Passage of electromagnetic waves through the critical surface

A study is made of the passage of electromagnetic waves through the critical surface at small angles between the plasma density gradient and the magnetic field. Expressions are derived for the transmission and reflection coefficients of electromagnetic oscillations that are periodic in the direction transverse to the density gradient. The penetration of wave beams is also analyzed. In the case of a wide beam, the incident and transmitted ray trajectories are shown to be mirror-image about the resonance surface. Behind the resonance surface, a narrow incident wave beam generates a beam propagating along the magnetic field.     

Dark Mode Driven Extra-narrow and Multiband Absorber

Recently, metamaterial absorbers have received tremendous amount of interest because of their remarkable ability to manipulate the amplitude, phase, and polarization of light. However, most absorbers rely on the direct coupling of electric or magnetic field with external excitation, which lead to inevitable energy leakage to the surrounding environment and depress the quality factor of the structure. In this work, we investigate the multiband absorption property by exciting dark plasmonic modes in reflective symmetric and asymmetric metamaterials. Theoretically, the existence of dark plasmonic modes in asymmetric metamaterials is unambiguously illustrated by the improved eigen-mode theory. With the introduction of asymmetry, dark modes in metamaterials can be easily excited by normal incident plane wave. Moreover, we also directly excite the dark modes in symmetric absorber with oblique incidence. The dark modes splitting mechanism is also clarified with the excitation of designer surface plasmon. Dominated by magnetic dipole or higher-order multipole, these dark modes possess high quality factors (Q). Numerical results indicate that the metamaterial absorber maintains high absorbance within a wide-angle incidence (0~50°). The high Q asymmetric metamaterial absorber can be an excellent candidate for multiband plasmonic sensor.     

Hybrid Plasmonic Waveguide Based on Tapered Dielectric Nanoribbon: Excitation and Focusing

The nanofocusing of light source was proposed and simulated using the dielectric-loaded surface plasmon polariton (SPP) model with various laterally tapered planar dielectric architectures on the top surface of the metal. By using finite-difference time-domain method, enhancement factor for the local electric field under distinctive incident polarization was analyzed with different taper apexes under various incident wavelengths and incident angles of the excitation laser. The SPP dispersion and the effect of dissipation on adiabatic nanofocusing of SPP in a sharp taper structure were used to predict the optimal taper angles of the structure and to explain the phenomena of SPP wave slowing down as it propagating toward the taper end. This SPP nanofocusing process was also experimentally realized by illuminating the structure of a tapered CdS nanoribbon deposited on the Ag surface. As the emission of the focused SPP at the taper end, the proposed plasmonic structure can be severed as a light nanosource emitter in the future optical integrated circuits.     

Optical Bifacial Transmission by Asymmetric Charge-Oscillation-Induced Light Transmission Through a Plasmonic Structure

From first-principles computation, we reveal that optical bifacial transmission can be induced within an asymmetric metallic subwavelength structure. This phenomenon can be explained by a concrete picture in which the intensity of the driving forces for surface plasmon or charge wave is asymmetric for the two incident directions. Two distinguished different numerical methods, finite difference time domain (FDTD), and rigorous coupled wave analysis (RCWA) are utilized to verify that optical bifacial transmission can exist for linear plasmonic metamaterial. Previous results are also reviewed to confirm the physical meaning of optical bifacial transmission for a planar linear metamaterial. The incident light can provide direct driving forces for surface plasmon in one direction. While in the opposite direction, forces provided by the light diffraction are quite feeble. With the asymmetric driving forces, the excitation, propagation, and light-charge conversion of surface plasmon give the rise of bifacial charge-oscillation-induced transmission. In periodic a structure, the excitation of surface plasmon polariton can lead to the spoof vanish of such phenomenon. The transmissions for two incident directions get the same in macroscopic while the bifacial still exists in microscale.     

Simultaneous Excitation of Surface Plasmon Polaritons on Both Interfaces in an Asymmetric Dielectric-Metal Corrugated Structure

The simultaneous excitation of plasmon polaritons on both surfaces of metal film was studied for asymmetric dielectric-metal-dielectric corrugated structures. Due to the small resonant absorption of the incident light on the transmission side of the structure, we investigated the enhancement of the surface plasmon polaritons on the mentioned side by controlling the structure parameters. When the illuminate light changes from normal incidence to non-normal incidence, the resonant absorption peak splits into a doublet. The simultaneous excitation of surface plasmon polaritons on both surfaces of the metal film can be achieved by controlling the incident angle. Since the wave vector matching condition is not satisfied, there is no coupling between the plasmon polaritons modes on the two surfaces of the corrugated metallic film. The excitation and control of the non-coupled surface plasmon polartions simultaneously propagating on the different interfaces of one metallic film have potential applications for designing novel compact and tunable nano-photonic devices at visible frequency.     

