simulated using TSMC CMOS 0.18 µm technology with a 1.8 V supply voltage. The simulation results show that the proposed TX can cover both narrowband and wideband UWB PSD, depending on the number of oscillations per emitted pulse. The TX output swing is 483mVpp, and the pulse duration is 0.8 ns for narrowband and 1.74 ns for wideband. For a pulse repetition frequency of 10 MHz, the pulse generator consumes 94.8 µW and accomplishes an energy efficiency of 23.4%, while the total energy consumption is 9.48 pJ/pulse.
• #### Theory and experimental verification of configurable computing with stochastic memristors.

(Scientific reports, Springer Science and Business Media LLC, 2021-02-19) [Article]
The inevitable variability within electronic devices causes strict constraints on operation, reliability and scalability of the circuit design. However, when a compromise arises among the different performance metrics, area, time and energy, variability then loosens the tight requirements and allows for further savings in an alternative design scope. To that end, unconventional computing approaches are revived in the form of approximate computing, particularly tuned for resource-constrained mobile computing. In this paper, a proof-of-concept of the approximate computing paradigm using memristors is demonstrated. Stochastic memristors are used as the main building block of probabilistic logic gates. As will be shown in this paper, the stochasticity of memristors' switching characteristics is tightly bound to the supply voltage and hence to power consumption. By scaling of the supply voltage to appropriate levels stochasticity gets increased. In order to guide the design process of approximate circuits based on memristors a realistic device model needs to be elaborated with explicit emphasis of the probabilistic switching behavior. Theoretical formulation, probabilistic analysis, and simulation of the underlying logic circuits and operations are introduced. Moreover, the expected output behavior is verified with the experimental measurements of valence change memory cells. Hence, it is shown how the precision of the output is varied for the sake of the attainable gains at different levels of available design metrics. This approach represents the first proposition along with physical verification and mapping to real devices that combines stochastic memristors into unconventional computing approaches.
• #### Shape-Tailored Deep Neural Networks

(arXiv, 2021-02-16) [Preprint]
We present Shape-Tailored Deep Neural Networks (ST-DNN). ST-DNN extend convolutional networks (CNN), which aggregate data from fixed shape (square) neighborhoods, to compute descriptors defined on arbitrarily shaped regions. This is natural for segmentation, where descriptors should describe regions (e.g., of objects) that have diverse shape. We formulate these descriptors through the Poisson partial differential equation (PDE), which can be used to generalize convolution to arbitrary regions. We stack multiple PDE layers to generalize a deep CNN to arbitrary regions, and apply it to segmentation. We show that ST-DNN are covariant to translations and rotations and robust to domain deformations, natural for segmentation, which existing CNN based methods lack. ST-DNN are 3-4 orders of magnitude smaller then CNNs used for segmentation. We show that they exceed segmentation performance compared to state-of-the-art CNN-based descriptors using 2-3 orders smaller training sets on the texture segmentation problem.
• #### Flexible Hall sensor made of laser-scribed graphene

(npj Flexible Electronics, Springer Science and Business Media LLC, 2021-02-15) [Article]
Graphene has shown considerable potential for sensing magnetic fields based on the Hall Effect, due to its high carrier mobility, low sheet carrier density, and low-temperature dependence. However, the cost of graphene in comparison to conventional materials has meant that its uptake in electronic manufacturing has been slow. To lower technological barriers and bring more widespread adoption of graphene Hall sensors, we are using a one-step laser scribing process that does not rely on multiple steps, toxic chemicals, and subsequent treatments. Laser-scribed graphene Hall sensors offer a linear response to magnetic fields with a normalized sensitivity of ~1.12 V/AT. They also exhibit a low constant noise voltage floor of ~ 50 nV/Hz−−−√ for a bias current of 100 µA at room temperature, which is comparable with state-of-the-art low-noise Hall sensors. The sensors combine a high bendability, come with high robustness and operating temperatures up to 400 °C. They enable device ideas in various areas, for instance, soft robotics. As an example, we combined a laser-scribed graphene sensor with a deformable elastomer and flexible magnet to realize low-cost, compliant, and customizable tactile sensors.
• #### Ultra-thin dark amorphous TiOx hollow nanotubes for full spectrum solar energy harvesting and conversion‡

