Nonlocal Optical Response Research Articles

Non-local optical response of a multi-phased quantum material

Non-local optical response of a multi-phased quantum material

Effect of the Contribution of the Local and Nonlocal Optical Response of an Isotropic Gyrotropic Medium on the Components of the Minkowski Energy–Momentum Tensor of the Electromagnetic Field of the Self-Focusing Beam

Effect of the Contribution of the Local and Nonlocal Optical Response of an Isotropic Gyrotropic Medium on the Components of the Minkowski Energy–Momentum Tensor of the Electromagnetic Field of the Self-Focusing Beam

Inhomogeneous and nonlocal optical response in magic-angle twisted trilayer graphene

Inhomogeneous and nonlocal optical response in magic-angle twisted trilayer graphene

Nonlocal optical response in asymmetric IMI plasmonic waveguides

In this paper, we investigate the nonlocal optical response of surface-plasmon polaritons (SPPs) in an asymmetric insulator-metal-insulator (A-IMI) waveguide, in which the two insulator materials have different permittivities. A nonlocal dispersion relation of SPPs in A-IMI waveguides is derived based on the generalized nonlocal response (GNOR) model including quantum pressure effects and induced charge diffusion kinetics in metal film, from which one can see that the nonlocal effects result in not only the disappearance of double-peak and step-like behaviors in the dispersion relation of A-IMI waveguides occurring at local response approximation, but also an obvious frequency blueshift and a loss enhancement. Furthermore, the influence of electron diffusion on the dispersion relation under different diffusion constants in the GNOR model as well as the variation in propagation length and penetration depth of SPPs with metal film thickness and frequency are provided, which are helpful to the future development of plasmonic waveguides.

Analysis of the Influence of Quantum Effects on Optical Characteristics of Plasmonic Nanoparticles Based on the Discrete Sources Method

The discrete sources method is adapted to the study of surface quantum effects based on mesoscopic boundary conditions with Feibelman parameters. A comparative analysis of the influence of bulk nonlocal effects and surface effects on optical characteristics of gold and silver nanoparticles is carried out using the generalized nonlocal optical response model. It is established that allowance for the nonlocal effect in the noble metals always leads to a reduced amplitude of the surface plasmon resonance (SPR) and its blue shift, while the surface effect depends substantially on the geometry of the particles. To a large degree, the mesoscopic boundary conditions recover the SPR amplitude as compared with the bulk nonlocal effect. This difference is especially noticeable in the field enhancement factor on the surface of the particles. Additionally, substantial differences in the SPR behavior for gold and silver particles are found in the case of mesoscopic boundary conditions.

Nonlocal Soft Plasmonics in Planar Homogeneous Multilayers

Plasmonics is the study of resonant oscillations of free electrons in metals caused by incident electromagnetic radiation. Surface plasmons can focus and steer light on the subwavelength scale. Apart from metals, plasmonic phenomena can be observed in soft matter systems such as electrolytes which we study here. Resonant charge oscillations can be induced for ions in solution, however, due to their larger mass, they are plasmon-active in a lower frequency regime and on a larger wavelength scale. Our investigation focuses on spatial confinement which allows increasingly strong charge interactions and gives rise to nonlocality or spatial dispersion effects. We derive and discuss the nonlocal optical response of ionic plasmons using a hydrodynamic two-fluid model in a planar homogeneous three-layer system with electrolyte-dielectric interfaces. As in metals, we observe the emergence of additional longitudinal propagation modes in electrolytes which causes plasmonic broadening. Studying such systems enables us to identify and understand plasmonic phenomena in biological and chemical systems.

