Nanoscale Wavelengths Research Articles

Exciting High‐Frequency Short‐Wavelength Spin Waves using High Harmonics of a Magnonic Cavity Mode

AbstractSpin waves (SWs) are promising objects for signal processing and future quantum technologies due to their high microwave frequencies with corresponding nanoscale wavelengths. However, the nano‐wavelength SWs generated so far are limited to low frequencies. In the paper, using micromagnetic simulations, it is shown that a microwave‐pumped SW mode confined to the cavity of a thin film magnonic crystal (MC) can be used to generate waves at tens of GHz and wavelengths well below 50 nm. These multi‐frequency harmonics of the fundamental cavity mode are generated when the amplitude of the pumping microwave field exceeds a threshold, and their intensities then scale linearly with the field intensity. The frequency of the cavity mode is equal to the ferromagnetic resonance frequency of the planar ferromagnetic film, which overlaps with the magnonic bandgap, providing an efficient mechanism for confinement and magnetic field tunability. The effect reaches saturation when the microstrip feed line covers the entire cavity, making the system feasible for realization.

A New N‐Type High Entropy Semiconductor AgBiPbSe2S with High Thermoelectric and Mechanical Properties

AbstractExploring N‐type High‐entropy materials with both high thermoelectric and mechanical properties is highly desirable for all‐high‐entropy thermoelectric generators (TEGs) since the thermoelectric and mechanical properties of N‐type one are largely behind its P‐type counterparts. Herein, a new rock‐salt structure N‐type high entropy thermoelectric AgBiPbSe2S is introduced with a bandgap of ≈0.43 eV. The atomic radii difference of each component results in a large lattice distortion of 0.246, leading to a low thermal conductivity of 0.36 W m−1 K−1 at 823 K. The figure of merit (ZT) reaches 0.6 for AgBiPbSe2S at 823 K. Moreover, Ag2Se precipitates are included in AgBiPbSe2S to filter low energy carriers for high Seebeck coefficients and to scatter phonons with nanoscale wavelength for ultralow lattice thermal conductivities. Consequently, a peak ZT of ≈1.18 at 823 K and an average ZT of 0.60 at 400–823 K are obtained for Ag1.02BiPbSe2S. More importantly, high mechanical properties are also obtained in Ag1.02BiPbSe2S, of which the Vickers hardness and flexural strength are ≈209 Hv and 32 MPa, respectively, originating from the enhanced lattice friction by chemical short‐range disorder (i.e., high entropy effect) and dispersion strengthening caused by Ag2Se nanoprecipitates.

Coherent Magnons with Giant Nonreciprocity at Nanoscale Wavelengths.

Nonreciprocal wave propagation arises in systems with broken time-reversal symmetry and is key to the functionality of devices, such as isolators or circulators, in microwave, photonic, and acoustic applications. In magnetic systems, collective wave excitations known as magnon quasiparticles have so far yielded moderate nonreciprocities, mainly observed by means of incoherent thermal magnon spectra, while their occurrence as coherent spin waves (magnon ensembles with identical phase) is yet to be demonstrated. Here, we report the direct observation of strongly nonreciprocal propagating coherent spin waves in a patterned element of a ferromagnetic bilayer stack with antiparallel magnetic orientations. We use time-resolved scanning transmission X-ray microscopy (TR-STXM) to directly image the layer-collective dynamics of spin waves with wavelengths ranging from 5 μm down to 100 nm emergent at frequencies between 500 MHz and 5 GHz. The experimentally observed nonreciprocity factor of these counter-propagating waves is greater than 10 with respect to both group velocities and specific wavelengths. Our experimental findings are supported by the results from an analytic theory, and their peculiarities are further discussed in terms of caustic spin-wave focusing.

Single-crystalline LiNbO3 film based wideband SAW devices with spurious-free responses for future RF front-ends

This work demonstrates super-high frequency wave-guided surface acoustic wave (SAW) devices based on LiNbO3/SiO2/Si layered substrates. SAW resonators with fundamental operating frequency to the 7-GHz range were developed, and wave modes like the Rayleigh, Shear-horizontal (SH), Pseudo-Bulk, etc., were observed. Furthermore, effective coupling coefficients (k2eff) and quality factor (Q) of the devices have been investigated systematically. Specifically, spurious-free SAW resonators with the SH wave frequency over 7.40 GHz and the high k2eff value of ∼7.8% were obtained at a nanoscale wavelength of 480 nm. Finally, high-performance filters with a bandwidth over 300 MHz were achieved. The demonstrated filters show sharp roll-off and spurious-free responses within the passband with insertion loss <−5 dB and a small temperature coefficient of frequency of ∼−45.8 ppm/K at a super-high frequency of 7.17 GHz. Upon further optimizations, high-performance SAW devices built on a single crystalline LiNbO3 film can potentially enable the low-loss and wideband signal processing functions, promising for the next-generation radio frequency front-end system applications.

