Showing 1–26 of 26 results for author: Rao, J

Image detection-based high-throughput sorting of particles using traveling surface acoustic waves in microscale flows

Authors: Nikhil Sethia, Joseph Sushil Rao, Amit Manicka, Michael L. Etheridge, Erik B. Finger, John C. Bischof, Cari S. Dutcher

Abstract: Large particle sorters have potential applications in sorting microplastics and large biomaterials (>50 micrometer), such as tissues, spheroids, organoids, and embryos. Though great advancements have been made in image-based sorting of cells and particles (<50 micrometer), their translation for high-throughput sorting of larger biomaterials and particles (>50 micrometer) has been more limited. An… ▽ More Large particle sorters have potential applications in sorting microplastics and large biomaterials (>50 micrometer), such as tissues, spheroids, organoids, and embryos. Though great advancements have been made in image-based sorting of cells and particles (<50 micrometer), their translation for high-throughput sorting of larger biomaterials and particles (>50 micrometer) has been more limited. An image-based detection technique is highly desirable due to richness of the data (including size, shape, color, morphology, and optical density) that can be extracted from live images of individualized biomaterials or particles. Such a detection technique is label-free and can be integrated with a contact-free actuation mechanism such as one based on traveling surface acoustic waves (TSAWs). Recent advances in using TSAWs for sorting cells and particles (<50 micrometer) have demonstrated short response times (<1 ms), high biocompatibility, and reduced energy requirements to actuate. Additionally, TSAW-based devices are miniaturized and easier to integrate with an image-based detection technique. In this work, a high-throughput image-detection based large particle microfluidic sorting technique is implemented. The technique is used to separate binary mixtures of small and large polyethylene particles (ranging between ~45-180 micrometer in size). All particles in flow were first optically interrogated for size, followed by actuations using momentum transfer from TSAW pulses, if they satisfied the size cutoff criterion. The effect of control parameters such as duration and power of TSAW actuation pulse, inlet flow rates, and sample dilution on sorting efficiency and throughput was observed. At the chosen conditions, this sorting technique can sort on average ~4.9-34.3 particles/s (perform ~2-3 actuations/s), depending on the initial sample composition and concentration. △ Less

Submitted 26 August, 2025; originally announced September 2025.

Comments: 31 pages, 5 figures

Ultrasensitive Magnetometer based on Cusp Points of the Photon-Magnon Synchronization Mode

Authors: Xinlin Mi, Jinwei Rao, Lijun Yan, Xudong Wang, Bingbing Lyu, Bimu Yao, Shishen Yan, Lihui Bai

Abstract: Ultrasensitive magnetometers based on spin resonances have led to remarkable achievements. However, the gyromagnetic ratios of these spin resonances that determine the responsivity of magnetometers to weak magnetic fields are inherently constrained by the Land$\acute{e}$ g-factor of particles, such as the electron, with a constant gyromagnetic ratio of $γ_e=2π\times28$ GHz/T. Here, we demonstrate… ▽ More Ultrasensitive magnetometers based on spin resonances have led to remarkable achievements. However, the gyromagnetic ratios of these spin resonances that determine the responsivity of magnetometers to weak magnetic fields are inherently constrained by the Land$\acute{e}$ g-factor of particles, such as the electron, with a constant gyromagnetic ratio of $γ_e=2π\times28$ GHz/T. Here, we demonstrate an ultrasensitive magnetometer based on the cusp point (CP) of photon-magnon synchronization modes (PMSMs). The PMSM's gyromagnetic ratio at the CP is enhanced to $37γ_e$ and further amplified to $236γ_e$ by utilizing the sixth-order oscillating mode of the PMSM. Moreover, the emission linewidth of the PMSM can be reduced to 0.06 Hz, resulting in excellent sensitivity to weak magnetic fields. These outstanding properties position our magnetometer to potentially achieve superior sensitivity to conventional magnetometers. Our work introduces a cost-effective prototype for the next generation of magnetometry, and may advance scientific research and technologies that rely on ultrasensitive magnetic field detection. △ Less

Submitted 8 July, 2025; originally announced July 2025.

Filling a gap in materials mechanics: Nanoindentation at high constant strain rates upto $10^5 s^{-1}$

Authors: Lalith Kumar Bhaskar, Dipali Sonawane, Hendrik Holz, Jeongin Paeng, Peter Schweizer, Jing Rao, Bárbara Bellón, Damian Frey, Aloshious Lambai, Laszlo Petho, Johann Michler, Jakob Schwiedrzik, Gaurav Mohanty, Gerhard Dehm, Rajaprakash Ramachandramoorthy