Nonlocal Hydrodynamic Model with Viscosive Damping and Generalized Drude–Lorentz Term

The response of plasmonic metal particles to an electromagnetic wave produces significant features at the nanoscale level. Different properties of the internal composition of a metal, such as its ionic background and the free electron gas, begin to manifest more prominently. As the dimensions of the nanostructures decrease, the classical local theory gradually becomes inadequate. Therefore, Maxwell’s equations need to be supplemented with a relationship determining the dynamics of current density which is the essence of nonlocal plasmonic models. In this field of physics, the standard (linearized) hydrodynamic model (HDM) has been widely adopted with great success, serving as the basis for a variety of simulation methods. However, ongoing efforts are also being made to expand and refine it. Recently, the GNOR (general nonlocal optical response) modification of the HDM has been used, with the intention of incorporating the influence of electron gas diffusion. Clearly, from the classical description of fluid dynamics, a close relationship between viscosive damping and diffusion arises. This offers a relevant motivation for introducing the GNOR modification in an alternative manner. The standard HDM and its existing GNOR modification also do not include the influence of interband electron transitions in the conduction band and other phenomena that are part of many refining modifications of the Drude–Lorentz and other models of metal permittivity. In this article, we present a modified version of GNOR-HDM that incorporates the viscosive damping of the electron gas and a generalized Drude–Lorentz term. In the selected simulations, we also introduce Landau damping, which corrects the magnitude of the standard damping constant of the electron gas based on the size of the nanoparticle. We have chosen a spherical particle as a suitable object for testing and comparing HD models and their modifications because it allows the calculation of precise analytical solutions for the interactions and, simultaneously, it is a relatively easily fabricated nanostructure in practice. Our contribution also includes our own analytical method for solving the HDM interaction of a plane wave with a spherical particle. This method forms the core of calculations of the characteristic quantities, such as the extinction cross-sections and the corresponding components of electric fields and current densities.

Nonlocal Optical Response of Particle Plasmons in Single Gold Nanorods.

The investigation of particle plasmons in metal nanoparticles has predominantly relied on local optical response approximations. However, as the nanoparticle size approaches the average distance of electrons to the metal surface, mesoscopic effects such as size-dependent plasmon line width broadening and resonance energy blue shifts are expected to become observable. In this work, we compared the experimental spectral characteristics with simulated values obtained by using a generalized nonlocal optical response theory-based local analogue model. Our results show that the nonlocal plasmon damping effects in single nanoparticles are less pronounced than those observed in plasmon-coupled systems. Furthermore, our research demonstrates that single-particle dark-field spectroscopy is an effective tool for investigating the nonlocal optical response of particle plasmons in single nanoparticles. These results have important implications for the rational design of novel nanophotonic devices.

Electrical generation of surface phonon polaritons

Abstract Efficient electrical generation of mid-infrared light is challenging because of the dearth of materials with natural dipole-active electronic transitions in this spectral region. One approach to solve this problem is through quantum-engineering of the electron dispersion to create artificial transitions, as in quantum cascade devices. In this work we propose an alternative method to generate mid-infrared light, utilizing the coupling between longitudinal and transverse degrees of freedom due to the nonlocal optical response of nanoscopic polar dielectric crystals. Polar crystals support sub-diffraction photonic modes in the mid-infrared. They also support longitudinal phonons, which couple efficiently with electrical currents through the Fröhlich interaction. As we have shown in previous theoretical and experimental works, these two degrees of freedom can hybridize forming longitudinal-transverse polaritons. Here we theoretically demonstrate that longitudinal-transverse polaritons can be efficiently generated by electrical currents, leading to resonant narrowband photonic emission. This approach can therefore be utilised to electrically generate far-field mid-infrared photons in the absence of dipole-active electronic transitions, potentially underpinning a novel generation of mid-infrared optoelectronic devices.

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.

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.

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.

