Novel Degree Of Freedom Research Articles

Temporal effect on tight focusing, optical force and spin torque of high-order vector-vortex beams

We theoretically study the dependence of the light field distributions, optical force and spin torque of high-order vector-vortex (HOVV) beams focused by a high numerical aperture objective on ultrafast variable time (0–400 fs). It is accomplished by the fast Fourier transform, time-dependent vectorial diffraction theory and Rayleigh scattering model. Firstly, it is shown that the light field patterns of tightly focused HOVV beams can be quasi-periodically reshaped by adjusting the intrinsic fleeting time. We further examine the three-dimensional (3D) distributions of optical force on a Rayleigh particle (with a size much smaller than the light wavelength) in the focal volume induced from tightly focused HOVV beams, which reveals that the distinct vector-vortex light fields as a function of ultrafast time endow the capability of the selective trapping for particles. Beyond that, we proceed to the 3D spin torque on the given particle, which makes the conversions between transverse and longitudinal spin torques possible, resulting in the 3D spatial rotation within an ultrafast timescale. Additionally, it is found that the vortex order plays a pivotal role in steering the focused light fields and related optical forces and spin torques. The proposed pathway and acquired results not only provide a novel degree of freedom for the ultrafast control of light fields, but also have potential applications in integrated spectroscopic analysis, ultrafast optical tweezers and spanners, and high-speed optical measurements.

Generalized quantum-classical correspondence for random walks on graphs

We introduce a minimal set of physically motivated postulates that the Hamiltonian H of a continuous-time quantum walk should satisfy in order to properly represent the quantum counterpart of the classical random walk on a given graph. We found that these conditions are satisfied by infinitely many quantum Hamiltonians, which provide novel degrees of freedom for quantum enhanced protocols, In particular, the on-site energies, i.e. the diagonal elements of H, and the phases of the off-diagonal elements are unconstrained on the quantum side. The diagonal elements represent a potential energy landscape for the quantum walk, and may be controlled by the interaction with a classical scalar field, whereas, for regular lattices in generic dimension, the off-diagonal phases of H may be tuned by the interaction with a classical gauge field residing on the edges, e.g., the electro-magnetic vector potential for a charged walker.

On the anomalous low-resistance state and exceptional Hall component in hard-magnetic Weyl nanoflakes

Magnetic topological materials, which combine magnetism and topology, are expected to host emerging topological states and exotic quantum phenomena. In this study, with the aid of greatly enhanced coercive fields in high-quality nanoflakes of the magnetic Weyl semimetal Co3Sn2S2, we investigate anomalous electronic transport properties that are difficult to reveal in bulk Co3Sn2S2 or other magnetic materials. When the magnetization is antiparallel to the applied magnetic field, the low longitudinal resistance state occurs, which is in sharp contrast to the high resistance state for the parallel case. Meanwhile, an exceptional Hall component that can be up to three times larger than conventional anomalous Hall resistivity is also observed for transverse transport. These anomalous transport behaviors can be further understood by considering nonlinear magnetic textures and the chiral magnetic field associated with Weyl fermions, extending the longitudinal and transverse transport physics and providing novel degrees of freedom in the spintronic applications of emerging topological magnets.

Quantum Cascade Lasers: Controlling and Phase‐Locking a THz Quantum Cascade Laser Frequency Comb by Small Optical Frequency Tuning (Laser Photonics Rev. 15(6)/2021)

In article number 2000417, Luigi Consolino and co-workers demonstrate a novel degree of freedom for controlling the emission of quantum cascade lasers (QCLs). It is based on small optical frequency tuning (SOFT) by means of a white LED. The slightly perturbative regime accessible in these conditions allows tweaking the semiconductor complex refractive index. The resulting additional tuning knob is capable of phase stabilizing the modes spacing of a QCL comb.

Dynamical control of directional nonlinear scattering from metallic nanoantennas by three-dimensional focal polarization orientation

We demonstrate that the directionality of far-field second harmonic (SH) emission generated from individual gold nanosphere can be flexibly engineered by manipulating three-dimensional (3D) focal polarization orientation of the excitation field, which is explained by the coherent interference between SH dipolar and quadrupolar emission modes. The SH dipolar emission mode is independent of the polarization direction of the fundamental field whereas the evolution of the focal polarization orientation can dramatically modify the quadrupolar emission pattern. Therefore, the resultant SH emission pattern has a polarization-dependent behavior and side scattering almost perpendicular to the propagation direction of the incident light can be observed under a specific condition. Our findings provide a novel degree of freedom for all-optical control of directional nonlinear scattering from single nanoantenna, thereby opening new possibilities for future potential applications.