Abstract: A central focus in high strain rate research is understanding the dynamic behavior of materials at strain rates where a strength upturn is observed. While strength upturns at strain rates of $10^3$ to $10^4~\mathrm{s}^{-1}$ have been widely reported in the literature, their occurrence in certain materials remains controversial, and the underlying physics driving this phenomenon is not yet fully un… ▽ More A central focus in high strain rate research is understanding the dynamic behavior of materials at strain rates where a strength upturn is observed. While strength upturns at strain rates of $10^3$ to $10^4~\mathrm{s}^{-1}$ have been widely reported in the literature, their occurrence in certain materials remains controversial, and the underlying physics driving this phenomenon is not yet fully understood. Current mechanical testing methods are limited, as no single technique spans the full strain rate range of $10^1$ to $10^5~\mathrm{s}^{-1}$ where this phenomenon is expected, and a unified technique would enable consistent post-deformation characterization with minimal error. To address this, we developed a customized piezoelectric in situ nanomechanical test setup, enabling constant indentation strain rates up to $10^5~\mathrm{s}^{-1}$ for the first time. Using this system, we examined rate-dependent hardness in single-crystalline molybdenum, nanocrystalline nickel, and amorphous fused silica over strain rates from $10^1$ to $10^5~\mathrm{s}^{-1}$, remarkably revealing a hardness upturn in all three materials. Further, post-deformation analysis of single-crystalline molybdenum revealed that the hardness upturn was primarily driven by increased dislocation density, with phonon drag -- traditionally considered a dominant contributor -- playing a minimal role. △ Less

Submitted 13 June, 2025; v1 submitted 10 February, 2025; originally announced February 2025.

Time-Varying Strong Coupling and Its Induced Time Diffraction of Magnon Modes

Authors: Jinwei Rao, Yi-Pu Wang, Zhijian Chen, Bimu Yao, Kaixin Zhao, Chunke Wei, Congyi Wang, Runze Li, Li-Hui Bai, Wei Lu

Abstract: Time-varying media break the temporal translation symmetry of wave propagation in materials, enabling advanced wave manipulations. However, this novel phenomenon has been rarely explored in magnonic systems due to the significant challenge of achieving a sudden and prominent change in magnon dispersion within materials. Here, we construct a time-varying strong coupling between two magnon modes, an… ▽ More Time-varying media break the temporal translation symmetry of wave propagation in materials, enabling advanced wave manipulations. However, this novel phenomenon has been rarely explored in magnonic systems due to the significant challenge of achieving a sudden and prominent change in magnon dispersion within materials. Here, we construct a time-varying strong coupling between two magnon modes, and observe a change in the beats of Rabi-like oscillations near the pulse edges. Using a frequency-comb spectroscopy technique developed in this work, we characterize the frequency conversion of magnon modes induced by the time-varying strong-coupling effect. Moreover, we construct time slits with adjacent time interfaces and demonstrate, for the first time, the double-slit time diffraction of magnon modes, analogous to the well-known Young's double-slit experiment. These findings rely solely on the time-varying strong magnon coupling, independent of device reconfiguration. Our results open avenues for applications such as all-magnetic mixers or on-chip GHz sources. △ Less

Submitted 18 December, 2024; v1 submitted 11 November, 2024; originally announced November 2024.

Microstructural features and hydrogen diffusion in bcc FeCr alloys: a comparison between the Kelvin probe- and nanohardness based- methods

Authors: Jing Rao, Binhan Sun, Arulkumar Ganapathi, Xizhen Dong, Anton Hohenwarter, Chun-Hung Wu, Michael Rohwerder, Gerhard Dehm, Maria Jazmin Duarte

Abstract: Hydrogen embrittlement can result in a sudden failure in metallic materials, which is particularly harmful in industrially relevant alloys, such as steels. A more comprehensive understanding of hydrogen interactions with microstructural features is critical for preventing hydrogen-induced damage and promoting a hydrogen-based environment-benign economy. We use the Kelvin probe-based potentiometric… ▽ More Hydrogen embrittlement can result in a sudden failure in metallic materials, which is particularly harmful in industrially relevant alloys, such as steels. A more comprehensive understanding of hydrogen interactions with microstructural features is critical for preventing hydrogen-induced damage and promoting a hydrogen-based environment-benign economy. We use the Kelvin probe-based potentiometric hydrogen electrode method and thermal desorption spectroscopy to investigate hydrogen interactions with different hydrogen traps in ferritic FeCr alloys with different chromium contents, dislocation densities, and grain sizes. In addition, we confirm the validity of a novel nanohardness-based diffusion coefficient approach by performing in situ nanoindentation testing. Simultaneous acquisition of the dynamic time-resolved mechanical response of FeCr alloys to hydrogen and the hydrogen diffusivities in these alloys is possible during continuous hydrogen supply. Dislocations, grain boundaries and Cr atoms induce reversible hydrogen trapping sites in these ferritic alloys, leading to the reduction of the hydrogen diffusion coefficients and the increase of the absorbed hydrogen. △ Less

Submitted 4 September, 2024; originally announced September 2024.

