New Degree Of Freedom Research Articles

PPO-based deployment and phase control for movable intelligent reflecting surface

Intelligent reflecting surface (IRS) stands as a promising technology to revolutionize wireless communication by manipulating incident signal amplitudes and phases to enhance system performance. While existing research primarily centers around optimizing the phase shifts of IRS, the deployment of IRS on movable platforms introduces a new degree of freedom in the design of IRS-assisted systems. Leveraging flexible deployment strategies for IRS holds the potential to further amplify network throughput and extend coverage. This paper addresses the challenging non-convex joint optimization problem of the movable IRS and proposes a dynamic optimization algorithm based on proximal policy optimization (PPO) for dynamically optimizing the aerial position and phase configuration of IRS. Simulation results show the effectiveness of the proposed approach, demonstrating significant performance improvements compared to communication schemes without IRS assistance and conventional static IRS-assisted methods.

Tunable Stealth Metasurface via Dual Geometric Phase Modulation

AbstractTunable metasurfaces are verified as an effective way to modulate electromagnetic (EM) waves within a thin interface. However, most tunable metasurfaces currently use electronic control methods, making it difficult to achieve large‐scale sample manufacturing. This article proposes a novel metasurface design approach based on germanium telluride (GeTe) film, which is used to achieve anomalous scattering of radar detection signals. By designing the composite units to form a 2‐bit dual geometric phase modulation coding array, low reflection characteristics in different frequency bands can be achieved by controlling the state of GeTe, which adds a new degree of freedom to array design. In this article, the phase characteristics of composite units and the EM scattering characteristics of different array arrangements are analyzed in detail. The simulation and experimental results indicate the designed metasurface has excellent dual geometric phase independent modulation ability, and the reflectivity reduction of the fabricated sample is less than ‐10 dB in the range of 11.6–15.2 GHz and 8.0–11.6 GHz before and after phase transition of GeTe film, respectively. This method has significant advantages in design, fabrication, and modulation, which determine that this research can be a good candidate for military stealth materials.

Strong transient magnetic fields induced by THz-driven plasmons in graphene disks

Strong circularly polarized excitation opens up the possibility to generate and control effective magnetic fields in solid state systems, e.g., via the optical inverse Faraday effect or the phonon inverse Faraday effect. While these effects rely on material properties that can be tailored only to a limited degree, plasmonic resonances can be fully controlled by choosing proper dimensions and carrier concentrations. Plasmon resonances provide new degrees of freedom that can be used to tune or enhance the light-induced magnetic field in engineered metamaterials. Here we employ graphene disks to demonstrate light-induced transient magnetic fields from a plasmonic circular current with extremely high efficiency. The effective magnetic field at the plasmon resonance frequency of the graphene disks (3.5 THz) is evidenced by a strong ( ~ 1°) ultrafast Faraday rotation ( ~ 20 ps). In accordance with reference measurements and simulations, we estimated the strength of the induced magnetic field to be on the order of 0.7 T under a moderate pump fluence of about 440 nJ cm−2.

Multipole engineering by displacement resonance: a new degree of freedom of Mie resonance

The canonical studies on Mie scattering unravel strong electric/magnetic optical responses in nanostructures, laying foundation for emerging meta-photonic applications. Conventionally, the morphology-sensitive resonances hinge on the normalized frequency, i.e. particle size over wavelength, but non-paraxial incidence symmetry is overlooked. Here, through confocal reflection microscopy with a tight focus scanning over silicon nanostructures, the scattering point spread functions unveil distinctive spatial patterns featuring that linear scattering efficiency is maximal when the focus is misaligned. The underlying physical mechanism is the excitation of higher-order multipolar modes, not accessible by plane wave irradiation, via displacement resonance, which showcases a significant reduction of nonlinear response threshold, sign flip in all-optical switching, and spatial resolution enhancement. Our result fundamentally extends the century-old light scattering theory, and suggests new dimensions to tailor Mie resonances.

