Plasmonic nanostructures, which are used to generate surface plasmon polaritions (SPPs), always involve sharp corners where the charges can accumulate. This can result in strong localized electromagnetic fields at the metallic corners, forming the hot spots. The influence of the hot spots on the propagating SPPs are investigated theoretically and experimentally in a metallic slit structure. It is found that the electromagnetic fields radiated from the hot spots, termed as the hot spot cylindrical wave (HSCW), can greatly manipulate the SPP launching in the slit structure. The physical mechanism behind the manipulation of the SPP launching with the HSCW is explicated by a semi-analytic model. By using the HSCW, unidirectional SPP launching is experimentally realized in an ultra-small metallic step-slit structure. The HSCW bridges the localized surface plasmons and the propagating surface plasmons in an integrated platform and thus may pave a new route to the design of plasmonic devices and circuits.
An ultra-small and broadband polarization splitter is numerically and experimentally demonstrated based on the double-slit interference in a polymer-film-coated double-slit structure. The hybrid slab waveguide(air-polymer-Au) supports both the transverse-magnetic and transverse-electric modes. The incident beam from the back side can excite these two guided modes of orthogonally polarized states in the hybrid structure. By exploiting the difference slit widths and the large mode birefringence, these two guided modes propagate to the opposite directions along the front metal surface. Moreover, the short interference length broadens the operation bandwidth. Experimentally, a polarization splitter with a lateral dimension of only about 1.6 μm and an operation bandwidth of 50 nm is realized. By designing the double-slit structure in a hybrid strip waveguide, the device dimension can be significant downscaled to about 0.3 × 1.3 μm2. Such an ultra-small and broadband polarization splitter may find important applications in the integrated photonic circuits.
Subwavelength plasmonic waveguides are the most promising candidates for developing planar photonic circuitry platforms. In this study a subwavelength metallic ridge waveguide is numerically and experimentally investigated. Differing from previous plasmonic waveguides, the metallic strip of the subwavelength ridge waveguide is placed on a thick metal film. It is found that the surface-plasmon-polariton (SPP) waveguide modes result from the coupling of the corner modes in the two ridge corners. The bottom metal film has a great influence on the SPP modes, and nearly all the evanescent fields of the SPP modes are tightly confined outside the ridge waveguide. Simulations show that 50% of the total power flow in the SPP mode can be confined outside the ridge waveguide with an area of only about λ 2/20. The propagation length is still about 10 times the plasmon wavelength. Through comparison with a metallic strip placed directly on the dielectric substrate, the proposed ridge waveguide exhibits a much higher sensing performance. This plasmonic ridge waveguide with deep-subwavelength outside-field confinements is of significance in a range of nano-optics applications, especially in nanosensing.