Near-field radiative heat transfer modulated by nontrivial topological surface states

Three-dimensional topological insulators (3D TIs), such as Bi2Se3 and Bi2Te3, possessing insulating bulk states and nontrivial topological surface states protected by time-reversal symmetry, have attracted considerable attention. In this paper, we investigate the near-field radiative heat transfer in 3D TIs, which can be strongly modulated by the non-trivial surface states. Full 3D TI properties including both the bulk and surface states are considered in our theoretical study. We show that the hybrid polaritons in single-material TI films, resulting from the coupling of Dirac plasmons on the top and bottom surfaces with the multiple phonon polariton modes supported by the bulk states including hyperbolic phonon polaritons (HPP), surface phonon polaritons (SPhP) and epsilon-near-pole (ENP) modes, can strongly mediate and enhance the near-field thermal radiation. The dispersions of hybrid polaritons intensely depend on the chemical potential of the surface and the film thickness, and by tuning both separately, we can control the coupling and competition between the surface and bulk modes, and thus modulate the radiative spectrum and heat flux. In addition, the nonlocal optical response of the surface states has a remarkable impact on the mode hybridization. This work demonstrates the potential of 3D TIs in manipulating near-field thermal radiation, which may provide a useful platform for energy conversion and thermal management.

Minkowski Energy–Momentum Tensor in the Nonlinear Optics of Media with a Nonlocal Optical Response

Minkowski Energy–Momentum Tensor in the Nonlinear Optics of Media with a Nonlocal Optical Response

Extremely confined gap plasmon modes: when nonlocality matters

Historically, the field of plasmonics has been relying on the framework of classical electrodynamics, with the local-response approximation of material response being applied even when dealing with nanoscale metallic structures. However, when the confinement of electromagnetic radiation approaches atomic scales, mesoscopic effects are anticipated to become observable, e.g., those associated with the nonlocal electrodynamic surface response of the electron gas. Here, we investigate nonlocal effects in propagating gap surface plasmon modes in ultrathin metal–dielectric–metal planar waveguides, exploiting monocrystalline gold flakes separated by atomic-layer-deposited aluminum oxide. We use scanning near-field optical microscopy to directly access the near-field of such confined gap plasmon modes and measure their dispersion relation via their complex-valued propagation constants. We compare our experimental findings with the predictions of the generalized nonlocal optical response theory to unveil signatures of nonlocal damping, which becomes appreciable for few-nanometer-sized dielectric gaps.

Single‐Molecule Fluorescence Enhancement by Plasmonic Core–Shell Nanostructures Incorporating Nonlocal Effects

AbstractMolecular fluorescence may strongly be affected by plasmonic environments, especially the ones with deep‐nanometer features, associated with the emergence of nonclassical effects. Such effects are largely investigated by a nonlocal hydrodynamic model with additional boundary conditions, describing the collective motion of the free electron gas in metals. The study of a fluorophore in the presence of the nano core–shell topology is performed, by employing the following models: the hard‐wall hydrodynamic model and the generalized nonlocal optical response. The analysis is conducted by investigating the fluorescence rates of the system, while considering the fluorophore located inside and outside the nanoparticle. It is demonstrated that the rates generated by the above models may considerably differ from the ones obtained via the classical approach. The optimal fluorescence enhancements for different size and material parameters are found both inside and outside the studied topology.

Effect of surface modification of metallic nanorod on spontaneous emission enhancement

Metal nanorods show excellent optical properties, since the plasmonic resonance frequency can be tuned by its aspect ratio and the optical field can be confined within a region of subwavelength, even within a nanometer region. It has the ability to flexibly modify the spontaneous emission properties of a nearby quantum emitter. However, it is unclear how the emission property changes when the metal nanorod has been deposited at the tips or coated on all sides with metal. In this work, the spontaneous emission enhancements of a two-level atom around a tailored nanorod with a wide variety of shapes, dimensions or materials are systematically investigated by the finite element method. Three different optical response models are adopted, including the classical local response approximation (LRA), the nonlocal hydrodynamic model (HDM), and the generalized nonlocal optical response model (GNOR). For a cylindrical nanorod with two endcaps, it is found that the resonance frequency shows large redshift and the emission enhancement peak increases as the endcap gradually changes from cone to cylinder of the same height. The resonance frequency shows small blueshift and the emission enhancement peak decreases slightly as the deposited metal of the conical endcaps changes from gold to silver. However, as the material of the cylinder also changes from gold to silver, becoming an all-silver nanostructure, an obvious blueshift can be detected at the resonance frequency and the emission enhancement peak rises sharply. For bimetal core-shell nanostructure, the shell can screen the surface plasmon of the core from being excited, and the plasmonic resonance associated with shell increases in proportion to the thickness of the shell. The emission enhancement peak for gold nanostructure appears to be blue-shifted when coated with silver. In contrast, it is red-shifted for silver nanostructure coated with gold.