Generation of dual-polarization orbital angular momentum vortex beams with reflection-type metasurface

Orbital angular momentum (OAM) vortex beams can provide a new degree of freedom for information modulation at a physical level, which has great prospects to expand channel capacity in the fields of wireless microwave and optical communication. In this paper, a reflection-type anisotropic metasurface is proposed to transform the incident plane wave into reflected vortex beams by modulating the phase of wavefront. Single vortex beam, multi-beam vortex beams with different mode beams are demonstrated. Under the incidence of orthogonal linearly polarized waves, independent vortex beams can be generated at 15 GHz for polarization multiplexing. Besides, the OAM spectrum decomposition is performed to analyze the purity of the OAM modes based on a Fourier transformation. The results presented in the paper demonstrate the feasibility of generating vortex beams on metasurface within a subwavelength thickness, which may promote the development of electromagnetic/optical integration devices with polarization multiplexing techniques.

Anti-resonance fiber propagating 86 orbital angular momentum modes with high purity for a long distance

Future advances in information transmission technology require better capacities for fiber optics communication. As a new degree of freedom, the orbital angular momentum (OAM) has attracted extensive attention in the field of mode division multiplexing. Herein, an anti-resonance fiber (ARF) is designed with three different materials, SF11, LaSF09, and SiO2, for the negative curvature tube. Systematic analysis and optimization are carried out to determine the number of layers of the negative curvature tubes in the cladding, the number of second-layer cladding tubes, and thickness of the annular area. The finite element analysis software COMSOL is utilized to calculate the effective refractive index difference, dispersion, limiting loss, and other characteristics. The ARF can stably transmit 86 OAM modes in the wavelength range between 1.2 and 2.0[Formula: see text][Formula: see text]m, and the minimum refractive index difference is greater than [Formula: see text], which effectively reduces the mode abridgment. The nonlinear coefficients of the HE[Formula: see text] mode and the OAM purity of HE[Formula: see text] mode are 0.34[Formula: see text]km[Formula: see text][Formula: see text]⋅[Formula: see text]W[Formula: see text] and 99.43%, respectively, thus facilitating stable transmission of OAM modes for a long transmission distance. The anti-resonance fiber has significant application potential in long-distance network information transmission, relay communication, and other communication areas.

Refractive Index Engineering as a New Degree of Freedom for Designing High‐Performance AlGaN‐Based Ultraviolet C Light‐Emitting Diodes

This study delves into a profound exploration, using simple yet effective Monte Carlo ray‐tracing simulations, of the influence of the refractive indices of multiple quantum wells (MQWs) and p‐type electron blocking layer (p‐EBL) on the photon propagation and subsequent light extraction behavior in AlGaN‐based ultraviolet‐C (UVC) light‐emitting diodes (LEDs), covering both transverse‐electric (TE) and transverse‐magnetic (TM) polarized light. Remarkably, the refractive index contrasts at the heterointerfaces of p‐EBL/MQWs/n‐AlGaN have a relatively minor impact on the extraction efficiency of TE‐polarized light but exert a significant influence on TM‐polarized light. This discrepancy arises from the substantial effect of total internal reflection at these heterointerfaces on the propagation of photons emitted at large angles. Thus, this observation elucidates the oversight of photon propagation in the conventional design of MQWs and p‐EBL in AlGaN‐based UVA and UVB LEDs, where TE‐polarized emission predominates. Building upon simulation results, a highly effective strategy for extracting TM‐polarized light in AlGaN‐based UVC LEDs is further demonstrated by combining refractive index engineering with inclined sidewalls. The findings suggest refractive index engineering can serve as a novel design parameter for AlGaN‐based UVC LEDs dominated by TM‐polarized emission, expanding the range of techniques available for enhancing device performance.

