Nonlocal Response Research Articles

Optimal optical Ferris wheel solitons in a nonlocal Rydberg medium.

We propose a scheme for the creation of stable optical Ferris wheel (OFW) solitons in a nonlocal Rydberg electromagnetically induced transparency (EIT) medium. Depending on a careful optimization of both the atomic density and the one-photon detuning, we obtain an appropriate nonlocal potential provided by the strong interatomic interaction in Rydberg states that can perfectly compensate for the diffraction of the probe OFW field. Numerical results show that the fidelity remains larger than 0.96, while the propagation distance has exceeded 160 diffraction lengths. Higher-order OFW solitons with arbitrary winding numbers are also discussed. Our study provides a straightforward route to generate spatial optical solitons in the nonlocal response region of cold Rydberg gases.

Nonlocal cubic and quintic nonlinear wave patterns in pure-quartic media

In this work, pure-quartic soliton (PQS)formation is investigated in the framework of a nonlinear Schrödinger equation with competing Kerr (cubic) and non-Kerr (quintic) nonlocal nonlinearities and quartic dispersion. In the process, the modulational instability (MI) phenomenon is activated under a suitable balance between the nonlocal nonlinearities and the quartic dispersion, both for exponential and rectangular nonlocal nonlinear responses. Interestingly, the maximum MI growth rate and bandwidth are reduced or can completely be suppressed for some specific values of the cubic and quintic nonlocality parameters, depending on the type of nonlocal response. The analytical results are confirmed via direct numerical simulations, where the instability supports the signature of pure-quartic dark and bright solitons. These results may provide a better understanding of PQS structures for their potential applications in the next generation of nonlinear optical devices.

Chirped spatial solitons on a continuous-wave background in weak nonlocal media with polynomial law of nonlinearity

Spatial soliton propagation in a cubic-quintic-septimal optical material with a weakly nonlocal response is examined. We demonstrate that bright and gray solitons on a continuous-wave background exist for the envelope equation describing their dynamis in the physical media. In addition, optical kink and anti-kink type solitons are also identified. We find that these waveforms exhibit a frequency chirping property which is inversely proportional to the beam intensity. A noteworthy characteristic is that the chirped soliton solutions include no free parameters and their properties such as their amplitude, spatial width and propagation constant are determined solely by the material parameters. The numerical examples are structured for illustrating the propagation dynamics of these solitons in the optical system. The role of weak nonlocality on the modulation instability gain has also been discussed.

Nonlocal Fourier modal method for analyzing nonlocal plasmonic periodic nanostructures

In accordance with growing scientific interests in nanoplasmonic structures, along with the increasing ability to fabricate them using proper nanotechnologies, we have developed a methodology to incorporate nonlocal optical responses, described with a simple hydrodynamic model, into the numerical Fourier modal method (FMM) technique to enable broadening of the simulation portfolio of such physical phenomena in plasmonic nanostructures. Here, we have newly incorporated the nonlocal response approximation (NRA) into the periodic FMM technique (called NonLocFMM) described with a proper hydrodynamic model, and we show in several examples that this implementation is capable of numerically analyzing periodic plasmonic systems, such as nonlocal periodic multilayers and resonant gratings. The implementation was performed in such a way that a future possible generalization of the hydrodynamic nonlocal description will be feasible, as well as the aperiodization of the technique. Such an extension is, indeed, viable and will be the subject of a subsequent paper.

Multipolar theory of bianisotropic response of meta-atoms

Bianisotropy of metaatoms is usually associated with their nonlocal response and the mutual coupling between electric and magnetic dipole moments induced by the incident field. In this work, we generalize the theory of bianisotropy beyond the dipole response to the cases of arbitrary high-order multipole resonances. We demonstrate that bianisotropy is exclusively caused by the absence of the inversion symmetry of metaatoms. The strength of the bianisotropy response is normally increased with the size of a meta-atom but its origin is fully connected to the symmetry of the structure. As an important example of bianisotropic particle, we consider a triangular prism and show how accounting for the higher-order multipoles prevents the violation of the Onsager-Casimir conditions for kinetic coefficients appearing in the dipole approximation. The developed theory is an important step towards a deeper insight into the scattering properties of nanoantennas and meta-atoms.

Nonlocal effects investigation via the coupling between localized and acoustic plasmons.

A scheme to investigate nonlocal effects in metal using the coupling between localized graphene plasmons (GPs) and acoustic plasmons (APs) is proposed. Because of the extremely strong field confinement property, the APs on a configuration consisting of monolayer graphene and a metal film have different dispersions when the nonlocal response is considered or not. A graphene nanoribbon array can efficiently couple incident light to the localized GPs on the ribbons and subsequently the APs. The strong coupling between the two kinds of plasmon, equivalent to electric field dipole interaction, is highly related to the acoustic plasmonic dispersion and induces different absorption spectra, depending on the dispersion. Using a very simple model, nonlocal effects can be extracted from the spectra. The investigation provides a promising platform to manipulate nanophotonics and study nonlocal effects.

