Excitation Of Magnetic Polaritons Research Articles

A TM polarization absorber based on a graphene-silver asymmetrical grating structure for near-infrared frequencies.

In this paper, a TM polarization multi-band absorber is achieved in a graphene-Ag asymmetrical grating structure. The proposed absorber can achieve perfect absorption at 1108 nm, 1254 nm, and 1712 nm (the absorption exceeds 98.4% at the three peaks). Results show that the perfect absorption effect originates from the excitation of magnetic polaritons (MPs) in the silver ridge grating; a LC equivalent circuit model is utilized to confirm the finite-difference-time-domain (FDTD) simulation. The influences of the incident angle, polarization angle, and geometrical size on the absorption spectrum are investigated. Moreover, a quadruple band absorber and a quintuple band absorber are also designed by introducing more silver grating ridges in one period. The proposed graphene-Ag asymmetrical structure has some advantages compared with other absorbers such as the ability to be independently tuned and a simple structure. Thus, the proposed structure can be applied in the areas of multiple absorption switches, near-infrared modulators, and sensors.

Chiral Absorbers Based on Polarization Conversion and Excitation of Magnetic Polaritons

Chiral Absorbers Based on Polarization Conversion and Excitation of Magnetic Polaritons

Dual broadband infrared absorptance enhanced by magnetic polaritons using graphene-covered compound metal gratings.

A dual broadband perfect absorber based on a graphene-covered compound silver (Ag) grating structure working in the infrared (0.8-2.1 µm) regime is proposed and investigated numerically. Two distinct absorption peaks approximately 1.0 are achieved by the excitation of magnetic polaritons over a large range of incident angles from 0 to 70 degrees. The physics underlying the structure is also explained by computing interactions of electromagnetic fields with the graphene and the Ag grating. In addition, it has shown that the absorption peaks can be tuned by changing geometric parameters of the structure; however, their spectral shape and absorption remain unchanged. Furthermore, the proposed compound grating with a graphene overlay provides potential applications for infrared absorbing devices.

Near-field thermophotovoltaic energy conversion by excitation of magnetic polariton inside nanometric vacuum gaps with nanostructured Drude emitter and backside reflector

In this work, we study the effect of magnetic polariton (MP) on the conversion performance of a near-field thermophotovoltaic (TPV) system made of a nanostructured Drude grating emitter and a nanometer-thick photovoltaic (PV) cell with a lossless metal as the backside reflector. Near-field radiative heat flux is calculated through scattering matrix theory coupled with rigorous coupled-wave analysis based on fluctuational electrodynamics. It is theoretically shown that MP can be excited inside the nanometric vacuum gap and the 100-nm PV cell to spectrally enhance the heat flux above the cell bandgap. The lossless metal backing, which could practically serve as electrode for charge collection, is employed to reflect photons back into the cell for recycling. Contour plots of energy transmission coefficient and an inductor-capacitor circuit model along with magnetic field distribution are used to verify and understand the MP excitation in the near-field. Effects of PV cell thickness and vacuum gap distances, are systematically investigated on the near-field radiative heat flux, electrical power output, and energy conversion efficiency. It is found that the conversion efficiency increases from 0.7% to 12.2% when the semi-infinite In0.18Ga0.82Sb cell is replaced by an ultra-thin PV cell supported by a backside reflector with the emitter at 1000 K and the cell at 300 K separated by 200 nm gap. The conversion efficiency can be also further improved to 17.6% using a nanostructured Drude grating emitter over a planar emitter. This work would open up a new way to improve the performance of TPV devices with nanostructures with near-field thermal radiation.

