Rayleigh Anomalies Research Articles

UV–visible broadband polarization-independent metamaterial absorber based on two-dimensional Au grating

A broadband metamaterial absorber (BMA) based on a thin metamaterial nanostructure consisting of 2D Au square grating on graphene-SiO2 film and Au substrate is proposed and designed. The optimized structure can achieve nearly perfect absorption with absorption of exceeding 90 % spanning a broad range from deep UV light to visible light (100 nm−700 nm). The excitation of Rayleigh anomaly (RA), cavity resonance (CR), surface plasmon polaritons (SPPs) and magnetic polaritons (MPs) are responsible for nearly perfect broadband absorption, which have been analysed clearly by finite-difference time-domain (FDTD) method and the LC circuit model. Furthermore, the BMA is polarization and angle insensitive for incident light even at large angle. This proposed BMA not only has a simple geometry and ordinary metal-based material composition, but also shows a high performance in both bandwidth and absorptivity, suggesting that great potential in solar cell and photodetector applications.

Light scattering by plasmonic disks and holes arrays: different or the same?

We suggest a strategy for designing regular 2D arrays of nanoholes (NHs) in metal films with far-field scattering properties similar to that of regular 2D arrays of nanodisks (NDs) with the same periodicity. Full-wave simulations for perfectly conducting, Ag and Au NDs and respectively designed arrays of NHs demonstrate a minor difference between far-field properties either at wavelengths corresponding to Wood–Rayleigh anomalies of the arrays or in a broad wavelength range, depending on the array periodicity and sizes of NDs (NHs). Our results have broad implications in plasmon-enhanced-driven applications, including optoelectronic and photovoltaic devices, where the NH arrays are preferable to be fabricated for nano-structured optics.

Preserving High‐Q Lattice Plasmon Resonances for Poor Spatial Coherence of Light: Application in Enhanced Second Harmonic Generation

AbstractSurface lattice plasmon resonances (SLPR) in periodic arrays of metal nanostructures can feature a narrow spectral linewidth in case of the spectral overlap between the localized plasmons and Rayleigh anomalies. However, because of strong angular dispersion in experimental realizations, the excitation of SLPR requires collimated light, which has high spatial coherence. Here, excellent quality (Q)‐factor preservation in 1D all‐metallic nanograting metasurfaces due to the unique angular dispersion is reported. A high Q‐factor around 400 is obtained even when poor collimation, a numerical aperture (NA) objective with NA = 0.4, is employed for focusing the excitation light. With the advantages of highly focused excitation and high Q‐factor of the SLPR, a strong enhancement of second harmonic generation is realized from the proposed metasurface integrated with a few‐layers of GaSe flake. The results demonstrate that a 1D all‐metallic nanograting metasurface can overcome the demand for collimated light for the SLP resonances and manipulate focused light beams. The avenue of research proposed in this paper can open the door for the use of plasmonic metamaterials with SLPRs in many practical applications involving poorly coherent light sources, like the optoelectronic devices utilizing solar and daylight illuminations.

Resonant Excitation Analysis of the Absorption Enhancement of L-Shaped Metasurfaces in Midinfrared Band

Surface plasmon waveguides have attracted a lot of attention in recent years due to their ability to conduct light in the subwavelength scale. A large number of metallic structures have been used as waveguides and applied in integrated photonic circuits. Among these structures, the metal-insulator-metal surface plasmon waveguide is considered to have unique advantages. Compared to other waveguide forms, it is more compact and thus easier to integrate into optical circuits. For these reasons, we investigated the transmission properties of surface plasmon waves in metal-insulator-metal waveguides and built a MIM absorber consisting of gold and dielectric. We developed an interference model for the MIM absorber and concluded that the interactions between the metal patch and the substrate are not negligible in the near field. In addition, magnetic resonance plays an equally important role in approaching uniform absorption. Absorption spectra with different structural parameters, incidence angles, and polarizations were investigated. A sharp absorption peak was also found to be caused by the Rayleigh anomaly. The study reported in this paper contributes to further understanding of the physical properties of the MIM absorber, and we expect to refine the theoretical model in the future to eliminate the bias caused by near-field coupling.