Plasmonic Properties of Gold Nanostructures on Gold Film

This paper reports on a systematic study of the plasmonic properties of periodic arrays of gold cylindrical nanoparticles in contact with a gold thin film. Depending on the gold film thickness, it observes several plasmon bands. Using a simple analytical model, it is able to assign all these modes and determine that they are due to the coupling of the grating diffraction orders with the propagating surface plasmons travelling along the film. With finite difference time domain (FDTD) simulations, it demonstrates that large field enhancement occurs at the surface of the nanocylinders due to the resonant excitation of these modes. By tilting the sample, it also observes the evolution of the spectral position of these modes and their tuning through nearly the whole visible range is possible. Such plasmonic substrates combining both advantages of the propagative and localised surface plasmons could have large applications in enhanced spectroscopies.

     

Numerical Study of a Nonplanar Two-Stage Surface Plasmonic Lens Illuminated by a Radially Polarized Beam

We design and fabricate a nonplanar two-stage surface plasmonic lens composed of concentric circular slits for exciting propagating surface plasmonic wave and a center-positioned cone-like nanoparticle for generating localized surface plasmonic waves. The numerical investigation based on the finite difference in time domain method is performed. It is found that, when a radially polarized beam illumination is applied, a highly confined electric field with full width half maximum of as small as 6 nm and the transmission enhancement factor of six orders higher than the incident beam is achievable. The optimization design is conducted through comparison of different conic angles and different materials of the cone-like nanoparticles.     

Design of a Slit-Groove Coupler for Unidirectional Excitation of the Guided Surface Plasmon Polaritons Through a Plasmonic Slot Waveguide

In this paper, a plasmonic-photonic nanostructure has been introduced for efficient unidirectional coupling of free-space radiation to surface plasmon polariton (SPP) waves under normal illumination on a subwavelength slit. The structure consists of a conventional metallic slit-groove nanostructure integrated with a plasmonic waveguide to support SPP waves along the desired direction with a remarkable lateral confinement. The unidirectional coupling is achieved by using an integrated plasmonic distributed reflector designed under Bragg condition. This reflector basically distributes part of the light coupled through the slit into the SPP modes of the waveguide. Numerical simulations show that up to 26 % of the normally incident light couples to the transversely localized field of the surface plasmon. In addition, the ratio of mode current density of the surface plasmon, launched in the desired direction, to that in the opposite direction can reach about 23 times. This structure shows a 2.5-fold improvement in coupling efficiency relative to a standard slit-groove structure. Also, the transmission distance for the new nanostructure is shown to be more than 8 times greater than that of the standard nanostructure.     

Three-Dimensional Manipulations of Surface Plasmon Polariton Wave Propagation

The plasmonic integrated circuit, a potential application of surface plasmon polaritons (SPPs), can manipulate an SPP wave propagating on a metal surface in a way similar to electronic circuits. Here, we propose the concept of three-dimensional (3D) SPP wave manipulation: control of an SPP wave propagating in both the horizontal direction and the vertical direction. A hole set in the film can guide an SPP wave in the vertical direction. In the horizontal direction, two holographic groove patterns are used to focus an incident SPP wave on one surface of the film to the hole and control the divergent SPP waves transmitted from the hole on the other metal surface, respectively. The holographic groove patterns are designed via the methodology of surface electromagnetic wave holography. 3D finite-difference time-domain method simulations show a good performance of the 3D manipulation via these designed holographic groove patterns.

     

Parametric excitation of surface waves at the boundary between a magnetized plasma and a metal

A study is made of the parametric excitation of potential surface waves propagating in a planar plasma-metal waveguide structure in a magnetic field perpendicular to the plasma-metal boundary. An external, spatially uniform, alternating electric field at the second harmonic of the excited wave is used as the source of parametric excitation. A set of equations is derived that describes the excitation of surface waves due to the onset of decay instability. Expressions for the growth rates in the linear stage of instability are obtained, and the threshold amplitudes of the external electric field above which the parametric instability can occur are found. Analytic expressions for the saturation amplitudes are derived with allowance for the self-interaction of each of the excited waves and the interaction between them. The effect of the plasma parameters and the strength of the external magnetic field on the saturation amplitude, growth rates, and the threshold amplitudes of the pump electric field are analyzed.     

