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Generation of even and odd high harmonics in resonant metasurfaces using single and multiple ultra-intense laser pulses

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arXiv 2020年

作者： Shcherbakov, Maxim R. Zhang, Haizhong Tripepi, Michael Sartorello, Giovanni Talisa, Noah AlShafey, Abdallah Fan, Zhiyuan Twardowski, Justin Krivitsky, Leonid A. Kuznetsov, Arseniy I. Chowdhury, Enam Shvets, Gennady School of Applied and Engineering Physics Cornell University IthacaNY14853 United States Singapore138634 Singapore Department of Physics Ohio State University ColumbusOH43210 United States Department of Material Science and Engineering Ohio State University ColumbusOH43210 United States Department of Electrical and Computer Engineering Ohio State University ColumbusOH43210 United States

High harmonic generation (HHG)1,2 opens a window on the fundamental science of strong-field light-mater interaction and serves as a key building block for attosecond optics and metrology3,4. Resonantly enhanced HHG from hot spots in nanostructures is an attractive route to overcoming the well-known limitations of gases and bulk solids5–10. We demonstrate a nanoscale platform for highly efficient HHG driven by strong mid-infrared laser pulses: an ultra-thin resonant gallium phosphide (GaP) metasurface. The wide bandgap and the lack of inversion symmetry of the GaP crystal enable the generation of even and odd harmonics covering a wide range of photon energies between 1.3 and 3 eV with minimal reabsorption. The resonantly enhanced conversion efficiency facilitates single-shot measurements that avoid material damage and pave the way to controllable transition between perturbative and non-perturbative regimes of light-matter interactions at the nanoscale. Copyright © 2020, The Authors. All rights reserved.

关键词： III-V semiconductors

Control synthesis from linear temporal logic specifications using model-free reinforcement learning

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arXiv 2019年

作者： Bozkurt, Alper Kamil Wang, Yu Zavlanos, Michael Pajic, Miroslav Department of Electrical and Computer Engineering Duke University DurhamNC27708 United States Department of Mechanical Engineering and Material Science Duke University DurhamNC27708 United States

We present a reinforcement learning (RL) framework to synthesize a control policy from a given linear temporal logic (LTL) specification in an unknown stochastic environment that can be modeled as a Markov Decision Process (MDP). Specifically, we learn a policy that maximizes the probability of satisfying the LTL formula without learning the transition probabilities. We introduce a novel rewarding and path-dependent discounting mechanism based on the LTL formula such that (i) an optimal policy maximizing the total discounted reward effectivelly maximizes the probabilities of satisfying LTL objectives, and (ii) a model-free RL algorithm using these rewards and discount factors is guaranteed to converge to such policy. Finally, we illustrate the applicability of our RL-based synthesis approach on two motion planning case studies. Copyright © 2019, The Authors. All rights reserved.

关键词： Markov processes

Tunable Magnetic Domain Walls for Therapeutic Neuromodulation at Cellular Level: Stimulating Neurons Through Magnetic Nanowires

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arXiv 2019年

作者： Su, Diqing Wu, Kai Saha, Renata Wang, Jian-Ping Department of Chemical Engineering and Material Science University of Minnesota MinneapolisMN55455 United States Department of Electrical and Computer Engineering University of Minnesota MinneapolisMN55455 United States

Cellular-level neuron stimulation has attracted much attention in the areas of prevention, diagnosis and treatment of neurological disorders. Herein, we propose a spintronic neurostimulator based on the domain wall movement inside stationary magnetic nanowires driven by the spin transfer torque. The electromotive forces generated by the domain wall motion can serve as highly localized stimulation signals for neuron cells. Our simulation results show that the induced electric field from the domain wall motion in permalloy nanowires can reach up to 14 V/m, which is well above the reported threshold stimulation signal for clinical applications. The proposed device operates on a current range of several μA which is 103 times lower compared to magnetic stimulation by microcoils. The duration and amplitude of the stimulating signal can be controlled by adjusting the applied current density, the geometry of the nanowire, and the magnetic properties of the nanowire material. Copyright © 2019, The Authors. All rights reserved.