Experimental Demonstration of the Effect of Magnetic Polaritons on the Radiative Properties of Deep Aluminum Gratings

Despite the abundant theoretical studies of magnetic polaritons (MPs) in tailoring the radiative properties of nanostructures, experimental investigation of MPs in deep metal gratings is still lacking. This work experimentally demonstrates the excitation of MP from several microfabricated aluminum gratings in the mid-infrared region by measuring the specular reflectance (zeroth-order diffraction) of the specimen using a Fourier-transform infrared (FTIR) spectrometer. The rigorous coupled-wave analysis (RCWA) and an LC-circuit model are employed to elucidate the mechanism of various resonant modes and their coupling effect. The influence of incidence angle, plane of incidence, polarization, and the trench depth on the spectral reflectance is also discussed. Moreover, the MP dispersion for off-plane layout has been investigated and demonstrated for the first time. The insight gained from this work may facilitate future design and applications of subwavelength periodic structures with desired radiative properties.

Unidirectional transmission based on polarization conversion and excitation of magnetic or surface polaritons

We propose in this work combing a uniaxial crystal slab with a one-dimensional grating to realize unidirectional transmission (UDT). The physical mechanism for the UDT is attributed to polarization conversion with uniaxial crystal slab and excitation of magnetic polaritons (MPs) or surface plasmon polaritons (SPPs) in the grating region. Numerical simulations were performed by taking hexagonal boron nitride as the uniaxial crystal. The results reveal that UDT can be achieved for both TE and TM waves in the mid-infrared and the optical regions if the grating material is respectively selected as silicon carbide (SiC) and silver (Ag) with properly chosen values of the structure’s geometric parameters. This work may provide important guidelines for design of novel unidirectional transmission devices.

Plasmonic localized heating beyond the diffraction limit via magnetic polariton excitation

Optical localized heating in the nanoscale has recently attracted great attention due to its unique small hot spot size with high energy. However, the hot spot size is conventionally constrained by the diffraction limit. Plasmonic localized heating can provide solutions to this limitation in nanoscale patterning, cancer treatment, and data storage. Plasmonic approaches to overcome the diffraction limit in hot spot size have mainly utilized the excitation of surface plasmon or localized surface plasmon resonance. However, achieving plasmonic localized heating by the excitation of magnetic polariton has not been researched extensively yet. In this work, we numerically investigated the optical response of a nanoscale metamaterial composed of a gold nanowire array and a gold film separated by an ultrathin polymer spacer using ANSYS High Frequency Structural Simulator. A strong absorption peak at the wavelength of 760 nm was exhibited, and the underlying physical mechanism for the strong absorption was verified via the local electromagnetic field distribution to be magnetic resonance excitation. An inductor-capacitor circuit model was used to predict the magnetic resonance wavelength and compare with the numerical results for varied geometrical parameters. Volume loss density due to the strong local optical energy confinement was transferred as heat generation to an ANSYS thermal solver to obtain the local temperature profile. The steady state temperature profile shows an average temperature of 145 °C confined in a local area as small as 33 nm within the spacer, with a full-width at half-maximum of 50 nm along the x-direction. Moreover, the temperature rise from ambient drops to half its maximum value at a distance of 5 nm from the top of the spacer along the z-direction. This clearly demonstrates plasmonic localized heating beyond the diffraction limit via magnetic polariton excitation. Furthermore, the transient temperature profile shows that the system reached steady state within ∼0.36 μs.

Tailoring thermal radiative properties with film-coupled concave grating metamaterials

This work numerically investigates the radiative properties of film-coupled metamaterials made of a two-dimensional metallic concave grating on a continuous metal film separated by an ultrathin dielectric spacer. Spectrally-selective absorption is demonstrated in the visible and near-infrared regime, and underlying mechanisms are elucidated to be either localized magnetic polaritons (MPs) or surface plasmon polaritons (SPPs). The unique behaviors of MPs and SPPs are explained with the help of electromagnetic field distributions at respective resonance frequencies. An inductor–capacitor model is utilized to further confirm the excitation of MP, while dispersion relation is used to understand the behaviors of different SPP modes. Geometric effects of ridge width and grating period on the resonance absorption peaks are discussed. Moreover, directional responses at oblique incidences for different polarization states are studied. Fundamental understanding gained here will facilitate the design of novel metamaterials in energy harvesting and sensing applications.