Recent progress on artificial propeller chirality and related circular dichroism engineering

<p indent="0mm">Chirality, a geometric asymmetry in which a structure cannot coincide with its own mirror image through rotation and translation, is one of the basic properties of nature. It has been involved in all aspects of our life, such as art, architectural design, medicine, pesticides, life, information, chiral optoelectronic functional materials and devices. The interactions of chiral substances with light can differ for left- and right-handed circularly polarized (LCP and RCP) light and rely on the handedness of the enantiomers. Since scientists discovered the optical activity of chiral matter in the 19th century, the study of chiral light-matter interactions has become one of the most cutting-edge research fields in optics. Circular dichroism (CD) is the difference in absorption of LCP and RCP light by chiral substances. Since the chiral optical response of natural chiral matter is weak and difficult to tune, researchers have creatively designed various artificial chiral structures to enhance chiral light-matter interactions and realize many novel physical phenomena. The most common example is the helical configuration, which can be seen as the result of the stretched superposition of multiple split resonator rings. It can obtain much larger CD than natural materials. However, the three-dimensional fabrication process of micro/nanoscale helical structures faces challenges, which limit its application research. Different from common helical chiral structures and based on the nano-kirigami fabrication technique, this review focuses on a new type of artificial propeller-like chiral micro/nanostructures and their applications in chiral optics. This artificial propeller chirality is quite different from the traditional chiral structure in terms of geometric morphology and physical responses, such as the novel out-of-plane stereo twist and the chiral competition mechanism enabled by the hybridization of unit cells with two different chiral centers. The CD responses can be significantly enhanced by the chiral Fano resonance that is caused by the surface lattice resonances (SLRs). The chiral SLRs are formed by the coupling of chiral electric quadrupole modes to the diffractive lattice mode (Rayleigh anomaly). These unique features enable outstanding advantages in the flexible manipulation of optical CD, including the sensitive changes in intensity, sign and wavelength upon structural perturbations. Specially, the counterintuitive sign reversal of CD can be realized under minor structural changes without inverting the handedness of artificial propeller chirality. The work in this review provides a novel design idea for new artificial chiral micro/nanostructures and an ideal platform for in-depth study of chiral physics.

Free-standing plasmonic nanoarrays for leaky optical waveguiding and sensing.

Flat optics nanogratings supported on thin free-standing membranes offer the opportunity to combine narrowband waveguided modes and Rayleigh anomalies for sensitive and tunable biosensing. At the surface of high-refractive index Si3N4 membranes we engineered lithographic nanogratings based on plasmonic nanostripes, demonstrating the excitation of sharp waveguided modes and lattice resonances. We achieved fine tuning of these optical modes over a broadband Visible and Near-Infrared spectrum, in full agreement with numerical calculations. This possibility allowed us to select sharp waveguided modes supporting strong near-field amplification, extending for hundreds of nanometres out of the grating and enabling versatile biosensing applications. We demonstrate the potential of this flat-optics platform by devising a proof-of-concept nanofluidic refractive index sensor exploiting the long-range waveguided mode operating at the sub-picoliter scale. This free-standing device configuration, that could be further engineered at the nanoscale, highlights the strong potential of flat-optics nanoarrays in optofluidics and nanofluidic biosensing.

The role of Rayleigh anomalies in the coupling process of plasmonic gratings and the control of the emission properties of organic molecules

We report the investigation of the influence of periodic metallic arrays on the emission properties of organic emitters. Beforehand, the study of the coupling process between nanoparticles through the analysis of the extinction spectra related to Rayleigh anomalies indicate the crucial role of those latter in defining the nature of the excited grating modes. The obtained results emphasis that Rayleigh Anomalies can be considered as the intermediate between individual plasmonic and collective photonic responses. Thereafter, the experimental and numerical studies of the lattice modes and their associated effects on the lifetime and emission directivity of nearby emitters indicate that tuning the geometrical grating parameters offers a possibility to select a particular coupling process from a localized effect to a far field response. Depending on the coupling strength, the emission can be strongly altered by increasing the density of states or providing diffractive orders. Eventually, this study reports that the Rayleigh Anomalies play the role of an excitation source which drives the nanoparticles to act as a set of diffractive objects for shaping the emission to be highly directive.

THz plasmonic metasurface based on a periodic array of InSb metamolecules with narrow resonances

We propose an indium antimonide (InSb) THz plasmonic metasurface featuring a surface lattice resonance (SLR) with the quality factor of Q ∼ 375 and theoretically investigate the plasmonic response of the ensemble of InSb metamolecules. The localized surface plasmon resonances (LSPRs) of individual metamolecules couple together through Rayleigh Anomaly (RA) diffraction, leading to the appearance of the SLR at 0.75THz , suitable for biophotonic applications. The major advantage of the proposed metasurface over metal-based ones in the THz regime is the tunability of SLR frequency by changing the carrier concentration. Apart from SLR, it is possible to achieve additional resonances by tuning the superstrate thickness exploiting Fabry–Perot (FP)-like cavity effects. Achieving a narrow-linewidth SLR and the resultant remarkable local-field enhancement in the InSb-based metasurface opens the door to a wide range of applications. Conducive to optical manipulation and sensing, the capability of the proposed metasurface is examined in plasmonic tweezing and medium-perturbation sensing fields.

