Siwei Zhu

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.

Surface plasmon resonance imaging of cell-substrate contacts with radially polarized beams

We demonstrate the proof-of-concept for surface plasmon resonance sensing and imaging via a virtual probe at the cell-substrate interface of a biological cell in aqueous media. The technique is based on the optical excitation by focused radially polarized beams of localized surface plasmons, which forms a virtual probe on the metal substrate. The intensity distribution at the back focal plane of the objective lens enables quantitative measurements to be made of the cell-substrate contact. The acquired data is then visualized in the form of a local refractive index map.

Large-scale optical traps on a chip for optical sorting

The authors present a power-efficient large-scale lensless optical traps on a chip (OTOCs) as an optofluidic element for optical sorting of microparticles. Based on the well-known Talbot self-imaging effect in the Fresnel region, the OTOC makes use of a two-dimensional microfabricated chessboardlike structure to create an optical lattice near its emergent plane. Simultaneous trapping of hundreds of microparticles in a regular array (>200×200μm2) is proved experimentally without adopting an external optical projection lens configuration. Furthermore, the authors demonstrate experimental results for large-scale sorting of microparticles by sizes using the OTOC.

Plasmonic Hybridization Induced Trapping and Manipulation of a Single Au Nanowire on a Metallic Surface

Hybridization in the narrow gaps between the surface plasmon polaritons (SPPs) along a metal surface and the localized surface plasmons on metallic nano-objects strongly enhance the electromagnetic field. Here, we employ plasmonic hybridization to achieve dynamic trapping and manipulation of a single metallic nanowire on a flat metal surface. We reveal that the plasmonic hybridization achieved by exciting plasmonic tweezers with a linearly polarized laser beam could induce strong trapping forces and large rotational torques on a single metallic nanowire. The position and orientation of the nanowire could dynamically be controlled by the hybridization-enhanced nonisotropic electric field in the gap. Experimental results further verify that a single Au nanowire could robustly be trapped at the center of an excited SPP field by the induced forces and then rotated by the torques. Finally, a plasmonic swallow tail structure is built to demonstrate its potential in the fabrication of lab-on-a-chip plasmonic devices.

Experimental approach to the microscopic phase-sensitive surface plasmon resonance biosensor

We designed and proposed a microscopic configuration of wide-dynamic-range phase-sensitive surface plasmon resonance biosensor based on differential interferometry between focused radially polarized and azimuthally polarized cylindrical vector beams recently (R. Wang et al., Opt. Lett. 37, 2091 (2012)). In this Letter, we follow the simulation results up with experimental verifications with a sensitivity of 7.385 × 10−7refractive index unit (RIU)/0.1°. It is also verified that the dynamic range of this system could be as large as 0.35 RIU in principle, which is only confined by numerical aperture of the microscopic objective lens.

Focused cylindrical vector beam assisted microscopic pSPR biosensor with an ultra wide dynamic range

A novel phase-sensitive surface plasmon resonance (pSPR) biosensor based on differential phase measurement between two cylindrical vector beams, namely radially polarized and azmuthally polarized beams, is proposed and studied in an inverted microscope. Different from a fixed angle or a relatively small angular range for SPR excitation in the attenuated total reflection (ATR) configuration, the signal beam focused by a total internal reflection fluorescence microscopic objective contains the entire angular range from 0 to the maximum angle given by the numerical aperture, leading to a dynamic range of 0.41 RIU which is over seven times wider than the best result of the ATR pSPR sensor. Moreover, with the technique of differential phase measurement between radial and azimuthal polarizations employed in our configuration, high sensitivity of ±9.05×10(-8) refractive index unit/1 deg can simultaneously be achieved in principle. The proposed technique maintains the unique advantages in terms of securing high imaging resolution and sensitivity with an ultra-wide dynamic range simultaneously.

Image edge enhancement in optical microscopy with a Bessel-like amplitude modulated spiral phase filter

We experimentally demonstrate that a Bessel-like amplitude modulated spiral phase filter can be used in a real-time spatial image edge enhancement system in optical microscopy for biological sample imaging. Compared with previous methods based on a conventional spiral phase filter, a dark-field spiral phase filter and the Laguerre–Gaussian modulated spiral phase filter, the proposed technique further reduces the imaging diffraction noise. Experimental verifications in edge enhancement are implemented by a phase-only spatial light modulator for realizing the amplitude modulated spiral phase. It is shown that the proposed technique is able to efficiently suppress the diffraction noise and achieve high quality edge enhancement images for biological samples.

A modified fractal zone plate with extended depth of focus in spectral domain optical coherence tomography

In optical coherence tomography (OCT) systems, there is a trade-off between the depth of focus (DOF) and lateral resolution when conventional lenses are used. We propose a new method that employs a modified fractal generalized zone plate (MFraGZP) combined with a conventional lens to improve the trade-off effect, with an extended DOF of about 2.5 mm (14 times larger) while the lateral resolution is maintained at ~9.5 μm. As an example, images of the calibration microspheres are obtained and demonstrated with the extended DOF in a spectral domain OCT system.

Quantitative analysis of rectal cancer by spectral domain optical coherence tomography

To quantify OCT images of rectal tissue for clinic diagnosis, the scattering coefficient of the tissue is extracted by curve fitting the OCT signals to a confocal single model. A total of 1000 measurements (half and half of normal and malignant tissues) were obtained from 16 recta. The normal rectal tissue has a larger scattering coefficient ranging from 1.09 to 5.41 mm⁻¹ with a mean value of 2.29 mm⁻¹ (std:±0.32), while the malignant group shows lower scattering property and the values ranging from 0.25 to 2.69 mm⁻¹ with a mean value of 1.41 mm⁻¹ (std:±0.18). The peri-cancer of recta has also been investigated to distinguish the difference between normal and malignant rectal tissue. The results demonstrate that the quantitative analysis of the rectal tissue can be used as a promising diagnostic criterion of early rectal cancer, which has great value for clinical medical applications.

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.