Near-field circular dichroism of single molecules.

Near-field images of molecules provide information about their excited orbitals, giving rise to photonic and chemical functions. Such information is crucial to the elucidation of the full potential of molecules as components in functional materials and devices at the nanoscale. However, direct imaging inside single molecules with a complex structure in the near-field is still challenging because it requires in situ observation at a higher resolution than the molecular scale. Here, using a proven theoretical method that has demonstrated sub-nanoscale resolution based on photoinduced force microscopy (PiFM) experiment [Nat. Commun.12, 3865 (2021)10.1038/s41467-021-24136-2], we propose an approach to obtaining the near-field imaging with spatial patterns of electronic transitions of single molecules. We use an extended discrete dipole approximation method that incorporates microscopic nonlocal optical response of molecules and demonstrate that PiFM can visualize circular-dichroism signal patterns at sub-nanometer scale for both optically allowed and forbidden transitions. The result will open the possibility for the direct observation of complex spatial patterns of electronic transitions in a single molecule, providing insight into the optical function of single molecules and helping realize new functional materials and devices.

Multiple actions of time-resolved short-pulsed metamaterials

Recently, it has been shown that temporal metamaterials based on impulsive modulations of the constitutive parameters (of duration much smaller than a characteristic electromagnetic timescale) may exhibit a nonlocal response that can be harnessed so as to perform elementary analog computing on an impinging wavepacket. These short-pulsed metamaterials can be viewed as the temporal analog of conventional (spatial) metasurfaces. Here, inspired by the analogy with cascaded metasurfaces, we leverage this concept and take it one step further, by showing that short-pulsed metamaterials can be utilized as elementary bricks for more complex computations. To this aim, we develop a simple, approximate approach to systematically model the multiple actions of time-resolved short-pulsed metamaterials. Via a number of representative examples, we illustrate the computational capabilities enabled by this approach, in terms of simple and composed operations, and validate it against a rigorous numerical solution. Our results indicate that the temporal dimension may provide new degrees of freedom and design approaches in the emerging field of computational metamaterials, in addition or as an alternative to conventional spatially variant platforms.

Pushing the boundaries of nanocrystal optoelectronics with structured photonic environments

This article discusses the prospect of advanced functionalities in the field of nanocrystal optoelectronics using structured photonic environments, with a particular emphasis on near-infrared light-emitting diodes. The ability to achieve full control of brightness, colour, phase and polarization is highlighted, as well as the importance of determining whether a local or non-local photonic response is required to achieve a given functionality. These developments have implications for various applications, including displays, communication protocols and sensors.

Pure-quartic solitons in presence of weak nonlocality

In this paper, we study the characteristics of pure-quartic optical solitons in a nonlinear medium having a weakly nonlocal response. We propose an nonlinear Schrödinger equation incorporating pure fourth-order diffraction, Kerr nonlinearity and weak nonlocality to describe the transmission of solitons in the system. We obtain analytic pure-quartic soliton solutions to this model in a variety of shapes and forms, unlike the case of the model without taking into account the weak nonlocal contribution. Besides, we find that the obtained localized structures have both bright and dark type waveforms in the transmission system. Interestingly, the functional forms of these solitons are expressed in terms of sech and tanh functions which do not possess oscillating tails, markedly different from the conventional pure-quartic solitons in the absence of weak nonlocality. These structures exist for both positive and negative quartic diffraction coefficient whereas pure-quartic solitons in the absence of nonlocality occur only for negative fourth-order dispersion. The numerical examples are presented for illustrating the soliton evolution dynamics in the optical waveguide.

Quantitative phase contrast imaging with a nonlocal angle-selective metasurface

Phase contrast microscopy has played a central role in the development of modern biology, geology, and nanotechnology. It can visualize the structure of translucent objects that remains hidden in regular optical microscopes. The optical layout of a phase contrast microscope is based on a 4 f image processing setup and has essentially remained unchanged since its invention by Zernike in the early 1930s. Here, we propose a conceptually new approach to phase contrast imaging that harnesses the non-local optical response of a guided-mode-resonator metasurface. We highlight its benefits and demonstrate the imaging of various phase objects, including biological cells, polymeric nanostructures, and transparent metasurfaces. Our results showcase that the addition of this non-local metasurface to a conventional microscope enables quantitative phase contrast imaging with a 0.02π phase accuracy. At a high level, this work adds to the growing body of research aimed at the use of metasurfaces for analog optical computing.

