Novel Degree Of Freedom Research Articles

Anti−Reflection Metamaterials with Phase Gradient Index Generate Surface Waves for Radar Stealth in the Microwave Regime

AbstractAlthough the conversion of propagating waves into surface waves is anticipated to introduce a novel degree of freedom for radar stealth materials, the manipulation of wavelength and traveling direction of surface waves at small phase gradients remains unclear. Here the aforementioned conversion is demonstrated utilizing a metamaterial composed of carbonyl iron powders, where the momentum mismatch is compensated by the material and the phase gradient index of the aligned meta−atoms. Surface waves are generated in the direction of the phase gradient within a phase span of 180°. The metamaterials with 5−level metastructure (5x:1x) generate surface waves with two wavelengths, and get the ξ/k0 value (ratio of the phase gradient index to the wave vectors of propagating wave) of 7.5 and 4.32, respectively. Furthermore, the dual phase gradient metastructure possesses the capability to alter the trajectory of the leaky microwave, converting it into a waveform resembling that of a vortex, while simultaneously preserving the integrity of surface waves. The sample achieves an effective absorption bandwidth of 5.67–7.96 and 20.19–21.10 GHz with an optimal absorption peak of −40.77 dB at 6.87 GHz. Present study develops a novel mechanism to improve the radar stealth properties of microwave absorption materials.

Real-time dynamics of angular momentum transfer from spin to acoustic chiral phonon in oxide heterostructures.

Chiral phonons have recently been explored as a novel degree of freedom in quantum materials. The angular momentum carried by these quasiparticles is generated by the breaking of chiral degeneracy of phonons, owing to the chiral lattice structure or the rotational motion of ions of the material. In ferromagnets, a mechanism for generating non-equilibrium chiral phonons has been suggested, but their temporal evolution, which obeys Bose-Einstein statistics, remains unclear. Here we report the real-time dynamics of thermalized chiral phonons in an artificial superlattice composed of ferromagnetic metallic SrRuO3 and non-magnetic insulating SrTiO3. Following the photo-induced ultrafast demagnetization in the SrRuO3 layer, we observed the appearance of a magneto-optic signal in the superlattice, which is absent in the SrRuO3 single films. This magneto-optic signal exhibits thermally driven dynamic properties and a clear correlation with the thickness of the non-magnetic SrTiO3 layer, implying that it originates from thermalized chiral phonons. We use numerical calculations considering the magneto-elastic coupling in SrRuO3 to validate our experimental observations and the angular momentum transfer mechanism between the lattice and spin systems in ferromagnetic systems and also to the non-magnetic system.

Atomic thermal fluctuation reduction method for robust local lattice structure identification in finite-temperature molecular dynamics

Classical molecular dynamics (MD) is extensively employed to explore the properties, deformations, and fractures of materials at the atomic scale. Identifying local structures is crucial for understanding the mechanisms behind material deformation and fracture. Nevertheless, analyzing the local lattice structure at high temperatures poses challenges due to atomic thermal fluctuations, which act as noise and potentially lead to misjudgment of the local lattice structure. To date, various strategies have been implemented to circumvent this issue. However, they cannot be a solution because it is unable to reproduce phenomena unique to high temperatures, whereas others require significant computational resources. This paper introduces an innovative method to reduce atomic thermal fluctuations using a straightforward algorithm, thereby facilitating accurate identification of local lattice structures even at high temperatures. Our approach incorporates novel degrees of freedom, termed ‘Markers,’ that are linked to atoms. By reducing the thermal fluctuation of these Markers, precise analysis of the local lattice structure becomes feasible. The efficacy of this method is validated through its thermal reducibility and Markers trackabilities to atoms. Utilizing common neighbor analysis, the error rate for structure identification with our method is nearly 0% at temperatures up to 1200 K in Fe, in contrast to approximately 5% without it. Furthermore, the average distance between atoms and Markers remains below 0.1 Å. Applying our method to phase transformations, we successfully observed the transition from face-centered cubic to body-centered cubic structure in Fe at 1200 K. This method holds promise for expanding the capabilities of MD simulations at high temperatures.