Applying Deep-Learning-Based Computer Vision to Wireless Communications: Methodologies, Opportunities, and Challenges

Deep learning (DL) has seen great success in the computer vision (CV) field, and related techniques have been used in security, healthcare, remote sensing, and many other areas. As a parallel development, visual data has become universal in daily life, easily generated by ubiquitous low-cost cameras. Therefore, exploring DL-based CV may yield useful information about objects, such as their number, locations, distribution, motion, etc. Intuitively, DL-based CV can also facilitate and improve the designs of wireless communications, especially in dynamic network scenarios. However, so far, such work is rare in the literature. The primary purpose of this article, then, is to introduce ideas about applying DL-based CV in wireless communications to bring some novel degrees of freedom to both theoretical research and engineering applications. To illustrate how DL-based CV can be applied in wireless communications, an example of using a DL-based CV with a millimeter-wave (mmWave) system is given to realize optimal mmWave multiple-input and multiple-output (MIMO) beamforming in mobile scenarios. In this example, we propose a framework to predict future beam indices from previously observed beam indices and images of street views using ResNet, 3-dimensional ResNext, and a long short-term memory network. The experimental results show that our frameworks achieve much higher accuracy than the baseline method, and that visual data can significantly improve the performance of the MIMO beamforming system. Finally, we discuss the opportunities and challenges of applying DL-based CV in wireless communications.

Signal-Disturbance Interfacing Elimination for Unbiased Model Parameter Identification of Lithium-Ion Battery

A precisely parameterized battery model is the prerequisite of the model-based management of lithium-ion battery. However, the unexpected sensing of noises may discount the identification of model parameters in practical applications. This article focuses on the noise effect compensation and online parameter identification for the widely used equivalent circuit model. A novel degree of freedom (DOF) eliminator is proposed and combined with the Frisch scheme in a recursive fashion, for the first time, to coestimate the noise statistics and unbiased model parameters. A computationally tractable numerical solver is further proposed for the DOF eliminator to improve the real-time performance. Simulations and experiments are performed to validate the proposed method from theoretical to practical perspective. Results show that the proposed method can effectively mitigate the noise-induced identification biases and outperform the existing methods in terms of the accuracy and the robustness to noise corruption.

A Novel Degree of Freedom (DoF) Link Scheduler for Full-Duplex Wireless Local Area Networks

We consider an Access Point (AP) equipped with multiple antenna elements and an in-band full-duplex radio. In each time slot, it needs to poll clients for uplink and/or carry out downlink transmissions, and to allocate its antenna elements for data transmissions and interference cancellation. We outline a Markov Decision Process (MDP) and use it to find the optimum polling policy that maximizes the average number of data streams. We also outline two novel Degree of Freedom (DoF)-based antenna allocation algorithms. The results show our policy is able to activate respectively up to 58% more data streams and within 83% of the theoretical upper bound of half-duplex and full-duplex polling-based methods that require perfect information of queue occupancy and interference at clients.

Noninverse dynamics of a quantum emitter coupled to a fully anisotropic environment

Anisotropic nanophotonic structures can couple the levels of a quantum emitter through the quantum interference effect. In this paper we study the coupling of quantum emitters excited states through the modes of a fully anisotropic structure: a structure for which all directions are physically nonequivalent. We consider an anisotropic metasurface as an illustrative example of such a structure. We point out a novel degree of freedom in controlling the temporal dynamics and spectral profiles of quantum emitters: namely, we show that a combination of the metasurface anisotropy and tilt of the emitter quantization axis with respect to the metasurface normal results in nonsymmetric dynamics between the transitions of electrons from left-circular state to the right-circular states and the inverse process. Our findings give an additional mechanism for control over the light emission by quantum systems and, vice versa, can be utilized for probing active transitions of quantum emitters.

Effect of thin heterogeneous functional nanolayers on electron transport in InGaAs-based quantum wells with high electron density (a review)

A prospective improvement of InGaAs-based quantum well heterostructures design is possible towards the quantum treatment of electron subbands states and electron transport processes in the real structures with thin spatially inhomogeneous functional nanolayers. Insertion of nanosized GaAs, AlAs and InAs layers into quantum well/spacer/barrier layers or applying a graded InyGa1-yAs channel can enhance electron transport properties and gives a novel degree of freedom for the quantum structures engineering. The review and original study for PHEMT/GaAs, HFET/GaAs and InP HEMT structures are discussed.

