Large-Scale Hot Spot Engineering for Quantitative SERS at the Single-Molecule Scale

Quantitative surface enhanced Raman spectroscopy (SERS) requires precise control of Raman enhancement factor and detection uniformity across the SERS substrate. Here, we show that alkanethiolate ligand-regulated silver (Ag) nanoparticle films can be used to achieve quantitative SERS measurements down to the single-molecule level. The two-dimensional hexagonal close-packed superlattices of Ag nanoparticles formed in these films allow for SERS detection over a large area with excellent uniformity and high Raman enhancement factor. In particular, the SERS signal from the thiolate ligands on Ag nanoparticle surfaces can be utilized as a stable internal calibration standard for reproducible quantitative measurements. We demonstrate the capability of quantitative SERS by measuring the areal densities of crystal violet molecules embedded in an ultrathin spin-on-glass detection “hot zone”, which is a planar and uniformly enhanced region several nanometers above the Ag nanoparticles. The Raman measurement results exhibit a linear response over a wide dynamic range of analyte concentration

Dual-Band Planar Plasmonic Unidirectional Launching in a Semiannular Apertures Array

Multiple-band, frequency-adjustable unidirectional launching of planar surface plasmons is of great concern in plasmonic devices and circuits. We have designed and demonstrated a novel dual-band planar unidirectional surface plasmon polaritons (SPPs) launcher with narrow bandwidth (∼5 nm) and large band gap (∼50 nm) using a semiannular apertures array milled in a gold film. Symmetry breaking of the semiannular aperture brings significant advantages for the unidirectional launching, based on the excited asymmetrically distributed cylindrical surface plasmon resonance modes. During the unidirectional launching, the individual semiannular apertures function as unidirectional quasi-point SPP sources, and the grating coherently stacking amplitude of unidirectional SPPs functions as an amplifier. By controlling the semiannular aperture size, we achieved large range modulations of wavelengths beyond 60 nm for both bands. This efficient unidirectional launching is experimentally demonstrated for 632 nm, showing good agreement with numerical results

Optical Observation of Plasmonic Nonlocal Effects in a 2D Superlattice of Ultrasmall Gold Nanoparticles

The advances in recent nanofabrication techniques have facilitated explorations of metal structures into nanometer scales, where the traditional local-response Drude model with hard-wall boundary conditions fails to accurately describe their optical responses. The emerging nonlocal effects in single ultrasmall silver nanoparticles have been experimentally observed in single-particle spectroscopy enabled by the unprecedented high spatial resolution of electron energy loss spectroscopy (EELS). However, the unambiguous optical observation of such new effects in gold nanoparticles has yet not been reported, due to the extremely weak scattering and the obscuring fingerprint of strong interband transitions. Here we present a nanosystem, a superlattice monolayer formed by sub-10 nm gold nanoparticles. Plasmon resonances are spectrally well-separated from interband transitions, while exhibiting clearly distinguishable blueshifts compared to predictions by the classical local-response model. Our far-field spectroscopy was performed by a standard optical transmission and reflection setup, and the results agreed excellently with the hydrodynamic nonlocal model, opening a simple and widely accessible way for addressing quantum effects in nanoplasmonic systems

Ultracompact Pseudowedge Plasmonic Lasers and Laser Arrays

Concentrating light at the deep subwavelength scale by utilizing plasmonic effects has been reported in various optoelectronic devices with intriguing phenomena and functionality. Plasmonic waveguides with a planar structure exhibit a two-dimensional degree of freedom for the surface plasmon; the degree of freedom can be further reduced by utilizing metallic nanostructures or nanoparticles for surface plasmon resonance. Reduction leads to different lightwave confinement capabilities, which can be utilized to construct plasmonic nanolaser cavities. However, most theoretical and experimental research efforts have focused on planar surface plasmon polariton (SPP) nanolasers. In this study, we combined nanometallic structures intersecting with ZnO nanowires and realized the first laser emission based on pseudowedge SPP waveguides. Relative to current plasmonic nanolasers, the pseudowedge plasmonic lasers reported in our study exhibit extremely small mode volumes, high group indices, high spontaneous emission factors, and high Purell factors beneficial for the strong interaction between light and matter. Furthermore, we demonstrated that compact plasmonic laser arrays can be constructed, which could benefit integrated plasmonic circuits