Luping Du

Focused plasmonic trapping of metallic particles

Scattering forces in focused light beams push away metallic particles. Thus, trapping metallic particles with conventional optical tweezers, especially those of Mie particle size, is difficult. Here we investigate a mechanism by which metallic particles are attracted and trapped by plasmonic tweezers when surface plasmons are excited and focused by a radially polarized beam in a high-numerical-aperture microscopic configuration. This contrasts the repulsion exerted in optical tweezers with the same configuration. We believe that different types of forces exerted on particles are responsible for this contrary trapping behaviour. Further, trapping with plasmonic tweezers is found not to be due to a gradient force balancing an opposing scattering force but results from the sum of both gradient and scattering forces acting in the same direction established by the strong coupling between the metallic particle and the highly focused plasmonic field. Theoretical analysis and simulations yield good agreement with experimental results.

Mapping plasmonic near-field profiles and interferences by surface-enhanced Raman scattering

Mapping near-field profiles and dynamics of surface plasmon polaritons is crucial for understanding their fundamental optical properties and designing miniaturized photonic devices. This requires a spatial resolution on the sub-wavelength scale because the effective polariton wavelength is shorter than free-space excitation wavelengths. Here by combining total internal reflection excitation with surface-enhanced Raman scattering imaging, we mapped at the sub-wavelength scale the spatial distribution of the dominant perpendicular component of surface plasmon fields in a metal nanoparticle-film system through spectrally selective and polarization-resolved excitation of the vertical gap mode. The lateral field-extension at the junction, which is determined by the gap-mode volume, is small enough to distinguish a spot size ~0.355λ0 generated by a focused radially polarized beam with high reproducibility. The same excitation and imaging schemes are also used to trace near-field nano-focusing and interferences of surface plasmon polaritons created by a variety of plasmon lenses.

On-chip photonic Fourier transform with surface plasmon polaritons

The Fourier transform (FT), a cornerstone of optical processing, enables rapid evaluation of fundamental mathematical operations, such as derivatives and integrals. Conventionally, a converging lens performs an optical FT in free space when light passes through it. The speed of the transformation is limited by the thickness and the focal length of the lens. By using the wave nature of surface plasmon polaritons (SPPs), here we demonstrate that the FT can be implemented in a planar configuration with a minimal propagation distance of around 10 μm, resulting in an increase of speed by four to five orders of magnitude. The photonic FT was tested by synthesizing intricate SPP waves with their Fourier components. The reduced dimensionality in the minuscule device allows the future development of an ultrafast on-chip photonic information processing platform for large-scale optical computing.

Emission pattern of surface-enhanced Raman scattering from single nanoparticle-film junction

Emission pattern of surface-enhanced Raman scattering (SERS) from the junction of single nanoparticle and a metal film was experimentally demonstrated. The presence of a thin metal film enables the excitation of surface plasmon polaritons (SPPs) to greatly improve the excitation efficiency of SERS, which is subsequently coupled back to SPPs and re-radiates into the substrate side with higher refractive index at SPP excitation angle. The so-called surface plasmon coupled emission can serve as a high sensitivity detection tool for SERS and particularly for the tip-enhanced Raman spectroscopy.

Perfect optical vortex enhanced surface plasmon excitation for plasmonic structured illumination microscopy imaging

We propose an all-optical technique for plasmonic structured illumination microscopy (PSIM) with perfect optical vortex (POV). POV can improve the efficiency of the excitation of surface plasma and reduce the background noise of the excited fluorescence. The plasmonic standing wave patterns are excited by POV with fractional topological charges for accurate phase shift of {−2π/3, 0, and 2π/3}. The imaging resolution of less than 200 nm was produced. This PSIM technique is expected to be used as a wide field, super resolution imaging technique in dynamic biological imaging.

Dynamic plasmonic tweezers enabled single-particle-film-system gap-mode Surface-enhanced Raman scattering

Based on numerical simulation and experiment, we demonstrate a dynamic single-particle-film Surface-enhanced Raman scattering (SERS) system enabled by manipulation of a single gold nanoparticle by plasmonic nano-tweezers (PNT). A corresponding dynamic plasmonic gap-mode is induced by the hybridization of the surface plasmon polaritons (SPPs) on the film and the localized surface plasmon of the particle. This gap-mode produces an additional enhancement of ∼104 compared to the bare SPPs without the particle, reaching a final SERS enhancement factor of ∼109. Enabled by nano-manipulation with PNT, this dynamic single-particle-film-system provides a promising route to controllable SERS detection in aqueous environments.

Mode-matching metasurfaces: coherent reconstruction and multiplexing of surface waves

Metasurfaces are promising two-dimensional metamaterials that are engineered to provide unique properties or functionalities absent in naturally occurring homogeneous surfaces. Here, we report a type of metasurface for tailored reconstruction of surface plasmon waves from light. The design is based on an array of slit antennas arranged in a way that it matches the complex field distribution of the desired surface plasmon wave. The approach is generic so that one can readily create more intricate designs that selectively generate different surface plasmon waves through simple variation of the wavelength or the polarization state of incident light. The ultra-thin metasurface demonstrated in this paper provides a versatile interface between the conventional free-space optics and a two-dimensional platform such as surface plasmonics.

Diffraction-Free Bloch Surface Waves

Here, we demonstrate a diffraction-free Bloch surface wave sustained on all-dielectric multilayers that does not diffract after being passed through three obstacles or across a single mode fiber. It can propagate in a straight line for distances longer than 110 μm at a wavelength of 633 nm and could be applied as an in-plane optical virtual probe both in air and in an aqueous environment. Its ability to be used in water, its long diffraction-free distance, and its tolerance to multiple obstacles make this wave ideal for certain applications in areas such as the biological sciences, where many measurements are made on glass surfaces or for which an aqueous environment is required, and for high-speed interconnections between chips, where low loss is necessary.

Generation and detection of broadband multi-channel orbital angular momentum by micrometer-scale meta-reflectarray

We theoretically demonstrate the generation and detection of broadband multi-channel Orbital Angular Momentum(OAM) by a micrometer-scale meta-reflectarray. The meta-reflectarray composed of patterned silicon bars on a silver ground plane can be designed to realize phase modulation and work as chip-level OAM devices. Compared to traditional methods of OAM generation and detection, our approach shows superiorities of very compact structure size, broadband working wavelength (1250-1750 nm), high diffraction efficiency (~70%), simultaneously handling multiplex OAMs, and tunable reflection angle (0-45°). These fascinating advantages provides great potential applications in photonic integrated devices and systems for high-capacity and multi-channel OAM communication.

Detection of microscope-excited surface plasmon polaritons with Rayleigh scattering from metal nanoparticles

We propose a mapping tool of surface plasmon polaritons (SPPs) excited using an optical microscope. By combining dark-field and confocal microscopy, we can efficiently extract metal nanoparticle-induced Rayleigh scattering from background radiation, thereby leading to state-of-the-art SPP measurements. The method is verified to be sensitive to the dominant perpendicular field component of SPPs and be of high accuracy. We also use this method to reveal the conversion of spin angular momentum of light to the orbital angular momentum of SPPs under tight-focusing conditions.