An Integrated Multistage Nanofocusing System

We demonstrate an integrated multistage nanofocusing system which combines a conventional objective, a surface plasmonic lens, and a center-positioned rounded-tip cone nanoparticle. The surface plasmonic lens, fabricated on the cover glass which has been mounted on the biological microscopic objective, is composed of several concentric annular slits for exciting propagating surface plasmonic wave. The rounded-tip cone nanoparticle is for further generating non-propagating localized surface plasmonic wave. It is revealed that the enhancement of the nanoscale optical field can be improved by carefully choosing the appropriate numerical aperture of the objective to match the specific nanostructure of the surface plasmonic lens and choosing the relatively big cone angle of the nanoparticle. The investigation shows that a highly confined electric field as small as 20 nm and an enhancement factor of 5 orders of magnitude can be achieved through this multistage nanofocusing system when the system is illuminated with a uniform radially polarized beam.     

Tunnel Light Through a Continuous Optically Thick Metal Film Utilizing Higher Order Magnetic Plasmon Resonance

In this letter, we investigate the extraordinary optical transmission behavior of a flat continuous metal film sandwiched by magnetic plasmonic structures. A new mechanism by utilizing higher order magnetic plasmon resonance is proposed to enhance the transmission. Numerical simulation results show that 80 % electromagnetic energy can be transmitted through the middle 50-nm-thick continuous gold film in near-infrared regime. The excitation of the second-order magnetic plasmons and the propagating surface plasmons, as well as the interaction between them accounts for such a high transmission. The interaction of magnetic plasmons and surface plasmons leads to new hybrid modes, and the coupled oscillator model is introduced to analyze this hybridization. This work extends the application range of higher order magnetic plasmons and may have potential in transparent electrode and electromagnetic energy transfer applications.     

Synthesis,Characterization, and Functionalization of Hybrid Au/CdS and Au/ZnS Core/Shell Nanoparticles

Plasmonic nanoparticles are an attractive material for light harvesting applications due to their easily modified surface, high surface area and large extinction coefficients which can be tuned across the visible spectrum. Research into the plasmonic enhancement of optical transitions has become popular, due to the possibility of altering and in some cases improving photo-absorption or emission properties of nearby chromophores such as molecular dyes or quantum dots. The electric field of the plasmon can couple with the excitation dipole of a chromophore, perturbing the electronic states involved in the transition and leading to increased absorption and emission rates. These enhancements can also be negated at close distances by energy transfer mechanism, making the spatial arrangement of the two species critical. Ultimately, enhancement of light harvesting efficiency in plasmonic solar cells could lead to thinner and, therefore, lower cost devices. The development of hybrid core/shell particles could offer a solution to this issue. The addition of a dielectric spacer between a gold nanoparticles and a chromophore is the proposed method to control the exciton plasmon coupling strength and thereby balance losses with the plasmonic gains. A detailed procedure for the coating of gold nanoparticles with CdS and ZnS semiconductor shells is presented. The nanoparticles show high uniformity with size control in both the core gold particles and shell species allowing for a more accurate investigation into the plasmonic enhancement of external chromophores.     

Effective Coupling of Incident Light Through an Air Region into an S-Shape Plasmonic Ag Nanowire Waveguide with Relatively Long Propagation Length

Coupling of incident light through an air region into an S-shape silver (Ag) plasmonic nanowire waveguide (SSAPNW) is a highly difficult challenge of light guiding on the surface of metal nanowire. In this paper, we numerically analyze the coupling effect of an SSAPNW which is covered by a dielectric medium using a finite element method. The coupling effect can be modulated by adjusting the Ag nanowire diameter and the covering dielectric medium width and wavelength of incident light, and the propagation length of surface plasmon (SP) coupling can be maximized. Simulation results reveal that the field confinement can be significantly improved and the majority of the electric field can be carried on the surface of a bending Ag nanowire. The effect of electric field transport along an SSAPNW due to SP coupling and Fabry-Perot resonance is investigated for different dimensions and lengths. Accordingly, long propagation lengths of about 41.5 μm for 10?×?SSAPNW at an incident wavelength of 810 nm and longer propagation length can be achieved if more sections of an SSAPNW are used. Simulation results offer an efficient method for optimizing SP coupling into bending metal nanowire waveguides and promote the realization of highly integrated plasmonic devices.     