Comparison of spin-wave transmission in parallel and antiparallel magnetic configurations

Parallel (P) and antiparallel (AP) configurations are widely applied in magnetic heterostructures and have significant impacts on the spin-wave transmission in magnonic devices. In the present study, a theoretical investigation was conducted into the transmission of exchange-dominated spin waves with nanoscale wavelengths in a type of heterostructure including two magnetic media, of which the magnetization state can be set to the P (AP) configuration by ferromagnetic (antiferromagnetic) interfacial exchange coupling (IEC). The boundary conditions in P and AP cases were derived, by which the transmission and reflection coefficients of spin waves were analytically given and numerically calculated. In the P configuration, a critical angle $\theta_{\textrm{c}}$ always exists and has a significant influence on the transmission. Spin waves are refracted and reflected when the incident angle $\theta_{\textrm{i}}$ is smaller than the critical angle ($\theta_{\textrm{i}} < \theta_{\textrm{c}}$), while total reflection occurs as $\theta_{\textrm{i}} \geq \theta_{\textrm{c}}$. In the AP configuration, the spin-wave polarizations of medium 1 and 2 are inverse, that is, right-handed (RH) and left-handed (LH), leading to the total reflection being independent of $\theta_{\textrm{i}}$. As demonstrated by the difference in spin-wave transmission properties between the P ($\theta_{\textrm{i}} < \theta_{\textrm{c}}$) and AP cases, there is a polarization-dependent scattering. However, as $\theta_{\textrm{i}}$ exceeds $\theta_{\textrm{c}}$, the P ($\theta_{\textrm{i}} > \theta_{\textrm{c}}$) case exhibits similarities with the AP case, where the transmitted waves are found to be evanescent in medium 2 and their decay lengths are investigated.

Sum Rate Maximization for Intelligent Reflecting Surface-Aided Visible Light Communications

With the promotion of the solid-state lighting industry, visible light communication (VLC) has attracted increasing interests these years. However, the blockage problems in VLC due to the nanoscale wavelength of the visible light are severe. In this letter, a VLC system deployed with the intelligent reflecting surface (IRS) is modeled under the point source assumption, where both line-of-sight and specular non-line-of-sight paths are considered. By introducing a discrete matrix, the resource allocation process is simplified into a binary programming problem, and then we propose a low-complexity algorithm to maximize the achievable sum rate. Moreover, numerical results demonstrate that IRS can improve the rate performance and ease blockage problems of VLC.

3D multi-stable structures with surface wrinkling patterns

Abstract This paper demonstrates how two-dimensional (2D) planar films transform into three-dimensional (3D) multi-stable structures with 3D surface wrinkling patterns by means of magnetron sputtering. The wrinkling patterns on metal films deposited on polydimethylsiloxane (PDMS) elastic substrates is studied. Surface wrinkling of the Al film results from compressive thermal stresses. The effects of film thickness on the wrinkling patterns regularities are investigated. It is found that 3D labyrinth hierarchical wrinkling patterns for large sputtering flux span scale wavelengths (e.g., from nanometer to micrometer). Moreover, the hydrophobicity of the wrinkled surface has a good improvement with the contact angle. Furthermore, when bottom constraints are removed, bilayer thin films also spontaneously curl, evolving into 3D multi-stable structures with large deformations and surface patterns for releasing inner stress, which present neutrally stable configurations with no stiffness. Square multi-stable structures eventually remain stable along the diagonal as the width decreases. Simultaneously, the surface of multi-stable structures is found only labyrinth wrinkling pattern with nanoscale wavelength due to unrestraint. In addition, the finite element method was performed to simulate experimental deformation. Deformable 3D structures with multiple scales hold considerable promise for practical applications in engineering.

Chiral Emission of Exchange Spin Waves by Magnetic Skyrmions.

