Browsing by Author "Jungfleisch, M. Benjamin"
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Item Band Engineering of ErAs:InGaAlBiAs Nanocomposite Materials for Terahertz Photoconductive Switches Pumped at 1550 nm(Advanced Functional Materials, 2024-04-18) Acuna, Wilder; Wu, Weipeng; Bork, James; Doty, Mathew F.; Jungfleisch, M. Benjamin; Gundlach, Lars; Zide, Joshua M. O.Terahertz technology has the potential to have a large impact in myriad fields, such as biomedical science, spectroscopy, and communications. Making these applications practical requires efficient, reliable, and low-cost devices. Photoconductive switches (PCS), devices capable of emitting and detecting terahertz pulses, are a technology that needs more efficiency when working at telecom wavelength excitation (1550 nm) to exploit the advantages this wavelength offers. ErAs:InGaAs is a semiconductor nanocomposite working at this energy; however, high dark resistivity is challenging due to a high electron concentration as the Fermi level lies in the conduction band. To increase dark resistivity, ErAs:InGaAlBiAs material is used as the active material in a PCS detecting Terahertz pulses. ErAs nanoparticles reduce the carrier lifetime to subpicosecond values required for short temporal resolution, while ErAs pins the effective Fermi level in the host material bandgap. Unlike InGaAs, InGaAlBiAs offers enough freedom for band engineering to have a material compatible with a 1550 nm pump and a Fermi level deep in the bandgap, meaning low carrier concentration and high dark resistivity. Band engineering is possible by incorporating aluminum to lift the conduction band edge to the Fermi level and bismuth to keep a bandgap compatible with 1550 nm excitation.Item Comparing spin injection in Fe75Co25/Bi2Te3 at GHz and optical excitations(Applied Physics Letters, 2023-02-13) Sharma, Vinay; Nepal, Rajeev; Wu, Weipeng; Pogue, E. A.; Kumar, Ravinder; Kolagani, Rajeswari; Gundlach, Lars; Jungfleisch, M. Benjamin; Budhani, Ramesh C.Spin-to-charge conversion (S2CC) processes in thin-film heterostructures have attracted much attention in recent years. Here, we describe the S2CC in a 3D topological insulator Bi2Te3 interfaced with an epitaxial film of Fe75Co25. The quantification of spin-to-charge conversion is made with two complementary techniques: ferromagnetic resonance based inverse spin Hall effect (ISHE) at GHz frequencies and femtosecond light-pulse induced emission of terahertz (THz) radiation. The role of spin rectification due to extrinsic effects like anisotropic magnetoresistance (AMR) and planar Hall effects (PHE) is pronounced at the GHz timescale, whereas the THz measurements do not show any detectible signal, which could be attributed to AMR or PHE. This result may be due to (i) homodyne rectification at GHz, which is absent in THz measurements and (ii) laser-induced thermal spin current generation and magnetic dipole radiation in THz measurements, which is completely absent in GHz range. The converted charge current has been analyzed using the spin diffusion model for the ISHE. We note that regardless of the differences in timescales, the spin diffusion length in the two cases is comparable. Our results aid in understanding the role of spin pumping timescales in the generation of ISHE signals.Item Direct probing of strong magnon–photon coupling in a planar geometry(Quantum Science and Technology, 2022-10-31) Kaffash, Mojtaba T.; Wagle, Dinesh; Rai, Anish; Meyer, Thomas; Xiao, John Q.; Jungfleisch, M. BenjaminWe demonstrate direct probing of strong magnon–photon coupling using Brillouin light scattering (BLS) spectroscopy in a planar geometry. The magnonic hybrid system comprises a split-ring resonator loaded with epitaxial yttrium iron garnet thin films of 200 nm and 2.46 μm thickness. The BLS measurements are combined with microwave spectroscopy measurements where both biasing magnetic field and microwave excitation frequency are varied. The cooperativity for the 200 nm-thick YIG films is 1.1, and larger cooperativity of 29.1 is found for the 2.46 μm-thick YIG film. We show that BLS is advantageous for probing the magnonic character of magnon–photon polaritons, while microwave absorption is more sensitive to the photonic character of the hybrid excitation. A miniaturized, planar device design is imperative for the potential integration of magnonic hybrid systems in future coherent information technologies, and our results are a first stepping stone in this regard. Furthermore, successfully detecting the magnonic hybrid excitation by BLS is an essential step for the up-conversion of quantum signals from the microwave to the optical regime in hybrid quantum systems.Item Light and microwave driven