Comments: 39 pages, 11 figures

Meso-scale size effects of material heterogeneities on crack propagation in brittle solids: Perspectives from phase-field simulations

Authors: Liuchi Li, Jack Rao, Todd Hufnagel, KT Ramesh

Abstract: Brittle solids are often toughened by adding a second-phase material. This practice often results in composites with material heterogeneities on the meso scale: large compared to the scale of the process zone but small compared to that of the application. The specific configuration (both geometrical and mechanical) of this mesoscale heterogeneity is generally recognized as important in determining… ▽ More Brittle solids are often toughened by adding a second-phase material. This practice often results in composites with material heterogeneities on the meso scale: large compared to the scale of the process zone but small compared to that of the application. The specific configuration (both geometrical and mechanical) of this mesoscale heterogeneity is generally recognized as important in determining crack propagation and, subsequently, the (effective) toughness of the composite. Here, we systematically investigate how dynamic crack propagation is affected by mesoscale heterogeneities taking the form of an array of inclusions. Using a variational phase-field approach, we compute the apparent crack speed and fracture energy dissipation rate to compare crack propagation under Mode-I loading across different configurations of these inclusions. If fixing the volume fraction of inclusions, matching the inclusion size to the K-dominance zone size gives rise to the best toughening outcome. Conversely, if varying the volume fraction of inclusions, a lower volume fraction configuration can lead to a better toughening outcome if and only if the inclusion size approaches from above the size of the K-dominance zone. Since the size of the K-dominance zone can be estimated \textit{a priori} given an understanding of the application scenario and material availability, we can, in principle, exploit this estimation to design a material's mesoscale heterogeneity that optimally balances the tradeoff between strength and toughness. This paves the way for realizing functional (meta-)materials against crack propagation in extreme environments. △ Less

Submitted 19 February, 2024; v1 submitted 22 September, 2023; originally announced September 2023.

Non-Hermitian Topological Magnonics

Authors: Tao Yu, Ji Zou, Bowen Zeng, J. W. Rao, Ke Xia

Abstract: Dissipation in mechanics, optics, acoustics, and electronic circuits is nowadays recognized to be not always detrimental but can be exploited to achieve non-Hermitian topological phases or properties with functionalities for potential device applications. As elementary excitations of ordered magnetic moments that exist in various magnetic materials, magnons are the information carriers in magnonic… ▽ More Dissipation in mechanics, optics, acoustics, and electronic circuits is nowadays recognized to be not always detrimental but can be exploited to achieve non-Hermitian topological phases or properties with functionalities for potential device applications. As elementary excitations of ordered magnetic moments that exist in various magnetic materials, magnons are the information carriers in magnonic devices with low-energy consumption for reprogrammable logic, non-reciprocal communication, and non-volatile memory functionalities. Non-Hermitian topological magnonics deals with the engineering of dissipation and/or gain for non-Hermitian topological phases or properties in magnets that are not achievable in the conventional Hermitian scenario, with associated functionalities cross-fertilized with their electronic, acoustic, optic, and mechanic counterparts, such as giant enhancement of magnonic frequency combs, magnon amplification, (quantum) sensing of the magnetic field with unprecedented sensitivity, magnon accumulation, and perfect absorption of microwaves. In this review article, we address the unified approach in constructing magnonic non-Hermitian Hamiltonian, introduce the basic non-Hermitian topological physics, and provide a comprehensive overview of the recent theoretical and experimental progress towards achieving distinct non-Hermitian topological phases or properties in magnonic devices, including exceptional points, exceptional nodal phases, non-Hermitian magnonic SSH model, and non-Hermitian skin effect. We emphasize the non-Hermitian Hamiltonian approach based on the Lindbladian or self-energy of the magnonic subsystem but address the physics beyond it as well, such as the crucial quantum jump effect in the quantum regime and non-Markovian dynamics. We provide a perspective for future opportunities and challenges before concluding this article. △ Less

Submitted 9 November, 2023; v1 submitted 7 June, 2023; originally announced June 2023.

Comments: 101 pages, 35 figures

Giant Enhancement of Magnonic Frequency Combs by Exceptional Points

Authors: Congyi Wang, Jinwei Rao, Zhijian Chen, Kaixin Zhao, Liaoxin Sun, Bimu Yao, Tao Yu, Yi-Pu Wang, Wei Lu

Abstract: With their incomparable time-frequency accuracy, frequency combs have significantly advanced precision spectroscopy, ultra-sensitive detection, and atomic clocks. Traditional methods to create photonic, phononic, and magnonic frequency combs hinge on material nonlinearities which are often weak, necessitating high power densities to surpass their initiation thresholds, which subsequently limits th… ▽ More With their incomparable time-frequency accuracy, frequency combs have significantly advanced precision spectroscopy, ultra-sensitive detection, and atomic clocks. Traditional methods to create photonic, phononic, and magnonic frequency combs hinge on material nonlinearities which are often weak, necessitating high power densities to surpass their initiation thresholds, which subsequently limits their applications. Here, we introduce a novel nonlinear process to efficiently generate magnonic frequency combs (MFCs) by exploiting exceptional points (EPs) in a coupled system comprising a pump-induced magnon mode and a Kittel mode. Even without any cavity, our method greatly improves the efficiency of nonlinear frequency conversion and achieves optimal MFCs at low pump power. Additionally, our novel nonlinear process enables excellent tunability of EPs using the polarization and power of the pump, simplifying MFC generation and manipulation. Our work establishes a synergistic relationship between non-Hermitian physics and MFCs, which is advantages for coherent/quantum information processing and ultra-sensitive detection. △ Less

Submitted 3 June, 2023; originally announced June 2023.

Comments: 7 pages, 4 figures

Perspective on non-Hermitian physics in magnetic systems

Authors: Tao Yu, J. W. Rao

Abstract: A perspective on non-Hermitian physics in magnetic systems is addressed in this short article, including exceptional points, exceptional nodal phases, the non-Hermitian SSH model, and the non-Hermitian skin effect. A perspective on non-Hermitian physics in magnetic systems is addressed in this short article, including exceptional points, exceptional nodal phases, the non-Hermitian SSH model, and the non-Hermitian skin effect. △ Less

Submitted 23 August, 2023; v1 submitted 24 April, 2023; originally announced April 2023.