(Nano Energy, Elsevier BV, 2021-02-15) [Article]
Dark titania (TiOx) have been widely used for solar energy harvesting and conversion applications due to its excellent light absorbing performance throughout the ultraviolet to near infrared wavelength band, low cost, and non-toxic nature. However, the synthesis methods of dark TiOx are usually complicated and time-consuming. Here we report a facile and rapid method to fabricate dark amorphous TiOx (am-TiOx) hollow nanotube arrays on nanoporous anodic alumina oxide (AAO) templates using atomic layer deposition. Systematic investigation was performed to demonstrate that Ti3+ and O- species in the am-TiOx ultra-thin films, as well as the spatial distribution of these am-TiOx ultra-thin films on the vertical side walls of AAO templates are two major mechanisms of the black color. Importantly, the film deposition took ~18 min only to produce the optimized ~4-nm-thick am-TiOx film. Representative applications were demonstrated using photocatalytic reduction of silver nitrate and photothermal solar vapor generation, revealing the potential of these ultra-thin dark am-TiOx/AAO structures for full spectrum solar energy harvesting and conversion.
• #### Material absorption-based carrier generation model for modeling optoelectronic devices

(arXiv, 2021-02-12) [Preprint]
The generation rate of photocarriers in optoelectronic materials is commonly calculated using the Poynting vector in the frequency domain. In time-domain approaches where the nonlinear coupling between electromagnetic (EM) waves and photocarriers can be accounted for, the Poynting vector model is no longer applicable. One main reason is that the photocurrent radiates low-frequency EM waves out of the spectrum of the source, e.g., terahertz (THz) waves are generated in THz photoconductive antennas. These frequency components do not contribute to the photocarrier generation since the corresponding photon energy is smaller than the optoelectronic material's bandgap energy. However, the instantaneous Poynting vector does not distinguish the power flux of different frequency components. This work proposes a material absorption-based model capable of calculating the carrier generation rate accurately in the time domain. Using the Lorentz dispersion model with poles reside in the optical frequency region, the instantaneous optical absorption, which corresponds to the power dissipation in the polarization, is calculated and used to calculate the generation rate. The Lorentz model is formulated with an auxiliary differential equation method that updates the polarization current density, from which the absorbed optical power corresponding to each Lorentz pole is directly calculated in the time domain. Examples show that the proposed model is more accurate than the Poynting vector-based model and is stable even when the generated low-frequency component is strong.
• #### Domain-Size-Dependent Residual Stress Governs the Phase-Transition and Photoluminescence Behavior of Methylammonium Lead Iodide

(Advanced Functional Materials, Wiley, 2021-02-11) [Article]
Methylammonium lead iodide (MAPbI3) perovskite has garnered significant interest as a versatile material for optoelectronic applications. The temperature-dependent photoluminescence (TDPL) and phase-transition behaviors revealed in previous studies have become standard indicators of defects, stability, charge carrier dynamics, and device performance. However, published reports abound with examples of irregular photoluminescence and phase-transition phenomena that are difficult to reconcile, posing major challenges in the correlation of those properties with the actual material state or with the subsequent device performance. In this paper, a unifying explanation for the seemingly inconsistent TDPL and phase-transition (orthorhombic-to-tetragonal) characteristics observed for MAPbI3 is presented. By investigating MAPbI3 perovskites with varying crystalline states, ranging from polycrystal to highly oriented crystal as well as single-crystals, key features in the TDPL and phase-transition behaviors are identified that are related to the extent of crystal domain-size-dependent residual stress and stem from the considerable volume difference (ΔV ≈ 4.5%) between the primitive unit cells of the orthorhombic (at 80 K) and tetragonal phases (at 300 K) of MAPbI3. This fundamental connection is essential for understanding the photophysics and material processing of soft perovskites.
• #### Reduction of the Beam Pointing Error for Improved Free-Space Optical Communication Link Performance

(arXiv, 2021-02-09) [Preprint]
Free-space optical communication is emerging as a low-power, low-cost, and high data rate alternative to radio-frequency communication in short-to medium-range applications. However, it requires a close-to-line-of-sight link between the transmitter and the receiver. This paper proposes a robust $\cHi$ control law for free-space optical (FSO) beam pointing error systems under controlled weak turbulence conditions. The objective is to maintain the transmitter-receiver line, which means the center of the optical beam as close as possible to the center of the receiving aperture within a prescribed disturbance attenuation level. First, we derive an augmented nonlinear discrete-time model for pointing error loss due to misalignment caused by weak atmospheric turbulence. We then investigate the $\cHi$-norm optimization problem that guarantees the closed-loop pointing error is stable and ensures the prescribed weak disturbance attenuation. Furthermore, we evaluate the closed-loop outage probability error and bit error rate (BER) that quantify the free-space optical communication performance in fading channels. Finally, the paper concludes with a numerical simulation of the proposed approach to the FSO link's error performance.
• #### Establishing and Maintaining a Reliable Optical Wireless Communication in Underwater Environment