Reconstruction of a Scale-dependent Gravitational Phase Transition

In this work we extend our earlier phenomenological model for a gravitational phase transition and its generalization to early times by letting the modifications in the linearly perturbed Einstein equations be scale dependent. These modifications are characterized as deviations of the parameters μ(z, k) and γ(z, k) from their values in general relativity (GR). The scale-dependent amplitudes of modified μ(z, k) and γ(z, k) and the parameters defining the phase transition, along with the standard cosmological parameters, are measured by various data combinations. Out of the perturbation parameters, we construct gravity eigenmodes that represent patterns of perturbations best detectable by data. We detect no significant deviation from GR in these parameters. However, the larger parameter space produced due to the new degrees of freedom allows for the reconciliation of various data sets that are in tension in ΛCDM. In particular, we find H 0 = 71.9 ± 9.2 from anisotropies of the cosmic microwave background as measured by Planck (and various measurements of the baryonic acoustic oscillations, in agreement with local Hubble measurements). We also find that the σ 8 tension between the measurements of the Dark Energy Survey and Planck is reduced to less than 1σ.

On-chip sorting of orbital angular momentum beams using Bloch surface wave structures

Owing to their unique optical properties and new degrees of freedom, orbital angular momentum (OAM) beams have been applied in various fields. Detection of the topological charges (TCs) of OAM beams is the key step for their applications. However, on-chip sorting of OAM beams with large TCs still remains a challenge. In this paper, Bloch surface wave (BSW) structures with five semi-ring shaped nanoslits are modeled. A spatial separation of 135 nm on the chip is obtained between two neighboring OAM states. OAM beams with TCs up to 35 can be successfully sorted by the BSW structures, which is much larger than that using metallic structures (only seven). BSW structures exhibit better OAM sorting performances than metallic structures. We systematically show how the lower attenuation of BSW structures leads to far superior separation ability compared to surface plasmons propagating on metallic structures. In addition, sorting of two OAM beams with different TCs simultaneously can be achieved in this way. Our results reveal that BSW structures should be an excellent solution for OAM sorting with large TCs, which is beneficial for applications in integrated on-chip devices and optical communications.

Generation of high-order focused acoustic vortices and multiplexed beams using Dual-Channel metamaterials

Focused vortex beams have attracted considerable research attention due to their unique acoustic properties, and have been explored in medical imaging, biomedical therapy, and particle manipulation. Here, an acoustic metamaterial vortex generator (AMVG) is designed to realize a multiplexed focusing acoustic vortex (MFAV) with multiple-order topological charges (TCs). The proposed AMVG is composed of several fanlike sectors with two high transmission channels per unit, which can flexibly modulate the transmitted phase shift in two separate frequency bands. In contrast to conventional acoustic metamaterial-based vortices, the design strategy provides a new degree of freedom, i.e., frequency, that may extend to the agile tailoring of multi-band independent vortices, significantly enhancing the information encoded in the acoustic metamaterial. Acoustic transmission and phase shift of the dual-channel unit are investigated theoretically and demonstrated in simulations using the finite element method. The focal position and TC of the MFAVs can be modulated by simply adjusting the cavity length of the unit cell and even only the sector order. Furthermore, we implement three-dimensional anechoic experiments to verify the effectiveness of the off-axial MFAVs by the designed AMVG. These multiplexed focusing vortices offer a technical advance to tackle the rising challenges in the fields of acoustic signal processing and communication systems.

A novel methodology to assist manufacturers in the heat exchanger sizing for variable-speed heat pumps based on part load conditions and economic assessment