Non-local multi-fields coupling response of a piezoelectric semiconductor nanofiber under shear force

Based on the nonlocal piezoelectric semiconductor theory, the transient multi-fields coupling response of a nanosized piezoelectric semiconductor composite fiber is studied. The one-dimensional phenomenological theory of piezoelectric semiconductors is used to derive the coupling constitutive relation. Combining nonlocal piezoelectric semiconductor theory and coupling constitutive relation, the size-dependent governing equations for bending, the charge equation of electrostatics and the conservation of charge for electrons are obtained, and the multi-fields coupling response of the nanofiber under shear force is solved. The bending vibration, the electric potential, and the concentration of electrons along the nanofiber with different nonlocal effects are analyzed. It is found that the change of electric potential and concentration from positive and negative in the nanofiber can be used to control the performance of electronic devices.

Plasmonic gain in current biased tilted Dirac nodes

Surface plasmons, which allow tight confinement of light, suffer from high intrinsic electronic losses. It has been shown that stimulated emission from excited electrons can transfer energy to plasmons and compensate for the high intrinsic losses. To-date, these realizations have relied on introducing an external gain media coupled to the surface plasmon. Here, we propose that plasmons in two-dimensional materials with closely located electron and hole Fermi pockets can be amplified, when an electrical current bias is applied along the displaced electron-hole pockets, without the need for an external gain media. As a prototypical example, we consider WTe2 from the family of 1T{}^{{prime} }-MX2 materials, whose electronic structure can be described within a type-II tilted massive Dirac model. We find that the nonlocal plasmonic response experiences prominent gain for experimentally accessible currents on the order of mAμm−1. Furthermore, the group velocity of the plasmon found from the isofrequency curves imply that the amplified plasmons are highly collimated along a direction perpendicular to the Dirac node tilt when the electrical current is applied along it.

Modulational instability in nonlinear saturable media with competing nonlocal nonlinearity.

The modulational instability (MI) phenomenon is addressed in a nonlocal medium under controllable saturation. The linear stability analysis of a plane-wave solution is used to derive an expression for the growth rate of MI that is exploited to parametrically discuss the possibility for the plane wave to disintegrate into nonlinear localized light patterns. The influence of the nonlocal parameter, the saturation coefficient, and the saturation index are mainly explored in the context of a Gaussian nonlocal response. It is pointed out that the instability spectrum, which tends to be quenched by the high nonlocality parameter, gets amplified under the right choices of the saturation parameters, especially the saturation index. Via direct numerical simulations, confirmations of analytical predictions are given, where competing nonlocal and saturable nonlinearities enable the emergence of trains of patterns as manifestations of MI. The comprehensive parametric analysis carried out throughout the numerical experiment reveals the robustness of the obtained rogue waves of A- and B-type Akhmediev breathers, as the nonlinear signature of MI, providing the saturation index as a suitable tool to manipulate nonlinear waves in nonlocal media.

Electron-Sheet Metasurfaces for Reconfigurable Beam Patterns

Here, a beam-steering structure based on the interaction of antenna radiation with an electron sheet with swiftly moving electrons with a constant velocity is proposed and studied theoretically. Such electron-sheet metasurface, while uniform and homogeneous in its composition, has a dynamic surface conductivity that exhibits nonlocal response that is dependent on the tangential component of the wave vector impinging on it, and may offer a nonsymmetric conductivity distribution with respect of this wave vector component. As a result, asymmetric radiation patterns may be generated from a single magnetic current line source in the presence of such a metastructure. This effect, introduced by the electron sheet with fast-moving electrons on the radiation properties, can be tailored by modifying the number density and the constant velocity of these electron sheets. We explore the effects of the electron-sheet metasurface on the far-zone radiated field in the presence and absence of a perfect electric conducting ground plane. Some of the electromagnetic features, such as the tilt of radiation pattern in the far zone and the field distribution in the near zone, are also discussed. We envision that this reconfigurable beam steering method may find potential applications in THz, microwave, and optical technologies.

A modified fractional-order thermo-viscoelastic model and its application in thermal-induced nonlocal response analysis of a microscale plate

A modified fractional-order thermo-viscoelastic model and its application in thermal-induced nonlocal response analysis of a microscale plate

Gain-Assisted Giant Third-Order Nonlinearity of Epsilon-Near-Zero Multilayered Metamaterials.