Comments: 5 pages. Submitted as a section of Magnonic Roadmap 2024

Meter-scale strong coupling between magnons and photons

Authors: Jinwei Rao, C. Y. Wang, Bimu Yao, Z. J. Chen, K. X. Zhao, Wei Lu

Abstract: We experimentally realize a meter-scale strong coupling effect between magnons and photons at room temperature, with a coherent coupling of 20 m and a dissipative coupling of 7.6 m. To this end, we integrate a saturable gain into a microwave cavity and then couple this active cavity to a magnon mode via a long coaxial cable. The gain compensates for the cavity dissipation, but preserves the cavity… ▽ More We experimentally realize a meter-scale strong coupling effect between magnons and photons at room temperature, with a coherent coupling of 20 m and a dissipative coupling of 7.6 m. To this end, we integrate a saturable gain into a microwave cavity and then couple this active cavity to a magnon mode via a long coaxial cable. The gain compensates for the cavity dissipation, but preserves the cavity radiation that mediates the indirect photon-magnon coupling. It thus enables the long-range strong photon-magnon coupling. With full access to traveling waves, we demonstrate a remote control of photon-magnon coupling by modulating the phase and amplitude of traveling waves, rather than reconfiguring subsystems themselves. Our method for realizing long-range strong coupling in cavity magnonics provides a general idea for other physical systems. Our experimental achievements may promote the construction of information networks based on cavity magnonics. △ Less

Submitted 9 August, 2023; v1 submitted 20 March, 2023; originally announced March 2023.

Coherent Microwave Emission of a Gain-Driven Polariton

Authors: Bimu Yao, Y. S. Gui, J. W. Rao, Y. H. Zhang, Wei Lu, C. -M. Hu

Abstract: By developing a gain-embedded cavity magnonics platform, we create gain-driven polariton (GDP) that is activated by an amplified electromagnetic field. Distinct effects of gain-driven light-matter interaction, such as polariton auto-oscillations, polariton phase singularity, self-selection of a polariton bright mode, and gain-induced magnon-photon synchronization, are theoretically studied and exp… ▽ More By developing a gain-embedded cavity magnonics platform, we create gain-driven polariton (GDP) that is activated by an amplified electromagnetic field. Distinct effects of gain-driven light-matter interaction, such as polariton auto-oscillations, polariton phase singularity, self-selection of a polariton bright mode, and gain-induced magnon-photon synchronization, are theoretically studied and experimentally manifested. Utilizing the gain-sustained photon coherence of the GDP, we demonstrate polariton-based coherent microwave amplication (~ 40 dB) and achieve high-quality coherent microwave emission (Q > 10^9). △ Less

Submitted 15 February, 2023; originally announced February 2023.

Comments: 6 pages, 4 figures

Control of the magnon-polariton hybridization with a microwave pump

Authors: C. Zhang, Jinwei Rao, C. Y. Wang, Z. J. Chen, K. X. Zhao, Bimu Yao, Xu-Guang Xu, Wei Lu

Abstract: Pump-induced magnon modes (PIMs) are recently discovered elementary excitations in ferrimagnets that offer significant tunability to spin dynamics. Here, we investigate the coupling between a PIM and cavity magnon polaritons (CMPs) by driving a cavity magnonic system away from equilibrium with a microwave pump. In our experiment, the Walker mode simultaneously couples with the PIM and cavity photo… ▽ More Pump-induced magnon modes (PIMs) are recently discovered elementary excitations in ferrimagnets that offer significant tunability to spin dynamics. Here, we investigate the coupling between a PIM and cavity magnon polaritons (CMPs) by driving a cavity magnonic system away from equilibrium with a microwave pump. In our experiment, the Walker mode simultaneously couples with the PIM and cavity photons and thus combines two strongly coherent coupling processes in a single cavity structure. Such a PIM-CMP hybridization system acquires complementary properties from both the PIM and CMPs, allowing it to be freely manipulated by the magnetic field, the pump power and the pump frequency. These coherent manipulations exhibit unique behaviors beyond the intrinsic properties limited by the material nature and electromagnetic boundary conditions, thereby creating opportunities for extending the control of hybrid devices. △ Less

Submitted 5 August, 2023; v1 submitted 16 February, 2023; originally announced February 2023.