关键词： Domain walls

Distributed online convex optimization with improved dynamic regret

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arXiv 2019年

作者： Zhang, Yan Ravier, Robert J. Tarokh, Vahid Zavlanos, Michael M. Department of Mechanical Engineering and Material Science Duke University DurhamNC27708 United States Department of Electrical and Computer Engineering Duke University DurhamNC27708 United States

We consider the problem of distributed online convex optimization, where a group of agents collaborate to track the trajectory of the global minimizers of sums of time-varying objective functions in an online manner. For general convex functions, the theoretical upper bounds of existing methods are given in terms of regularity measures associated with the dynamical system as well as the time horizon. It is thus of interest to determine whether the explicit time horizon dependence can be removed as in the case of centralized optimization. In this work, we propose a novel distributed online gradient descent algorithm and show that the dynamic regret bound of this algorithm has no explicit dependence on the time horizon. Instead, it depends on a new regularity measure quantifying the total change in gradients at the optimal points at each time. The main driving force of our algorithm is an online adaptation of the gradient tracking technique used in static optimization. Since, in many applications, time-varying objective functions and the corresponding optimal points follow a non-adversarial dynamical system, we also consider the role of prediction assuming that the optimal points evolve according to a linear dynamical system. We present numerical experiments that show that our proposed algorithm outperforms the existing distributed mirror descent-based state of the art methods in term of the optimizer tracking performance. We also present an empirical example suggesting that the analysis of our algorithm is optimal in the sense that the regularity measures in the theoretical bounds cannot be removed. Copyright © 2019, The Authors. All rights reserved.

关键词： Dynamical systems

Deterministic field-free switching of a perpendicularly magnetized ferromagnetic layer via the joint effects of Dzyaloshinskii-Moriya Interaction and field-like Spin-Orbit Torque: An appraisal

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arXiv 2019年

作者： Wu, Kai Su, Diqing Saha, Renata Wang, Jian-Ping Department of Electrical and Computer Engineering University of Minnesota MinneapolisMN55455 United States Department of Chemical Engineering and Material Science University of Minnesota MinneapolisMN55455 United States

Field-free switching of perpendicularly magnetized ferromagnetic layer by spin orbit torque (SOT) from the spin Hall effect (SHE) is of great interest in the applications of magnetic memory devices. In this paper, we investigate the deterministic SOT switching through the joint effects of Dzyaloshinskii-Moriya Interaction (DMI) and field-like torque (FLT) by micromagnetic simulations. We confirmed that within a certain range of DMI values and charge current densities, it is possible to deterministically switch the magnetization without the assistance external magnetic field. We show that the FLT could play an adverse role in blocking and slowing down the magnetization switching under certain cases of DMI and charge current-driven field-free switching. However, in other cases, FLT can assist DMI on the deterministic field-free SOT switching. In addition, it is found that FLT can effectively expand the current density window for a deterministic field-free SOT switching in the presence of DMI. Copyright © 2019, The Authors. All rights reserved.

关键词： Spin Hall effect

Investigating the effect of magnetic dipole-dipole interaction on Magnetic Particle Spectroscopy (MPS): Implications for magnetic nanoparticle-based bioassays and Magnetic Particle Imaging (MPI)

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arXiv 2019年

作者： Wu, Kai Su, Diqing Saha, Renata Liu, Jinming Wang, Jian-Ping Department of Electrical and Computer Engineering University of Minnesota MinneapolisMN55455 United States Department of Chemical Engineering and Material Science University of Minnesota MinneapolisMN55455 United States

Superparamagnetic iron oxide nanoparticles (SPIONs), with comparable size to biomolecules (such as proteins, nucleic acids, etc.) and unique magnetic properties, good biocompatibility, low toxicity, potent catalytic behavior, are promising candidates for many biomedical applications. There is one property present in most SPION systems, yet it has not been fully exploited, which is the dipole-dipole interaction (also called dipolar interaction) between the SPIONs. It is known that the magnetic dynamics of an ensemble of SPIONs are substantially influenced by the dipolar interactions. However, the exact way it affects the performance of magnetic particle-based bioassays and magnetic particle imaging (MPI) is still an open question. The purpose of this paper is to give a partial answer to this question. This is accomplished by numerical simulations on the dipolar interactions between two nearby SPIONs and experimental measurements on an ensemble of SPIONs using our lab-based magnetic particle spectroscopy (MPS) system. Our results show that even moderate changes in the SPION concentration may have substantial effects on the magnetic dynamics of the SPION system and the harmonic signal magnitudes can be increased or decreased by 60%, depending on the values of MPS system parameters. Copyright © 2019, The Authors. All rights reserved.