Low-frequency forbidden bandgap engineering via a cascade of multiple 1D superlattices

Low-frequency forbidden bandgap (LFB) in solid-fluid superlattice (SFSL) provides a novel degree of freedom to control the propagation of low frequency acoustic waves. However, its narrow angle range seriously restricts its practical applications. To engineer the angle range of LFB, the transmission coefficient of a cascade of multiple 1D superlattices was calculated using the transfer matrix method and also verified by finite element simulations. It was found that the LFB with ultra-wide angle range can be created using only 4 cells of a two-phased cascade structure and can be adjusted by changing the solid filling fraction of each sub-SFSL. By introducing two or more 1D superlattices into the layered cascade structure, the LFB with multiple angle ranges and/or ultra-wide angle range can be engineered for low frequency acoustic waves whose wavelengths are much larger than the lattice constant. Such acoustic low-frequency forbidden bandgap materials are useful for making perfect acoustic low-frequency filters with broad bandwidth in selective angle ranges, which are useful in subwavelength acoustic devices.

Scaling the Graft Length and Graft Density of Irradiation‐Grafted Copolymers

AbstractIrradiation‐induced graft copolymerization is often the easiest route to combine the functionality of two polymers and, furthermore, the average composition of the copolymer is straightforward to control. However, control of the graft length and the graft density is more difficult and has been considered to be infeasible. To test the hypothesis that the graft density and graft length may be increased or decreased via increasing or decreasing the dose of irradiation and graft level, polymer electrolytes are characterized in terms of water sorption, proton conductivity, and nanostructure. Based on the premise that graft density and graft length define ion transport and phase segregation, impedance spectroscopy and small‐angle scattering support the hypothesis. The method therefore represents a novel degree of freedom in the design and synthesis of radiation grafted copolymers.

Backcoupling manipulation in silicon ring resonators

In this paper, we theoretically propose and experimentally demonstrate the manipulation of a novel degree of freedom in ring resonators, which is the coupling from the clockwise input to the counterclockwise propagating mode (and vice versa). We name this mechanism backcoupling, in contrast with the normal forward-coupling of a directional coupler. It is well known that internal reflections will cause peak splitting in a ring resonator. Our previous research demonstrated that the peak asymmetry will be strongly influenced by the backcoupling. Thus, it is worth manipulating the backcoupling in order to gain full control of a split resonance for the benefit of various resonance-splitting-based applications. While it is difficult to directly manipulate the backcoupling of a conventional directional coupler, here we design a circuit explicitly for manipulating the backcoupling. It can be potentially developed for applications such as single sideband filter, resonance splitting elimination, Fano resonance, and ultrahigh-Q and finesse.

Controlling hybrid nonlinearities in transparent conducting oxides via two-colour excitation

Nanophotonics and metamaterials have revolutionized the way we think about optical space (ɛ,μ), enabling us to engineer the refractive index almost at will, to confine light to the smallest of the volumes, and to manipulate optical signals with extremely small footprints and energy requirements. Significant efforts are now devoted to finding suitable materials and strategies for the dynamic control of the optical properties. Transparent conductive oxides exhibit large ultrafast nonlinearities under both interband and intraband excitations. Here we show that combining these two effects in aluminium-doped zinc oxide via a two-colour laser field discloses new material functionalities. Owing to the independence of the two nonlinearities, the ultrafast temporal dynamics of the material permittivity can be designed by acting on the amplitude and delay of the two fields. We demonstrate the potential applications of this novel degree of freedom by dynamically addressing the modulation bandwidth and optical spectral tuning of a probe optical pulse.

