Effect of symmetry breaking on localized and delocalized surface plasmons in monolayer hexagonal-close-packed metallic truncated nanoshells

Gold cauldrons as efficient candidates for plasmonic tweezers

In this report, gold cauldrons are proposed and proved as efficient candidates for plasmonic tweezers. Gold cauldrons benefit from high field localization in the vicinity of their apertures, leading to particle trapping by a reasonably low power source. The plasmonic trapping capability of a single gold cauldron and a cauldrons cluster are studied by investigating the plasmon-induced variations of the optical trap stiffness in a conventional optical tweezers configuration. This study shows that the localized plasmonic fields and the consequent plasmonic forces lead to enhanced trap stiffness in the vicinity of the cauldrons. This observation is pronounced for the cauldrons cluster, due to the additive plasmonic fields of the neighboring cauldrons. Strong direct plasmonic tweezing by the gold cauldrons cluster is also investigated and confirmed by our simulations and experimental results. In addition to the presented plasmonic trapping behavior, gold cauldrons benefit from a low cost and simple fabrication process with acceptable controllability over the structural average dimensions and plasmonic behavior, making them attractive for emerging lab-on-a-chip optophoresis applications.

Dye-doped spheres with plasmonic semi-shells: Lasing modes and scattering at realistic gain levels

We numerically simulate the compensation of absorption, the near-field enhancement as well as the differential far-field scattering cross section for dye-doped polystyrene spheres (radius 195 nm), which are half-covered by a silver layer of 10-40 nm thickness. Such silver capped spheres are interesting candidates for nanoplasmonic lasers, so-called spasers. We find that spasing requires gain levels less than 3.7 times higher than those in commercially available dye-doped spheres. However, commercially available concentrations are already apt to achieve negative absorption, and to narrow and enhance scattering by higher order modes. Narrowing of the plasmonic modes by gain also makes visible higher order modes, which are normally obscured by the broad spectral features of the lower order modes. We further show that the angular distribution of the far-field scattering of the spasing modes is by no means dipole-like and is very sensitive to the geometry of the structure.

Dual broadband near-infrared perfect absorber based on a hybrid plasmonic-photonic microstructure

High performance light absorber with a broad bandwidth is particularly desirable for near-infrared photodetection and optical interconnects. Here we demonstrate a dual broadband perfect absorber in the near-infrared regime, which is based on a hybrid plasmonic-photonic microstructure. Such a microstructure is fabricated by self-assembling a monolayer colloidal crystal on an optically opaque metal film followed by depositing a thin metallic half-shell on the top of the colloidal particles. Both experimental and numerical simulation results show that the simply designed absorbers have dual broadband with absorption exceeding 90% in the near-infrared region with the absorption bands being scalable by tuning the size of the colloidal particles. Moreover, the absorption efficiency shows an extremely slight dispersion for the incident angles up to 50 degrees, benefit from the high symmetry as well as the highly modulated plasmonic microstructures that lead to a weak polarization dependence of these two absorption bands. The relative ease of growing high-quality colloidal crystals and the low cost of fabricating such plasmonic-photonic microstructures with high reproducibility could promise applicability of the light absorber in the field of photodetectors, thermal emitters and photovoltaics.

Multiple Fano resonances in monolayer hexagonal non-close-packed metallic shells

In this study, we first numerically investigate the appearance and properties of multiple Fano resonances in two-dimensional hexagonal non-close-packed arrays of symmetric metallic shells. The coexistence of broad sphere-like plasmon modes formed from the near-field interaction between the individual sphere plasmons and substantially narrower void plasmon modes supported by the inner surface of the individual shell resonant over the same range of energies can produce such Fano resonances. In particular, void and sphere-like plasmon modes of different angular momentum could directly interact without the need of symmetry breaking in the structure. A cost-effective colloidal crystal templating method is utilized to prepare the arrays of the metallic shells with small openings. The effect of the symmetry breaking on the Fano resonances in metallic cup arrays is experimentally and numerically investigated. Further tunability on the Fano resonances is gained by changing the size of the inner dielectric core, hence changing the moment of the void plasmon modes and consequently the resonance frequency. By adopting the polymer dielectric core with gain materials, our study may offer realizable experimental opportunities towards subwavelength low threshold plasmonic lasing.

Optical transmission of corrugated metal films on a two-dimensional hetero-colloidal crystal

The near infrared transmission of corrugated metal films deposited on hetero-colloidal crystals is investigated. The transmission response of the quasi-three-dimensional (quasi-3D) metal film is modified by controlling the nominal thickness of a dielectric layer pre-deposited on the top surface of the colloidal crystal to form a new hetero-colloidal crystal. An extraordinary optical transmission (EOT) phenomenon could be presented in such metallodielectric (MD) architectures. We have found that the main transmission peak is suppressed as the thickness of the intercalated dielectric layer is increased. We propose that the observed EOT is a result of constructive interference between a localized sphere-like plasmon mode and an index-guided eigen mode mainly confined in the colloidal crystal, which is confirmed by our numerical simulations. Based on the MD microstructures, a distinct plasmon sensitivity response difference is achieved, which indicates potential applications for biochemical sensing.

A stretch-tunable plasmonic structure with a polarization-dependent response

We experimentally demonstrate a stretchable plasmonic structure composed of a monolayer array of gold semishells with dielectric cores on an elastic PDMS substrate. The composite structure is fabricated using simple and inexpensive self-assembly and transfer-printing techniques, and it supports Bragg-type surface plasmon resonances whose frequencies are sensitive to the arrangement of the metallic semishells. Under uniaxial stretching, the lattice symmetry of this plasmonic structure can be reconfigured from hexagonal to monoclinic, leading to resonance frequency shifts from 200 THz to 191 THz for the TM polarization and from 200 THz to 198 THz for the TE polarization with a strain up to 20%, respectively. Compared with previously reported tunable plasmonic structures, the reconfiguration of lattice symmetry offers a promising approach to tune the surface plasmon resonance with a polarization-dependent response at the standard telecommunication band, and such tunable plasmonic structure might be exploited in realizing photonic devices such as sensors, switches and filters.