Rayleigh Anomalies Research Articles

Research on multi-resonance mechanism to achieve ultra-wideband high absorption of a metamaterial absorber in the UV to MIR range

Wideband metamaterial perfect absorbers have been extensively researched due to their excellent properties. A four-layer ultra-broadband metamaterial perfect absorber engraved with a square cavity is proposed. A clever combination of grating and square cavity allows the absorber to manifests an overall absorption of over 92 % and an average absorption rate of 97.27 % within the operational band from 280 to 5400 nm. Through analysis, it can be concluded that the coupling effects of Rayleigh anomaly (RA), magnetic resonance (MR), cavity resonance (CR), propagating surface plasmon resonance (PSPR) and local surface plasmon resonance (LSPR) result in perfect absorption. The absorber is insensitive to polarization and large angle incidence. It is worth mentioning that the average absorption rate of our structure reaches 93.32 % at a large incidence angle of 60° in TE mode, which is much higher than that of other absorbers. At 1000 K, the total photothermal conversion efficiency of our absorber is still above 90 %. The designed structure has the following advantages: high absorption, ultra-wide bandwidth, high tolerance to fabrication errors and insensitivity to large angle incidence. Thus, the absorber can be used in the solar energy harvesting and perfect stealth. Additionally, it also can be served as a reference in thermophotovoltaics, thermal imaging and infrared detection.

Influence of material and geometry parameters on resonance linewidths of plasmonic modes in gratings made from highly doped Ge1− x Sn x

Highly doped group IV semiconductors such as Ge or GeSn are promising candidates for plasmonic mid infrared applications. The lower effective mass of GeSn alloys in comparison to pure Ge can result in lower plasma wavelengths and extend the application wavelength range. Devices made from doped GeSn alloys, therefore, are one interesting route towards plasmonic applications in the mid-infrared wavelength range, possibly extending to the NIR. Here, we specifically explore how spectrally narrow plasmonic resonances can be obtained in comb-like grating antennas by combining aspects of material growth with geometry optimization. We investigate both in simulation and in experiment how the interplay of localised surface plasmon resonances and Rayleigh anomalies can be tuned to achieve narrow extinction peaks originating from the resulting surface lattice resonances generated in our antennas made from highly doped Ge1−x Sn x .

A high-efficiency terahertz sensor based on surface lattice resonance metasurface for biochemical detection

Metasurfaces have demonstrated their sensing capabilities in the terahertz (THz) region. However, the practical application of these technologies is constrained by the complexity of their structural design and the randomness of their sensing performance. In this work, we propose a novel THz sensor based on a surface lattice resonance (SLR) metasurface with high refractive index (RI) sensitivity (502.8 GHz/RIU), high-quality factor (Q) (164.4), and simple structural design. The high sensing performance is attributed to the SLR generated by the coupling of Rayleigh anomaly (RA) diffraction and spoof localized surface plasmons in the metasurface. In the experiment, 0.9 % (0.1538 mol/L) sodium chloride (NaCl) solution with human physiological concentration can cause a 246.8 GHz shift of the SLR peak. Furthermore, the sensor demonstrates a blue-shift of the SLR peak with the decrease of the structural period of the metasurface, and meanwhile, its RI sensitivity increases with regularity. This work provides a new method for detecting health indicators of human physiological concentration by designing highly sensitive THz metasurface for a specific frequency.

Angular Dispersion of Free-Electron-Light Coupling in an Optical Fiber-Integrated Metagrating.