Tunable Plasmonic Dispersion and Strong Coupling in Graphene Ribbon and Double Layer Sheets Structure

Based on the interplay between propagating surface plasmon polaritons (PSPs) in graphene ribbon and double layer sheets structure, we theoretically demonstrate a tunable strong coupling mechanism significantly different from reported conventional noble metal nanostructures. The strong electromagnetic coupling between the low order antisymmetric and high order symmetric PSPs modes occurs due to the intersections of dispersion curves, which leads to a modification of plasmonic dispersion and multiple significant anti-crossing regions. Of particular, this strong coupling is controllable through external gate voltage of graphene sheets or ribbon. The results offer an effective regime to dynamically tune the interaction of graphene PSPs, which may find applications in the field of nanophotonic devices in the mid-infrared range.     

Plasmonic Band Gap: Role of the Slit Width in 1D Metallic Grating on Higher Refractive Index Substrate

This paper reports the excitation of surface plasmon polaritons (SPPs) and associated plasmonic band gap (PBG) while using TM plane wave interacting with 1D metallic grating on higher refractive index GaP substrate. A simple method is introduced to estimate the PBG which is crucial for many plasmonic devices. The PBG is estimated by measuring the transmission spectra obtained through the plasmonic grating structures when slit width is varied while periodicity and the thickness of the gold (Au) film remained fixed. The PBG is observed for the grating devices whose slit width is less than one third of the periodicity which is caused by the presence of a higher plasmonic mode. The PBG is absent for the grating device whose slit width is slightly less than half and greater than one third of the periodicity. Such grating devices support only a fundamental plasmonic mode because the profile/shape of the slit in the grating device is more like a sinusoidal nature. Furthermore, such grating offers intermediate scattering to the incident light and the SPP as well which in turn couple more incident energy to the SPPs. Far-field modelling results also support the results obtained through experiment.

     

Efficient and Directional Excitation of Surface Plasmon Polaritons by Oblique Incidence on Metallic Ridges

For many years, the search for efficient surface plasmon polariton (SPP) excitation mechanisms has been a recurring matter in the development of compact plasmonic devices. In this work, we excited SPPs illuminating a subwavelength metallic ridge with a focused spot to characterize the coupling efficiency by varying the incidence angle of the excitation beam from ??50 to 50°. The intensity distribution of the excited SPPs was measured using leakage radiation microscopy to determine the relative coupling efficiency in the wavelength interval from 740 to 840 nm. We modeled the excitation efficiency as a function of the incidence angle using a simple analytical diffraction model. Two ridges of different width (200 and 500 nm) were used to compare results and validate the model. The experimental results show a higher coupling efficiency at oblique incidence, where the coupling was enhanced by factors of 2× for the 500-nm-wide ridge, and 3× for the 200-nm-wide ridge, as well as unidirectional SPP excitation. The experimental results are in good agreement with the proposed model.     

Tunable Multi-Port Surface Plasmon Polariton Excitation with Nanostructures

Surface plasmon polaritons (SPPs) have appealing features such as tighter spatial confinement and higher local field intensity. Manipulation of surface plasmon polaritons on metal/dielectric interface is an important aspect in the achievement of integrated plasmonic circuit beyond the diffraction limit. Here, we introduce a design of pin cushion structure and a holographic groove pattern structure for tunable multi-port SPPs excitation and focusing. Free space light is coupled into SPPs through momentum matching conditions. Both nanostructures are capable of tunably controlling of SPPs depending on the incident polarizations, while the holographic method provides more flexibility of wavelength-dependent excitations. Furthermore, a quantitative method is applied to calculate the efficiencies of excitation for both nanostructures under different conditions, including radially polarized incident beams. These results can work as a guidance and be helpful to further choice of the suitable design strategies for variable plasmonic applications such as beam splitter, on-chip spectroscopy, and plasmonic detectors.     

Active Focal Length Control of Terahertz Slitted Plane Lenses by Magnetoplasmons

Active plasmonic devices are mostly designed at visible frequencies. Here, we propose an active terahertz (THz) plasmonic lens tuned by an external magnetic field. Unlike other tunable devices where the tuning is achieved by changing the plasma frequency of materials, the proposed active lens is tuned by changing the cyclotron frequency through manipulating magnetoplasmons (MPs). We have theoretically investigated the dispersion relation of MPs of a semiconductor?Cinsulator?Csemiconductor structure in the Voigt configuration and systematically designed several lenses realized with a doped semiconductor slab perforated with sub-wavelength slits. It is shown through finite?Cdifference time?Cdomain simulations that THz wave propagating through the designed structure can be focused to a small size spot via the control of MPs. The tuning range of the focal length under the applied magnetic field (up to 1?T) is ??3??, about 50% of the original focal length. Various lenses, including one with two focal spots and a tunable lens for dipole source imaging, are realized for the proposed structure, demonstrating the flexibility of the design approach. The proposed tunable THz plasmonic lenses may find applications in THz science and technology such as THz imaging.