Matts Bäckstöm assessed the selection criteria of heat exchangers for single-speed heat pumps in 1940, and his methodology and conclusions have been kept practically unchanged for almost a century. However, heat pump systems have evolved, especially with the introduction of variable-speed heat pumps. These new systems introduced a new degree of freedom to the heat pump design – the compressor speed – which is not considered by traditional design criteria.This study proposes novel design criteria to optimize the heat exchanger size when the heat pump unit can work under part-load conditions. The proposed method models how heat exchangers and variable-speed compressors perform under different loads and sizes. It considers part-load requirements based on climatic data (which are typically available in the standards) and considers economic factors such as initial investment and operation costs (which are dependent on the components's size). Given the mentioned information, a multivariable optimization algorithm is implemented to find the optimum heat exchanger size that minimizes the total cost. To validate the proposed approach's effectiveness, it was exemplified by determining the optimum size of a coil and a plate heat exchanger for a domestic variable-speed heat pump. The results demonstrate that using traditional criteria could lead to over-dimensioning heat exchangers in variable-speed heat pumps which could incur in an increase of costs of 5% for the studied case.

Weyl conformal geometry vs Weyl anomaly

Weyl conformal geometry is a gauge theory of scale invariance that naturally brings together the Standard Model (SM) and Einstein gravity. The SM embedding in this geometry is possible without new degrees of freedom beyond SM and Weyl geometry, while Einstein gravity is generated by the broken phase of this symmetry. This follows a Stueckelberg breaking mechanism in which the Weyl gauge boson becomes massive and decouples, as discussed in the past [1–3]. However, Weyl anomaly could break explicitly this gauge symmetry, hence we study it in Weyl geometry. We first note that in Weyl geometry metricity can be restored with respect to a new differential operator ( hat{nabla} ) that also enforces simultaneously a Weyl-covariant formulation. This leads to a metric-like Weyl gauge invariant formalism that enables one to do quantum calculations directly in Weyl geometry, rather than use a Riemannian (metric) geometry picture. The result is the Weyl-covariance in d dimensions of all geometric operators ( hat{R} , etc) and of their derivatives ( hat{nabla} μ hat{R} , etc), including the Euler-Gauss-Bonnet term. A natural (geometric) Weyl-invariant dimensional regularisation of quantum corrections exists and Weyl gauge symmetry is then maintained and manifest at the quantum level. This is related to a non-trivial current of this symmetry, the divergence of which cancels the trace of the energy-momentum tensor. The “usual” Weyl anomaly and Riemannian geometry are recovered in the (spontaneously) broken phase. The relation to holographic Weyl anomaly is discussed.

Photonic spin Hall effect in a graphene-incorporated quasi-PT symmetric system and the miniaturization of nanophotonic devices.

By inserting the monolayer graphene between the balanced gain and loss layers, the graphene-incorporated quasi-parity-time (PT) symmetric structure is established. In this contribution, the introduction of graphene provides a new degree of freedom to manipulate the optical performance as well as the photonic spin Hall effect (SHE). The coherent perfect absorption (CPA)-laser mode still remains in the graphene-incorporated quasi-PT symmetric system, and the spin shift of transmitted light can be significantly enhanced (i.e.,up to its upper limitation) in the vicinity of CPA-laser mode, which is 18 times larger than the value of a simple PT symmetric structure. In addition, the excitation of the CPA-laser mode and the huge spin shift of transmitted light can be achieved with the thin gain/loss layers, which will be conducive to the miniaturization of nanophotonic devices based on the photonic SHE in the future.

Thermodynamic properties of M− C 60 dimers immersed in an inert gas

When fullerenes capture an alkali metal they acquire a powerful permanent dipole. This suggests that if the system is in an inert atmosphere, like Argon, they tend to form dimers producing a gas whose thermodynamical properties can be studied. On this work, we focus on the degrees of freedom that appear when the dimer has formed. The fraction of that can be created is calculated by using the standard physical statistics and can be graphed against the temperature on the 273.15 K and 373.15 K interval. The usual procedure to obtain the Keesom energy is revised to show that it can only be valid for weakly interacting dipoles or when the separation distance between them is considerable. To solve the problem we use the correction to the Keesom energy obtained by Battezzati and Magnasco in order to introduce it into the partition function for the new degree of freedom. The energy, the Helmholtz energy and the entropy is calculated through both means. The results were represented on graphs against the distance of separation between the These results show that using the Keesom term alone is unsuitable when the separation distances are lower than 0.7 nm. Lastly, the calculations of the Helmholtz, Gibbs, and enthalpy as functions of temperature show that the dimer formation is both spontaneous and exothermic, as well as stable and an important fraction of the total amount of gas molecules in the system.

