Corrugated Metal Surface Research Articles

Photoexcited Hot Electron Catalysis in Plasmon-Resonant Grating Structures with Platinum, Nickel, and Ruthenium Coatings.

We report the electrochemical potential dependence of photocatalysis produced by hot electrons in plasmon-resonant grating structures. Here, corrugated metal surfaces with a period of 520 nm are illuminated with 785 nm wavelength laser light swept as a function of incident angle. At incident angles corresponding to plasmon-resonant excitation, we observe sharp peaks in the electrochemical photocurrent and dips in the photoreflectance consistent with the conditions under which there is wavevector matching between the incident light and the spacing between the lines in the grating. In addition to the bare plasmonic metal surface (i.e., Au), which is catalytically inert, we have measured grating structures with a thin layer of Pt, Ru, and Ni catalyst coatings. For the bare Au grating, we observe that the plasmon-resonant photocurrent remains relatively featureless over the applied potential range from -0.8 to +1.2 V vs NHE. For the Pt-coated grating, we observe a sharp peak around -0.3 V vs NHE, three times larger than the bare Au grating, and near complete suppression of the oxidation half-reaction, reflecting the reducing nature of Pt as a good hydrogen evolution reaction catalyst. The photocurrent associated with the Pt-coated grating is less noisy and produces higher photocurrents than the bare Au grating due to the faster kinetics (i.e., charge transfer) associated with the Pt-coated surface. The plasmon-resonant grating structures enable us to compare plasmon-resonant excitation with that of bulk metal interband absorption simply by rotating the polarization of the light while leaving all other parameters of the experiment fixed (i.e., wavelength, potential, electrochemical solution, sample surface, etc.). A 64X plasmon-resonant enhancement (i.e., p-to-s polarized photocurrent ratio) is observed for the Pt-coated grating compared to 28X for the bare grating. The nickel-coated grating shows an increase in the hot-electron photocurrent enhancement in both oxidation and reduction half-reactions. Similarly, Ru-coated gratings show an increase in hot-electron photocurrents in the oxidation half-reaction compared to the bare Au grating. Plasmon-resonant enhancement factors of 36X and 15X are observed in the p-to-s polarized photocurrent ratio for the Ni and Ru gratings, respectively.

Slow motion medium for broadband lens antennas based on corrugated metal surfaces

A slowing medium based on parallel corrugated metal surfaces. The study of the moderating properties of the medium was carried out using numerical method in the Ansys HFSS software environment. It is shown that the medium has a strong spatial natural and weak frequency dispersion. The latter circumstance allows it to be used in as a material for broadband lenses of the VHF-microwave ranges of electromagnetic waves. As an example of such use, a lens-based antenna was synthesized and analyzed Mikaelyan.

Delaying Medium for Broadband Lens Antennas Based on Corrugated Metal Surfaces

Delaying Medium for Broadband Lens Antennas Based on Corrugated Metal Surfaces

Excitation of surface plasmon polaritons by diffraction-free and vector beams.

Surface plasmon polaritons (SPPs) are traditionally excited by plane waves within the Rayleigh range of a focused transverse-magnetic (TM) Gaussian beam. Here we investigate and confirm the coupling between SPPs and two-dimensional Gaussian and Bessel-Gauss wave packets, as well as one-dimensional light sheets and space-time wave packets. We encode the incoming wavefronts with spatially varying states of polarization; then we couple the respective TM components of radial and azimuthal vector beam profiles to confirm polarization-correlation and spatial-mode selectivity. Our results do not require material optimization or multi-dimensional confinement via periodically corrugated metal surfaces to achieve coupling at a greater extent, hereby outlining a pivotal, yet commonly overlooked, path towards the development of long-range biosensors and all-optical integrated plasmonic circuits.