spin pumping across FeGaB–BiSb interface(Physical Review Materials, 2021-12-16) Sharma, Vinay; Wu, Weipeng; Bajracharya, Prabesh; To, Duy Quang; Johnson, Anthony; Janotti, Anderson; Bryant, Garnett W.; Gundlach, Lars; Jungfleisch, M. Benjamin; Budhani, Ramesh C.Three-dimensional (3D) topological insulators (TIs) with large spin Hall conductivity have emerged as potential candidates for spintronic applications. Here, we report spin to charge conversion in bilayers of amorphous ferromagnet (FM) Fe78Ga13B9 (FeGaB) and 3D TI Bi85Sb15 (BiSb) activated by two complementary techniques: spin pumping and ultrafast spin-current injection. DC magnetization measurements establish the soft magnetic character of FeGaB films, which remains unaltered in the heterostructures of FeGaB-BiSb. Broadband ferromagnetic resonance (FMR) studies reveal enhanced damping of precessing magnetization and large value of spin mixing conductance (5.03×1019m–2) as the spin angular momentum leaks into the TI layer. Magnetic field controlled bipolar DC voltage generated across the TI layer by inverse spin Hall effect is analyzed to extract the values of spin Hall angle and spin diffusion length of BiSb. The spin pumping parameters derived from the measurements of the femtosecond light-pulse-induced terahertz emission are consistent with the result of FMR. The Kubo-Bastin formula and tight-binding model calculations shed light on the thickness-dependent spin-Hall conductivity of the TI films, with predictions that are in remarkable agreement with the experimental data. Our results suggest that room temperature deposited amorphous and polycrystalline heterostructures provide a promising platform for creating novel spin orbit torque devices.Item MEMS-actuated terahertz metamaterials driven by phase-transition materials(Frontiers of Optoelectronics, 2024-05-27) Huang, Zhixiang; Wu, Weipeng; Herrmann, Eric; Ma, Ke; Chase, Zizwe A.; Searles, Thomas A.; Jungfleisch, M. Benjamin; Wang, XiThe non-ionizing and penetrative characteristics of terahertz (THz) radiation have recently led to its adoption across a variety of applications. To effectively utilize THz radiation, modulators with precise control are imperative. While most recent THz modulators manipulate the amplitude, frequency, or phase of incident THz radiation, considerably less progress has been made toward THz polarization modulation. Conventional methods for polarization control suffer from high driving voltages, restricted modulation depth, and narrow band capabilities, which hinder device performance and broader applications. Consequently, an ideal THz modulator that offers high modulation depth along with ease of processing and operation is required. In this paper, we propose and realize a THz metamaterial comprised of microelectromechanical systems (MEMS) actuated by the phase-transition material vanadium dioxide (VO2). Simulation and experimental results of the three-dimensional metamaterials show that by leveraging the unique phase-transition attributes of VO2, our THz polarization modulator offers notable advancements over existing designs, including broad operation spectrum, high modulation depth, ease of fabrication, ease of operation condition, and continuous modulation capabilities. These enhanced features make the system a viable candidate for a range of THz applications, including telecommunications, imaging, and radar systems. Graphical Abstract available at: https://doi.org/10.1007/s12200-024-00116-4Item Nonlinear multi-magnon scattering in artificial spin ice(Nature Communications, 2023-06-09) Lendinez, Sergi; Kaffash, Mojtaba T.; Heinonen, Olle G.; Gliga, Sebastian; Iacocca, Ezio; Jungfleisch, M. BenjaminMagnons, the quantum-mechanical fundamental excitations of magnetic solids, are bosons whose number does not need to be conserved in scattering processes. Microwave-induced parametric magnon processes, often called Suhl instabilities, have been believed to occur in magnetic thin films only, where quasi-continuous magnon bands exist. Here, we reveal the existence of such nonlinear magnon-magnon scattering processes and their coherence in ensembles of magnetic nanostructures known as artificial spin ice. We find that these systems exhibit effective scattering processes akin to those observed in continuous magnetic thin films. We utilize a combined microwave and microfocused Brillouin light scattering measurement approach to investigate the evolution of their modes. Scattering events occur between resonance frequencies that are determined by each nanomagnet’s mode volume and profile. Comparison with numerical simulations reveals that frequency doubling is enabled by exciting a subset of nanomagnets that, in turn, act as nanosized