In-Plane Excitation of Wedge Plasmon Polariton Modes in a Double Nanohole for Trapping and Sensing Nanoparticles

Abstract

Double nanohole (DNH) optical tweezers with high gradient forces at the cusps have been extensively used to trap and analyze single nanoparticles (NPs), such as DNA and proteins, and their interactions in real-time, without the need for labels or tethers. To excite plasmonic modes in DNHs fabricated on a flat thin gold film, a large-scale free-space optical setup with a well-aligned laser focus is needed. Fabricating DNHs at the fiber’s end removes the need for a bulky microscope setup, but complicates the fabrication process and loses its integrability to realize lab-on-a-chip devices. Here, an integrated DNH-based plasmonic tweezer is proposed in which the wedge plasmon polariton (WPP) modes at the cusps of a DNH are locally excited by a photonic tapered planar waveguide that is compatible with other integrated devices. Numerical results confirm the excitation of WPP modes under certain conditions and a good sensitivity of the transmitted/reflected power in the presence of trapped NPs to optically sense them. Also, comparing the optical forces calculated by Maxwell stress tensor (MST) and point-dipole approximation reveals that the proposed structure benefits from the self-induced back-action (SIBA) effect to trap NPs with lower light intensity. Moreover, due to the different excitation methods, the proposed structure can provide the opportunity to apply an electric field along the DNH’s cusp axis to analyze trapped NPs electrically, apply an electric field during optical measurements, apply dielectrophoresis forces, and integrate miniaturized parallel DNH tweezers with individual operation on a single chip. Numerical simulations give deterministic guidance and potential applications of the proposed excitation scheme to develop integrated DNH-based optical tweezers.