Sliding Ferroelectricity Engineered Coupling between Spin Hall Effect and Layertronics in 2D Lattice.

Coupling the spin Hall effect with novel degrees of freedom of electrons is central to the rich phenomena observed in condensed-matter physics. Here, using symmetry analysis and a low-energy k·p model, we report the sliding ferroelectricity engineered coupling between spin Hall effect and emerging layertronics, thereby generating the layer spin Hall effect (LSHE), in a 2D lattice. The physics is rooted in a pair of T-symmetry connected valleys, which experience spin splitting accompanied by large Berry curvature under spin-orbit coupling. The interaction between the out-of-plane ferroelectricity and electronic properties gives rise to the layer-locked Berry curvature and thus layer-polarized spin Hall effect (LP-SHE) in the bilayers. Such LP-SHE is strongly coupled with sliding ferroelectricity, enabling it to be ferroelectrically reversible. Using first-principles calculations, the mechanism is further demonstrated in a series of real bilayer systems, including MoS2, MoTe2, WSe2, MoSi2P4, and MoSi2As4. These phenomena and insights open a new direction for spin Hall effect.

Modulating electronic structure by interlayer spacing and twist on bilayer bismuthene.

Modulation of the electronic structure has played a crucial role in advancing the field of two-dimensional materials, but there are still many unexplored directions, such as the twist angle for a novel degree of freedom, for modulating the properties of heterostructures. We observed a distinct pattern in the energy bands of bilayer bismuthene, demonstrating that modulating the twist angle and interlayer spacing significantly influences interlayer interactions. Our study of various interlayer spacings and twist angles revealed a close relationship between bandgap size and interlayer spacing, while the twist angle notably affects the shape of the energy bands. Furthermore, we observed a synergistic effect between these two factors. As the twist angle decreases, the energy bands become flat, and flat bands can be generated without requiring a specific angle on bilayer bismuthene. Our results suggest a promising way to tailor the energy band structure of bilayer 2D materials by varying the interlayer spacing and twist angle.

Advancements of Intense Terahertz Field Focusing on Metallic Nanoarchitectures for Monitoring Hidden Interatomic Gas-Matter Interactions.

With the advancements of nanotechnology, innovative photonic designs coupled with functional materials provide a unique way to acquire, share, and respond effectively to information. It is found that the simple deposition of a 30nm-thick palladium nanofilm on a terahertz (THz) metasurface chip with a 14nm-wide effective nanogap of asymmetric materials and geometries allows the tracking of both interatomic and interfacial gas-matter interactions, including gas adsorption, hydrogenation (or dehydrogenation), metal phase changes, and unique water-forming reactions. Combinatorial analyses by simulation and experimental measurements demonstrate the distinct nanostructures, which leads to significant light-matter interactions and corresponding THz absorption in a real-time, highly repeatable, and reliable manner. The complex lattice dynamics and intrinsic properties of metals influenced by hydrogen gas exposure are also thoroughly examined using systematically controlled ternary gas mixture devices that mimic normal temperature and pressure. Furthermore, the novel degrees of freedom are utilized to analyze various physical phenomena, and thus, analytical methods that enable the tracking of unknown hidden stages of water-forming reactions resulting in water growth are introduced. A single exposure of the wave spectrum emphasizes the robustness of the proposed THz nanoscopic probe, bridging the gap between fundamental laboratory research and industry.

Structure-independent flat bands induced by discontinuity plasma-air interface in photonic crystals

The band structures of transverse electric (TE) polarized dispersive photonic crystals are generally more complicated than those of transverse magnetic (TM) polarized photonic crystals. Here, by simplifying the governing equations of a TE polarized dispersion system and conducting the band structure analysis, we deduce the Hermitian eigenstates of the vector electric field. The results exhibit the double degeneracy eigenstates in the triangular lattice for lower eigenfrequencies and numerous plasmon-induced flat band modes in the interface of the plasma and air for higher eigenfrequencies. Moreover, we illustrate that the surface plasmon modes tend to present the quantized eigenfrequencies, which is due to the boundary condition imposed by the cylindrical geometry. However, when the eigenfrequency approaches the surface plasma frequency, the localization of the field on the boundary becomes stronger, which weakens the coupling between sites and reduces the dependence of the flat bands on the lattice structure. Unlike the generation mechanism of the lattice-induced flat bands, such plasma frequency-dependent localization at the interface enables a robust flat band characteristic immune to the lattice disorder and provides a novel degree of freedom to control the energy band. Our findings are expected to be useful for electromagnetic wave manipulation and field enhancement.

