Oberflächenverstärkte Infrarot-Spektroskopie mittels Gold-Nanoantennen

Im Rahmen dieser Arbeit wurden elektrochemisch präparierte Gold-Nanodrähte und lithographisch hergestellte Gold-Nanostreifen mit Durchmessern von ungefähr 100nm an der Synchrotronstrahlungsquelle ANKA im Forschungszentrum Karlsruhe hinsichtlich ihrer infrarot optischen Eigenschaften untersucht. Unabhängig vom Durchmesser derNanoantennen und deren kristalliner Qualität wurden antennenartige Plasmonenresonanzengefunden, deren spektrale Eigenschaften von der Antennenlänge dominiert werden. Ein erhöhter Fernfeld-Extinktionsquerschnitt von Nanoantennen weist auf ihre Fähigkeit hin, elektromagnetische Strahlung auf ein nanoskaliges Volumen zu konzentrieren, ein Effekt, den wir uns bei der oberflächenverstärkten IR-Spektroskopie (SEIRS) zu Nutze machen. Zur Demonstration der SEIRS wird eine Oktadekanthiol- (ODT) Monolage auf Gold-Nanoantennen adsorbiert. Abhängig von der Resonanzfrequenz im Verhältnis zu der Adsorbatschwingung (2855 Wellenzahlen und 2927 Wellenzahlen) können Verstärkungsfaktoren bis zu 500 000 erzielt werden, welche die Spektroskopie von weniger als 100 Attogramm ODT ermöglicht. Im Vergleich zu herkömmlichen oberflächenverstärkten Infrarot Absorptions-Techniken eine um zwei Größenordnungen höhere Verstärkung. Resultate aus elektromagnetischen Streusimulationen (boundary element method) verifizieren die experimentell beobachteten asymmetrischen Linienformen der Adsorbatschwingungen, welche durch die Wechselwirkung von plasmonischer und molekularer Anregungen zu Stande kommen. Ein Nachweis, dass die Wechselwirkung elektromagnetischer Natur ist

Real-Time Tracking of Coherent Oscillations of Electrons in a Nanodevice by Photo-assisted Tunnelling

Coherent collective oscillations of electrons excited in metallic nanostructures (localized surface plasmons) can confine incident light to atomic scales and enable strong light-matter interactions, which depend nonlinearly on the local field. Direct sampling of such collective electron oscillations in real-time is crucial to performing petahertz scale optical modulation, control, and readout in a quantum nanodevice. Here, we demonstrate real-time tracking of collective electron oscillations in an Au bowtie nanoantenna, by recording photo-assisted tunnelling currents generated by such oscillations in this quantum nanodevice. The collective electron oscillations show a noninstantaneous response to the driving laser fields with a decay time of nearly 10 femtoseconds. The temporal evolution of nonlinear electron oscillations resulting from the coherent nonlinear optical response of the nanodevice were also traced in real-time. The contributions of linear and nonlinear electron oscillations in the generated tunnelling currents in the nanodevice were precisely determined. A coherent control of electron oscillations in the nanodevice is illustrated directly in the time domain. Functioning in ambient conditions, the excitation, coherent control, and read-out of coherent electron oscillations pave the way toward on-chip light-wave electronics in quantum nanodevices

Nearly diffraction limited FTIR mapping using an ultrastable broadband femtosecond laser tunable from 1.33 to 8 µm

Micro-Fourier-transform infrared (FTIR) spectroscopy is a widespread technique that enables broadband measurements of infrared active molecular vibrations at high sensitivity. SiC globars are often applied as light sources in tabletop systems, typically covering a spectral range from about 1 to 20 µm (10 000 - 500 cm−1) in FTIR spectrometers. However, measuring sample areas below 40x40 µm2 requires very long integration times due to their inherently low brilliance. This hampers the detection of ultrasmall samples, such as minute amounts of molecules or single nanoparticles. In this publication we extend the current limits of FTIR spectroscopy in terms of measurable sample areas, detection limit and speed by utilizing a broadband, tabletop laser system with MHz repetition rate and femtosecond pulse duration that covers the spectral region between 1250 - 7520 cm−1 (1.33 - 8 µm). We demonstrate mapping of a 150x150 µm2 sample of 100 nm thick molecule layers at 1430 cm−1 (7 µm) with 10x10 µm2 spatial resolution and a scan speed of 3.5 µm/sec. Compared to a similar globar measurement an order of magnitude lower noise is achieved, due to an excellent long-term wavelength and power stability, as well as an orders of magnitude higher brilliance

Electrochemically-controlled metasurfaces with high-contrast switching at visible frequencies

Recently in nanophotonics, a rigorous evolution from passive to active metasurfaces has been witnessed. This advancement not only brings forward interesting physical phenomena but also elicits opportunities for practical applications. However, active metasurfaces operating at visible frequencies often exhibit low performance due to design and fabrication restrictions at the nanoscale. In this work, we demonstrate electrochemically controlled metasurfaces with high intensity contrast, fast switching rate, and excellent reversibility at visible frequencies. We use a conducting polymer, polyaniline (PANI), that can be locally conjugated on preselected gold nanorods to actively control the phase profiles of the metasurfaces. The optical responses of the metasurfaces can be in situ monitored and optimized by controlling the PANI growth of subwavelength dimension during the electrochemical process. We showcase electrochemically controlled anomalous transmission and holography with good switching performance. Such electrochemically powered optical metasurfaces lay a solid basis to develop metasurface devices for real-world optical applications

Defect-induced activation of symmetry forbidden infrared resonances in individual metallic nanorods

International audienceWe report on the observation of second-order infrared (IR) plasmon resonances in lithographically prepared gold nanorods investigated by means of far-field microscopic IR spectroscopy. In addition to the fundamental antennalike mode, even and odd higher order resonances are observed under normal incidence of light. The activation of even-order modes under normal incidence is surprising since even orders are dipole-forbidden because of their centrosymmetric charge density oscillation. Performing atomic force microscopy and calculations with the boundary element method, we determine that excitation of even modes is enabled by symmetry breaking by structural deviations of the rods from an ideal, straight shape. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3437093

Strong magnetic response of submicron Silicon particles in the infrared

High-permittivity dielectric particles with resonant magnetic properties are being explored as constitutive elements of new metamaterials and devices in the microwave regime. Magnetic properties of low-loss dielectric nanoparticles in the visible or infrared are not expected due to intrinsic low refractive index of optical materials in these regimes. Here we analyze the dipolar electric and magnetic response of loss-less dielectric spheres made of moderate permittivity materials. For low material refractive index there are no sharp resonances due to strong overlapping between different multipole contributions. However, we find that Silicon particles with refractive index 3.5 and radius approx. 200nm present a dipolar and strong magnetic resonant response in telecom and near-infrared frequencies, (i.e. at wavelengths approx. 1.2-2 micrometer). Moreover, the light scattered by these Si particles can be perfectly described by dipolar electric and magnetic fields, quadrupolar and higher order contributions being negligible.Comment: 10 pages, 5 figure

Angstrom-Scale Distance Dependence of Antenna-Enhanced Vibrational Signals

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Optical Nanoantennas for Multiband Surface-Enhanced Infrared and Raman Spectroscopy

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