Mirror-backed dielectric metasurface sensor with ultrahigh figure of merit based on a super-narrow Rayleigh anomaly.

Plasmonic nanostructures with large local field enhancement have been extensively investigated for sensing applications. However, the quality factor and thus the sensing figure of merit are limited due to relatively high ohmic loss. Here we propose a novel, to the best of our knowledge, plasmonic sensor with an ultrahigh figure of merit based on a super-narrow Rayleigh anomaly (RA) in a mirror-backed dielectric metasurface. Simulation results show that the RA in such a metasurface can have a super-high quality factor of 16,000 in the visible regime, which is an order of magnitude larger than the highest value of reported plasmonic nanostructures. We attribute this striking performance to the enhanced electric fields far away from the metal film. The super-high quality factor and the greatly enhanced field confined to the superstrate region make the mirror-backed dielectric metasurface an ideal platform for sensing. We show that the figure of merit of this RA-based metasurface sensor can be as high as 15,930/refractive index units. Additionally, we reveal that RA-based plasmonic sensors share some typical characteristics, providing guidance for the structure design. We expect this work to advance the development of high-performance plasmonic metasurface sensors.

Quality factor of plasmonic monopartite and bipartite surface lattice resonances

Surface lattice resonance (SLR) is the collective excitation of nanoparticle resonances arising from the hybridization between localized surface plasmons (LSPs) and propagating Rayleigh anomalies (RAs). When comparing with the corresponding LSPs, SLRs exhibit a much higher quality factor. In fact, as the quality factor depends on the constituting resonances and their hybridization, how one can parametrize it in an analytic form is an important issue. We have studied the SLRs arising from two-dimensional Au monopartite nanoparticle arrays by angle- and polarization-resolved reflectivity spectroscopy, temporal coupled mode theory (CMT), and finite-difference time-domain simulation. The scattering matrix of the SLRs is formulated, revealing the importance of the spectral detuning and the interaction strengths between the LSP and the RAs in governing the quality factor. We then extend the CMT approach to study bipartite arrays where a nanoparticle dimer is employed and find the coupling between two LSPs plays a major role in further boosting the quality factor. Specifically, the coupling takes part in controlling the detuning factor as well as determining whether the coupled bright or dark mode is hybridized with the RAs. The dark mode hybridization can strongly enhance the quality factor, which is otherwise not possible in the monopartite counterparts.

Comment on "Electronic, vibrational and torsional couplings in N-methylpyrrole: Ground, first excited and cation states" [J. Chem. Phys. 154, 224305 (2021)

Two-color (1 + 1') zero-electron-kinetic-energy (ZEKE) and photoionization efficiency (PIE) spectra are reported via different levels in the S1 ← S0 (Ã1A2←X̃1A1) one-photon transition of jet-cooled N-methylpyrrole. The laser radiation is produced using two dye lasers, one with an 1800 l/mm grating and one with 2400 l/mm. We report spectra where the excitation and ionization radiation are produced with both combinations of the dye lasers; these spectra differ markedly. This is attributed to Wood's anomalies with the 2400 l/mm grating: one aspect is a loss in light intensity over a range of wavelengths, attributed to a resonance anomaly. Another is the appearance of a "shadow" ZEKE spectrum and PIE curve at apparently higher ionization wavenumbers; under some conditions, a third ZEKE spectrum was observed-these latter observations arise from higher-order dispersion effects, likely caused by a Rayleigh anomaly. We comment on these observations and report more representative ZEKE and PIE spectra than those presented in a recent paper by our group [A. R. Davies, D. J. Kemp, and T. G. Wright, J. Chem. Phys. 154, 224305 (2021)] for four intermediate S1 levels.

Lattice effect for enhanced hot-electron generation in nanoelectrodes

A stronger electric field in metal nanostructures can be realized by exciting nanoparticle plasmonic resonances to enhance hot electron generation. One can alter the nanoparticle shape, size, material, and/or the refractive index of the surrounding medium to achieve higher efficiency. Here, we report the nanostructure design that enhances the generation of plasmonic hot electrons from the periodically arranged gold nanoelectrodes. The periodic arrangement results in the excitation of collective lattice resonances in proximity to the Rayleigh anomalies (diffraction order transitions). We show how to select a lattice period that gives the highest field enhancement and the potential for the most efficient generation of plasmonic hot electrons, which are injected into the water environment from gold nanoelectrodes. Our study can serve as a general guideline in designing plasmonic nanostructures with nanoelectrodes injecting hot electrons into an aqueous environment.