Monolithically integrated photonic crystal surface emitters on silicon with a vortex beam by using bound states in the continuum.

Optical resonant cavities with high quality factor (Q-factor) are widely used in science and technology for their capabilities of strong confinement of light and enhanced light-matter interaction. The 2D photonic crystal structure with bound states in the continuum (BICs) is a novel concept for resonators with ultra-compact device size, which can be used to generate surface emitting vortex beams based on symmetry-protected BICs at the Γ point. Here, to the best of our knowledge, we demonstrate the first photonic crystal surface emitter with a vortex beam by using BICs monolithically grown on CMOS-compatible silicon substrate. The fabricated quantum-dot BICs-based surface emitter operates at 1.3 µm under room temperature (RT) with a low continuous wave (CW) optically pumped condition. We also reveal the BIC's amplified spontaneous emission with the property of a polarization vortex beam, which is promising to provide a novel degree of freedom in classical and quantum realms.

Contactless manipulation of subwavelength-structured acoustic metasurfaces

The use of acoustic waves to actuate materials without physical contact is relevant in a variety of fields, including assembly, imaging, and holography. Current acoustic manipulation techniques can produce high-precision, non-invasive and non-contact forces, but they are generally limited to small (subwavelength) objects due to the nature of acoustic trapping potentials. In this study, we describe an approach for overcoming these limitations by using metasurfaces to control the refraction of sound waves. We present theory, simulations, and experimental results showing how a deliberately engineered metasurface can steer the momentum of sound waves and therefore control the intensity and the direction of acoustic radiation pressure. To illustrate the potential applications of this concept, we demonstrate acoustic metasurfaces which exhibit dynamical phenomena such as self-guidance and contactless acoustic pulling. We expand this analysis to survey how this concept can be implemented in different metasurface unit cell topologies and how it scales with different acoustic frequencies. Our results combine actuation with acoustic metamaterial physics to provide novel degrees of freedom to control acoustic forces beyond the limits of traditional wave-matter interactions. Nat. Commun. 13, 6533 (2022).

Enhanced Spin Current in Ni81 Fe19 /Cu-CuOx Bilayer with Top and Sideways Oxidization.

Generation and manipulation of spin current are the cores of spintronic devices, which are intensely pursued. Heavy metals with strong spin-orbit coupling are commonly used for the generation of spin current, but areincompatible with the mass production of devices, and the polarization of spin current is limited to be in-plane. Here, it is shown that the spin current with strong out-of-plane polarization component can be generated and transmitted in Ni81 Fe19 /Cu-CuOx bilayer with sideways and top oxidizations. The charge-to-spin current conversion efficiency can be enhanced through the spin currents consisting of both out-of-plane polarization (σz ) and in-plane polarization (σy ) induced by spin-vorticity coupling. Such a spin current is demonstrated to be closely related to the lateral oxidization gradient and can be controlled by changing the temperatures and times of annealing. The finding here provides a novel degree of freedom to produce and control the spin current in spintronic devices.

Metastability of photonic spin meron lattices in the presence of perturbed spin-orbit coupling.

Photonic skyrmions and merons are topological quasiparticles characterized by nontrivial electromagnetic textures, which have received increasing research attention recently, providing novel degree of freedom to manipulate light-matter interactions and exhibiting excellent potential in deep-subwavelength imaging and nanometrology. Here, the topological stability of photonic spin meron lattices, which indicates the invariance of skyrmion number and robustness of spin texture under a continuous deformation of the field configuration, is demonstrated by inducing a perturbation to break the C4 symmetry in the presence spin-orbit coupling in an optical field. We revealed that amplitude perturbation would result in an amplitude-dependent shift of spin center, while phase perturbation leads to the deformation of domain walls, manifesting the metastability of photonic meron. Such spin topology is verified through the interference of plasmonic vortices with a broken rotational symmetry. The results provide new insights on optical topological quasiparticles, which may pave the way towards applications in topological photonics, optical information storage and transfer.

