Tunable optical forces enhanced by plasmonic modes hybridization in optical trapping of gold nanorods with plasmonic nanocavity.

The optomechanical interaction between a plasmonic nanocavity and a gold nanorod through optical forces is demonstrated. It is revealed that strong localized plasmon resonance mode hybridization induced by a gold nanorod results in the resonance mode of the nanocavity splitting into two different plasmon resonance modes (bonding plasmon resonance mode and antibonding plasmon resonance mode). When the whole system (gold nanorod and gold nanocavity) is excited at the antibonding plasmon mode, the gold nanorod can receive an optical pushing force and be pushed away from the gold nanocavity. On the other hand, an optical pulling force acts on the gold nanorod and the gold nanorod can be trapped by the gold nanocavity when the plasmonic tweezers work at the bonding mode. The optical pulling force acting on the gold nanorod can be enhanced by two orders of magnitude larger than that of the same sized dielectric nanorod, which benefits from the strong resonant nearfield interaction between the gold nanorod and the gold nanocavity. More importantly, the shape and the position of the optical potential can be tuned by tailoring the wavelength of the laser used in the optical trapping, which can be used to manipulate the gold nanorod within a nanoscale region. Our findings have important implications for optical trapping, manipulation, sorting, and sieving of plasmonic nanoparticles with plasmonic tweezers.

Tunable Anderson localization in disorder graphene sheet arrays.

We study the electromagnetic wave propagating in coupled monolayer graphene waveguide arrays (CMGWAs). It is found that Anderson localization exists when the disorder in the coupling strength between adjacent waveguides is introduced. We find that changing the statistical parameters of the disorder coupling strength between waveguides can be used to tailor the properties of the Anderson localization modes in the strong coupling region which is beyond the coupled-mode theory. Benefiting from the electric tunable surface conductivity, we further demonstrate via the full vectorial simulation that the localization strength of the Anderson localized mode can be manipulated by changing the applied gate voltage on the CMGWAs. Our results might facilitate the manipulation of electromagnetic wave propagation in the coupled waveguide array system.

Single protein sensing with asymmetric plasmonic hexamer via Fano resonance enhanced two-photon luminescence.

Fano resonances in plasmonic systems have been proved to facilitate various sensing applications in the nanoscale. In this work, we propose an experimental scheme to realize a single protein sensing by utilizing its two-photon luminescence enhanced by a plasmonic Fano resonance system. The asymmetric gold hexamer supporting polarization-dependent Fano resonances and plasmonic modes without in-plane rotational symmetry is used as a referenced spatial coordinate for bio-sensing. We demonstrate via the full-vectorial three-dimensional simulation that the moving direction and the spatial location of a protein can be detected via its two-photon luminescence, which benefits from the resonant near-field interaction with the electromagnetic hot-spots. The sensitivity to changes in position of our method is substantially better compared with the conventional linear sensing approach. Our strategy would facilitate the sensing, tracking and imaging of a single biomolecule in deep sub-wavelength scale and with a small optical extinction cross-section.

Efficient three-photon luminescence with strong polarization dependence from a scintillating silicate glass co-doped with Gd3+ and Tb3+.

Efficient three-photon luminescence (3PL) from a scintillating silicate glass co-doped with Gd(3+) and Tb(3+) was generated by using a focused femtosecond laser beam at 800 nm. Four emission bands centered at 496, 541, 583, and 620 nm were identified as the electronic transitions between the energy levels of Tb(3+) followed by three-photon absorption (3PA) in Gd(3+) and Tb(3+) and the resonant energy transfer from Gd(3+) to Tb(3+). More interestingly, a strong polarization dependence of the 3PL was observed and it is ascribed to the polarization dependent 3PA in Gd(3+) and Tb(3+) and/or the angular distribution of photogenerated electrons in the glass.

Size dependent competition between second harmonic generation and two-photon luminescence observed in gold nanoparticles.

We investigate systematically the competition between the second harmonic generation (SHG) and two-photon-induced luminescence (TPL) that are simultaneously present in Au nanoparticles excited by using a femtosecond (fs) laser. For a large-sized (length ~ 800 nm, diameter ~ 200 nm) Au nanorod, the SHG appears to be much stronger than the TPL. However, the situation is completely reversed when the Au nanorod is fragmented into many Au nanoparticles by the fs laser. In sharp contrast, only the TPL is observed in small-sized (length ~ 40 nm, diameter ~ 10 nm) Au nanorods. When a number of the small-sized Au nanorods are optically trapped and fused into a large-sized Au cluster by focused fs laser light, the strong TPL is reduced while the weak SHG increases significantly. In both cases, the morphology change is characterized by scanning electron microscope. In addition, the modification of the scattering and absorption cross sections due to the morphology change is calculated by using the discrete dipole approximation method. It is revealed that SHG is dominant in the case when the scattering is much larger than the absorption. When the absorption becomes comparable to or larger than the scattering, the TPL increases dramatically and will eventually become dominant. Since the relative strengths of scattering and absorption depend strongly on the size of the Au nanoparticles, the competition between SHG and TPL is found to be size dependent.

Role of interfering optical fields in the trapping and melting of gold nanorods and related clusters.

We investigate the simultaneous trapping and melting of a large number of gold (Au) nanorods by using a single focused laser beam at 800 nm which is in resonance with the longitudinal surface plasmon resonance of Au nanorods. The trapping and melting processes were monitored by the two-photon luminescence of Au nanorods. A multi-ring-shaped pattern was observed in the steady state of the trapping process. In addition, optical trapping of clusters of Au nanorods in the orbits circling the focus was observed. The morphology of the structure after trapping and melting of Au nanorods was characterized by scanning electron microscope. It was revealed that Au nanorods were selectively melted in the trapping region. While Au nanorods distributed in the dark rings were completely melted, those located in the bright rings remain unmelted. The multi-ring-shaped pattern formed by the interference between the incident light and the scattered light plays an important role in the trapping and melting of Au nanorods.

Assembling of three-dimensional crystals by optical depletion force induced by a single focused laser beam.

We proposed a method to assemble microspheres into a three-dimensional crystal by utilizing the giant nonequilibrium depletion force produced by nanoparticles. Such assembling was demonstrated in a colloid formed by suitably mixing silica microspheres and magnetic nanoparticles. The giant nonequilibrium depletion force was generated by quickly driving magnetic nanoparticles out of the focusing region of a laser light through both optical force and thermophoresis. The thermophoretic binding of silica beads is so tight that a colloidal photonic crystal can be achieved after complete evaporation of solvent. This technique could be employed for fabrication of colloidal photonic crystals and molecular sieves.

All-optical switching mediated by magnetic nanoparticles.

We demonstrate the switching of light in the near-infrared region (1.55 microm) through the manipulation of magnetic nanoparticles in a magnetic fluid by using another light in the visible region (0.532 microm). The formation of a photonic gap is found in the magnetic fluid when a laser light or a magnetic field is applied. A shift of the photonic gap to longer wavelengths is observed with increasing laser power or magnetic field strength.