Remarkable Color Gamut Enhancement of Dye Lacquers Using Corrugated Surfaces

The visual appearance of any colorized object is usually determined by the spectral absorption of either pigments or dyes. One can however also colorize objects with structural colors that typically have an iridescent appearance, as seen on many beetles. The former colorization mechanism is based on the chemical absorption of the used pigments or dye, while the latter mechanism exploits optical/physical effects such as interference, diffraction, or scattering. Both mechanisms are usually engineered separately and by different specialists. Actually, they can be combined synergistically and it is shown that absorption of three different dye lacquers is significantly enhanced when applied on a corrugated metal surface. Inspired by works on emission enhancement for molecules on top of grating surfaces, these dye lacquers are applied conformally on metallic crossed gratings. We vary thickness and extinction coefficient of the dye lacquers and find that absorption can be enhanced locally by more than a factor of 20. Simulations reveal the underlying mechanisms of this enhancement and the huge color gamut that can be realized by combining otherwise colorless dye lacquers with a corrugated surface is revealed. This approach has potential for applications in sensing with focus on visual inspection like the pregnancy test.

Multipole resonance and Vernier effect in compact and flexible plasmonic structures

Spoof surface plasmons in corrugated metal surfaces allow tight field confinement and guiding even at low frequencies and are promising for compact microwave photonic devices. Here, we use metal-ink printing on flexible substrates to construct compact spoof plasmon resonators. We clearly observe multipole resonances in the microwave frequencies and demonstrate that they are still maintained even under significant bending. Moreover, by combining two resonators of slightly different sizes, we demonstrate spectral filtering via the Vernier effect. We selectively address a target higher-order resonance while suppressing the other modes. Finally, we investigate the index-sensing capability of printed plasmonic resonators. In the Vernier structure, we can control the resonance amplitude and frequency by adjusting a resonance overlap between two coupled resonators. The transmission amplitude can be maximized at a target refractive index, and this can provide more functionalities and increased design flexibility. The metal-ink printing of microwave photonic structures can be applied to various flexible devices. Therefore, we expect that the compact, flexible plasmonic structures demonstrated in this study may be useful for highly functional elements that can enable tight field confinement and manipulation.

Tailoring surface enhanced Raman scattering platform based on pulsed laser deposited MoS2 - Ag hybrid nanostructure

Surface enhanced Raman scattering (SERS), is a widely used sensing technique in which Raman scattering by molecules is significantly enhanced by adsorbing the molecules onto corrugated metal surfaces enabling even single molecular detection. In the present work, we focus on the development of SERS substrate based on MoS2 film fabricated using pulsed laser deposition (PLD) technique taking advantage of the charge transfer interaction between MoS2 film and the probe molecule. Rhodamine 6G (R6G) is used as the probe molecule for the qualitative evaluation of SERS performance of the fabricated substrates. Additionally, silver nanoparticles (Ag NPs) are deposited on the PLD grown MoS2 film and is developed as SERS substrate. SERS performance is optimised by varying deposition time during the fabrication of MoS2 film and also by altering the concentration of Ag NPs. The enhancement depends on both the number of MoS2 layers and on the distribution of Ag NPs. Simulation based on finite difference time domain (FDTD) method is used to validate this result. Enhancement factor of the order of 105 and 108 is obtained for the monolayer MoS2 film and for the optimised MoS2 - Ag hybrid films respectively.

Full-Wave Leaky-Wave Analysis of 1-D Periodic Corrugated Metal Surface Antennas

A full-wave leaky-wave analysis of 1-D periodically corrugated metallic surfaces with wide gaps is presented. Such wide gaps, in the order of half-wavelength, are employed in the most cited designs for corrugated metallic antenna implementation due to the high attainable gain performance. A full-wave periodic unit cell method is described and implemented on a corrugated metal surface design for the first time. The results are compared with the known matrix pencil method. The complex dispersion characteristics verify that these structures operate as leaky-wave antennas. The calculated wavenumber of the leaky mode is then used for the analytical calculation of the radiation patterns of a finite-size antenna, based on a simple array factor approach and a near-field inspection of the electric fields of the simulated unit cell. The calculated patterns are compared to those obtained from a commercial simulation software (CST Microwave Studio), exhibiting a very good agreement.