Spectroscopic Mueller matrix ellipsometry of a gap surface plasmon array at conical incidences

Spectroscopic Mueller matrix ellipsometry is used to study an array of rectangular Au patches on a S i O 2 film backed by optically thick Au. The array supports resonances related to gap surface plasmons and Rayleigh anomalies, and these are mapped out by full rotation of the azimuthal angle of incidence. The finite element method is used to model the system, and it is found that the Ti adhesion layers at S i O 2 / A u interfaces used in the manufacture process must be included in the model for accurate results. We show how oxidation of the Ti layer beneath the Au patch causes the optical response to drift in time, and we demonstrate an extreme sensitivity of the Mueller matrix to the dimensions of the patch.

Direction‐Sensitive Magnetophotonic Surface Crystals

Nanometer‐thin rare‐earth–transition‐metal (RE–TM) alloys with precisely controlled compositions and out‐of‐plane magnetic anisotropy are currently in the focus for ultrafast magnetophotonic applications. However, achieving lateral nanoscale dimensions, crucial for potential device downscaling, while maintaining designable optomagnetic functionality and out‐of‐plane magnetic anisotropy is extremely challenging. Herein, nanosized Tb18Co82 ferrimagnetic alloys, having strong out‐of‐plane magnetic anisotropy, within a gold plasmonic nanoantenna array to design a micrometer‐scale magnetophotonic crystal that exhibits abrupt and narrow magneto‐optical (MO) spectral features that are both magnetic field and light incidence direction controlled are integrated. The narrow Fano‐type resonance arises through the interference of the individual nanoantenna's surface plasmons and a Rayleigh anomaly of the whole nanoantenna array, in both optical and MO spectra, which are demonstrated and explained using Maxwell theory simulations. This robust magnetophotonic crystal opens the way for conceptually new high‐resolution light incidence direction sensors, as well as for building blocks for plasmon‐assisted all‐optical magnetization switching in ferrimagnetic RE–TM alloys.

Multi-Angular Colorimetric Responses of Uni- and Omni-Directional Femtosecond Laser-Induced Periodic Surface Structures on Metals.

We investigated the colorimetric behaviors of metal surfaces with unidirectional low-spatial-frequency laser-induced periodic surface structures (UD-LSFLs) and omnidirectional LSFLs (OD-LSFLs) fabricated using femtosecond laser pulse irradiation. With the CIE standard illuminant D65, incident at −45°, we show that UD-LSFLs on metals transform polished metals to gonio-apparent materials with a unique behavior of colorimetric responses, depending on both the detection and rotation angles, whereas OD-LSFLs have nearly uniform gonio-apparent colors at each detection angle, regardless of their rotation. These colorimetric behaviors can be observed not only at the angles of diffraction but also near the angle of reflection, and we find that the power redistribution due to Rayleigh anomalies also plays an important role in the colorimetric responses of UD- and OD-LSFLs, in addition to diffraction.

Ultra-narrow-band metamaterial perfect absorber based on surface lattice resonance in a WS2 nanodisk array.

Metamaterial perfect absorbers (MPAs) are attractive platforms for the unique manipulation of electromagnetic waves from near-field to far-field. Narrow-band MPAs are particularly intriguing for their potential applications as thermal emitters or biosensors. In this work, we proposed ultra-narrow-band MPAs based on surface lattice resonance (SLR) modes of WS2 nanodisk arrays on gold films. The SLR modes stem from the coupling between the magnetic dipole modes of individual nanodisks and the Rayleigh anomaly of the array giving rise to high quality-factor resonances. With proper design of the nanodisk array, an ultra-narrow-band of 15 nm is achieved in the near infrared wavelength range. The underneath gold film provides the loss channel converting the incident light within the narrow band into heat in the gold film, effectively creating a perfect absorber. Systematic numerical simulations were performed to investigate the effects of the geometrical parameters on their optical properties, demonstrating the great tunability of this type of MPAs as well as their potential for engineering light-matter interactions.