Influence of Spatial Dispersion on the Electromagnetic Properties of Magnetoplasmonic Nanostructures.

Magnetoplasmonics based on composite nanostructures is widely used in many biomedical applications. Nanostructures, consisting of a magnetic core and a gold shell, exhibit plasmonic properties, that allow the concentration of electromagnetic energy in ultra-small volumes when used, for example, in imaging and therapy. Magnetoplasmonic nanostructures have become an indispensable tool in nanomedicine. The gold shell protects the core from oxidation and corrosion, providing a biocompatible platform for tumor imaging and cancer treatment. By adjusting the size of the core and the shell thickness, the maximum energy concentration can be shifted from the ultraviolet to the near infrared, where the depth of light penetration is maximum due to low scattering and absorption by tissues. A decrease in the thickness of the gold shell to several nanometers leads to the appearance of the quantum effect of spatial dispersion in the metal. The presence of the quantum effect can cause both a significant decrease in the level of energy concentration by plasmon particles and a shift of the maxima to the short-wavelength region, thereby reducing the expected therapeutic effect. In this study, to describe the influence of the quantum effect of spatial dispersion, we used the discrete sources method, which incorporates the generalized non-local optical response theory. This approach made it possible to account for the influence of the nonlocal effect on the optical properties of composite nanoparticles, including the impact of the asymmetry of the core-shell structure on the energy characteristics. It was found that taking spatial dispersion into account leads to a decrease in the maximum value of the concentration of electromagnetic energy up to 25%, while the blue shift can reach 15 nm.

Discrete sources method for modeling of the influence of the non-local effect on the absorption of bimetallic core-shell non-spherical plasmonic nanoparticles

Core-shell bimetallic particles are of great interest, since their plasmonic properties can differ significantly from the properties of their constituent components. In chemistry, they are widely used to improve various photovoltaic energy conversion processes, because they have good absorption and heat transfer properties. In this paper, the generalized non-local optical response theory is used in conjunction with the discrete sources method to examine the impact of spatial nonlocality on the absorption properties of spheroidal core-shell particles composed of two plasmonic metals. It is demonstrated that variation of the core diameter and the thickness of the shell, as well as, as the particle elongation increases the absorption and shifts its maximum to longer wavelengths. Besides, it was found that accounting for spatial dispersion in both metals will reduce the absorption to 65 % of its value without including the nonlocal effect and the corresponding blue shift can exceed 15 nm.

Formulation of resonant optical force based on the microscopic structure of chiral molecules.

Optical manipulation, exemplified by Ashkin's optical tweezers, is a promising technique in the fields of bioscience and chemistry, as it enables the non-destructive and non-contact selective transport or manipulation of small particles. To realize the separation of chiral molecules, several researchers have reported on the use of light and discussed feasibility of selection. Although the separation of micrometer-sized chiral molecules has been experimentally demonstrated, the separation of nanometer-sized chiral molecules, which are considerably smaller than the wavelength of light, remains challenging. Therefore, we formulated an optical force under electronic resonance to enhance the optical force and enable selective manipulation. In particular, we incorporated the microscopic structures of molecular dipoles into the nonlocal optical response theory. The analytical expression of optical force could clarify the mechanism of selection exertion of the resonant optical force on chiral molecules. Furthermore, we quantitatively evaluated the light intensity and light exposure time required to separate a single molecule in a solvent. The results can facilitate the design of future schemes for the selective optical manipulation of chiral molecules.