Optical manipulation of valley pseudospin

The coherent manipulation of spin and pseudospin underlies existing and emerging quantum technologies, including NMR, quantum communication, and quantum computation. Valley polarization, associated with the occupancy of degenerate, but quantum mechanically distinct valleys in momentum space, closely resembles spin polarization and has been proposed as a pseudospin carrier for the future quantum electronics. Valley exciton polarization has been created in the transition metal dichalcogenide (TMDC) monolayers using excitation by circularly polarized light and has been detected both optically and electrically. In addition, the existence of coherence in the valley pseudospin has been identified experimentally. The manipulation of such valley coherence has, however, remained out of reach. Here we demonstrate an all-optical control of the valley coherence by means of the pseudomagnetic field associated with the optical Stark effect. Using below-bandgap circularly polarized light, we experimentally rotate the valley exciton pseudospin in monolayer WSe2 on the femtosecond time scale. Both the direction and speed of the rotation can be optically manipulated by tuning the dynamic phase of excitons in opposite valleys. This study completes the generation-manipulation-detection paradigm for valley pseudospin, enabling the platform of excitons in 2D materials for the control of this novel degree of freedom in solids.

Near-field focus steering along arbitrary trajectory via multi-lined distributed nanoslits.

The modulation of near-field signals has recently attracted considerable interest because of demands for the development of nano-scale optical devices that are capable of overcoming the diffraction limit of light. In this paper, we propose a new type of tuneable plasmonic lens that permits the foci of surface plasmon polariton (SPP) signals to be continuously steered by adjusting the input polarization state. The proposed structure consists of multi-lined nanoslit arrays, in which each array is tilted at a different angle to provide polarization sensitivity and the nanoslit size is adjusted to balance the relative amplitudes of the excited SPPs from each line. The nanoslits of each line are designed to focus SPPs at different positions; hence, the SPP focal length can be tuned by modifying the incident polarization state. Unlike in previously reported studies, our method enables plasmonic foci to be continuously varied with a smooth change in the incident linear polarization state. The proposed structures provide a novel degree of freedom in the multiplexing of near fields. Such characteristics are expected to enable the realization of active SPP modulation that can be applied in near-field imaging, optical tweezing systems, and integrated nano-devices.

Tuning Valley Polarization in aWSe2Monolayer with a Tiny Magnetic Field

A recently discovered class of two-dimensional semiconductors exhibits a novel degree of freedom known as valley pseudospin. New results show that a weak magnetic field can significantly extend the depolarization time of this pseudospin.

Antiferromagnetic Spin Wave Field-Effect Transistor.

In a collinear antiferromagnet with easy-axis anisotropy, symmetry dictates that the spin wave modes must be doubly degenerate. Theses two modes, distinguished by their opposite polarization and available only in antiferromagnets, give rise to a novel degree of freedom to encode and process information. We show that the spin wave polarization can be manipulated by an electric field induced Dzyaloshinskii-Moriya interaction and magnetic anisotropy. We propose a prototype spin wave field-effect transistor which realizes a gate-tunable magnonic analog of the Faraday effect, and demonstrate its application in THz signal modulation. Our findings open up the exciting possibility of digital data processing utilizing antiferromagnetic spin waves and enable the direct projection of optical computing concepts onto the mesoscopic scale.

Phononic bandgaps peculiar to solid–fluid superlattices

We theoretically study the phonons propagating through a superlattice consisting of alternating layers of an elastic solid and a fluid. In this structure, there exist phononic bandgaps not originating from Bragg reflections. We examine the origin of these non-Bragg gaps and show that they are peculiar to the solid–fluid superlattices, where the number of allowed modes varies periodically. Even a single solid layer immersed in fluid contains discrete frequencies at which incident waves are perfectly reflected. We demonstrate the resonant reflection process at these frequencies. In the multilayered structure, these transmission zeros are gathered and form a bandgap. This is similar to the relation between atomic levels and an electronic energy band, though the allowed and forbidden states are interchanged. This non-Bragg gap introduces novel degrees of freedom to the design of phononic bandgap structures.

Variations on an aethereal theme

We consider a class of Lorentz-violating theories of gravity involving a timelike unit vector field (the aether) coupled to a metric, two examples being Einstein-aether theory and Ho\v{r}ava gravity. The action always includes the Ricci scalar of the metric and the invariants quadratic in covariant derivatives of the aether, but the theories differ in how the aether is constructed from other fields, and whether those fields are varied in the action. Fields that are not varied define background structures breaking diffeomorphsim invariance, including threadings, folations, and clocks, which generally produce novel degrees of freedom arising from the violation of what would otherwise be initial value constraints. The principal aims of this paper are to survey the nature of the theories that arise, and to understand the consequences of breaking diffeomorphism invariance in this setting. In a companion paper [arXiv:1504.03305], we address some of the phenomenology of the "ponderable aether" case in which the presence of a background clock endows the aether with a variable internal energy density that behaves in some respects like dark matter.