Free electrons can couple to optical material excitations on nanometer-length and attosecond-time scales, opening-up unique opportunities for both the generation of radiation and the manipulation of the electron wave function. Here, we exploit the Smith-Purcell effect to experimentally study the coherent coupling of free electrons and light in a circular metallo-dielectric metagrating that is fabricated onto the input facet of a multimode optical fiber. Using hyperspectral angle-resolved (HSAR) far-field imaging inside a scanning electron microscope, we probe the angular dispersion of Smith-Purcell radiation (SPR) that is simultaneously generated in free space and inside the fiber by an electron beam that grazes the metagrating at a nanoscale distance. Furthermore, we analyze the spectral distribution of SPR that is emitted into guided optical modes and correlate it with the numerical aperture of the fiber. By varying the electron energy between 5 and 30 keV, we observe the emission of SPR from the ultraviolet to the near-infrared spectral range, and up to the third emission order. In addition, we detect incoherent cathodoluminescence that is generated by electrons penetrating the input facet of the fiber and scattering inelastically. As a result, our HSAR measurements reveal a Fano resonance that is coupled to a Rayleigh anomaly of the metagrating, and that overlaps with the angular dispersion of second-order SPR at 20 keV. Our findings demonstrate the potential of optical fiber-integrated metasurfaces as a versatile platform to implement novel ultrafast light sources and to synthesize complex free-electron quantum states with light.

Asymmetric Transmission in Diffractive Chiral Metasurfaces Consisting of Nanoantennas

We have studied the asymmetric transmission in diffractive chiral metasurfaces consisting of nanoantennas. The study was made of lattice plasmon modes on the phenomenon of asymmetric transmission in chiral two dimensional arrays of plasmonic nanoparticles. It was demonstrated that asymmetric transmission resulted from contribution of higher order diffracted waves. It was shown that the isolated nanostructures has a fourfold rotational symmetry, the diffractive metasurfaces exhibited asymmetric transmission for normal incidence light within spectral range for which lattice plasmon modes were supported. It was found that lattice plasmon modes played role in enabling symmetric transmission in diffractive chiral metasurfaces. The symmetric transmission mechanism is due to different lattice plasmon mode excitation efficiencies of left circularly polarized and right circularly polarized and right circularly polarized light. The excitation efficiencies are controlled by tailoring the nano particles in plan distribution of scattered light for circularly polarized excitation and its alignment with the inplane diffraction orders of the metasurfaces. The phenomenon for a metasurface composed of an array of a chiral nanoparticles consisting of four nanoantennas. It was shown that the Rayleigh anomaly condition where the asymmetric transmission effect was strongest and metasurfaces supported lattice plasmon modes. It was found that the difference in the inplane scattered intensity varied as a function of the inplane angle around the nanostructure, rotating the nanostructure in the plane of the metasurface. The obtained results found in good agreement with previously obtained results.

Lattice Mie resonances and emissivity enhancement in mid-infrared iron pyrite metasurfaces.

High-refractive-index antennas with characteristic dimensions comparable to wavelength have a remarkable ability to support pronounces electric and magnetic dipole resonances. Furthermore, periodic arrangements of such resonant antennas result in narrow and strong lattice resonances facilitated by the lattice. We design iron pyrite antennas operating in the mid-infrared spectral range due to the material's low-energy bandgap and high refractive index. We utilize Kirchhoff's law, stating that emissivity and absorptance are equal to each other in equilibrium, and we apply it to improve the thermal properties of the iron pyrite metasurface. Through the excitation of collective resonances and manipulation of the antenna lattice's period, we demonstrate our capacity to control emissivity peaks. These peaks stem from the resonant excitation of electric and magnetic dipoles within proximity to the Rayleigh anomalies. In the lattice of truncated-cone antennas, we observe Rabi splitting of electric and magnetic dipole lattice resonances originating from the antennas' broken symmetry. We demonstrate that the truncated-cone antenna lattices support strong out-of-plane magnetic dipole lattice resonances at oblique incidence. We show that the truncated-cone antennas, as opposed to disks or cones, facilitate a particularly strong resonance and bound state in the continuum at the normal incidence. Our work demonstrates the effective manipulation of emissivity peaks in iron pyrite metasurfaces through controlled lattice resonances and antenna design, offering promising avenues for mid-infrared spectral engineering.

Dual Control of Enhanced Quasi-Bound States in the Continuum Emission from Resonant c-Si Metasurfaces.