Study of magnetic polaritons in deep gratings for thermal emission control

Recently, it has been shown that convex cavities or 2D grating structures can enhance thermal emission for energy conversion systems. The mechanisms, however, cannot be well explained by either the conventional cavity resonance modes or surface plasmon polaritons. The present study elucidates the fundamental mechanism by considering the excitation of magnetic polaritons (MPs) in deep gratings. Rigorous coupled-wave analysis (RCWA) is employed to calculate the radiative properties by solving Maxwell's equations numerically. The LC-circuit model is employed to predict the resonance conditions. The current and field distributions further confirm the excitation of magnetic resonances. It is shown that MPs and cavity modes agree with each other when the kinetic inductance is negligibly small. However, when the kinetic inductance is sufficiently large, the maximum resonance wavelength can be more than twice that predicted by the cavity mode. Furthermore, different materials are considered and the frequency range is extended from the near-infrared to the microwave region to illustrate the scalability of the MPs. This study clarifies one of the underlying mechanisms of optical resonance in deep gratings and will benefit the design of wavelength-selective thermal emitters.

A polarization-insensitive and omnidirectional broadband terahertz metamaterial absorber based on coplanar multi-squares films

A simple design of terahertz metamaterial absorber (MA) based on a periodic array of square copper films was investigated numerically. The perfect narrow absorption mainly originates from magnetic polariton excitation and perfect impedance-match. The perfect absorbing properties of this simple design could be tunable by changing the side length based on equivalent LC resonance circuit mode. The bandwidth of the perfect absorption can be effectively enhanced by simply patterning different dimension elements with appropriate geometrical parameters on a coplanar. Finally, the absorption of the composite structure MA is greater than 90% and near perfect impedance-match to the free space in the frequency ranges of 6.24–7.04THz. Further numerical simulations also demonstrate that the MA could achieve very high absorptivity at wide angles of incidence and polarization for both TE and TM waves.

Surface and magnetic polaritons on two-dimensional nanoslab-aligned multilayer structure

The present study theoretically investigates the radiative properties of a two-dimensional (2-D) multilayer structure that has a dielectric spacer between a metallic substrate and square cross-sectional metallic gratings. Differently from the one-dimensional metallic strips coated on a dielectric spacer atop an opaque metallic film [Opt. Express 16, 11328 (2008)], the 2-D metallic gratings can support the localized surface plasmon in addition to the propagating surface plasmon along the metal-dielectric interface. Moreover, the presence of a dielectric spacer also allows the excitation of magnetic polaritons. Underlying mechanisms of the surface and magnetic polartions on the proposed structure are elucidated by employing the 2-D rigorous coupled-wave analysis. The results obtained in this study will advance our fundamental understanding of light-matter interaction at the nanometer scale and will facilitate the development of engineered nanostructures for real-world applications, such as thermophotovoltaic and photovoltaic devices.

Coherent thermal emission by excitation of magnetic polaritons between periodic strips and a metallic film

The present paper theoretically demonstrates coherent thermal emission in the infrared region by exciting magnetic polaritons between metallic gratings and an opaque metallic film, separated by a dielectric spacer. The coupling of the metallic strips and the film induces a magnetic response that is characterized by a negative permeability and positive permittivity. On the other hand, the metallic film intrinsically exhibits a negative permittivity and positive permeability in the near infrared. This artificial structure is equivalent to a pair of single-negative materials. By exciting surface magnetic polaritons, large emissivity peaks can be achieved at the resonance frequencies and are almost independent of the emission angle. The resonance frequency of the magnetic response can be predicted by an analogy to an inductor and capacitor circuit. The proposed structure can be easily constructed using micro/nanofabrication.

Excitation of magnetic polaritons in plates of FeBO 3

The magnetic polaritons in free plates of a weak ferromagnet FeBO 3 are excited at 77 K with frequencies of 30–75 GHz. Polariton modes on both sides of the line of uniform antiferromagnetic resonance are observed in a thick plate which is magnetized in the basal plane. The experimental method is described. The nature of resonances for volume and surface polariton modes in iron borate is considered.