Mode thermo-optic coefficient engineering of sub-wavelength gratings and its application for a mode-insensitive switch.

It is shown that the thermo-optic (TO) coefficients of various waveguide modes of a sub-wavelength grating (SWG)-assisted strip waveguide is closely dependent on the various waveguide parameters with different dependencies, including the SWG width, strip waveguide width, duty cycle, and pitch. This offers what we believe to be new degrees of freedom in the design of TO coefficients for integrated-optic waveguides, opening the door to engineering the TO coefficients of individual spatial modes or polarization states using sub-wavelength structures. Such a capability is expected to offer new design possibilities for a variety of integrated photonic, thermo-optic devices. To demonstrate the application of the concept, a mode-insensitive switch on silicon-on-insulator using a TO coefficient-engineered SWG as a mode-independent, thermo-optic phase shifter is designed and experimentally demonstrated. The experimental results show that the switching powers of the TE0-TE2 modes are only ∼29 mW, and the maximum extinction ratios for the cross (bar) states are 38.2 dB (31 dB), 37.9 dB (37 dB), and 31.9 dB (20.5 dB) for the TE0-TE2 modes, respectively, at the wavelength of 1550 nm.

Nonzero spatial curvature in symmetric teleparallel cosmology

We consider the symmetric teleparallel fQ-gravity in Friedmann–Lemaître–Robertson–Walker cosmology with nonzero spatial curvature. For a nonlinear fQ model there exist always the limit of General Relativity with or without the cosmological constant term. The de Sitter solution is always provided by the theory and for the specific models fAQ≃Qαα−1,fBQ≃Q+f1Qαα−1 and fCQ≃Q+f1QlnQ it was found to be the unique attractor. Consequently small deviations from STGR can solve the flatness problem and lead to a de Sitter expansion without introduce a cosmological constant term. This result is different from that given by the power-law theories for the other two scalar of the trinity of General Relativity. What makes the nonlinear symmetric teleparallel theory to stand out are the new degree of freedom provided by the connection defined in the non-coincidence frame which describes the nonzero spatial curvature.

Bessel–Gaussian beam-based orbital angular momentum holography

Orbital angular momentum (OAM), as a new degree of freedom, has recently been applied in holography technology. Due to the infinite helical mode index of OAM mode, a large number of holographic images can be reconstructed from an OAM-multiplexing hologram. However, the traditional design of an OAM hologram is constrained by the helical mode index of the selected OAM mode, for a larger helical mode index OAM mode has a bigger sampling distance, and the crosstalk is produced for different sampling distances for different OAM modes. In this paper, we present the design of the OAM hologram based on a Bessel–Gaussian beam, which is non-diffractive and has a self-healing property during its propagation. The Fourier transform of the Bessel–Gaussian beam is the perfect vortex mode that has the fixed ring radius for different OAM modes. The results of simulation and experiment have demonstrated the feasibility of the generation of the OAM hologram with the Bessel–Gaussian beam. The quality of the reconstructed holographic image is increased, and the security is enhanced. Additionally, the anti-interference property is improved owing to its self-healing property of the Bessel-OAM holography.

