Second harmonic generation in gallium phosphide nano-waveguides.

We designed, fabricated and tested gallium phosphide (GaP) nano-waveguides for second harmonic generation (SHG). We demonstrate SHG in the visible range around 655 nm using modal phase matching. We observe phase matched SHG for different combinations of interacting modes by varying the widths of the waveguides and tuning the wavelength of the pump. We achieved a normalized internal SHG conversion efficiency of 0.4% W-1cm-2 for a continuous-wave pump at wavelength of 1283.5 nm, the highest reported in the literature for a GaP waveguide. We also demonstrated temperature tuning of the SHG wavelength with a slope of 0.17 nm/°C. The presented results contribute to the development of integrated photonic platforms with efficient nonlinear wave-mixing processes for classical and quantum applications.

Heterophase fcc-2H-fcc gold nanorods.

The crystal phase-based heterostructures of noble metal nanomaterials are of great research interest for various applications, such as plasmonics and catalysis. However, the synthesis of unusual crystal phases of noble metals still remains a great challenge, making the construction of heterophase noble metal nanostructures difficult. Here, we report a one-pot wet-chemical synthesis of well-defined heterophase fcc-2H-fcc gold nanorods (fcc: face-centred cubic; 2H: hexagonal close-packed with stacking sequence of "AB") at mild conditions. Single particle-level experiments and theoretical investigations reveal that the heterophase gold nanorods demonstrate a distinct optical property compared to that of the conventional fcc gold nanorods. Moreover, the heterophase gold nanorods possess superior electrocatalytic activity for the carbon dioxide reduction reaction over their fcc counterparts under ambient conditions. First-principles calculations suggest that the boosted catalytic performance stems from the energetically favourable adsorption of reaction intermediates, endowed by the unique heterophase characteristic of gold nanorods.

Optical resonances in Kretschmann and Otto configurations.

This Letter revises the resonance theory of Kretschmann and Otto configurations, where enhanced electromagnetic fields and strong reflectance dips observed under total internal reflection are conventionally attributed to the resonance on surface plasmon-polaritons, the subsystem eigenwaves supported by a single metal/dielectric interface. The rigorous mathematical analysis demonstrates that the field enhancement and reflection decrease, in fact, are two independent physical phenomena that occur under different conditions. Moreover, neither of them coincides with the resonance on surface plasmon-polaritons that manifests in Kretschmann and Otto configurations through other distinct optical effect.

Highly efficient tunable and localized on-chip electrical plasmon source using protruded metal-insulator-metal structure.

A compact and highly efficient tunable and localized source of propagating surface plasmon-polaritons is proposed based on a protruded metal-insulator-metal (pMIM) structure. The protrusion along a segment of the pMIM forms a nanometer gap and allows a low voltage bias to generate a localized tunneling current. The tunneling current excited plasmons can be fully coupled to the metal-insulator-metal (MIM) waveguiding segment of the pMIM without leakage and propagate a long distance as the gap in the MIM waveguiding segment is much larger than the gap in the protruded segment of the pMIM. Eigenmode and numerical analyses show that by using MIM structures as a benchmark, the pMIM structure enhances the total amount of average power that is transferred from the tunneling current into the excitation of intrinsic eigenmodes of the MIM waveguiding segment. Depending on the magnitude of the applied voltage bias, the pMIM structure supports single, dual and multi modes for a typical Au-SiO2-Au design with a 500 nm-thick SiO2. Among all excited modes, the single mode operation allows highly efficient excitation of long travelling surface plasmon-polaritons (SPPs) of up to 30 µm. The electrical excitation of SPPs using pMIM structures opens up the possibility of integrating plasmon sources into nanoscale optoelectronic circuits to facilitate on-chip data communications.

Stabilization of 4H hexagonal phase in gold nanoribbons.

Gold, silver, platinum and palladium typically crystallize with the face-centred cubic structure. Here we report the high-yield solution synthesis of gold nanoribbons in the 4H hexagonal polytype, a previously unreported metastable phase of gold. These gold nanoribbons undergo a phase transition from the original 4H hexagonal to face-centred cubic structure on ligand exchange under ambient conditions. Using monochromated electron energy-loss spectroscopy, the strong infrared plasmon absorption of single 4H gold nanoribbons is observed. Furthermore, the 4H hexagonal phases of silver, palladium and platinum can be readily stabilized through direct epitaxial growth of these metals on the 4H gold nanoribbon surface. Our findings may open up new strategies for the crystal phase-controlled synthesis of advanced noble metal nanomaterials.

Photoluminescence via gap plasmons between single silver nanowires and a thin gold film.

In this work, we investigate the one-photon photoluminescence from a system consisting of an Ag nanowire on an Au film with a ~6 nm dielectric spacer that supports a localized resonance known as the gap plasmon mode. Although the Ag nanowire and Au film exhibit weak photoluminescence on their own, the combined system produces an enhanced PL emission. Our analysis reveals that the strong PL emission in this system was due to the enhanced laser absorption in, and PL emission from Au, with the Ag nanowire acting as an efficient antenna. The PL due to the gap mode is sensitive to the polarization of the laser excitation with respect to the nanowire orientation, and shows a clear dipolar emission profile. PL emission wavelengths were found to depend only on the nanowire width, and independent of its length. These observations were supported by simulation results indicating that gap plasmons are excited by light polarized transverse to the nanowire length.

Submicrometer radius and highly confined plasmonic ring resonator filters based on hybrid metal-oxide-semiconductor waveguide.

We numerically report the submicrometer radius (0.5 µm) and high confinement (mode area ~λ(2)/1200) plasmonic ring resonators for both all-pass and add-drop filters based on the hybrid metal-oxide-semiconductor (Ag-SiO(2)-Si) waveguide platform. The best tradeoff between the propagation length and the confinement of this hybrid plasmonic waveguide platform is also discussed and compared to the dielectric-loaded plasmonic waveguide counterpart. We show that the ring resonator all-pass filter features an extinction ratio as high as 23 dB with a transmission loss of 1.5 dB, and a wide free spectral range of 168 nm with a bandwidth of 14 nm. Moreover, the demonstrated add-drop filter achieves an extinction ratio larger than 12 dB with a channel isolation between the through and drop channels of 13.5 dB at the resonant wavelength. These demonstrated plasmonic devices reveal as potential building blocks for future nanoscale electronic-photonic integrated circuits.