Spin waves or their quanta magnons raise the prospect to act as information carriers in the absence of Joule heating. The challenge to excite spin waves with nanoscale wavelengths free of nanolithography becomes a critical bottleneck for the application of nanomagnonics. Magnetic skyrmions are chiral magnetic textures at the nanoscale. In this work, short-wavelength exchange spin waves are demonstrated to be chirally emitted in a low damping magnetic insulating thin film by magnetic skyrmions. The spin-wave chirality originates from the chiral spin pumping effect and is determined by the cross product of the magnetization orientation and the film normal direction. The Halbach effect explains the enhancement or attenuation of the spin-wave amplitude with a reversed sign of the Dyzaloshinskii-Moriya interaction. Controllable spin-wave propagation is demonstrated by rotating a moderate applied field. Our findings are key for building compact low-power nanomagnonic devices based on intrinsic nanoscale magnetic textures.

Model of nanostructural layers formation at long-term operation of rails

A mathematical model was developed and a mechanism was proposed for the formation of nanoscale structural-phase states on the example of rail steel at long-term operation. It was believed that during intense plastic deformations, the material behaves like a viscous incompressible fluid. In order to take into account the sliding of the wheel relative to the rail, a two-layer fluid model was proposed, the top layer of which slides at a certain speed relative to the first. In this case, the Kelvin-Helmholtz instability develops. For each layer, we have written the Navier-Stokes equations and kinematic and dynamic boundary conditions. Solution of the obtained system in the form of normal perturbation modes was carried out on the basis of assumption of the viscous-potential material flow. In this approximation, it was believed that viscosity effects occur only at the layer interface. A dispersion equation was derived, which was analyzed using a graphical representation of the functions included in the analytical solution. A range of characteristics of the material and parameters of the external influence (the velocity of the layer) was established, at which two peaks are observed in dependence of disturbances growth rate on the wave number. The first (hydrodynamic) maximum is due to the motion of the layers relative to each other; the second is associated with the effects of fluid viscosity. Approximate formulas were obtained for dependence of the growth rate of perturbations on the wave number. Conditions for realization of only one maximum were found. The viscously determined maximum at slip velocities of the order of 1 m/s can be in the nanoscale wavelength range. Assuming that the white layer in the rails during long-term operation is formed mainly due to the action of intense plastic deformations, we believe that the obtained results detail the mechanism of white layers formation in the rails in this case.

Chiral Magnonics: Reprogrammable Nanoscale Spin Wave Networks Based on Chiral Domain Walls.

SummarySpin waves offer promising perspectives as information carriers for future computational architectures beyond conventional complementary metal-oxide-semiconductor (CMOS) technology, owing to their benefits for device minimizations and low-ohmic losses. Although plenty of magnonic devices have been proposed previously, scalable nanoscale networks based on spin waves are still missing. Here, we demonstrate a reprogrammable two-dimensional spin wave network by combining the chiral exchange spin waves and chiral domain walls. The spin-wave network can be extended to two dimensions and offers unprecedented control of exchange spin waves. Each cell in the network can excite, transmit, and detect spin waves independently in the chiral domain wall, and spin-wave logics are also demonstrated. Our results open up perspectives for integrating spin waves into future logic and computing circuits and networks.

Massively Multiplexed Submicron Particle Patterning in Acoustically Driven Oscillating Nanocavities.

Nanoacoustic fields are a promising method for particle actuation at the nanoscale, though THz frequencies are typically required to create nanoscale wavelengths. In this work, the generation of robust nanoscale force gradients is demonstrated using MHz driving frequencies via acoustic-structure interactions. A structured elastic layer at the interface between a microfluidic channel and a traveling surface acoustic wave (SAW) device results in submicron acoustic traps, each of which can trap individual submicron particles. The acoustically driven deformation of nanocavities gives rise to time-averaged acoustic fields which direct suspended particles toward, and trap them within, the nanocavities. The use of SAWs permits massively multiplexed particle manipulation with deterministic patterning at the single-particle level. In this work, 300 nm diameter particles are acoustically trapped in 500 nm diameter cavities using traveling SAWs with wavelengths in the range of 20-80 µm with one particle per cavity. On-demand generation of nanoscale acoustic force gradients has wide applications in nanoparticle manipulation, including bioparticle enrichment and enhanced catalytic reactions for industrial applications.

Optically Inspired Nanomagnonics with Nonreciprocal Spin Waves in Synthetic Antiferromagnets.