Long-Range Spoof Surface Plasmons(LRSSP) on the Asymmetric Double Metal Gratings

Spoof surface plasmons (SSP) on the corrugated metal surfaces have been widely studied over the past decade in both microwave and Terahertz (THz) band. Previous studies show that SSP mode on the metal grating suffers from high dissipative losses in high THz band especially near asymptotic frequency which may limit its real applications. The issue can be addressed by utilizing long-range SSP mode (LRSSP). LRSSP is a special low-loss symmetric propagating surface wave mode along double-layer corrugated metallic waveguides. In this paper, the asymmetric double metal gratings are proposed to support LRSSP which can significantly reduce the damping losses of the other short-range SSP mode (SRSSP) on the conventional doubly-corrugated metallic surfaces, thus enabling new possibilities to utilizing this SRSSP mode within long-range manner. According to our dispersion theory and the numerical calculations, the propagation loss of LRSSP is still lower than that of SRSSP within asymmetric regime, which demonstrates good tolerance to the asymmetry of the double-layer corrugated structure. However, its long-range propagation superiority of LRSSP vanishes gradually as the degree of asymmetry is enlarged. This work provides an alternative approach to increase the propagation length of SRSSP mode on the doubly-corrugated metallic surfaces and can open up new avenues to develop some novel LRSSP-based plasmonic devices such as waveguides, filters, sensors and active powerful radiation sources induced by energy beam below optical spectrum.

Asymmetric propagation of spoof surface plasmons along doubly corrugated metal surfaces

Spoof surface plasmons (SSPs) on the doubly corrugated metal surfaces can find a variety of applications, such as waveguides, filters, sensors, communications, and other high-performance active devices in terahertz (THz) and microwave bands. However, these studies so far are mostly on the perfect symmetric structure. In this paper, the asymmetric doubly corrugated metal surfaces are proposed to support and propagate SSP modes, which is inherently free from the conventional structure. The analytical dispersion theory of SSP modes on the structure is presented by a simplified field expansion method, which is also verified by a finite integration method. Based on the given modal analysis, the dispersion relations, propagation losses, and field profiles of the SSP modes with various geometric parameters for both 2D and 3D structures are investigated and analyzed in THz frequencies. By introducing an asymmetry of different upper and lower groove depths, the asymptotical frequency of the symmetric SSP mode at the Brillouin boundary can be largely tuned compared with the conventional symmetric structure. However, the asymptotical frequency of anti-symmetric SSPs on the structure is almost unmovable for the given structural parameters. The symmetric SSP mode demonstrates a larger propagation loss on the structure with the increased degree of asymmetry, while the anti-symmetric mode is inversely lower. By increasing the gap size between these two asymmetric corrugated metal surfaces, the propagation losses decrease for both symmetric and anti-symmetric SSP modes. SSP modes experience a larger distance on the asymmetric structure with an increased unit period and, thus, the damping losses are also enlarged. The effect of 3D structure parameters on the propagation characteristics of SSP modes with a closed sidewall is also considered for practical applications. The presented studies on the SSP modes of the asymmetric doubly corrugated metal surfaces provide new avenues to develop plenty of devices such as low-loss waveguides and filters and many other compact active devices at THz frequencies.

Surface plasma discharge generated by spoof surface plasmon polariton excitation: a computational modeling study

We discuss a computational modeling study of surface plasma generation by spoof surface plasmon polariton (SSPP) mode excitation of a corrugated metal surface. The SSPP mode is a highly localized electromagnetic wave propagating along the interface between a conductor and dielectric. The corrugations are defined by an array of rectangular cavities (comb structures) in the metal at the location of the interface. Spoof plasmon resonances occurring at each cavity couple with one another at the interface to produce localized surface wave structures whose wavelength can be much smaller than the incident wave. The electromagnetic wave structure for a finite length of the interface (metasurface) is analytically estimated and its correlation to the resonance frequency of the infinite metasurface is studied numerically. Strong local enhancement of the electromagnetic fields at the metasurface is used to initiate the plasma breakdown of pure argon gas at 10 Torr. Confined microdischarges produced initially at each of the comb structure evolve rapidly to create an extended structure of high density ~1019 m−3 surface plasma whose properties are comparable to surface streamers generated in dielectric barrier discharges.

