Fano-resonant aluminum and gold nanostructures created with a tunable, up-scalable process

An ultranarrow SPR linewidth in the UV region for plasmonic sensing

Conventional surface plasmon resonance (SPR) modes based on gold and silver nanostructures only operate in the visible and near-infrared (NIR) regions. Nowadays, with the rapid development of strong coupling between molecules and plasmonic nanostructures and surface enhanced spectroscopy, it is highly desired to modulate the SPR modes with a narrow linewidth toward the ultraviolet (UV) wavelength region through a low cost and reproducible fabrication method. Herein, laser interference lithography is utilized to manufacture stable Al plasmonic arrays with well-controlled and tunable geometries. Importantly, an ultranarrow linewidth of SPR modes as narrow as 14 nm has been successfully obtained in the near UV region. The fabricated Al plasmonic arrays show a high sensitivity toward 485 nm RIU^-1 when it is used as a refractive index sensor. The results reported here make a valuable extension of plasmonic resonant modes spanning visible and NIR into the UV region, and it may provide a robust way to achieve alternative plasmonic materials for plasmon-enhanced molecular sensing, plasmonic nanolasers, non-linear optics, strong coupling and surface enhanced spectroscopy in the UV regions.

Lateral sorting of chiral nanoparticles using Fano-enhanced chiral force in visible region

Chiral gradient force allows a passive separation of an enantiomer since its direction is dependent on the handedness of its chiral entities. However, chiral polarisability is much weaker than electric polarisability. As a consequence, the non-chiral gradient force dominates over chiral force, which makes enantioselective sorting challenging. We present here, both numerically and analytically, that the chiral gradient force acting on chiral nanoparticles can overcome the non-chiral force when specimens are placed in a Fano-enhanced chiral gradient near-field using a plasmonic nanoaperture. Under circularly polarized light illumination, the interaction between the resonant modes of symmetric outer and asymmetric inner Au split-rings results in a splitting of the modal energies, which excites multipolar interference Fano resonances (FRs). This enables a local aperture between the two split-rings to possess very large optical chirality gradients while maintaining low gradients of electromagnetic energy density around the FRs from the visible region. By way of the lateral resultant force composed of both chiral and non-chiral gradient forces, we can accomplish a helicity-dependent transverse deflection of the chiral nanoparticles positioned above the aperture, which may offer a good platform for all-optical enantiopure compounds.

Plasmonic plano-semi-cylindrical nanocavities with high-efficiency local-field confinement

Plasmonic nanocavity arrays were achieved by producing isolated silver semi-cylindrical nanoshells periodically on a continuous planar gold film. Hybridization between localized surface plasmon resonance (LSPR) in the Ag semi-cylindrical nanoshells (SCNS) and surface plasmon polaritons (SPP) in the gold film was observed as split bonding and anti-bonding resonance modes located at different spectral positions. This led to strong local field enhancement and confinement in the plano-concave nanocavites. Narrow-band optical extinction with an amplitude as high as 1.5 OD, corresponding to 97% reduction in the transmission, was achieved in the visible spectrum. The resonance spectra of this hybrid device can be extended from the visible to the near infrared by adjusting the structural parameters.

Fano resonant Ge2Sb2Te5 nanoparticles realize switchable lateral optical force

Sophisticated optical micromanipulation of small biomolecules usually relies on complex light, e.g., structured light, highly non-paraxial light, or chiral light. One emerging technique is to employ chiral light to drive the chiral nanoparticle along the direction perpendicular to the propagation of the light, i.e., the lateral optical force. Here, we theoretically study the lateral optical force exerted by a entirely Gaussian beam. For the very first time we demonstrate that the Fano resonances (FRs) of the Ge2Sb2Te5 (GST) phase-change nanoparticles encapsulated with Au shells could enable a conventional Gaussian laser to exert a lateral force on such a dielectric GST nanoparticle, attributed to the strongly asymmetric energy flow around the sphere in the dipole-quadrupole FRs. More interestingly, the direction of this lateral force could be reversible during the state transition (i.e., from amorphous to crystalline). By bonding small biomolecules to the outer surface of the phase-change nanoparticle, the particle behaves as a direction-selective vehicle to transport biomolecules along opposite directions, at pre-assessed states of the Ge2Sb2Te5 core correspondingly. Importantly, the origin of the reversal of the lateral optical force is further unveiled by the optical singularity of the Poynting vector. Our mechanism of tailoring the FRs of phase-change nanoparticles, not just limited to GST, may bring a new twist to optical micromanipulation and biomedical applications.