Beampattern synthesis for active RIS-assisted radar with sidelobe level minimization

To enhance the performance, reconfigurable intelligent surface (RIS) is now employed in monostatic radars since it creates a new path between the radar and the target by artificially reflecting radar signals and thus provides novel degrees of freedom. This correspondence studies the transmit beampattern synthesis for an active RIS-assisted monostatic radar via minimizing the sidelobe levels. The proposed beampattern synthesis is coined as a nonconvex and nonlinear optimization problem which is solved by jointly optimizing the beamforming weights of the transmitter and the phase shift and amplification matrix of the active RIS. The auxiliary variables are first introduced to simplify the nonconvex constraints, and two variables are then alternatively optimized by the alternating direction method of multipliers (ADMM) where their close-form optimal solutions are derived. Numerical results show that the active RIS-assisted radar has a significantly lower sidelobe level than conventional monostatic radars.

Twist angle modulated electronic properties and band alignments of hydrogen-terminated diamond (1 1 1)/hexagonal boron nitride heterostructures

The weak van der Waals (vdW) coupling between hexagonal boron nitride (hBN) and hydrogen-terminated diamond (H-diamond) provides the foundation for introducing the twist angle, as a novel degree of freedom, to modulate the properties of heterostructures. Herein, the influence of four specific twist angles on the carrier behaviors and band alignments of H-diamond (111)/hBN vdW heterostructures was investigated based on the density functional theory combined with the non-equilibrium Green’s function method. The results showed that the additional free orbitals generated by the spontaneous relaxation of hBN under large twist-induced strain could enhance the vdW coupling between hBN and H-diamond, this would promote the charge transfer at the interface, thus weakening the surface impurity scattering and increasing the hole accumulation at the H-diamond surface. Although the band gap and valence band offset of the heterostructure were reduced under large twist-induced strain, the leakage current was shielded greatly by the large invalid transmission gap and low transmission coefficient. Additionally, in configurations with the same or similar strains, the twist angle could modify the properties of the heterostructure by tuning the stacking patterns. This study proposed a feasible approach to modulate the performance of vdW heterostructures based devices.

Spin‐Orbit Modal Optical Vortex Beam Shaping from Dielectric Metasurfaces

AbstractOptical information and communication technologies are essentially based on the ability to shape the light field, either at the transmission or at the reception of the optical signal. Nowadays, wavelength and polarization state are already implemented in current telecom architectures and adding the spatial structuring of the light field as a novel degree of freedom to enhance data rate is desirable. In this context, the development of optical devices to shape optical modes belonging to orthogonal basis remains challenging. Here, in the context of singular optics, this work reports on the design, fabrication, and characterization of modal optical vortex beam shaping of paraxial light in the visible domain, in the framework of the Laguerre–Gauss basis. Both the azimuthal and radial degrees of freedom are univocally shaped via optical spin‐orbit interaction mediated by dielectric metasurfaces by combining the spatial modulation of amplitude, geometric phase, and dynamic phase. These results not only demonstrate the realization of new optical components, but also highlight how metasurface‐based technologies can contribute to optical information processing in the classical or quantum regime.

Disforming to conformal symmetry

We analyse the dynamical properties of disformally transformed theories of gravity. We show that disformal transformation typically introduces novel degrees of freedom, equivalent to the mimetic dark matter, which possesses a Weyl-invariant formulation. We demonstrate that this phenomenon occurs in a wider variety of disformal transformations than previously thought.

