x5 x0.5 remote excitation emission remote detection nano-bead far-field fluorescence image two-wire transmission line 1μmPlasmonic waveguides are key elements in nanophotonic devices serving as optical interconnects between nanoscale light sources and detectors. Multimode operation in plasmonic two-wire transmission lines promises important degrees of freedom for near-field manipulation and information encoding. However, highly confined plasmon propagation in gold nanostructures is typically limited to the near-infrared region due to ohmic losses, excluding all visible quantum emitters from plasmonic circuitry. Here, we report on top-down fabrication of complex plasmonic nanostructures in single-crystalline silver plates. We demonstrate controlled remote excitation of a small ensemble of fluorophores by a set of waveguide modes and emission of the visible luminescence into the waveguide with high efficiency. This approach opens up the study of nanoscale lightmatter interaction between complex plasmonic waveguides and a large variety of quantum emitters available in the visible spectral range.Propagating surface plasmon polaritons can break the diffraction limit 1 and find applications in optical communication, sensing and quantum information 2,3 . Whereas single nanowires of noble metals are extensively studied as plasmonic waveguides 4,5 , recent research focuses on multimode operation in more complex geometries like two-wire transmission lines. These waveguides offer two fundamental modes which can be selectively excited with an optical antenna and detected by means of a mode detector 6 . Such waveguides find application in polarization manipulation 7 , local mode-conversion 8 , coherent control 9 and spin-dependent flow 10 . In the last years such complex plasmonic and high-quality nanostructures have been mainly fabricated by focused ion beam milling of single-crystalline gold plates 11-14 .Long-range plasmonic waveguiding in gold structures is however limited to the near-infrared spectral range due to ohmic losses in the visible range 15 . This drastically restricts the choice of emitters available for quantum plasmonic experiments [16][17][18] . In contrast to gold, silver leads to reduced losses in the visible spectral range 19-21 although care has to be taken to avoid surface degradation 22 . Chemically grown silver nanowires have been successfully coupled with visible quantum emitters 4,23,24 . More complex shapes need to be manufactured by focused ion beam milling of single-crystalline silver films or flakes 25,26 .Let us start by demonstrating the advantages of silver plasmonics with the example of a two-wire transmission line, formed by two parallel nanowires separated by a gap of a few tens of nanometers ( Fig. 1). We label the two fundamental eigen-modes by the symmetry of their a) Electronic mail: markus.lippitz@uni-bayreuth.de 550 650 750 600 700 wavelength λ (nm) propagation length (µm) 0 2 4 6 8 Ag as Ag s Ag ho guided leaky mode as mode ho E x (arb.u.) max min 0 mode s + -+ + -glass air Al 2 O ...
Plasmonic waveguides offer the unique possibility to confine light far below the diffraction limit. Past room temperature experiments focused on efficient generation of single waveguide plasmons by a quantum emitter. However, only the simultaneous interaction of the emitter with multiple plasmonic fields would lead to functionality in a plasmonic circuit. Here, we demonstrate the nonlinear optical interaction of a single molecule and propagating plasmons. An individual terrylene diimide (TDI) molecule is placed in the nanogap between two single-crystalline silver nanowires. A visible wavelength pump pulse and a red-shifted depletion pulse travel along the waveguide, leading to stimulated emission depletion (STED) in the observed fluorescence. The efficiency increases by up to a factor of 50 compared to far-field excitation. Our study thus demonstrates remote nonlinear four-wave mixing at a single molecule with propagating plasmons. It paves the way toward functional quantum plasmonic circuits and improved nonlinear single-molecule spectroscopy.
Active optical waveguides based on functional small organic molecules in micro/nano regime have attracted great interest for their potential applications in high speed miniaturized photonic integrations. Here, we report on the active waveguiding properties of millimeter sized single crystals of a newly synthesized thiophene-based oligomer. These large crystals exhibit low optical loss compared to other organic nanostructures, and optical losses depend on the emission energy. Moreover, we find that the coupling of photoluminescence to waveguide modes is very efficient, typically greater than 40%. These features indicate that such perfect single crystals with a low density of defects and extremely smooth surfaces exhibit low propagation loss, which makes them good candidates for the design and the fabrication of novel organic optical fibers and lasers.
Plasmonic nanoparticles in close vicinity to a metal surface confine light to nanoscale volumes within the insulating gap. With gap sizes in the range of a few nanometers or below, atomic-scale dynamical phenomena within the nanogap come into reach. However, at these tiny scales, an ultra-smooth material is a crucial requirement. Here, we demonstrate large-scale (50 μm) single-crystalline silver flakes with a truly atomically smooth surface, which are an ideal platform for vertically assembled silver plasmonic nanoresonators. We investigate crystalline silver nanowires in a sub-2 nm separation to the silver surface and observe narrow plasmonic resonances with a quality factor Q of about 20. We propose a concept toward the observation of the spectral diffusion of the lowest-frequency cavity plasmon resonance and present first measurements. Our study demonstrates the benefit of using purely crystalline silver for plasmonic nanoparticle-on-mirror resonators and further paves the way toward the observation of dynamic phenomena within a nanoscale gap.
We demonstrate the read-out of the conformational state of photoswitchable molecules, without modifying that state by the read-out process itself. The ring-opening and ring-closing reactions change not only the absorption spectrum of a molecular layer but also its index of refraction. A thin layer of a dithienylethene derivative was combined with welldesigned gold nanostructures. The particle plasmon resonance of these nanostructures is extremely sensitive to the photoinduced change in the environment and can therefore be used to probe the state of the photochromic switch. The probing wavelength now interrogates the plasmonic structure, not the molecular film, and can thus be conveniently placed in a transparent spectral range, e.g., the near-infrared. We find good agreement between experiments and numerical simulations with regard to spectral signatures of the plasmon resonance.
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