Near-field vector imaging of plasmonic nanostructures

The optical local field enhancement on nanometer length scales provides the basis for plasmonic metal nanostructures to serve as molecular sensors and as nanophotonic devices. Understanding the correlation between particle size and shape, its spectral plasmon response, and the related details of the local field distribution is the goal of this project. Using interferometric homodyne scattering Scanning Near-field Optical Microscopy (s-SNOM), we spatially map the strong field variations around plasmonic metal nanostructures and optical antenna geometries from the visible to the mid-infrared. [more]

Topography (a) and corresponding s-SNOM image (b) for a small rounded single-crystalline triangular Ag nanoprism. This illustrates a single dipolar surface plasmon polariton excitation. The in-plane electric field vector has been probed under in-plane optical excitation conditions as indicated by the double-arrow.

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Domain formation in correlated electron materials

Correlated electron systems are often associated with electronic and structural phase transitions with ordering and domain formation on nanometer length scale. Here we investigate the ferroelectric order parameter in multiferroic materials such as in rare-earth manganites (RMnO3), and the metal-to-insulator transition in VO2.

The newly discovered multiferroic materials present the possibility to control magnetization by means of an electric field and vice versa which has potential for nonvolatile data storage and nanoelectronic devices. This makes a detailed comprehension of the microscopic phase behavior and in particular of the ferroelectric domain structure very desirable. [more]

Ferroelectic domain topology of intrinsic multiferroic single crystalline YMnO3. Near-field SHG imaging of the 180° domains in YMnO3, together with two color map of the domain structure. Ferroelectric domains within the sample plane and elongated along the z-direction are observed with length of 1- 2 μm and typical width of 200 nm.

VO2 undergoes a first-order phase transition from a semiconducting to metallic state which is coupled to structural changes of the crystal lattice as the temperature is raised through the transition temperature at 341K. These changes of the structural and electrical properties of VO2 are accompanied by significant differences in the optical properties.

Here we use infrared scattering scanning near-field optical microscopy (s-SNOM) to study the domain behavior of the metal-insulator transition (MIT) in VO2 with spatial resolution down the nanometer length scale. This research is in collaboration with the group of Prof. David Cobden (Department of Physics at UW).

Mapping of the MIT in a VO2 single crystal wire as it is heated through the transition temperature. a) Illustration of the experimental setup, b) topography, c)-f) corresponding optical mappings of the near-field distribution under illumination with 10.6μm light for temperatures 332.5K, 340.8K, 345K, 349.1K, respectively showing the emergence and growth of the metallic domains (bright stripes).

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A nano-confined plasmonic light source

Plasmonic scanning probe tips play a central role in many optical scanning probe experiments. Ongoing experiments are dedicated to improve our understanding and enhancing the optical response of metallic tips. This includes the development of novel nanoscopic light light sources using optical antenna design concepts in combination with nanofabrication techniques. [more]

Surface plasmon polariton response of a Au scanning probe tip showing its capabilities for resonant light scattering and local-field enhancement in the visible spectral region. Inset: electron micrograph of tip (scale bar: 100 nm).

Nanofabrication by focused-ion-beam milling allows for the targeted design of new nanoconfined, intense, and ultrashort light sources. For example, grating-coupling surface plasmons onto the tip shaft allows for propagation and subsequent conversion of the surface plasmon into a localized excitation at the tip apex of only nanometer dimensions. [more]

A nanoconfined light source: By illuminating on the grating coupler written on the shaft of the tip (left), this will launch surface plasmon polaritoins that propagate adiabatically towards the tip. The resulting optical field concentration and enhancement is seen at the apex where the field is reemitted due to the broken symmetry - resulting in a nanoconfined light source.

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Ultrafast electron dynamics on the nanoscale

The electronic dephasing of metallic nanostructures determines such interesting properties as the photochemistry on metal surfaces or the effectiveness of plasmon polaritons for device applications. The dephasing time is determined by both, the intrinsic electron scattering time in the metal, and extrinsic effects due to the nanoparticle shape, size, and chemical environment. In order to distinguish different relaxation channels contributing to this dephasing time probing on the single particle level is desirable to avoid effects due to the inherent heterogeneity of any ensemble. [more]

Spectrally integrated interferometric two-pulse second-harmonic correlation response of a Au nanotip. From the simulation using a three-level optical Bloch model plasmon dephasing times of 3-5 fs are derived.

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Raman spectroscopy on the nanoscale

Tip-enhanced Raman spectroscopy (TERS) has shown great potential as a nano-analytical tool with diverse applications in material and surface science as well as analytical chemistry. Drawing on the enhancement and confinement of the electric field due to the plasmonic coupling between a scanning probe tip and a metallic sample, this technique allows for Raman signal enhancement of up to 109 and all-optical resolution down to 10 nm. Having previously demonstrated the capability of TERS for single molecule sensitivity, we are currently exploring the extension of this technique to the study of crystalline nanostructures. [more]

The spectral diffusion, blinking, and peak amplitude fluctuations observed in time series of successive tip-scattered Raman spectra are characteristic for single molecule Raman emission.

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Infrared-vibrational probing of organic nanocomposites

In this project we have advanced infrared microscopy for chemical analysis achieving unprecedented spatial resolution better than 10 nm and a sensitivity down to 10-20 mole or just 103 functional groups. This result demonstrates that infrared spectroscopy with access to intramolecular dimensions is within reach. [more]

Infrared scattering-type near-field imaging of nanodomains of a block-copolymer surface. Contrast is obtained due to the spectral differences between the C-H stretch vibrational resonances of the respective polymer constituents.

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Nonlinear optics on the nanoscale

The higher symmetry selectivity of the nonlinear optical response compared to the linear one can be used for in new and unique ways for optical probing on the nanoscale. We derived new selection rules for asymmetric nanostructures that allow for the distinction of different source polarizations and enable simultaneous surface and bulk investigations. [more]

Second-harmonic generation (SHG) of individual nanoscopic metal tips. As a partially asymmetric nanostructure the tips allow for the distinction of otherwise inseparable local surface and nonlocal bulk SH source polarizations via their different polarization characteristics for emission.

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