We investigate the third-order nonlinear optical properties of epsilon-near-zero (ENZ) Au/dye-doped fused silica multilayered metamaterials in the visible spectral range for TM incident by using nonlocal effective medium theory at different incidence angles. The nonlocal response affects the permittivity of anisotropic metamaterials when the thickness of the layer cannot be much smaller than the incident wavelength. By doping pump dye gain material within the dielectric layer to compensate for the metal loss, the imaginary part of the effective permittivity is reduced to 10−4, and the optical nonlinear refractive index and nonlinear absorption coefficient are enhanced. The real and imaginary parts of the permittivity are simultaneously minimized when the central emission wavelength of the gain material is close to the ENZ wavelength, and the nonlinear refraction coefficient reaches the order of 10−5 cm2/W, which is five orders of magnitude larger than that of the nonlinear response of the metamaterial without the gain medium. Our results demonstrate that a smaller imaginary part of the permittivity can be obtained by doping gain materials within the dielectric layer; it offers the promise of designing metamaterials with large nonlinearity at arbitrary wavelengths.

The biogeophysical effects of idealized land cover and land management changes in Earth system models

Abstract. Land cover and land management change (LCLMC) has been highlighted for its critical role in mitigation scenarios in terms of both global mitigation and local adaptation. Yet, the climate effect of individual LCLMC options, their dependence on the background climate, and the local vs. non-local responses are still poorly understood across different Earth system models (ESMs). Here we simulate the climatic effects of LCLMC using three state-of-the-art ESMs, including the Community Earth System Model (CESM), the Max Planck Institute for Meteorology Earth System Model (MPI-ESM), and the European Consortium Earth System Model (EC-EARTH). We assess the LCLMC effects using four idealized experiments: (i) a fully afforested world, (ii) a world fully covered by cropland, (iii) a fully afforested world with extensive wood harvesting, and (iv) a full cropland world with extensive irrigation. In these idealized sensitivity experiments performed under present-day climate conditions, the effects of the different LCLMC strategies represent an upper bound for the potential of global mitigation and local adaptation. To disentangle the local and non-local effects from the LCLMC, a checkerboard-like LCLMC perturbation, i.e. alternating grid boxes with and without LCLMC, is applied. The local effects of deforestation on surface temperature are largely consistent across the ESMs and the observations, with a cooling in boreal latitudes and a warming in the tropics. However, the energy balance components driving the change in surface temperature show less consistency across the ESMs and the observations. Additionally, some biases exist in specific ESMs, such as a strong albedo response in CESM mid-latitudes and a soil-thawing-driven warming in boreal latitudes in EC-EARTH. The non-local effects on surface temperature are broadly consistent across ESMs for afforestation, though larger model uncertainty exists for cropland expansion. Irrigation clearly induces a cooling effect; however, the ESMs disagree regarding whether these are mainly local or non-local effects. Wood harvesting is found to have no discernible biogeophysical effects on climate. Our results overall underline the potential of ensemble simulations to inform decision-making regarding future climate consequences of land-based mitigation and adaptation strategies.

Noncontact Friction in Electric Force Microscopy over a Conductor with Nonlocal Dielectric Response.

Electric force microscopy, in which a charged probe moves above a surface, can measure thermally generated electric field fluctuations from the sample. Noncontact friction measurements of energy loss from the probe have been performed over insulators, semiconductors, and conductors and have been interpreted in terms of the dielectric response of the sample. Noncontact friction over metal surfaces has recently been ascribed to dielectric relaxation of adsorbed molecules, motivating the examination of the baseline noncontact friction over a bare metal surface. The noncontact friction for a thin conducting film is calculated for a wavevector-dependent dielectric function, complementing previous calculations for insulators and semiconductors employing a dielectric continuum representation. Inclusion of the wavevector dependence in the dielectric response of a conductor enhances the friction and alters its dependence on tip-sample separation, relative to the continuum treatment.

A CNN-based surrogate model of isogeometric analysis in nonlocal flexoelectric problems

We proposed a convolutional neural network (CNN)-based surrogate model to predict the nonlocal response for flexoelectric structures with complex topologies. The input, i.e. the binary images, for the CNN is obtained by converting geometries into pixels, while the output comes from simulations of an isogeometric (IGA) flexoelectric model, which in turn exploits the higher-order continuity of the underlying non-uniform rational B-splines (NURBS) basis functions to fast computing of flexoelectric parameters, e.g., electric gradient, mechanical displacement, strain, and strain gradient. To generate the dataset of porous flexoelectric cantilevers, we developed a NURBS trimming technique based on the IGA model. As for CNN construction, the key factors were optimized based on the IGA dataset, including activation functions, dropout layers, and optimizers. Then the cross-validation was conducted to test the CNN’s generalization ability. Last but not least, the potential of the CNN performance has been explored under different model output sizes and the corresponding possible optimal model layout is proposed. The results can be instructive for studies on deep learning of other nonlocal mech-physical simulations.