Unidirectional Excitation Of Surface Plasmon Polaritons Research Articles

Polarization-controlled efficient and unidirectional surface plasmon polariton excitation enabled by metagratings in a generalized Kretschmann configuration.

In this paper, we propose a generalized Kretschmann configuration that employs a metagrating to replace the prism, realizing polarization-controlled efficient and unidirectional surface plasmon polariton (SPP) excitation. This dielectric phase gradient metagrating on the top surface of a silica substrate is designed to deflect incident light, which subsequently launches SPP wave by means of momentum matching on the metal film coated on the bottom surface. A series of metagratings is designed to enable the SPP excitation by circularly or linearly polarized incident light. The flexibility and tunability of this design to efficiently control SPPs show potential to find wide applications in diverse integrated optics and SPP devices.

Directional Excitation of Surface Plasmon Polaritons by Circularly Polarized Vortex Beams

We investigate the excitation of surface plasmon polaritons (SPPs) using a metallic nanoaperture array illuminated by circularly polarized Laguerre-Gaussian (LG) vortex beams. The direction of SPP excitation is tunable by changing the circular polarization and topological charge of LG beams. The left- or right-handed circular polarization determines SPP propagation on either side of the nanoaperture array. Furthermore, varying the topological charge of LG beam will result in beam splitting of SPPs. We also utilize a composite nanoaperture array with different periods to achieve unidirectional excitation of SPPs. The study provides an interesting approach to control the excitation direction of SPPs and may find great applications in SPP generators and optical switches.

A Review of Unidirectional Surface Plasmon Polariton Metacouplers

Efficient and unidirectional excitation of surface plasmon polaritons (SPPs) and their low-frequency counterparts (i.e., spoof SPPs) is highly desired in the research area of photonics and plasmonics, while being at the same time rather challenging. Recent progress in gradient metasurfaces, ultrathin, and planar materials composed of sub-wavelength unit cells, has opened up new possibilities for efficiently coupling of free-space propagating waves to surface waves. In this paper, we will discuss the recent achievements in gradient metasurface-based unidirectional SPP couplers (i.e., metacouplers). Starting with explaining the basic concept of generalized Snell's law, we then introduce some typical examples of unidirectional SPP metacouplers, followed by an overview of polarization-controlled tunable SPP metacouplers. We also describe metacouplers that also feature other functionalities, beyond the unidirectional SPP excitation. The review is concluded with a short summary and perspective for future developments.

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.

Versatile and tunable surface plasmon polariton excitation over a broad bandwidth with a simple metaline by external polarization modulation.

Surface plasmon polariton (SPP) sources and launchers are highly demanded in various applications of nanophotonics. Here, we propose a general approach that can realize complete control of the complex extinction ratio (including amplitude and phase) of any two linearly independent SPP modes excited by any elementary SPP excitation architecture just by manipulating the incident polarization state. In an optical system, it suffices to simply tune the orientation angles of a linear polarizer and a quarter wave plate, which may greatly simplify the design and application of SPP launchers and diversify their functionalities. As an example to show the broad application prospect of this method, we design and realize a metaline consisting of Δ-shaped plasmonic nanoantennas, which can effectively realize dual functionalities, i.e., the tunable directional SPP excitation at an arbitrarily chosen wavelength and the complete unidirectional SPP excitation over a broad bandwidth. This general approach can also be extended to the control of the complex extinction ratio of any two linearly independent excited modes in many other linear optical systems, such as two modes in a waveguide or two diffraction orders in a grating, over a broad bandwidth.

Broad-angle and efficient unidirectional excitations of surface plasmons with dielectric-coated subwavelength metallic periodic nanoslits

We experimentally demonstrate a mechanism of broad-angle and efficient unidirectional excitations of surface plasmon polaritons (SPPs). By combining the plasmonic critical angle phenomenon and grating effects, unidirectional excitation of SPPs with broad incident angular width up to 25° and high extinction ratio better than 10 dB was experimentally demonstrated. A peak extinction ratio of 17 dB was achieved. The proposed structure was also numerically simulated showing good agreement with experimental measurement. The operation principle was numerically and experimentally clarified through compare and analysis.

Unidirectional surface plasmon-polariton excitation by a compact slot partially filled with dielectric

We propose a new scheme on unidirectional surface plasmon-polariton (SPP) excitation with the following advantages: ultracompact size, working at arbitrary incidence angle and over a wide spectrum. The proposed structure utilizes a partially filled metallic slot with dielectric to realize unidirectional SPP excitation via direct field manipulation. We theoretically and numerically show that unidirectional SPP excitation with a ratio of 93% can be achieved by a structure with a 50 nm slot. The proposed structure keeps its functional capability over incident angles from -80° to 80°, and has a broadband working spectrum of more than 70 nm.

Helicity dependent directional surface plasmon polariton excitation using a metasurface with interfacial phase discontinuity

Surface plasmon polaritons (SPPs) have been widely exploited in various scientific communities, ranging from physics, chemistry to biology, due to the strong confinement of light to the metal surface. For many applications, it is important that the free space photon can be coupled to SPPs in a controllable manner. In this Letter, we apply the concept of interfacial phase discontinuity for circularly polarizations on a metasurface to the design of a novel type of polarization-dependent SPP unidirectional excitation at normal incidence. Selective unidirectional excitation of SPPs along opposite directions is experimentally demonstrated at optical frequencies by simply switching the helicity of the incident light. This approach, in conjunction with dynamic polarization modulation techniques, opens gateway towards integrated plasmonic circuits with electrically reconfigurable functionalities.

Experimental demonstration of tunable directional excitation of surface plasmon polaritons with a subwavelength metallic double slit

We demonstrate experimentally the directional excitation of surface plasmon polaritons (SPPs) on a metal film by a subwavelength double slit under backside illumination, based on the interference of SPPs generated by the two slits. By varying the incident angle, the SPPs can be tunably directed into two opposite propagating directions with a predetermined splitting ratio. Under certain incident angle, unidirectional SPP excitation can be achieved. This compact directional SPP coupler is potentially useful for many on-chip applications. As an example, we show the integration of the double-slit couplers with SPP Bragg mirrors, which can effectively realize selective coupling of SPPs into different ports in an integrated plasmonic chip.

Unidirectional SPP excitation at asymmetrical two-layered metal gratings

A new configuration of a reflective one-dimensional (1D) surface-relief metallic-grating structure for unidirectional excitation of surface plasmon polaritons (SPPs) is proposed and numerically explored for plane wave normal incidence. The structure is embedded between two different dielectric media and composed of two layers, each of them consisting of periodically placed rectangular metal stripes. It is shown that even a small horizontal shift between these two layers, or a change in dielectric contrast of the grating fillings, may redirect energy flow propagation in the vicinity of the grating structure. From these two cases of counter propagating energy flows, only one shows the field behaviour typical of SPP excitation. This evident directivity phenomenon exists together with a high concentration of the electromagnetic field at the periodic structure. The configurations analysed may be useful in designing optical devices like photodetectors or, in general, in any case where efficient control of energy propagation directivity is of primary importance.

Unidirectional Surface Plasmon Polariton Excitation on Single Slit with Oblique Backside Illumination

We have theoretically investigated the unidirectional surface plasmon polariton (SPP) excitation on single slits with oblique backside illumination. An aperture diffraction method is devised, from which the conditions of slit width and beam illumination angle for the unidirectional SPP excitation are formulated analytically. The derived unidirectional conditions are validated with vectorial electromagnetic simulation using the rigorous coupled wave analysis.