Mapping the electric field distribution of tightly focused cylindrical vector beams with gold nanorings.

In this paper gold nanorings (NRs) are applied as particularly well-suited sensing elements for mapping the radially symmetric electric fields in the high numerical aperture focus of cylindrical vector beams. The optical properties of gold nanorings are analyzed by a combination of extinction and single particle dark field spectroscopy as well as confocal photoluminescence (PL) imaging. The results are compared to numerical calculations. The in-plane components in the focus of the cylindrical vector beams are estimated through the PL intensity distributions of the NRs. The optimum overlap between the structure and excitation is visualized by a narrow centre spot in the far-field PL scan.

Compact plasmonic optical biosensors based on nanostructured gradient index lenses integrated into microfluidic cells.

We report on a compact and cost-effective integrated label-free biosensor configuration which is based on the refractive index sensitivity of the localized surface plasmon resonance (LSPR) of gold nanostructures. Aiming for compactification and miniaturization of the sensor, arrays of nanodiscs were fabricated on the planar surface of a gradient index (GRIN) lens, which acts as a substrate as well as an imaging objective for the light scattered by the gold structures. Integration of the lens into a microfluidic flow cell enabled the controlled exchange of liquid media at the sensor surface. The light scattered by the nanostructures was investigated spatially and spectrally resolved making use of the imaging properties of the GRIN lens. Dynamic spectral analysis during refractive index changes was conducted, revealing high sensitivities of up to 372 nm per refractive index unit for the shift of the LSPR. Biosensing capabilities were demonstrated by the detection of binding of an analyte by means of a testosterone-immunoassay.

Dispersion forces between ultracold atoms and a carbon nanotube.

Dispersion forces are long-range interactions between polarizable objects that arise from fluctuations in the electromagnetic field between them. Dispersion forces have been observed between microscopic objects such as atoms and molecules (the van der Waals interaction), between macroscopic objects (the Casimir interaction) and between an atom and a macroscopic object (the Casimir-Polder interaction). Dispersion forces are known to increase the attractive forces between the components in nanomechanical devices, to influence adsorption rates onto nanostructures, and to influence the interactions between biomolecules in biological systems. In recent years, there has been growing interest in studying dispersion forces in nanoscale systems and in exploring the interactions between carbon nanotubes and cold atoms. However, there are considerable difficulties in developing dispersion force theories for general, finite geometries such as nanostructures. Thus, there is a need for new experimental methods that are able to go beyond measurements of planar surfaces and nanoscale gratings and make measurements on isolated nanostructures. Here, we measure the dispersion force between a rubidium atom and a multiwalled carbon nanotube by inserting the nanotube into a cloud of ultracold rubidium atoms and monitoring the loss of atoms from the cloud as a function of time. We perform these experiments with both thermal clouds of ultracold atoms and with Bose-Einstein condensates. The results obtained with this approach will aid the development of theories describing quantum fields near nanostructures, and hybrid cold-atom/solid-state devices may also prove useful for applications in quantum sensing and quantum information.

Cold-atom scanning probe microscopy.

Scanning probe microscopes are widely used to study surfaces with atomic resolution in many areas of nanoscience. Ultracold atomic gases trapped in electromagnetic potentials can be used to study electromagnetic interactions between the atoms and nearby surfaces in chip-based systems. Here we demonstrate a new type of scanning probe microscope that combines these two areas of research by using an ultracold gas as the tip in a scanning probe microscope. This cold-atom scanning probe microscope offers a large scanning volume, an ultrasoft tip of well-defined shape and high purity, and sensitivity to electromagnetic forces (including dispersion forces near nanostructured surfaces). We use the cold-atom scanning probe microscope to non-destructively measure the position and height of carbon nanotube structures and individual free-standing nanotubes. Cooling the atoms in the gas to form a Bose-Einstein condensate increases the resolution of the device.

Tailoring gold nanostructures for near-field optical applications.

A method of combined thin-film deposition, electron beam lithography, and ion milling is presented for the fabrication of gold and silver nanostructures. The flexibility of lithographical processes for the variation of geometric parameters is combined with three-dimensional control over the surface evolution. Depending on the etching angle, different shapes ranging from cones over rods to cups can be achieved. These size- and shape-tunable structures present a toolbox for nano-optical investigations. As an example, optical properties of systematically varying structures are examined in a parabolic mirror confocal microscope.

Characterization of the morphology and composition of commercial negative resists used for lithographic processes.

We present a spectroscopic and microscopic characterization of the chemical composition, structure, and morphology of two commercial negative resists using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). For this purpose, films of a novolak-based resist (ma-N 2400) and hydrogen silsesquioxane (HSQ) are treated under different conditions (temperature, deep ultraviolet (DUV) exposure, CHF(3) plasma). Topographic AFM images show that both heating and DUV exposure strongly affect the surface morphology of as-prepared ma-N 2400 resist films. These different treatment conditions also lead to decreasing roughnesses, which indicates structural reorganization. Furthermore, the decrease of the photoactive compound (bisazide) in the ma-N 2400 resist films, observed in FTIR spectra, suggests cross-linking of the resist after CHF(3) plasma treatment, heating, or DUV exposure. XPS measurements on different CHF(3) plasma-treated surfaces reveal that a structurally homogeneous fluorine-containing polymer is generated that is responsible for an enhanced etch resistance. FTIR measurements of HSQ films show a correlation between the degree of HSQ cross-linking and baking time.

High-Resolution Imaging by Fourier Transform X-ray Holography.

Fourier transform x-ray holography has been used to image gold test objects with submicrometer structure, resolving features as small as 60 nanometers. The hologram-recording instrument uses coherent 3.4-nanometer radiation from the soft x-ray undulator beamline X1A at the National Synchrotron Light Source. The specimen to be imaged is placed near the first-order focal spot produced by a Fresnel zone plate; the other orders, chiefly the zeroth, illuminate the specimen. The wave scattered by the specimen interferes with the spherical reference wave from the focal spot, forming a hologram with fringes of low spatial frequency. The hologram is recorded in digital form by a charge-coupled device camera, and the specimen image is obtained by numerical reconstruction.