(arXiv, 2021-02-09) [Preprint]
This paper proposes the trajectory tracking problem between an autonomous underwater vehicle (AUV) and a mobile surface ship, both equipped with optical communication transceivers. The challenging issue is to maintain stable connectivity between the two autonomous vehicles within an optical communication range. We define a directed optical line-of-sight (LoS) link between the two-vehicle systems. The transmitter is mounted on the AUV while the surface ship is equipped with an optical receiver. However, this optical communication channel needs to preserve a stable transmitter-receiver position to reinforce service quality, which typically includes a bit rate and bit error rates. A cone-shaped beam region of the optical receiver is approximated based on the channel model; then, a minimum bit rate is ensured if the AUV transmitter remains inside of this region. Additionally, we design two control algorithms for the transmitter to drive the AUV and maintain it in the cone-shaped beam region under an uncertain oceanic environment. Lyapunov function-based analysis that ensures asymptotic stability of the resulting closed-loop tracking error is used to design the proposed NLPD controller. Numerical simulations are performed using MATLAB/Simulink to show the controllers' ability to achieve favorable tracking in the presence of the solar background noise within competitive times. Finally, results demonstrate the proposed NLPD controller improves the tracking error performance more than $70\%$ under nominal conditions and $35\%$ with model uncertainties and disturbances compared to the original PD strategy.
• #### Time scale state feedback h-stabilisation of linear systems under Lipschitz-type disturbances

(International Journal of Systems Science, Informa UK Limited, 2021-02-08) [Article]
This paper studies the h-stabilisation problem of certain classes of perturbed systems on time scales. Sufficient conditions for the control law design are proposed to ensure the h-stability of the closed-loop dynamical system under Lipschitz-type disturbances. Using the Gronwall inequality approach with time scale theory, the h-stability of the closed-loop system is investigated in non-uniform time domains with bounded graininess. Some numerical examples are provided to show the feasibility of the obtained results using the proposed approach for systems evolving on some arbitrary time scales.
• #### Adaptive ensemble optimal interpolation for efficient data assimilation in the red sea

(Journal of Computational Science, Elsevier BV, 2021-02-06) [Article]
Ensemble optimal interpolation (EnOI) is a variant of the ensemble Kalman filter (EnKF) that operates with a static ensemble to drastically reduce its computational cost. The idea is to use a pre-selected ensemble to parameterize the background covariance matrix, which avoids the costly integration of the ensemble members with the dynamical model during the forecast step of the filtering process. To better represent the pronounced time-varying circulation of the Red Sea, we propose a new adaptive EnOI approach in which the ensemble members are adaptively selected at every assimilation cycle from a large dictionary of ocean states describing the Red Sea variability. We implement and test different schemes to select the ensemble members (i) based on the similarity to the forecast state according to some criteria, or (ii) in term of best representation of the forecast in an ensemble subspace using an Orthogonal Matching Pursuit (OMP) algorithm. The relevance of the schemes is first demonstrated with the Lorenz 63 and Lorenz 96 models. Then results of numerical experiments assimilating real remote sensing data into a high resolution MIT general circulation model (MITgcm) of the Red Sea using the Data Assimilation Research Testbed (DART) system are presented and discussed.
• #### DOA Estimation with Non-Uniform Linear Arrays: A Phase-Difference Projection Approach

(arXiv, 2021-02-06) [Preprint]
Phase wrapping is a major problem in direction-of-arrival (DOA) estimation using phase-difference observations. For a sensor pair with an inter-sensor spacing greater than half of the wavelength ($\lambda/2$) of the signal, phase wrapping occurs at certain DOA angles leading to phase-difference ambiguities. Existing phase unwrapping methods exploit either frequency or spatial diversity. These techniques work by imposing restrictions on the utilized frequencies or the receiver array geometry. In addition to sensitivity to noise and calibration errors, these methods may also have high computational complexity. We propose a grid-less \emph{phase-difference projection} (PDP) DOA algorithm to overcome these issues. The concept of \emph{wrapped phased-difference pattern} (WPDP) is introduced, which allows the proposed algorithm to compute most of the parameters required for DOA estimation in an offline manner, hence resulting in a superior computational speed in realtime. Simulation results demonstrate the excellent performance of the proposed algorithm, both in terms of accuracy and speed.
• #### Theory and practice of orbital angular momentum and beyond

(Accepted by Wiley, 2021-02) [Book Chapter]
Nearly three decades since its discovery, orbital angular momentum (OAM) has proven to be highly versatile for a wide range of applications. It is an indispensable tool in quantum optics, has made a significant impact in optical tweezing, enabled higher contrast and more detailed imaging, and offers a convenient way to harness the space degree of freedom in telecommunications. In this paper, we present a review of a wide range of applications of OAM as well as describing the creation and detection of OAM modes, with a focus on the use of OAM in communications. In addition, we detail various similar higher-order optical modes, such as vector vortex modes, and provide an introduction to the use of OAM in quantum optics, pitched for readers new to the field.