Optical bound states in the continuum (BICs) offer strong interactions with quantum emitters and have been extensively studied for manipulating spontaneous emission, lasing, and polariton Bose-Einstein condensation. However, the out-coupling efficiency of quasi-BIC emission, crucial for practical light-emitting devices, has received less attention. Here, we report an adaptable approach for enhancing quasi-BIC emission from a resonant monocrystalline silicon (c-Si) metasurface through lattice and multipolar engineering. We identify dual-BICs originating from electric quadrupoles (EQ) and out-of-plane magnetic dipoles, with EQ quasi-BICs exhibiting concentrated near-fields near the c-Si nanodisks. The enhanced fractional radiative local density of states of EQ quasi-BICs overlaps spatially with the emitters, promoting efficient out-coupling. Furthermore, coupling the EQ quasi-BICs with Rayleigh anomalies enhances directional emission intensity, and we observe inherent opposite topological charges in the multipolarly controlled dual-BICs. These findings provide valuable insights for developing efficient nanophotonic devices based on quasi-BICs.

Toroidal electric dipole enabled chiral surface lattice resonances in stereo propeller metasurfaces

Surface lattice resonances (SLRs) are the coherent collective interactions between periodically arranged nanoparticles, which are generally considered to be formed by the resonant electric dipole, magnetic dipole, or electric quadrupole moments of a single nanoparticle coupled with the Rayleigh anomaly (RA). Here we reveal the first observation of the chiral SLRs that are formed by the coupling of the chiral toroidal electric dipole (TED) moment and RA mode through the theoretical design and experimental fabrication of a nano-kirigami based propeller metasurface. By engineering the rotational symmetry of the propeller, e.g., from C2 (C3) symmetry to C4 symmetry, we find that the electric dipole (electric quadrupolar) chiral SLRs have evolved into the TED associated chiral SLRs. Furthermore, it is found that the excitation amplitude of the TED moment can be tailored by controlling the stereo twisted height of the propeller and the spin of the incident light. Finally, the chiral TED moment enhanced circular dichroism is verified in the near-infrared wavelength region. Our study provides an effective yet simple scheme to manipulate the TED-dependent chiral SLRs, paving the way toward exploring the unconventional physical properties of TED and advanced chiroptical physics.

Rayleigh's and Wood's anomalies at the edge-type phononic structure

Anomalies in reflection from periodic structures, well known in optics, result from either the transition from specular reflection to diffraction of light (Rayleigh's anomaly) or by coupling to the leaky surface mode supported by the structure (Wood's anomaly). Experimental observation and detailed analysis of both kinds of anomalies are presented here in the case of antisymmetric Lamb wave (A0) incidence at the 1D periodic structure located along the edge of the elastic plate. A structure with a strong phononic effect has been chosen, which enables the separation of both effects in terms of frequency. Another distinct feature of elastic waves in the periodic structure considered in this work is the existence of several localized modes, which leads to the multiple Wood's anomalies. Here, we report the experimental observation of double Wood's anomaly in addition to the Rayleigh's type of anomaly.

Dual-Function Meta-Grating Based on Tunable Fano Resonance for Reflective Filter and Sensor Applications.

Localized surface plasmon resonance (LSPR)-based sensors exhibit enormous potential in the areas of medical diagnosis, food safety regulation and environmental monitoring. However, the broadband spectral lineshape of LSPR hampers the observation of wavelength shifts in sensing processes, thus preventing its widespread applications in sensors. Here, we describe an improved plasmonic sensor based on Fano resonances between LSPR and the Rayleigh anomaly (RA) in a metal-insulator-metal (MIM) meta-grating, which is composed of silver nanoshell array, an isolation grating mask and a continuous gold film. The MIM configuration offers more freedom to control the optical properties of LSPR, RA and the Fano resonance between them. Strong couplings between LSPR and RA formed a series of narrowband reflection peaks (with a linewidth of ~20 nm in full width at half maximum (FWHM) and a reflectivity nearing 100%) within an LSPR-based broadband extinction window in the experiment, making the meta-grating promising for applications of high-efficiency reflective filters. A Fano resonance that is well optimized between LSPR and RA by carefully adjusting the angles of incident light can switch such a nano-device to an improved biological/chemical sensor with a figure of merit (FOM) larger than 57 and capability of detecting the local refractive index changes caused by the bonding of target molecules on the surface of the nano-device. The figure of merit of the hybrid sensor in the detection of target molecules is 6 and 15 times higher than that of the simple RA- and LSPR-based sensors, respectively.