Improvement of terahertz beam modulation efficiency for baseless all-dielectric coded gratings

Optical metasurfaces are two-dimensional ultrathin devices based on single-layer or multilayer arrays of subwavelength nanostructures. They can achieve precise control of phase, amplitude, and polarization on the subwavelength scale. In this paper, a substrate-free all-silicon coded grating is designed, which can realize the phase control of the outgoing beam after the y-polarized plane wave is vertically incident on the metasurface at 0.1 THz. Through a single-layer silicon nanoarray structure, a low-reflection anomalous transmission metasurface is realized, and a variety of different beam deflectors are designed based on these encoded gratings. We propose a coded grating addition principle, which adds and subtracts two traditional coded grating sequences to obtain a new coded grating sequence. The encoded supergrating can flexibly control the scattering angle, and the designed substrate-free all-silicon encoded grating can achieve a deflection angle of 48.59°. In order to verify the principle of coded grating addition, we experimented with cascade operation of two coded sequence gratings to obtain the flexible control of the terahertz beam of the composite supergrating. The principle of grating addition provides a new degree of freedom for the flexible regulation of the terahertz wavefront. At the same time, this method can be extended to the optical band or microwave band, opening up new ways for electromagnetic wave manipulation and beam scanning.

Interlayer Coupling in Anisotropic/Isotropic Van der Waals Heterostructures of ReS2 and WS2.

Van der Waals (vdW) heterostructures are composed of atomically thin layers assembled through weak (vdW) force, which have opened a new era for integrating materials with distinct properties and specific applications. However, few studies have focused on whether and how anisotropic materials affect heterostructure system. The study introduces anisotropic and isotropic materials in a heterojunction system to change the in-plane symmetry, offering a new degree of freedom for modulating its properties. The sample is fabricated by manually stacking ReS2 and WS2 flakes prepared by mechanical exfoliation. Raman spectra and photoluminescence measurements confirm the formation of an effective heterojunction, indicating interlayer coupling of the system. The anisotropy and asymmetry of the WS2 -ReS2 heterostructure system can be adjusted by the introduction of isotropic WS2 and anisotropic ReS2 , which can be proved by the change of the polarized Raman pattern. In the transient absorption measurement, the transient absorption spectra of WS2 -ReS2 heterostructure are red-shifted compared to those of WS2 monolayer, and the charge transfer is observed in the heterostructure. These results show the potential of anisotropic 2D materials in anisotropy modulation of heterostructures, which may promote future electronic or photonic application.

Voltage-controlled magnetic double-skyrmion states in magnetoelectric elliptical nanostructures by phase-field model

The magnetic skyrmions are promising candidates as information carriers for future energy-efficient spintronic devices (e.g. memory, sensor, logic). Moreover, manipulating elliptically distorted skyrmions can result in a variety of novel functions that differ from those of conventional circular skyrmions. Despite the tremendous prospects, achieving voltage-driven dynamics of the elliptically distorted skyrmions remains a key challenge. Here, we employ dynamical phase-field simulations that consider the strain-mediated magnetoelectric coupling to demonstrate electric field manipulates the dynamic behavior of the elliptically distorted skyrmions. This study finds that the elliptically distorted skyrmions can split into smaller-sized circular skyrmions and reversibly switch between the single domain - single skyrmion and single domain - double skyrmion on elliptical nano-islands. The stabilization of magnetic skyrmions and double skyrmions in multiferroic heterostructures essentially depends on the ratio of the major axis and the minor axis of the elliptical nono-islands. Intriguingly, the operation is non-volatile and exhibits a multistate character. The temporal behavior of energy density variations during the evolution of magnetic domain structures indicates that the electric-field manipulation of skyrmions originates from the competition between elastic, demagnetization, and anisotropic energies. Our work shows that deterministic switching of multiple domain structures on elliptical nano-islands is achievable, indicating the morphology is a new degree of freedom for manipulating the voltage-driven dynamics of skyrmions, providing a novel avenue to develop magnetoelectric coupling-based memory devices, logic devices, sensor devices, etc., at the nanoscale. Impact statementThis research aims to construct a unique multi-state storage device as well as to indicate the morphology is a new degree of freedom for manipulating the voltage-driven dynamics of skyrmions, giving a fresh way to create nanoscale magnetoelectric coupling-based memory, logic, sensor, and other devices.