Study of Spoof Plasmon Formed on Periodically Corrugated Metal Surface Based on Cavity Resonance Method

Study of Spoof Plasmon Formed on Periodically Corrugated Metal Surface Based on Cavity Resonance Method

Terahertz spoof surface-plasmon-polariton subwavelength waveguide

Surface plasmon polaritons (SPPs) with the features of subwavelength confinement and strong enhancements have sparked enormous interest. However, in the terahertz regime, due to the perfect conductivities of most metals, it is hard to realize the strong confinement of SPPs, even though the propagation loss could be sufficiently low. One main approach to circumvent this problem is to exploit spoof SPPs, which are expected to exhibit useful subwavelength confinement and relative low propagation loss at terahertz frequencies. Here we report the design, fabrication, and characterization of terahertz spoof SPP waveguides based on corrugated metal surfaces. The various waveguide components, including a straight waveguide, an S-bend waveguide, a Y-splitter, and a directional coupler, were experimentally demonstrated using scanning near-field terahertz microscopy. The proposed waveguide indeed enables propagation, bending, splitting, and coupling of terahertz SPPs and thus paves a new way for the development of flexible and compact plasmonic circuits operating at terahertz frequencies.

Prediction of Green Phosphorus with Tunable Direct Band Gap and High Mobility.

Black phosphorus is an emerging material in nanoelectronics and nanophotonics due to its high carrier mobility and anisotropic in-plane properties. In addition, the polymorphism of phosphorus leads to numerous searches for new allotropes that are more attractive than black phosphorus in a variety of applications. On the basis of ab initio evolutionary crystal structure search computation, we report the prediction of a phosphorus allotrope called green phosphorus (λ-P), which exhibits direct band gaps ranging from 0.7 to 2.4 eV and strong anisotropy in optical and transport properties. Free-energy calculations show that a single-layer form, termed green phosphorene, is energetically more stable than blue phosphorene, and a phase transition from black to green phosphorene can occur at temperatures above 87 K. We suggest that green phosphorene can be synthesized on corrugated metal surfaces rather than clean surfaces due to its buckled structure, providing guidance to achieving epitaxial growth.

Corrugation-assisted metal-coated angled fiber facet for wavelength-dependent off-axis directional beaming.

We propose a fiber-optic-plasmonic hybrid device that is based on a corrugation-assisted metal-coated angled fiber facet (CA-MCAFF) for wavelength-dependent off-axis directional beaming (WODB). The device breaks into two key structures: One is the MCAFF structure, which is a modified Kretschmann configuration implemented onto a fiber platform, thereby being able to generate a unidirectional surface plasmon with dramatically enhanced properties in terms of non-confined diffracted radiation loss and operational bandwidth. The other is the periodic corrugation structure put on the MCAFF, thereby enabling WODB functionality out of the whole structures. The corrugated metal surface out-couples the surface plasmon mode to free-space optical radiation into a direction that varies with the wavelength of the optical radiation with excellent linearity. We perform extensive numerical investigations based on the finite-element-method and analyze the out-coupling efficiency (OCEout) and spectral bandwidth (SBout) of the proposed device for various designs and conditions. We determine the seven structural parameters of the device via taking sequential optimization steps. We deduce two optimal conditions particularly for the fiber-facet angle, in terms of the averaged OCEout or the SBout in the whole visible wavelength range (400 - 700 nm), which eventually leads to OCEout = 30.4% and SBout = 230 nm or to OCEout = 24.5% and SBout = 245 nm, respectively. These results suggest substantial enhancements in both OCEout and SBout, in comparison with the performance properties of a typical nano-slit-based device having a similar type of WODB functionality. The proposed CA-MCAFF is a simple, compact and efficient WODB device that is fully compatible with the state-of-the-art optical fiber technology.

Terahertz Electronic Source Based on Spoof Surface Plasmons on the Doubly Corrugated Metallic Waveguide

Spoof surface plasmons (SSP) has become an active research topic in microwave and terahertz (THz) spectrum since its extraordinary optical and physical properties. The strong near field of SSP mode on the corrugated metal surfaces makes it especially attractive for developing a THz electronic source. A THz electronic source based on the efficient generation of SSP modes on the doubly corrugated metallic waveguide is proposed and studied in this paper. The analytical dispersion relations of SSP modes are obtained based on a modal expansion method and the field profiles of SSP modes are also presented by the finite integration method. Besides, the interaction between SSP and injected electron beam is modeled and implemented by particle-in-cell (PIC) simulation based on finite difference time domain algorithm. The gap size between the doubly corrugated metal surfaces can significantly influence the output power and PIC simulation results reveal that output power can be increased from 272 mW to 36.5 W when the gap size decreases from 90 to $40~\mu \text{m}$ at the frequency near 1 THz by the 19.55 kV, 1 A injected electron beam within 4.5-mm interaction length. The dependencies of the output performance on electron beam parameters are also investigated and we find that there is an optimized beam voltage for the given operation frequency. Various electron beams of pulse and direct current electron beam are studied and we find that half pulsewidth of periodical electron beam is more preferable than other emissive shape of injected electron beam for the given structure. Our studies on the efficient generation of SSP modes on the doubly corrugated metallic waveguide may provide a new way to develop THz electronic sources.