Engineering Plasmonic Colloidal Meta‐Molecules for Tunable Photonic Supercrystals

AbstractOrdered arrays of metal nanoparticles offer new opportunities to engineer light–matter interactions through the hybridization of Rayleigh anomalies and localized surface plasmons. The generated surface lattice resonances exhibit much higher quality factors compared to those observed in isolated metal nanostructures. Template‐induced colloidal self‐assembly has already shown a great potential for the scalable fabrication of 2D plasmonic meta‐molecule arrays, but the experimental challenge of controlling optical losses within the repeating units has so far prevented this approach to compete with more standard fabrication methods in the production of high‐quality factor resonances. In this manuscript, the optical properties of plasmonic arrays are investigated by varying the lattice parameter (between 200 and 600 nm) as well as the diameter of the gold colloidal building‐blocks (between 11 ± 1 and 98 ± 6 nm). It is systematically studied how the internal architecture of the repeating gold‐nanoparticle meta‐molecules influences the optical response of the plasmonic supercrystals. Combining both experimental measurements and simulations, it is demonstrated how, reducing the size of the gold nanoparticles it is possible to switch from strong near‐field plasmonic architectures to high‐quality factors (&gt;60) for lattice plasmon resonances located in the visible spectral range.

Broadband, High‐Efficiency and Wide‐Incident‐Angle Anomalous Reflection in Groove Metagratings

AbstractIn this paper, some nonplanar metagratings with multilevel asymmetric grooves are demonstrated to produce broadband and high‐efficiency anomalous reflection in wide‐incident‐angle range. First, bipartite metagrating with two asymmetric grooves per unit cell is proposed and optimized by machine learning. It can not only realize a high efficiency (≈88.8%) and large‐angle (≈70°) anomalous reflection under normal incidence, but also preserve the high performance over broad bandwidth (12.0–20.0 GHz) and wide‐incident‐angle range. Furthermore, to improve the large‐angle anomalous reflection at low frequencies, a tripartite metagrating is designed. The participation of more cavity modes makes the metagrating achieve the anomalous reflection with an efficiency greater than 90% over a broader bandwidth from ±1st Rayleigh anomalies to 20.0 GHz (even higher) and a wide range of incident angles from −70° to 70°, which is verified by the experimental measurement. Especially at an incident angle of 20°, the high‐efficiency anomalous reflection is sustained over an ultrabroad frequency range from 9.7 to 21.2 GHz, i.e., a fractional bandwidth of 74.4%. The excellent performance of anomalous reflection and relatively simple structures endow the asymmetric groove metagratings with abundant functions, such as ideal three‐channel retroreflector and abnormal reflector, and make them attractive in highly efficient and extreme wave manipulation.

Singular integral equations in plane wave scattering by infinite graphene strip grating with brake of periodicity

Abstract In this paper, the solution of the H-polarized wave scattering problem by infinite graphene strip grating is obtained. The structure is periodic except two neighboring strips. The distance between these two strips is arbitrary. In particular, such a problem allows to quantify the mutual interaction of graphene strips in the array. The total field is represented as a superposition of the field of currents on the ideally-periodic grating and correction currents induced by the shift of the strips. The analysis is based on the convergent method of singular integral equations. It enables us to study the influence of the correction currents in a wide range from 10 GHz to 6 THz. It is shown that the interaction between graphene strips is strong near plasmon resonances and near the Rayleigh anomaly.

Coherent Networks of Plasmonic Dipole Domains: Long-Range Optical Coupling of Phase-Correlated Packages of Metallic Nanoparticles

Through the use of experimental and numerical techniques, we study coherent networks of plasmonic phase-correlated dipole domains that can spread coherent properties along ultralong distances. The phase-correlated domains occur in regions containing closely packed metallic nanoparticles with random positions, shapes, and sizes. We demonstrate that when such regions are periodically arranged, forming two-dimensional arrays, optical diffraction can coherently excite and align the plasmonic dipoles of the nanoparticles within each region, forming delocalized collective plasmon resonances associated with the in-phase coupling of the dipoles. We study the impact of the number of metallic nanoparticles in each region, determining the limit at which it becomes a phase-correlated dipole domain and establishes coherent interdomain coupling. Our results show how such a coupling can form coherent networks of packages of delocalized states with the same phase information. $\mathrm{Au}$ nanoislands with different packings, sizes, and shapes are considered as experimental models to explore phase-correlated dipole domains and interdomain coupling. The conditions in which the coherent interdomain coupling occurs via parallel or orthogonal hybridization of the Rayleigh anomaly with the delocalized plasmon resonances supported by the domains are explored. The outcomes offer alternative techniques for coherent transport of energy and information, in which phase-correlated dipole domains serve as units with large coherent spatial extension.