Unveiling a Pump-Induced Magnon Mode via its Strong Interaction with Walker Modes

Authors: J. W. Rao, Bimu Yao, C. Y. Wang, C. Zhang, Tao Yu, Wei Lu

Abstract: We observe a power-dependent anticrossing of Walker spin-wave modes under microwave pumping when a ferrimagnet is placed in a microwave waveguide that does not support any discrete photon mode. We interpret this unexpected anticrossing as the generation of a pump-induced magnon mode that couples strongly to the Walker modes of the ferrimagnet. This anticrossing inherits an excellent tunability fro… ▽ More We observe a power-dependent anticrossing of Walker spin-wave modes under microwave pumping when a ferrimagnet is placed in a microwave waveguide that does not support any discrete photon mode. We interpret this unexpected anticrossing as the generation of a pump-induced magnon mode that couples strongly to the Walker modes of the ferrimagnet. This anticrossing inherits an excellent tunability from the pump, which allows us to control the anticrossing via the pump power, frequency, and waveform. Further, we realize a remarkable functionality of this anticrossing, namely, a microwave frequency comb, in terms of the nonlinear interaction that mixes the pump and probe frequencies. Such a frequency comb originates from the magnetic dynamics and thereby does not suffer from the charge noise. The unveiled hybrid magnonics driven away from its equilibrium enriches the utilization of anticrossing for coherent information processing. △ Less

Submitted 5 August, 2023; v1 submitted 9 April, 2022; originally announced April 2022.

Comments: 7 pages, 4 figures

Quantitative reconstruction of defects in multi-layered bonded composites using fully convolutional network-based ultrasonic inversion

Authors: Jing Rao, Fangshu Yang, Huadong Mo, Stefan Kollmannsberger, Ernst Rank

Abstract: Ultrasonic methods have great potential applications to detect and characterize defects in multi-layered bonded composites. However, it remains challenging to quantitatively reconstruct defects, such as disbonds and kissing bonds, that influence the integrity of adhesive bonds and seriously reduce the strength of assemblies. In this work, an ultrasonic method based on the supervised fully convolut… ▽ More Ultrasonic methods have great potential applications to detect and characterize defects in multi-layered bonded composites. However, it remains challenging to quantitatively reconstruct defects, such as disbonds and kissing bonds, that influence the integrity of adhesive bonds and seriously reduce the strength of assemblies. In this work, an ultrasonic method based on the supervised fully convolutional network (FCN) is proposed to quantitatively reconstruct defects hidden in multi-layered bonded composites. In the training process of this method, an FCN establishes a non-linear mapping from measured ultrasonic data to the corresponding velocity models of multi-layered bonded composites. In the predicting process, the trained network obtained from the training process is used to directly reconstruct the velocity models from the new measured ultrasonic data of adhesively bonded composites. The presented FCN-based inversion method can automatically extract useful features in multi-layered composites. Although this method is computationally expensive in the training process, the prediction itself in the online phase takes only seconds. The numerical results show that the FCN-based ultrasonic inversion method is capable to accurately reconstruct ultrasonic velocity models of the high contrast defects, which has great potential for online detection of adhesively bonded composites. △ Less

Submitted 11 September, 2021; originally announced September 2021.

Journal ref: Journal of Sound and Vibration, 2023

Void Engineering in Epitaxially Regrown GaAs-Based Photonic Crystal Surface Emitting Lasers by Grating Profile Design

Authors: Adam F. McKenzie, Ben C. King, Katherine J. Rae, Stephen Thoms, Neil D. Gerrard, Jonathan Orchard, Kenishi Nishi, Keizo Takemasa, Mitsuru Sugawara, Richard J. E. Taylor, David T. D. Childs, Donald A. McLaren, Richard A. Hogg

Abstract: We report the engineering of air-voids embedded in GaAs-based photonic crystal surface emitting lasers realised by metalorganic vapour-phase epitaxy regrowth. Two distinct void geometries are obtained by modifying the photonic crystal grating profile within the reactor prior to regrowth. The mechanism of void formation is inferred from scanning transmission electron microscopy analysis, with the e… ▽ More We report the engineering of air-voids embedded in GaAs-based photonic crystal surface emitting lasers realised by metalorganic vapour-phase epitaxy regrowth. Two distinct void geometries are obtained by modifying the photonic crystal grating profile within the reactor prior to regrowth. The mechanism of void formation is inferred from scanning transmission electron microscopy analysis, with the evolution of the growth front illustrated though the use of an AlAs/GaAs superlattice structure. Competition between rapid lateral growth of the (100) surface and slow diffusion across higher index planes is exploited in order to increase void volume, leading to an order of magnitude reduction in threshold current and an increase in output power through an increase in the associated grating coupling strength. △ Less

Submitted 9 November, 2020; originally announced November 2020.

Comments: 7 pages, 4 figures

Unconventional Singularity in Anti-Parity-Time Symmetric Cavity Magnonics

Authors: Y. Yang, Yi-Pu Wang, J. W. Rao, Y. S. Gui, B. M. Yao, W. Lu, C. -M. Hu

Abstract: By engineering an anti-parity-time (anti-PT) symmetric cavity magnonics system with precise eigenspace controllability, we observe two different singularities in the same system. One type of singularity, the exceptional point (EP), is produced by tuning the magnon damping. Between two EPs, the maximal coherent superposition of photon and magnon states is robustly sustained by the preserved anti-PT… ▽ More By engineering an anti-parity-time (anti-PT) symmetric cavity magnonics system with precise eigenspace controllability, we observe two different singularities in the same system. One type of singularity, the exceptional point (EP), is produced by tuning the magnon damping. Between two EPs, the maximal coherent superposition of photon and magnon states is robustly sustained by the preserved anti-PT symmetry. The other type of singularity, arising from the dissipative coupling of two anti-resonances, is an unconventional bound state in the continuum (BIC). At the settings of BICs, the coupled system exhibits infinite discontinuities in the group delay. We find that both singularities co-exist at the equator of the Bloch sphere, which reveals a unique hybrid state that simultaneously exhibits the maximal coherent superposition and slow light capability. △ Less

Submitted 8 September, 2020; originally announced September 2020.