Equivalent Circuit Models of a Bifunctional Optical Metasurface for Beam Splitting and Refractive Index Sensing

Metasurfaces have attracted extensive attention in the micro/nano-optics field depending on their significant ability to modulate optical parameters. However, the current numerical simulation technology cannot meet the demand of the design and analysis of metasurfaces efficiently due to consuming substantial calculating time and memory. Besides, they cannot illustrate the physical mechanism straightforwardly behind the optical responses. Herein, we propose and demonstrate two equivalent circuit models systematically for a bifunctional metasurface with metal–dielectric–metal structural meta-atoms based on polarization multiplexing in the visible band. In the y-polarization state, the equivalent circuit model is established to interpret the phase shift exactly with a high goodness of fit of 0.931, resulting in the beam splitting function from the anomalous reflection phenomenon. The polychromatic light splits from 34 to 69° to produce the grating-type high-saturation structural colors with a large gamut of about 170.07% of the DCI-P3 standard. In the x-polarization state, the metasurface produces the surface lattice resonance that is suitable for the refractive index sensing function due to the high sensitivity to environmental changes. The second equivalent circuit model simulates the linewidth narrowing and red-shift phenomenon with a sensitivity relative error of 7.00%. We introduce a branch with a narrow-band-pass filter and a capacitor in series into the model to mimic the characteristic of Rayleigh anomalies. Both models extend the application of equivalent circuit theory in optics further and provide a crucial approach to enhance the insights into the mechanism of metasurfaces.

Excitation of out-of-plane lattice plasmons in dislocated nanostructures through near-field coupling.

Out-of-plane lattice plasmons (OLPs) show great potential in realizing high-quality factors due to the strong interparticle coupling. However, the strict conditions of oblique incidence bring challenges to experimental observation. This Letter proposes a new, to the best of our knowledge, mechanism to generate OLPs: through near-field coupling. Notably, with specially designed nanostructure dislocation, the strongest OLP can be achieved at normal incidence. The direction of energy flux of the OLPs is mainly determined by the wave vectors of Rayleigh anomalies. We further found that the OLP exhibits symmetry-protected bound states in the continuum characteristic, which explains the failure of previously reported symmetric structures to excite OLPs at normal incidence. Our work extends the understanding of the OLP and brings benefit to promote the flexible design of functional plasmonic devices.

Multi-scale optical simulation of crystalline silicon solar cells by combining ray and wave optics.

Optical simulations allow the evaluation of the absorption, reflection, and transmission of each functional layer of solar cells and, therefore, are of great importance for the design of high-efficiency crystalline silicon (c-Si) solar cells. Here, a multi-scale simulation method (MSM) based on ray and wave optics is proposed to investigate the optical characteristics of c-Si solar cells. The ray and wave optical methods are first independently employed on inverted pyramid glass sheets, where the latter one can describe the size-dependent interfacial scattering characteristics more accurately. Then the optical properties of a c-Si solar cell with a tunnel oxide passivated carrier-selective contact configuration are studied by employing the MSM, where scattering at the interfaces is acquired by a finite-difference time-domain method (wave optics). Since the MSM can accurately simulate optical modes such as the Rayleigh anomaly, Bloch mode, and Mie resonances, the reflection and transmission spectra of the whole device are in good agreement with the measured data. The proposed MSM has proven to be accurate for structures with functional thin films, which can be extended to hybrid tandem devices with top-level cells consisting of stacks of layers with similar dimensions.

