Polarization‐free directional coupling of surface plasmon polaritons

Abstract

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 (<λ 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.

Brightening and Guiding Single‐Photon Emission by Plasmonic Waveguide–Slit Structures on a Metallic Substrate

“…The electromagnetic field of the SPP waveguide mode is mainly distributed in the low‐index air ( n air = 1.0), and thus the SPP waveguide mode has long propagation lengths ( L SPP > 450 µm for r ≤ 300 nm), as displayed using the blue line with circle symbols in Figure a. This phenomenon is similar to that in the wedge plasmon polariton (WPP) and ridge waveguide, which have been widely used in nanofocusers, modulators, and beam splitters . More importantly, the plasmonic waveguide structure with the silver nanowire placed on the silver substrate has no cutoff radii, which can provide a large selection range of the silver nanowire radii to brighten the single‐photon emission.…”

“…The green dashed line in Figure a is the effective refractive index ( nʹ eff = 1.0041) of the SPPs on the silver–air interface. Since n eff is greater than nʹ eff , there is no cutoff radius for the SPP waveguide mode in this plasmonic waveguide . This provides a large selection range of radii to support the resonant mode in the metallic nanoslit.…”

Controlling surface-plasmon-polaritons launching with hot spot cylindrical waves in a metallic slit structure

“…Different from the quasi-CW, which is a surface wave of a p-polarized state, [34][35][36] HSCW is a radiated wave in the free space and is independent of the polarizations. [29] As shown in Figure 2(c), the right-and left-propagating SPP amplitudes (denoted by H R and H L , respectively) are composed of two parts: the pure-SPP parts (HSP R and HSP L ) and the hot-spot parts (HHS R and HHS L ). Herein, the pure-SPP parts refers to the SPPs that are generated purely from SPP modes, such as the waveguide modes in the lower-slit and the SPPs along the upper-slit.…”

“…This is attributed to the anisotropic emission of the HSCW at the sharp corners. The HSCW will mainly radiate to the diagonal direction of the rectangular corner in the free space, [29] and thus the HSCW amplitude propagating from the right hot spot to the left corner of the step-slit structure is smaller than the HSCW amplitude propagating from the left hot spot to the right corner. To sum up, the SPP launching in the step-slit structure can be influenced by the HSCW, and thus the SPP launching can be manipulated with the modulation of the HSCW.…”

“…As a result, the localized electromagnetic fields become extremely strong at the sharp corners, which are usually termed as hot spots. [23][24][25][26][27][28][29] Because of the strong field enhancement, hot spots have been applied in enormous areas of the second harmonics, Raman enhancement, surface-enhanced optical sensing, spectroscopy, and microscopy. [23][24][25][26][27][28] However, seldom works have done to concern the conversion of the localized hot spots to the nearby propagating SPPs in a platform.…”