Comments: 6 pages, 4 figures

Journal ref: Phys. Rev. Lett. 125, 147202 (2020)

Electrical detection of magnon-photon interaction via auxiliary spin wave mode

Authors: Peng-Chao Xu, J. W. Rao, Y. Wang, Y. S. Gui, John Q. Xiao, Xiaofeng Jin, C. -M. Hu

Abstract: We report on the electrical detection of a hybrid magnon-photon system, which is comprised of a magnetic sample coupled to a planar cavity. While the uniform Kittel mode has the largest coupling strength among all the magnon modes, it only generates a modest voltage signal by means of inverse spin-Hall effect. We have found that the generated voltage can be significantly enhanced by introducing a… ▽ More We report on the electrical detection of a hybrid magnon-photon system, which is comprised of a magnetic sample coupled to a planar cavity. While the uniform Kittel mode has the largest coupling strength among all the magnon modes, it only generates a modest voltage signal by means of inverse spin-Hall effect. We have found that the generated voltage can be significantly enhanced by introducing a higher order magnon mode, which possesses a much higher spin pumping efficiency and furthermore, it is nearly degenerated with the Kittel mode. The experimental results can be explained by our theoretical model, and suggest that the use of an auxiliary magnon mode can realize the configuration of a magnon-photon system with both strong coupling and large spin current. △ Less

Submitted 8 May, 2020; originally announced May 2020.

Journal ref: Phys. Rev. B 102, 014453 (2020)

Synergistically creating sulfur vacancies in semimetal-supported amorphous MoS2 for efficient hydrogen evolution

Authors: Guowei Li, Chenguang Fu, Jiquan Wu, Jiancun Rao, Sz-Chian Liou, Xijin Xu, Baiqi Shao, Kai Liu, Enke Liu, Nitesh Kumar, Xianjie Liu, Mats Fahlman, Johannes Gooth, Gudrun Auffermann, Yan Sun, Claudia Felser, Baomin Zhang

Abstract: The presence of elemental vacancies in materials is inevitable according to statistical thermodynamics, which will decide the chemical and physical properties of the investigated system. However, the controlled manipulation of vacancies for specific applications is a challenge. Here we report a facile method for creating large concentrations of S vacancies in the inert basal plane of MoS2 supporte… ▽ More The presence of elemental vacancies in materials is inevitable according to statistical thermodynamics, which will decide the chemical and physical properties of the investigated system. However, the controlled manipulation of vacancies for specific applications is a challenge. Here we report a facile method for creating large concentrations of S vacancies in the inert basal plane of MoS2 supported on semimetal CoMoP2. With a small applied potential, S atoms can be removed in the form of H2S due to the optimized free energy of formation. The existence of vacancies favors electron injection from the electrode to the active site by decreasing the contact resistance. As a consequence, the activity is increased by 221 % with the vacancy-rich MoS2 as electrocatalyst for hydrogen evolution reaction (HER). A small overpotential of 75 mV is needed to deliver a current density of 10 mA cm-2, which is considered among the best values achieved for MoS2. It is envisaged that this work may provide a new strategy for utilizing the semimetal phase for structuring MoS2 into a multi-functional material. △ Less

Submitted 25 March, 2020; originally announced March 2020.

Travelling photons mediated interactions between a magnon mode and a cavity photon mode

Authors: J. W. Rao, Y. P. Wang, Y. Yang, T. Yu, Y. S. Gui, X. L. Fan, D. S. Xue, C. -M. Hu

Abstract: We systematically study the indirect interaction between a magnon mode and a cavity photon mode mediated by travelling photons of a waveguide. From a general Hamiltonian, we derive the effective coupling strength between two separated modes, and obtain the theoretical expression of system's transmission. Accordingly, we design an experimental set-up consisting of a shield cavity photon mode, micro… ▽ More We systematically study the indirect interaction between a magnon mode and a cavity photon mode mediated by travelling photons of a waveguide. From a general Hamiltonian, we derive the effective coupling strength between two separated modes, and obtain the theoretical expression of system's transmission. Accordingly, we design an experimental set-up consisting of a shield cavity photon mode, microstrip line and a magnon system to test our theoretical predictions. From measured transmission spectra, indirect interaction, as well as mode hybridization, between two modes can be observed. All experimental observations support our theoretical predictions. In this work, we clarify the mechanism of travelling photon mediated interactions between two separate modes. Even without spatial mode overlap, two separated modes can still couple with each other through their correlated dissipations into a mutual travelling photon bus. This conclusion may help us understand the recently discovered dissipative coupling effect in cavity magnonics systems. Additionally, the physics and technique developed in this work may benefit us in designing new hybrid systems based on the waveguide magnonics. △ Less

Submitted 11 December, 2019; originally announced December 2019.