关键词： Biocompatibility

Detection of magnetic nanoparticles (MNPs) using spin current nano-oscillator (SCNO) biosensor: A frequencybased rapid, ultra-sensitive, magnetic bioassay

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arXiv 2019年

作者： Saha, Renata Wu, Kai Su, Diqing Wang, Jian-Ping Department of Electrical and Computer Engineering University of Minnesota MinneapolisMN55455 United States Department of Chemical Engineering and Material Science University of Minnesota MinneapolisMN55455 United States

Spin-orbit torque and spin hall effect-based cellular level therapeutic neuromodulators: Modulating neuron activities through spintronic nanodevices

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arXiv 2019年

Artificial modulation of a neuronal subset through ion channels activation can initiate firing patterns of an entire neural circuit in vivo. As nanovalves in the cell membrane, voltage-gated ion channels can be artificially controlled by the electric field gradient that caused by externally applied time varying magnetic fields. Herein, we theoretically investigate the feasibility of modulating neural activities by using magnetic spintronic nanostructures. An antiferromagnet/ferromagnet (AFM/FM) structure is explored as neuromodulator. For FM layer with perpendicular magnetization, stable bidirectional magnetization switching can be achieved by applying in-plane currents through AFM layer to induce the spin-orbit torque (SOT) due to the spin Hall effect (SHE). This Spin-orbit Torque Neurostimulator (SOTNS) utilizes in-plane charge current pulses to switch the magnetization in FM layer. The time changing magnetic stray field induces electric field that modulates the surrounding neurons. The Object Oriented Micromagnetic Framework (OOMMF) is used to calculate space and time dependent magnetic dynamics of SOTNS structure. The current driven magnetization dynamics in SOTNS has no mechanically moving parts. Furthermore, the size of SOTNS can be down to tens of nanometers, thus, arrays of SOTNSs could be fabricated, integrated together and patterned on a flexible substrate, which gives us much more flexible control of the neuromodulation with cellular resolution. Copyright © 2019, The Authors. All rights reserved.

关键词： Spin Hall effect

Triggering phase-coherent spin packets by pulsed electrical spin injection across an Fe/GaAs Schottky barrier

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arXiv 2021年

作者： Schreiber, L.R. Schwark, C. Güntherodt, G. Lepsa, M. Adelmann, C. Palmstrøm, C.J. Lou, X. Crowell, P.A. Beschoten, Bernd 2nd Institute of Physics and JARA-FIT RWTH Aachen University Aachen52074 Germany JARA-FIT Institute for Quantum Information Forschungszentrum Jülich GmbH RWTH Aachen University Aachen52074 Germany Forschungszentrum Jülich GmbH Jülich52425 Germany Department of Chemical Engineering and Material Science University of Minnesota Minneapolis55455 United States IMEC Leuven3001 Belgium Departments of Electrical and Computer Engineering and Materials University of California Santa BarbaraCA93106 United States School of Physics and Astronomy University of Minnesota MinneapolisMN55455 United States

The precise control of spins in semiconductor spintronic devices requires electrical means for generating spin packets with a well-defined initial phase. We demonstrate a pulsed electrical scheme that triggers the spin ensemble phase in a similar way as circularly-polarized optical pulses are generating phase coherent spin packets. Here, we use fast current pulses to initialize phase coherent spin packets, which are injected across an Fe/GaAs Schottky barrier into n-GaAs. By means of time-resolved Faraday rotation, we demonstrate phase coherence by the observation of multiple Larmor precession cycles for current pulse widths down to 500 ps at 17 K. We show that the current pulses are broadened by the charging and discharging time of the Schottky barrier. At high frequencies, the observable spin coherence is limited only by the finite band width of the current pulses, which is on the order of 2 GHz. These results therefore demonstrate that all-electrical injection and phase control of electron spin packets at microwave frequencies is possible in metallic-ferromagnet/semiconductor heterostructures. Copyright © 2021, The Authors. All rights reserved.

关键词： Schottky barrier diodes