Efficient Microparticle Trapping with Plasmonic Annular Apertures Arrays

In this work, we demonstrate trapping of microparticles using a plasmonic tweezers based on arrays of annular apertures. The transmission spectra and the E- field distribution are simulated to calibrate the arrays. Theoretically, we observe sharp peaks in the transmission spectra for dipole resonance modes and these are redshifted as the size of the annular aperture is reduced. We also expect an absorption peak at approximately 1,115 um for the localised plasmon resonance. Using a laser frequency between the two resonances, multiple plasmonic hotspots are created and used to trap and transport micron and submicron particles. Experimentally, we demonstrate trapping of individual 0.5 um and 1 um polystyrene particles and particle transportation over the surface of the annular apertures using less than 1.5 mW/um2 incident laser intensity at 980 nm

Optical nanofiber-based cavity induced by periodic air-nanohole arrays

We experimentally realized an optical nanofiber-based cavity by combining a 1-D photonic crystal and Bragg grating structures. The cavity morphology comprises a periodic, triplex air-cube introduced at the waist of the nanofiber. The cavity has been theoretically characterized using FDTD simulations to obtain the reflection and transmission spectra. We have also experimentally measured the transmission spectra and a Q-factor of ~784(87) for a very short periodic structure has been observed. The structure provides strong confinement of the cavity field and its potential for optical network integration makes it an ideal candidate for use in nanophotonic and quantum information systems

Efficient microparticle trapping with plasmonic annular apertures arrays

In this work, we demonstrate trapping of microparticles using plasmonic tweezers based on arrays of annular apertures. The transmission spectra and the electric-field distribution are simulated to calibrate the arrays. Theoretically, we observe sharp peaks in the transmission spectra for dipole resonance modes and these are red-shifted as the size of the annular aperture is reduced. We also expect an absorption peak at approximately 1115 m for the localised plasmon resonance. Using a laser frequency between the two resonances, multiple plasmonic hot spots are created and used to trap and transport micron and submicron particles. Experimentally, we demonstrate trapping of individual 0.5 μm and 1 μm polystyrene particles and the feasibility of particle transportation over the surface of the annular apertures using less than 1.5 mW μm⁻² incident laser intensity at 980 nm

Fano-Resonant, Asymmetric, Metamaterial-Assisted Tweezers for Single Nanoparticle Trapping

Plasmonic nanostructures can overcome Abbe's diffraction limit to generate strong gradient fields, enabling efficient optical trapping of nano-sized particles. However, it remains challenging to achieve stable trapping with low incident laser intensity. Here, we demonstrate a Fano resonance-assisted plasmonic optical tweezers (FAPOT), for single nanoparticle trapping in an array of asymmetrical split nano-apertures, milled on a 50 nm gold thin film. Stable trapping is achieved by tuning the trapping wavelength and varying the incident trapping laser intensity. A very large normalized trap stiffness of 8.65 fN/nm/mW for 20 nm polystyrene particles at a near-resonance trapping wavelength of 930 nm was achieved. We show that trap stiffness on resonance is enhanced by a factor of 63 compared to off-resonance conditions. This can be attributed to the ultra-small mode volume, which enables large near-field strengths and a cavity Purcell effect contribution. These results should facilitate strong trapping with low incident trapping laser intensity, thereby providing new options for studying transition paths of single molecules, such as proteins, DNA, or viruses.Comment: 28 pages, 7 figure

Chiral force of guided light on an atom

We calculate the force of a near-resonant guided light field of an ultrathin optical fiber on a two-level atom. We show that, if the atomic dipole rotates in the meridional plane, the magnitude of the force of the guided light depends on the field propagation direction. The chirality of the force arises as a consequence of the directional dependencies of the Rabi frequency of the guided driving field and the spontaneous emission from the atom. This provides a unique method for controlling atomic motion in the vicinity of an ultrathin fiber.Comment: text and figures were revised, and a new discussion was adde

Asymmetric split-ring plasmonic nanostructures for optical sensing of Escherichia coli

Strategies for in-liquid micro-organism detection are crucial for the clinical and pharmaceutical industries. While Raman spectroscopy is a promising label-free technique for micro-organism detection, it remains challenging due to the weak bacterial Raman signals. In this work, we exploit the unique electromagnetic properties of metamaterials to identify bacterial components in liquid using an array of Fano-resonant metamolecules. This Fano-enhanced Raman scattering (FERS) platform is designed to exhibit a Fano resonance close to the protein amide group fingerprint around 6030 nm. Raman signatures of Escherichia coli were recorded at several locations on the metamaterial under off-resonance laser excitation at 530 nm, where the photodamage effect is minimized. As the sizes of the Escherichia coli are comparable to the micro-gaps, i.e 0.41 {\mu}m, of the metamaterials, its local immobilisation leads to an increase in the Raman sensitivity. We also observed that the time-dependent FERS signal related to bacterial amide peaks increased during the bacteria's mid-exponential phase while it decreased during the stationary phase. This work provides a new set of opportunities for developing ultrasensitive FERS platforms suitable for large-scale applications and could be particularly useful for diagnostics and environmental studies at off-resonance excitation.Comment: 15 pages, 5 figure