Comments: 6 papges and 4 figures

Journal ref: Phys. Rev. B 101, 064404 (2020)

Nonreciprocity and Unidirectional Invisibility in Cavity Magnonics

Authors: Yi-Pu Wang, J. W. Rao, Y. Yang, Peng-Chao Xu, Y. S. Gui, B. M. Yao, J. Q. You, C. -M. Hu

Abstract: We reveal the cooperative effect of coherent and dissipative magnon-photon couplings in an open cavity magnonic system, which leads to nonreciprocity with a considerably large isolation ratio and flexible controllability. Furthermore, we discover unidirectional invisibility for microwave propagation, which appears at the zero-damping condition for hybrid magnon-photon modes. A simple model is deve… ▽ More We reveal the cooperative effect of coherent and dissipative magnon-photon couplings in an open cavity magnonic system, which leads to nonreciprocity with a considerably large isolation ratio and flexible controllability. Furthermore, we discover unidirectional invisibility for microwave propagation, which appears at the zero-damping condition for hybrid magnon-photon modes. A simple model is developed to capture the generic physics of the interference between coherent and dissipative couplings, which accurately reproduces the observations over a broad range of parameters. This general scheme could inspire methods to achieve nonreciprocity in other systems. △ Less

Submitted 21 August, 2019; originally announced August 2019.

Comments: 6 pages, 4 figures

Cavity mediated dissipative coupling of distant magnetic moments: theory and experiment

Authors: Peng-Chao Xu, J. W. Rao, Y. S. Gui, Xiaofeng Jin, C. -M. Hu

Abstract: We investigate long-range coherent and dissipative coupling between two spatially separated magnets while both are coupled to a microwave cavity. A careful examination of the system shows that the indirect interaction between two magnon modes is dependent on their individual mechanisms of direct coupling to the cavity. If both magnon modes share the same form of coupling to the cavity (either cohe… ▽ More We investigate long-range coherent and dissipative coupling between two spatially separated magnets while both are coupled to a microwave cavity. A careful examination of the system shows that the indirect interaction between two magnon modes is dependent on their individual mechanisms of direct coupling to the cavity. If both magnon modes share the same form of coupling to the cavity (either coherent or dissipative), then the indirect coupling between them will produce level repulsion. Conversely, if the magnon modes have different forms of coupling to the cavity (one coherent and one dissipative), then their indirect coupling will produce level attraction. We further demonstrate the cavity-mediate nature of the indirect interaction through investigating the dependence of the indirect coupling strength on the frequency detuning between the magnon and cavity modes. Our work theoretically and experimentally explores indirect cavity mediate interactions in systems exhibiting both coherent and dissipative coupling, which opens a new avenue for controlling and utilizing light-matter interactions. △ Less

Submitted 15 July, 2019; originally announced July 2019.

Journal ref: Phys. Rev. B 100, 094415 (2019)

Coherent control of magnon radiative damping with local photon states

Authors: B. M. Yao, T. Yu, Y. S. Gui, J. W. Rao, Y. T. Zhao, W. Lu, C. -M. Hu

Abstract: The collective excitation of ordered spins, known as spin waves or magnons, can in principle radiate by emitting travelling photons to an open system when decaying to the ground state. However, in contrast to the electric dipoles, magnetic dipoles contributed by magnons are more isolated from electromagnetic environment with negligible radiation in the vacuum, limiting their application in coheren… ▽ More The collective excitation of ordered spins, known as spin waves or magnons, can in principle radiate by emitting travelling photons to an open system when decaying to the ground state. However, in contrast to the electric dipoles, magnetic dipoles contributed by magnons are more isolated from electromagnetic environment with negligible radiation in the vacuum, limiting their application in coherent communication by photons. Recently, strong interaction between cavity standing-wave photons and magnons has been reported, indicating the possible manipulation of magnon radiation via tailoring photon states. Here, with loading an yttrium iron garnet sphere in a one-dimensional circular waveguide cavity in the presence of both travelling and standing photon modes, we demonstrate an efficient photon emissions from magnon and a significant magnon radiative damping with radiation rate found to be proportional to the local density of states (LDOS) of photon. By modulating the LDOS including its magnitude and/or polarization, we can flexibly tune the photon emission and magnon radiative damping on demand. Our findings provide a general way in manipulating photon emission from magnon radiation for harnessing energy and angular momentum generation, transfer and storage modulated by magnon in the cavity and waveguide electrodynamics. △ Less

Submitted 10 September, 2019; v1 submitted 18 February, 2019; originally announced February 2019.

Comments: 9+7 pages, 4+2 figures

Control of the magnon-photon level attraction in a planar cavity

Authors: Y. Yang, J. W. Rao, Y. S. Gui, B. M. Yao, W. Lu, C. -M. Hu

Abstract: A resistive coupling circuit is used to model the recently discovered dissipative coupling in a hybridized cavity photon-magnon system. With this model as a basis we have designed a planar cavity in which a controllable transition between level attraction and level repulsion can be achieved. This behaviour can be quantitatively understood using an LCR circuit model with a complex coupling strength… ▽ More A resistive coupling circuit is used to model the recently discovered dissipative coupling in a hybridized cavity photon-magnon system. With this model as a basis we have designed a planar cavity in which a controllable transition between level attraction and level repulsion can be achieved. This behaviour can be quantitatively understood using an LCR circuit model with a complex coupling strength. Our work therefore develops and verifies a circuit method to model level repulsion and level attraction and confirms the universality of dissipative coupling in the cavity photon-magnon system. The realization of both coherent and dissipative couplings in a planar cavity may provide new avenues for the design and adaptation of dissipatively coupled systems for practical applications in information processing. △ Less

Submitted 8 April, 2019; v1 submitted 22 January, 2019; originally announced January 2019.