Integrated optically inspired wave-based processing is envisioned to outperform digital architectures in specific tasks, such as image processing and speech recognition. In this view, spin waves represent a promising route due to their nanoscale wavelength in the gigahertz frequency range and rich phenomenology. Here, a versatile, optically inspired platform using spin waves is realized, demonstrating the wavefront engineering, focusing, and robust interference of spin waves with nanoscale wavelength. In particular, magnonic nanoantennas based on tailored spin textures are used for launching spatially shaped coherent wavefronts, diffraction-limited spin-wave beams, and generating robust multi-beam interference patterns, which spatially extend for several times the spin-wave wavelength. Furthermore, it is shown that intriguing features, such as resilience to back reflection, naturally arise from the spin-wave nonreciprocity in synthetic antiferromagnets, preserving the high quality of the interference patterns from spurious counterpropagating modes. This work represents a fundamental step toward the realization of nanoscale optically inspired devices based on spin waves.

Ultrasmall broadband wavelength and polarization router based on hybrid waveguide of monolithic-LiNbO3

The nanoscale wavelength and polarization router, which can simultaneously separate wavelength and polarization modes, is an essential component of on-chip nanophotonic devices. Here, an on-chip wavelength and polarization router is realized experimentally based on a three-layer hybrid waveguide of Au-SiO2-LiNbO3 etched with asymmetric nano-cavities. The central area size of the device is only 1.60×1.96 μm2. A broad operation band covers from 500nm to 1150nm with low cross talk of under 10dB. The monolithic-LiNbO3is introduced for the first time, to the best of our knowledge, to on-chip multichannel wavelength and polarization routers. This work plays a key role for dense chip integration, visible light displays, and communications, and can inspire LiNbO3-based nanophotonic devices.

Optically Detecting Acoustic Oscillations at the Nanoscale: Exploring Techniques Suitable for Studying Elastic Wave Propagation

Nanoacoustics is the study of acoustic wave propagation on a submicron scale. Research in this area is motivated by fundamental [1] and technological interests [2]. On the fundamental side, nanoscale objects can provide information about the impact of size, geometry, and surface conditions on elastic properties, elastic energy dissipation, and the validity of continuum mechanics models close to the atomic scale. On the technological side, high-frequency (MHz-THz) nanomechanical oscillators and nanoelectromechanical systems are promising tools for electromechanical computers and information storage, sensors for ultraweak chemicals and biological agents and ultralow forces and masses, radio frequency mobile phone f ilters, and artificial brains for self-aware autonomous vehicles. Furthermore, acoustic microscopy techniques [3]-[6] that rely on elastic waves with nanoscale wavelengths have great potential for materials characterization, failure analysis, and nondestructive imaging of 3D integrated structures with nanoscale scale spatial resolution. Surface acoustic waves (SAWs), which propagate in the near surface of a material with mechanical oscillations confined to a fraction of their wavelength from the surface, have nanoscale wavelengths at frequencies in the hundreds-of-GHz range. The nanoscale confinement of these elastic waves can facilitate studies of ultrasonic properties of individual grain boundaries in nanocrystalline materials [7], applicable to material fracture and nanoscale thermal transport [8].

Emission and propagation of 1D and 2D spin waves with nanoscale wavelengths in anisotropic spin textures.

Spin waves offer intriguing perspectives for computing and signal processing, because their damping can be lower than the ohmic losses in conventional complementary metal-oxide-semiconductor (CMOS) circuits. Magnetic domain walls show considerable potential as magnonic waveguides for on-chip control of the spatial extent and propagation of spin waves. However, low-loss guidance of spin waves with nanoscale wavelengths and around angled tracks remains to be shown. Here, we demonstrate spin wave control using natural anisotropic features of magnetic order in an interlayer exchange-coupled ferromagnetic bilayer. We employ scanning transmission X-ray microscopy to image the generation of spin waves and their propagation across distances exceeding multiples of the wavelength. Spin waves propagate in extended planar geometries as well as along straight or curved one-dimensional domain walls. We observe wavelengths between 1 μm and 150 nm, with excitation frequencies ranging from 250 MHz to 3 GHz. Our results show routes towards the practical implementation of magnonic waveguides in the form of domain walls in future spin wave logic and computational circuits.