Strongly enhanced sensitivities of CMOS compatible plasmonic titanium nitride nanohole arrays for refractive index sensing under oblique incidence.

Titanium nitride (TiN) is a complementary metal-oxide-semiconductor (CMOS) compatible material with large potential for the fabrication of plasmonic structures suited for device integration. However, the comparatively large optical losses can be detrimental for application. This work reports a CMOS compatible TiN nanohole array (NHA) on top of a multilayer stack for potential use in integrated refractive index sensing with high sensitivities at wavelengths between 800 and 1500 nm. The stack, consisting of the TiN NHA on a silicon dioxide (SiO2) layer with Si as substrate (TiN NHA/SiO2/Si), is prepared using an industrial CMOS compatible process. The TiN NHA/SiO2/Si shows Fano resonances in reflectance spectra under oblique excitation, which are well reproduced by simulation using both finite difference time domain (FDTD) and rigorous coupled-wave analysis (RCWA) methods. The sensitivities derived from spectroscopic characterizations increase with the increasing incident angle and match well with the simulated sensitivities. Our systematic simulation-based investigation of the sensitivity of the TiN NHA/SiO2/Si stack under varied conditions reveals that very large sensitivities up to 2305 nm per refractive index unit (nm RIU-1) are predicted when the refractive index of superstrate is similar to that of the SiO2 layer. We analyze in detail how the interplay between plasmonic and photonic resonances such as surface plasmon polaritons (SPPs), localized surface plasmon resonances (LSPRs), Rayleigh Anomalies (RAs), and photonic microcavity modes (Fabry-Pérot resonances) contributes to this result. This work not only reveals the tunability of TiN nanostructures for plasmonic applications but also paves the way to explore efficient devices for sensing in broad conditions.

Reconfigurable and nonvolatile terahertz lithography-free photonic devices based on phase change films

High-performance terahertz (THz) devices with reconfigurable features are highly desirable in many promising THz applications. However, most of the existing reconfigurable THz elements are still limited to volatile responses, single functionality, and time-consuming multistep manufacturing procedures. In this paper, we report a lithography-free approach to create reconfigurable and nonvolatile THz components by exploring the reversible, nonvolatile, and continuous THz modulation capability of the phase change material Ge2Sb2Te5 . As a proof of concept, THz gratings with significant Rayleigh anomalies and diffraction as well as ultrathin THz flat lenses with subwavelength and ultra-broadband focusing capabilities are designed and fabricated on ultrathin Ge2Sb2Te5 films using the presented photo-imprint strategy. Moreover, such a method can also be adopted to create more complex THz devices, such as Pancharatnam–Berry phase metasurfaces and grayscale holographic plates. With these findings, the proposed method will provide a promising solution to realize reconfigurable and nonvolatile THz elements.

Ultra-sensitive terahertz sensing based on Rayleigh anomaly in hyperbolic metamaterial gratings

Terahertz sensors are promising for biomedical, environmental and security applications. Challenges in terahertz sensing have been the limited sensitivity and small figure of merit (FOM). Here we propose an ultra-sensitive terahertz sensing platform based on the Rayleigh anomaly (RA) in hyperbolic metamaterial gratings, which are composed of periodic slits patterned in paired metal-dielectric multilayers. Simulation results show that, under normal incidence with transverse-magnetic polarization and designed with proper parameters, the proposed structure has a super-high quality factor of 516, an ultra-high sensitivity of 1.56 THz RIU−1 (or nm RIU−1), and extremely large FOM of 355 RIU−1 at frequency of 2.272 THz, corresponding to a large normalized sensitivity of 0.69 RIU−1. By comparing with other multilayered gratings, we attribute this exceptionally high sensing performance to the strong local field enhancement over an extremely large area in the sensing region at the RA frequency, which is found to be unique for hyperbolic metamaterial gratings. We further show that the resonance frequency decreases linearly with the grating period, following the RA equation, whereas the large normalized sensitivity keeps constant. We expect this work provides a novel strategy to design extremely sensitive sensors in terahertz as well as other frequency regimes.