Comments: 7 papges, 4 figures and 2 additional figures in the appendix

Journal ref: Phys. Rev. Applied 11, 054023 (2019)

Level Attraction Due to Dissipative Magnon-Photon Coupling

Authors: M. Harder, Y. Yang, B. M. Yao, C. H. Yu, J. W. Rao, Y. S. Gui, R. L. Stamps, C. -M. Hu

Abstract: We report dissipative magnon-photon coupling caused by cavity Lenz effect, where the magnons in a magnet induce a rf current in the cavity, leading to a cavity back action that impedes the magnetization dynamics. This effect is revealed in our experiment as level attraction with a coalescence of hybridized magnon-photon modes, which is distinctly different from level repulsion with mode anticrossi… ▽ More We report dissipative magnon-photon coupling caused by cavity Lenz effect, where the magnons in a magnet induce a rf current in the cavity, leading to a cavity back action that impedes the magnetization dynamics. This effect is revealed in our experiment as level attraction with a coalescence of hybridized magnon-photon modes, which is distinctly different from level repulsion with mode anticrossing caused by coherent magnon-photon coupling. We develop a method to control the in- terpolation of coherent and dissipative magnon-photon coupling, and observe a matching condition where the two effects cancel. Our work sheds light on the so-far hidden side of magnon-photon coupling, opening a new avenue for controlling and utilizing light-matter interactions. △ Less

Submitted 4 September, 2018; originally announced September 2018.

Comments: 5 pages, 4 figures

Bright sub-20 nm cathodoluminescent nanoprobes for multicolor electron microscopy

Authors: Maxim B. Prigozhin, Peter C. Maurer, Alexandra M. Courtis, Nian Liu, Michael D. Wisser, Chris Siefe, Bining Tian, Emory Chan, Guosheng Song, Stefan Fischer, Shaul Aloni, D. Frank Ogletree, Edward S. Barnard, Lydia-Marie Joubert, Jianghong Rao, A. Paul Alivisatos, Roger M. Macfarlane, Bruce E. Cohen, Yi Cui, Jennifer A. Dionne, Steven Chu

Abstract: Electron microscopy (EM) has been instrumental in our understanding of biological systems ranging from subcellular structures to complex organisms. Although EM reveals cellular morphology with nanoscale resolution, it does not provide information on the location of proteins within a cellular context. An EM-based bioimaging technology capable of localizing individual proteins and resolving protein-… ▽ More Electron microscopy (EM) has been instrumental in our understanding of biological systems ranging from subcellular structures to complex organisms. Although EM reveals cellular morphology with nanoscale resolution, it does not provide information on the location of proteins within a cellular context. An EM-based bioimaging technology capable of localizing individual proteins and resolving protein-protein interactions with respect to cellular ultrastructure would provide important insights into the molecular biology of a cell. Here, we report on the development of luminescent nanoprobes potentially suitable for labeling biomolecules in a multicolor EM modality. In this approach, the labels are based on lanthanide-doped nanoparticles that emit light under electron excitation in a process known as cathodoluminescence (CL). Our results suggest that the optimization of nanoparticle composition, synthesis protocols and electron imaging conditions could enable high signal-to-noise localization of biomolecules with a sub-20-nm resolution, limited only by the nanoparticle size. In ensemble measurements, these luminescent labels exhibit narrow spectra of nine distinct colors that are characteristic of the corresponding rare-earth dopant type. △ Less

Submitted 30 May, 2018; originally announced June 2018.

Effect of intense, ultrashort laser pulses on DNA plasmids in their native state: strand breakages induced by {\it in-situ} electrons

Authors: J. S. D'Souza, J. A. Dharmdhikari, A. K. Dharmdhikair, B. J. Rao, D. Mathur

Abstract: Single strand breaks are induced in DNA plasmids, pBR322 and pUC19, in aqueous media by intense ultrashort laser pulses (820 nm wavelength, 45 fs pulse duration, 1 kHz repetition rate) at intensities of 1-12 TW cm$^{-2}$. The intense laser radiation generates, {\it in situ}, electrons that induce transformation of supercoiled DNA into relaxed DNA. The extent of electron-mediated relaxation of DNA… ▽ More Single strand breaks are induced in DNA plasmids, pBR322 and pUC19, in aqueous media by intense ultrashort laser pulses (820 nm wavelength, 45 fs pulse duration, 1 kHz repetition rate) at intensities of 1-12 TW cm$^{-2}$. The intense laser radiation generates, {\it in situ}, electrons that induce transformation of supercoiled DNA into relaxed DNA. The extent of electron-mediated relaxation of DNA structure is quantified. Introduction of electron and radical scavengers inhibits DNA damage. △ Less

Submitted 2 June, 2010; originally announced June 2010.

Comments: Part of an invited paper presented at the 3rd International Symposium on Filamentation, 31 May - 5 June, 2010, Crete, Greece