Electronic Structural Insight into High‐Performance Quantum Dot Light‐Emitting Diodes

AbstractQuantum dot light‐emitting diodes (QD‐LEDs) are highly promising light sources with excellent figures of merit. Although great successes have been achieved in elevating some key parameters to an ideal level, QD‐LEDs with superior performance in all aspects have rarely been realized. Herein, by exploring crystalline structure‐dependent electronic properties, it is shown that QD‐LEDs can simultaneously exhibit high external quantum efficiency, roll‐off‐free under high brightness, and dramatically improved operational stability. This improved performance stems from the crystal phase engineering of QD. Reduced structure symmetry in the wurtzite phase introduces an unneglectable internal crystal field compared with the zinc‐blende one, which raises both the conduction and valence band energy levels, thus, facilitating a more balanced charge injection. The crystal phase‐optimized superior‐comprehensive performance in the QD‐LED offers a novel degree of freedom for device engineering and promotes commercial applications for the upcoming display and illumination technologies.

Disorder Effects in One-Dimensionally Periodic Extraordinary Transmission Structures

Extraordinary transmission structures show promising capabilities for highly selective filters in both frequency and angle of incidence. However, their realistic response once manufacturing limitations are taken into account remains unexplored. In this manuscript, we explore a novel degree of freedom: disorder. We expand on previously developed highly efficient method of moment (MoM) implementations to study the effects on the transmission properties of large but finite chains of slots due to different families of disorder: lateral and vertical displacements as well as scatterer manipulation (modifying sizes and rotation angles), showing their effects on the transmission spectrum.

Defect-Polymorphism-Controlled Electrophoretic Propulsion of Anisometric Microparticles in a Nematic Liquid Crystal

Nontrivial shape of colloidal particles create complex elastic distortions and topological defects in liquid crystals and play a key role in governing their electrophoretic propulsion through the medium. Here, we report experimental results on defects and electrophoretic transport of anisometric (snowman-shaped) dielectric particles subjected to an alternating electric field perpendicular to the director in a nematic liquid crystal. We demonstrate that the shape asymmetry gives rise to defect-polymorphism by nucleating point or ring defects at multiple locations on the particle and controls the direction as well as the magnitude of the electrophoretic propulsion. Our findings unveil a novel degree of freedom in translocating microparticles in liquid crystals for applications in microfluidics, controlled transport and assembly.

3D Engineering of Orbital Angular Momentum Beams via Liquid‐Crystal Geometric Phase

AbstractOrbital angular momentum (OAM) provides a novel degree of freedom for light, deeply inspiring versatile light‐matter interactions and large‐density multiplexing computing. Recently, multimode and multichannel control of OAM beams have aroused extensive curiosity, whereas their flexible engineering in 3D space still remains challenging. Here, a new type of liquid‐crystal geometric phase optical elements is demonstrated to achieve on‐demand 3D tailoring of OAM beams. Via integrating binary and continuous phase patterns into photo‐aligned liquid crystals, customized 3D lattices of identical or propagation‐variant OAM beams are generated in an electrically tunable broadband manner. By altering the incident spin, these volumetric OAM beams can be switched among different states on the higher‐order Poincaré sphere. This work demonstrates a practical approach toward efficient OAM harnessing with large channel capacity and high spatial mode diversity, holding great potential for 3D optical manipulation, recording, and microscopy.

Efficient valley polarization of charged excitons and resident carriers in Molybdenum disulfide monolayers by optical pumping

The roadmap of future innovative device developments foresees the reduction of material dimensions down to nanometer scale and the incorporation of novel degrees of freedom. For instance, electrons and holes in 2D semiconductors like MoS2 monolayers exhibit a unique coupling between the spin and the crystal momentum, also referred to as the valley. A crucial requirement for future applications is therefore the possibility to initialise the spin/valley degree of freedom in these materials. Here we investigate the optical initialisation of the valley degree of freedom in charge-tunable MoS2 monolayers encapsulated with hexagonal boron nitride at cryogenic temperatures. We report in photoluminescence a large steady state valley polarization of the different excitonic complexes following circularly-polarized laser excitation. We reveal efficient valley initialisation of positively-charged excitons, which have so far proved to be elusive in non-encapsulated monolayers due to defect and laser-induced large electron doping. We find that negatively-charged excitons present a polarization of 70% which is unusually large for non-resonant excitation. We attribute this large valley polarization to the particular band structure of MoS2. In addition, we demonstrate that circular excitation induces a dynamical polarization of resident electrons and holes––as recently shown in tungsten-based monolayers.