Spoof surface plasmon modes on doubly corrugated metal surfaces at terahertz frequencies

Spoof surface plasmons (SSPs) have many potential applications such as imaging and sensing, communications, innovative leaky wave antenna and many other passive devices in the microwave and terahertz (THz) spectrum. The extraordinary properties of SSPs (e.g. extremely strong near field, enhanced beam–wave interaction) make them especially attractive for developing novel THz electronic sources. SSP modes on doubly corrugated metal surfaces are investigated and analyzed both theoretically and numerically in this paper. The analytical SSP dispersion expressions of symmetric and anti-symmetric modes are obtained with a simplified modal field expansion method; the results are also verified by the finite integration method. Additionally, the propagation losses are also considered for real copper surfaces with a limited constant conductivity in a THz regime. It is shown that the asymptotical frequency of the symmetric mode at the Brillouin boundary decreases along with the decreased gap size between these two corrugated metal surfaces while the asymptotical frequency increases for the anti-symmetric mode. The anti-symmetric mode demonstrates larger propagation losses than the symmetric mode. Further, the losses for both symmetric and anti-symmetric modes decrease when this gap size enlarges. By decreasing groove depth, the asymptotical frequency increases for both the symmetric and the anti-symmetric mode, but the variation of propagation losses is more complicated. Propagation losses increase along with the increased period. Our studies on the dispersion characteristics and propagation losses of SSP modes on this doubly corrugated metallic structure with various parameters is instructive for numerous applications such as waveguides, circuitry systems with high integration, filters and powerful electronic sources in the THz regime.

Long-range spoof surface plasmons on the doubly corrugated metal surfaces

In this paper, symmetric spoof surface plasmon (SSP) mode on the doubly corrugated metal surfaces is indentified as long-range spoof surface plasmon (LRSSP) because of its extreme low propagation loss and symmetric dominant field profile so as short-range SSP (SRSSP) for anti-symmetric mode. Based on theoretical calculation and numerical simulation of finite integration method, symmetric and anti-symmetric SSP modes with various gap sizes between these two identical corrugated metal surfaces are investigated in terahertz (THz) regime and good agreement is realized. Besides, the low loss superiority of LRSSP diminishes along with the increased gap size. This work opens up new avenues to utilize this long-range surface mode in far-infrared, THz or lower frequency band and can find many potential applications such as low-loss waveguide, filters and novel electronic sources.

Trapping of surface plasmon wave through gradient corrugated strip with underlayer ground and manipulating its propagation

Corrugated metal surface with underlayer metal as ground is designed as spoof surface plasmons polaritons (SSPPs) structure in microwave frequencies. Efficient conversion from guided wave to SSPP is required for energy feeding into and signal extracting from such plasmonic structure. In this paper, first a high efficient transition design is presented by using gradient corrugated strip with underlayer metal as ground and by using the impedance matching theory. The SSPP wave is highly confined within the teeth part of the corrugated surface. By using this characteristic, then the simple wire-based metamaterial is added below the strip to manipulate the SSPP wave within the propagating band. Two aforementioned devices are designed and fabricated. The simulated and measured results on the scattering coefficients demonstrate the excellent conversion and excellent manipulating of SSPP transmitting. Such results have very important value to develop advanced plasmonic integrated circuits in the microwave frequencies.

Hermitian two-band model for one-dimensional plasmonic crystals

We propose an analytical two-band model for surface plasmon polaritons on corrugated metal surfaces. The generalized eigenvalue equation is derived from Maxwell's equation and matching conditions for electromagnetic fields at the metal surface. Our basic equation is Hermitian for the bands outside of the light cone and perturbation theory is therefore justified. We solve the equation and show the dispersion of the one-dimensional plasmonic crystal, and compare the behaviors of eigenfrequencies with previous works for analytic plasmonic crystals and the band theory for electrons in solids.