Flat-optics hybrid MoS2/polymer films for photochemical conversion.

Novel light harvesting platforms and strategies are crucial to develop renewable photon to energy conversion technologies that overcome the current global energy and environmental challenges. Two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductor layers are particularly attractive for photoconversion applications but new ultra-compact photon harvesting schemes are urgently required to mitigate their poor photon absorption properties. Here, we propose a flat-optics scheme based on nanogrooved ultra-thin MoS2 layers conformally grown onto large area (cm2 scale) nanopatterned templates. The subwavelength re-shaping of the 2D-TMD layers promotes the excitation of photonic Rayleigh anomaly (RA) modes, uniquely boosting a strong in-plane electromagnetic confinement. By tailoring the illumination conditions, we demonstrate effective tuning of the photonic anomalies over a broadband visible spectrum across the absorption band of relevant polluting dye molecules. Thanks to the strong photonic in-plane confinement, we achieve a resonant enhancement of the photodissociation rate of methylene blue (MB) molecules, well above a factor of 2. These results highlight the potential of flat-optics photon harvesting schemes for boosting photoconversion efficiency in large-scale hybrid 2D-TMD/polymer layers, with a strong impact in various applications ranging from new-generation photonics to waste water remediation and renewable energy storage.

Hybrid MoS2/polymer nanoarrays for large-scale photon harvesting and enhanced molecular photo-bleaching

Header Two-dimensional (2D) Transition Metal Dichalcogenide semiconductors (TMDs) are a promising platform in view of developing a novel generation of optoelectronics devices and renewable photon to energy conversion technologies. However new ultra-compact photon harvesting schemes are urgently required to mitigate their poor photon absorption properties. In this work we propose a novel flat-optic solution where a few layers MoS2 film is conformally deposited on a large area (cm2) nanogrooved template. The subwavelength reshaping of the TMDs film induces the excitation of photonic Rayleigh Anomalies (RA). The latter promote a strong in-plane electromagnetic confinement and can be easily tuned over a broadband spectrum by tailoring the illumination conditions. As a demonstration of the potential of this large-area surface functionalization we employed the hybrid 2D nanopatterns as a photocatalyst in a prototype photobleaching reaction of Methylene Blue (MB) molecules. We demonstrate a strong enhancement of the photochemical MB degradation, well above a factor 2, by effectively tuning the RA mode in resonance to the molecular absorption band. Therefore, these findings pave the way to flat-optics photon harvesting schemes for boosting photoconversion efficiency in large-scale hybrid 2D-TMD/polymer layers, with a strong impact in applications ranging from waste-water remediation to new-generation photonics and renewable energy storage

Terahertz high-Q magnetic dipole resonance induced by coherent Fano interactions

High Q-factor resonance holds great promise for bio-chemical sensing and enhanced light–matter interaction. However, terahertz (THz) magnetic resonances usually demonstrate low Q-factors, resulting in huge energy radiation loss particularly in high frequency bands. Here, we show that high Q-factor magnetic dipole resonance at THz frequencies can be achieved by exploiting the coherent Fano interactions with strong field enhancements in an array composed of single metallic split-ring resonators, working at Wood–Rayleigh anomalies. It can give rise to ultrahigh Q-factor beyond 104 in the THz regime. Experimentally, the measured Q-factor of dominant magnetic dipole resonance can achieve no less than a level of ∼261 by Lorentzian fitting to the experimental data. In addition, a high Q-factor of the fundamental-order magnetic dipole resonance is demonstrated beyond 30. High-Q magnetic dipole resonance is closely associated with ultralow-damping and negative permeability in the THz band. The measurements of magnetic dipole resonances are in good agreement with the theoretical analyses. Our scheme suggests a feasible route to suppress radiative loss for enhanced THz field-matter interaction.

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.