An ultra‐low‐cost smartphone octochannel spectrometer for mobile health diagnostics

With the rapid development and proliferation of mobile devices with powerful computing power and the ability of integrating sensors into mobile devices, the potential impact of mobile health (mHealth) diagnostics on the public health is drawing researchers' attention. We developed a Smartphone Octo-channel Spectrometer (SOS) as a mHealth diagnostic tool. The SOS has nanoscale wavelength resolution, is self-illuminated from the smartphone itself, and is ultra-low cost (less than $20). A user interface controls the optical sensing parameters and precise alignment. After calibrating and testing the SOS by quantifying protein concentrations, we clinically validated the SOS by comparing the diagnostic performance of our device with that of a clinical spectrophotometer. About 180 serum samples from de-identified patients with 4 types of autoantibodies were blindly read the ELISA results. The accuracy of the SOS achieved 100% across the clinical reportable range compared with the FDA-approved instrument. Furthermore, the self-illuminated SOS only requires about half of the light intensity of the FDA-approved instrument to achieve clinical-level sensitivity. The low-energy-consumption and low-cost SOS enables point-of-care spectrophotometric sensing in low-resource areas, and can be integrated into point-of-care diagnostic systems for rapid multiplex readout and analysis at patient bedside or at home.

Optical band-stop filter and multi-wavelength channel selector with plasmonic complementary aperture embedded in double-ring resonator

A compact nanoscale wavelength band-stop filter with aperture embedded in double-ring resonator is proposed and numerically investigated by using Finite-Difference Time-Domain (FDTD) method. With a narrow aperture created between embedded double rings, the modes of the split-ring cavity can be modulated by the aperture in different manners when the parameters of the aperture are changed. Furthermore, the absorption peaks of resonator modes can be selectively inhibited by altering the positions of the aperture without changing outer size of the resonator. Based on above characteristics, a 1×2 multiple-contact channel selector is designed with a rotating aperture which can select the output waveguide. The proposed filter and selector have potential applications in highly integrated optical circuits.

Approaching soft X-ray wavelengths in nanomagnet-based microwave technology.

Seven decades after the discovery of collective spin excitations in microwave-irradiated ferromagnets, there has been a rebirth of magnonics. However, magnetic nanodevices will enable smart GHz-to-THz devices at low power consumption only, if such spin waves (magnons) are generated and manipulated on the sub-100 nm scale. Here we show how magnons with a wavelength of a few 10 nm are exploited by combining the functionality of insulating yttrium iron garnet and nanodisks from different ferromagnets. We demonstrate magnonic devices at wavelengths of 88 nm written/read by conventional coplanar waveguides. Our microwave-to-magnon transducers are reconfigurable and thereby provide additional functionalities. The results pave the way for a multi-functional GHz technology with unprecedented miniaturization exploiting nanoscale wavelengths that are otherwise relevant for soft X-rays. Nanomagnonics integrated with broadband microwave circuitry offer applications that are wide ranging, from nanoscale microwave components to nonlinear data processing, image reconstruction and wave-based logic.

Simulation of Excitation and Propagation of Pico-Second Ultrasound

This paper presents an analytic and numerical simulation of the generation and propagation of pico-second ultrasound with nano-scale wavelength, enabling the production of bulk waves in thin films. An analytic model of laser-matter interaction and elasto-dynamic wave propagation is introduced to calculate the elastic strain pulse in microstructures. The model includes the laser-pulse absorption on the material surface, heat transfer from a photon to the elastic energy of a phonon, and acoustic wave propagation to formulate the governing equations of ultra-short ultrasound. The excitation and propagation of acoustic pulses produced by ultra-short laser pulses are numerically simulated for an aluminum substrate using the finite-difference method and compared with the analytical solution. Furthermore, Fourier analysis was performed to investigate the frequency spectrum of the simulated elastic wave pulse. It is concluded that a pico-second bulk wave with a very high frequency of up to hundreds of gigahertz is successfully generated in metals using a 100-fs laser pulse and that it can be propagated in the direction of thickness for thickness less than 100 ㎚.

Kinetics of spinodal decomposition and strain energy effects in Cu-Ni(Fe) nanolaminates

ABSTRACTThe phase transformation of spinodal decomposition proceeds without nucleation and is affected by the alloy composition, temperature, interfaces and gradient energy, as well as the presence of lattice strain. As a consequence, a coherent spinodal can be depressed well below the chemical spinodal within the miscibility gap. Phase separation from a solid solution within the spinodal leads to the formation of characteristic composition wavelengths. In the nickel-based alloy system, a nanolaminate structure is used to initially create an artificial composition fluctuation with unique nanoscale wavelengths. The direct measurement of diffusivity at low temperatures in Cu-Ni and Cu-Ni(Fe), from the spinodal towards room temperature, requires sensitivity to the nanoscale fluctuations in composition. For this purpose, x-ray diffraction scans are used to assess changes in the short-range order of the composition fluctuation and the corresponding changes in the gradient energy, from which an evaluation of lattice distortion effects reveals a peak in strain energy for 2-3 nm composition wavelengths.