Polarization-Independent Multiple Fano Resonances in Plasmonic Nonamers for Multimode-Matching Enhanced Multiband Second-Harmonic Generation.

Plasmonic oligomers composed of metallic nanoparticles are one class of the most promising platforms for generating Fano resonances with unprecedented optical properties for enhancing various linear and nonlinear optical processes. For efficient generation of second-harmonic emissions at multiple wavelength bands, it is critical to design a plasmonic oligomer concurrently having multiple Fano resonances spectrally matching the fundamental excitation wavelengths and multiple plasmon resonance modes coinciding with the harmonic wavelengths. Thus far, the realization of such a plasmonic oligomer remains a challenge. This study demonstrates both theoretically and experimentally that a plasmonic nonamer consisting of a gold nanocross surrounded by eight nanorods simultaneously sustains multiple polarization-independent Fano resonances in the near-infrared region and several higher-order plasmon resonances in the visible spectrum. Due to coherent amplification of the nonlinear excitation sources by the Fano resonances and efficient scattering-enhanced outcoupling by the higher-order modes, the second-harmonic emission of the nonamer is significantly increased at multiple spectral bands, and their spectral positions and radiation patterns can be flexibly manipulated by easily tuning the length of the surrounding nanorods in the nonamer. These results provide us with important implications for realizing ultrafast multichannel nonlinear optoelectronic devices.

Surface Plasmon Polariton Cross-Coupling Enhanced Forward Emission from Insulator-Metal-Capped ZnO Films.

Increasing light extraction efficiency in the forward direction is being extensively pursued due to its crucial role in realizing top-emitting organic and inorganic light emitting devices. Various surface plasmon polariton (SPP)-based strategies for emission enhancement and light extraction have been developed to improve the top-emitting efficiency of these devices. However, the role of surface roughness of both semiconductor film and metal electrode in improving the emission efficiency of a practical device has not been thoroughly studied yet. In this work, the influence of surface roughness of a top metal electrode on the photoluminescence enhancement of a ZnO thin film is investigated experimentally and numerically based on an insulator-metal-semiconductor system. It is found that the generic surface roughness of the metal electrode plays an encouraging role in increasing the forward-emission intensity by facilitating cross-coupling of SPPs on the two opposite sides of the metal layer. More importantly, the forward emission can be further enhanced by capping a high-index polymer layer on the metal electrode to bridge the momentum mismatch between the two SPPs modes. The experimental observations are well explained by the SPPs cross-coupling mechanism that models the radiation power of a dipolar emitter underneath the metal electrode as a function of the metal surface roughness. Our work opens up the possibility of using cross-coupling of SPPs as an effective means to fabricate high-brightness top-emitting devices without the need of complicated nanoscale patterning.

Dressing plasmon resonance with particle-microcavity architecture for efficient nano-optical trapping and sensing.

We propose a particle-microcavity scheme for efficient optical trapping and sensing. When a resonant plasmonic nanoparticle (NP) is placed inside a microcavity with high Q-factor, sensitivity is enhanced in the far-field extinction while near-field around the NP is barely affected. Stable near-field and high sensitivity for optical trapping and ultrasensitive detection of nanosized targets are therefore realized simultaneously. Such a particle-microcavity system opens up a new hybrid nanophotonic device platform that combines the unique merits of conventional and plasmonic integrated photonics.

Interplay between absorption and radiative decay rates of surface plasmon polaritons for field enhancement in periodic arrays.

We studied the effects of absorption and radiative decay rates of surface plasmon polaritons on the field enhancement in periodic metallic arrays by temporal coupled mode theory and finite-difference time-domain simulation. When two rates are equal, the field enhancement is the strongest and the peak height of the orthogonal reflectivity reaches 0.25. To demonstrate this fact, we fabricated two series of two-dimensional Au and Ag nanohole arrays with different geometries and measured their corresponding reflectivity and decay rates. The experimental results agree well with the analytical and numerical results.

Determination of absorption and radiative decay rates of surface plasmon polaritons from nanohole array.

We have devised a simple method to determine the absorption and radiative decay rates of surface plasmon polaritons in an Au nanohole array by combining polarization-resolved reflectivity spectroscopy and temporal coupled-mode theory. The dependence of two rates on wavelength has been measured and they are found to agree with finite-difference time-domain simulations. As both absorption and radiative decay rates play a key role in several plasmonic applications, our approach offers a simple and effective means in determining them.

Decay rates modification through coupling of degenerate surface plasmon modes.

We measured the decay rates of two degenerate surface plasmon modes in Au nanohole arrays with different hole sizes by angle-resolved reflectivity spectroscopy. For each hole size, at the spectral region where resonant coupling occurs, we observed a large modification in decay rates, leading to the formation of dark and bright modes. The change in decay rates is well explained by temporal coupled mode theory. The deduced coupling constant is found to increase with increasing hole diameter. This study provides us a simple and effective means to control the decay rates of dark and bright modes, which are useful in plasmonic applications.

Numerical and analytical evaluations of the sensing sensitivity of waveguide mode in one-dimensional metallic gratings.

We study numerically and analytically the refractive index sensing sensitivities of surface plasmon (S(SP)) and waveguide (S(WG)) modes arising from one-dimensional Au gratings. By using rigorous coupled wave analysis, we find that while S(SP) is mainly controlled by the periodicity of the grating, the shape of the groove governs S(WG). As a result, it is possible to increase S(WG) to 1000 nm/RIU and figure of merit to 24 by tailoring the height and width of the groove. Finally, a simple analytical expression is derived to describe S(WG) and it agrees well with the numerical data. This easy-to-use expression not only reveals the origin of waveguide mode sensitivity, but also provides useful guidance for the theoretical design and experimental realization of high-sensitivity metallic-gratings-based biosensors.

Plasmonic optical trap having very large active volume realized with nano-ring structure.

The feasibility of using gold nano-rings as plasmonic nano-optical tweezers is investigated. We found that at a resonant wavelength of λ=785 nm, the nano-ring produces a maximum trapping potential of ~32k(B)T on gold nanoparticles. The existence of multiple potential wells results in a very large active volume of ~10(6) nm(3) for trapping the target particles. The report nano-ring design provides an effective approach for manipulating nano-objects in very low concentration into the high-field region and is well suited for integration with microfluidics for lab-on-a-chip applications.