antennas, an effect that is akin to scattering in continuous films. Moreover, our results suggest that tunable directional scattering is possible in these structures.Item Observation of ultrafast ballistic orbital transport(Nature Nanotechnology, 2023-08-07) Jungfleisch, M. BenjaminTerahertz emission spectroscopy reveals long-distance ballistic orbital-angular-momentum transport in tungsten. While most electronic devices so far are based on the electron’s charge or its spin degree of freedom, electrons can also carry orbital angular momentum. Orbitronics (orbital electronics), which focuses on the electron’s orbital angular momentum1, is much less explored than the field of spintronics, especially at terahertz (THz) frequencies2,3. However, orbitronics promises higher-density information transfer over longer distances in many materials than would be possible with spin currents. Furthermore, utilizing the electron’s orbital angular momentum L offers distinct advantages: (1) orbital current is an emergent property from Bloch states in a solid, comprising many atoms and, hence, orbital angular momentum transfer can be arbitrarily large1, whereas the spin angular momentum S of one electron is limited to 1/2h. This may hinder efficient transport and control of information in spintronic devices. (2) The conversion of orbital angular momentum to charge currents does not rely on spin–orbit coupling, suggesting that many more materials could potentially be harnessed for interfacing angular-momentum-based devices with charge-based devices4. Despite these advantages, it has been experimentally challenging to unambiguously distinguish L and S transport and their conversion into charge currents. Furthermore, it has been unclear if L transport could be used similarly to S transport at ultrafast timescales, potentially leading to efficient THz devices5,6.Item Spin currents with unusual spin orientations in noncollinear Weyl antiferromagnetic Mn3Sn(Physical Review Materials, 2023-03-10) Wang, Xinhao; Hossain, Mohammad Tomal; Thapaliya, T. R.; Khadka, Durga; Lendinez, Sergi; Chen, Hang; Doty, Matthew F.; Jungfleisch, M. Benjamin; Huang, S. X.; Fan, Xin; Xiao, John Q.There are intensive efforts to search for mechanisms that lead to spin-orbit torque with unusual spin orientation, particularly out-of-plane spin orientation which can efficiently switch perpendicular magnetizations. Such a phenomenon has been observed in materials with low structural symmetry, ferromagnetic materials, and antiferromagnets with noncollinear spin structures. Here, we demonstrate the observation of, in addition to out-of-plane spin orientation, spin orientation along the charge current direction in Mn3Sn, a noncollinear antiferromagnet and Weyl semimetal. The mechanism arises from noncollinear spin structure with spin-orbit coupling and it can be viewed as spin rotation around the octupole moment, the lowest order of cluster multipole moment pertaining to the Mn3Sn crystal group.Item Ultrastrong magnon-magnon coupling and chiral spin-texture control in a dipolar 3D multilayered artificial spin-vortex ice(Nature Communications, 2024-05-14) Dion, Troy; Stenning, Kilian D.; Vanstone, Alex; Holder, Holly H.; Sultana, Rawnak; Alatteili, Ghanem; Martinez, Victoria; Kaffash, Mojtaba Taghipour; Kimura, Takashi; Oulton, Rupert F.; Branford, Will R.; Kurebayashi, Hidekazu; Iacocca, Ezio; Jungfleisch, M. Benjamin; Gartside, Jack C.Strongly-interacting nanomagnetic arrays are ideal systems for exploring reconfigurable magnonics. They provide huge microstate spaces and integrated solutions for storage and neuromorphic computing alongside GHz functionality. These systems may be broadly assessed by their range of reliably accessible states and the strength of magnon coupling phenomena and nonlinearities. Increasingly, nanomagnetic systems are expanding into three-dimensional architectures. This has enhanced the range of available magnetic microstates and functional behaviours, but engineering control over 3D states and dynamics remains challenging. Here, we introduce a 3D magnonic metamaterial composed from multilayered artificial spin ice nanoarrays. Comprising two magnetic layers separated by a non-magnetic spacer, each nanoisland may assume four macrospin or vortex states per magnetic layer. This creates a system with a rich 16N microstate space and intense static and dynamic dipolar magnetic coupling. The system exhibits a broad range of emergent phenomena driven by the strong inter-layer dipolar interaction, including ultrastrong magnon-magnon coupling with normalised coupling rates of Δf/ν =0.57, GHz mode shifts in zero applied field and chirality-control of magnetic vortex microstates with corresponding magnonic spectra.