In addition to surface plasmon polaritons (SPPs), quasi-cylindrical waves (CWs) are also important components of electromagnetic fields on metal surfaces. In this study, we present a closed-form expression for CW fields on a dielectric-film-coated metal surface. It is found that the effective refractive index of CWs roughly keeps unchanged when the coated dielectric film becomes thicker, while the effective refractive index of SPPs increases rapidly. These different responses are explained by referring to a waveguide perspective, in which the SPP and CW waves are, respectively, identified as a waveguide mode and superposition of radiation modes. The results and related analyses with the waveguide perspective explicitly show the different physical natures of SPPs and CWs. (C) 2015 Optical Society of America
Surface plasmon polaritons (SPPs) have sparked enormous interest on nanophotonics beyond the diffraction limit for their remarkable capabilities of subwavelength confinements and strong enhancements. Due to the inherent polarization sensitivity of the SPPs [transverse-magnetic (TM) polarization], it is a great challenge to couple the s-polarized free-space light to the SPPs. Here, an ultrasmall defect aperture (<(2)/2) is designed to directionally couple both the p- and s-polarized incident beams to the single SPP mode in a broad bandwidth, which is guided by a subwavelength plasmonic waveguide. Simulations show that hot spots emerge at the sharp corners of the defect aperture when the incident beams illuminate it from the back side. The strong radiative fields from the hot spots are directionally coupled to the SPP mode because of the symmetry breaking of the defect aperture. By adjusting the structural parameters, both the unidirectional and bidirectional SPP coupling from the two orthogonal linear-polarization incident beams are experimentally demonstrated. The polarization-free coupling of the SPPs is of importance in circuits for fully optical processing of information with a deep-subwavelength footprint.
Launching the free-space light to the surface plasmon polaritons (SPPs) in a broad bandwidth is of importance for the future plasmonic circuits. Based on the interference of the pure SPP component, the bandwidths of the unidirectional SPP launching is difficult to be further broadened. By greatly manipulating the SPP intensities with the quasi-cylindrical waves (Quasi-CWs), an ultra-broadband unidirectional SPP launcher is experimentally realized in a submicron asymmetric slit. In the nanogroove of the asymmetric slit, the excited Quasi-CWs are not totally damped, and they can be scattered into the SPPs along the metal surface. This brings additional interference and thus greatly manipulates the SPP launching. Consequently, a broadband unidirectional SPP launcher is realized in the asymmetric slit. More importantly, it is found that this principle can be extended to the three-dimensional subwavelength plasmonic waveguide, in which the excited Quasi-CWs in the aperture could be effectively converted to the tightly guided SPP mode along the subwavelength plasmonic waveguide. In the large wavelength range from about 600 nm to 1300 nm, the SPP mode mainly propagates to one direction along the plasmonic waveguide, revealing an ultra-broad (about 700 nm) operation bandwidth of the unidirectional SPP launching.
Directional light scattering is important in basic research and real applications. This area has been successfully downscaled to wavelength and subwavelength scales With the development of optical antennas, especially single-element nano-antennas. Here, by adding an auxiliary resonant structure to a single-element plasmonic nanoantenna, we show that the highly efficient lowest-order antenna mode can be effectively transferred into inactive higher-order modes. On the basis of this mode conversion, scattered optical fields can be well manipulated by utilizing the interference between different antenna modes. Both broadband directional excitation of surface plasmon polaritons (SPPs) and inversion of SPP launching direction at different wavelengths are experimentally demonstrated as typical examples. The proposed strategy based on mode conversion and mode interference provides new opportunities for the design of nanoscale optical devices, especially directional nanoantennas.
Colloidal CdS nanorods similar to 4.9 nm in diameter and similar to 60 nm long were positioned in gold bow-tie electrodes with a gap of similar to 50 nm by an AC dielectrophoresis process to construct optoelectronic devices. The fabricated devices exhibited an excellent photoresponse to white and blue light, but no response to green light. However, the response of the devices to white light could be degraded by green light. This is considered to be related to surface plasmon polaritons suppressing the generation of photo-carriers in the CdS nanorods. The results indicate that surface plasmons do not always benefit nano-optoeletronic devices. (C) 2015 The Japan Society of Applied Physics
Miniaturizing optical devices beyond the diffraction limit is of great importance for high-integration photonic circuits. By directly fabricating a double-slit aperture structure of different sizes in a subwavelength plasmonic waveguide, an ultra-small plasmonic wavelength splitter is realized experimentally. Due to the different slit widths, the surface plasmon polaritons (SPPs) in the opposite directions exhibit anti-phase interferences. As a result, the SPPs excited at different wavelengths can be split to propagate in the opposite directions along the subwavelength plasmonic waveguide. The plasmonic wavelength splitter only occupies a footprint of about 1.4 mu m(2) on the metal surface, and the splitting wavelengths and their separation can be easily varied by adjusting the structural parameters. This provides it with important applications in the areas of the optical modulating, sensing, and computing networks in highly integrated plasmonic circuits. (C) 2015 Optical Society of America
A sharp trapped resonance is numerically predicted in a single metal-insulator-metal (MIM) resonator by exciting the anti-symmetric waveguide mode. This MIM resonator consists of a wide-gap MIM structure which is connected with a narrow-gap MIM waveguide. By introducing a small structural break in the plasmonic resonator, both of the symmetric and anti-symmetric waveguide modes are excited in the wide-gap MIM structure. By reducing the propagation loss as well as taking advantage of the different field distribution of the anti-symmetric waveguide mode, a strongly trapped resonance with a quality factor as high as about Q a parts per thousand 570 emerges in the MIM resonator. This quality factor is significantly greater than that of the MIM resonators based on the widely used symmetric waveguide mode, which has much longer propagation lengths. The utilization of the anti-symmetric mode in the MIM waveguide provides a new possibility for designing high-performance plasmonic devices.
Using a double-slit structure fabricated on a gold film or a subwavelength (300 nm) plasmonic waveguide, high-contrast and broadband plasmonic sensors based on the interference of surface plasmon polaritons (SPPs) are experimentally demonstrated on chips. By adjusting the focused spot position of the p-polarized incident light on the double-slit structure to compensate for the propagation loss of the SPPs, the interfering SPPs from the two slits have nearly equal intensities. As a result, nearly completely destructive interference can be experimentally achieved in a broad bandwidth (>200 nm), revealing the robust design and fabrication of the double-slit structure. More importantly, a high sensing figure of merit (FOM*) of >1 x 10(4) RIU-1 (refractive index unit), which is much greater than the previous experimental results, is obtained at the destructive wavelength because of a high contrast ratio (C = 0.96). The high-contrast and broadband on-chip sensor fabricated on the subwavelength plasmonic waveguide may find important applications in the real-time sensing of particles and molecules.