Toward Surface Plasmon-Enhanced Optical Parametric Amplification (SPOPA) with Engineered Nanoparticles: A Nanoscale Tunable Infrared Source.

Active optical processes such as amplification and stimulated emission promise to play just as important a role in nanoscale optics as they have in mainstream modern optics. The ability of metallic nanostructures to enhance optical nonlinearities at the nanoscale has been shown for a number of nonlinear and active processes; however, one important process yet to be seen is optical parametric amplification. Here, we report the demonstration of surface plasmon-enhanced difference frequency generation by integration of a nonlinear optical medium, BaTiO3, in nanocrystalline form within a plasmonic nanocavity. These nanoengineered composite structures support resonances at pump, signal, and idler frequencies, providing large enhancements of the confined fields and efficient coupling of the wavelength-converted idler radiation to the far-field. This nanocomplex works as a nanoscale tunable infrared light source and paves the way for the design and fabrication of a surface plasmon-enhanced optical parametric amplifier.

Standing wave plasmon modes interact in an antenna-coupled nanowire.

In a standing wave optical cavity, the coupling of cavity modes, for example, through a nonlinear medium, results in a rich variety of nonlinear dynamical phenomena, such as frequency pushing and pulling, mode-locking and pulsing, modal instabilities, even complex chaotic behavior. Metallic nanowires of finite length support a hierarchy of longitudinal surface plasmon modes with standing wave properties: the plasmonic analog of a Fabry-Pérot cavity. Here we show that positioning the nanowire within the gap of a plasmonic nanoantenna introduces a passive, hybridization-based coupling of the standing-wave nanowire plasmon modes with the antenna structure, mediating an interaction between the nanowire plasmon modes themselves. Frequency pushing and pulling, and the enhancement and suppression of specific plasmon modes, can be controlled and manipulated by nanoantenna position and shape.

Coherent anti-Stokes Raman scattering with single-molecule sensitivity using a plasmonic Fano resonance.

Plasmonic nanostructures are of particular interest as substrates for the spectroscopic detection and identification of individual molecules. Single-molecule sensitivity Raman detection has been achieved by combining resonant molecular excitation with large electromagnetic field enhancements experienced by a molecule associated with an interparticle junction. Detection of molecules with extremely small Raman cross-sections (~10(-30) cm(2) sr(-1)), however, has remained elusive. Here we show that coherent anti-Stokes Raman spectroscopy (CARS), a nonlinear spectroscopy of great utility and potential for molecular sensing, can be used to obtain single-molecule detection sensitivity, by exploiting the unique light harvesting properties of plasmonic Fano resonances. The CARS signal is enhanced by ~11 orders of magnitude relative to spontaneous Raman scattering, enabling the detection of single molecules, which is verified using a statistically rigorous bi-analyte method. This approach combines unprecedented single-molecule spectral sensitivity with plasmonic substrates that can be fabricated using top-down lithographic strategies.

Fluorescence enhancement of molecules inside a gold nanomatryoshka.

Metallic nanoparticles exhibiting plasmonic Fano resonances can provide large enhancements of their internal electric near field. Here we show that nanomatryoshkas, nanoparticles consisting of an Au core, an interstitial nanoscale SiO2 layer, and an Au shell layer, can selectively provide either a strong enhancement or a quenching of the spontaneous emission of fluorophores dispersed within their internal dielectric layer. This behavior can be understood by taking into account the near-field enhancement induced by the Fano resonance of the nanomatryoshka, which is responsible for enhanced absorption of the fluorophores incorporated into the nanocomplex. The combination of compact size and enhanced light emission with internal encapsulation of the fluorophores for increased biocompatibility suggests outstanding potential for this type of nanoparticle complex in biomedical applications.

Nanostructure-mediated launching and detection of 2D surface plasmons.

Au nanoparticles deposited on a metallic film act as nanoantenna receivers and transmitters for the coupling of free-space radiation into, and out of, 2D surface plasmons. Nanosteps, sub-10-nm gaps between metallic films of differing thickness, can also launch and detect surface plasmons. Here we use both types of structures to locally launch propagating surface plasmon waves and probe their properties. Nanoparticle-launched surface plasmons emerge as two lobes of nominally 90 degree angular width, propagating along the direction of incident polarization. Alternatively, plasmons can be launched unidirectionally, by asymmetric illumination of a nanoparticle receiver.