Patrik Hoffmann

Gold elliptical nanoantennas as probes for near field optical microscopy

We investigate the light scattering by individual nanometer-sized gold particles attached at the apex of fiber-based probes for near field optical microscopy. The dependence of the light scattering by the gold nanoparticle on the wavelength, the shape, and the surrounding medium dielectric profile are theoretically described and experimentally investigated, demonstrating that the tuning of the particle’s size and shape plays a crucial role in the light scattering process. In the case of gold spherical nanostructures, the plasmon resonance occurs at 540 nm in air, and 600 nm in water. A higher surrounding medium refraction index leads to a redshift of the plasmon resonance in the gold particle. Moreover, for elliptical structures, the orientation of the polarization of the incident field, as well as the relative ratio of the ellipse dimensions along its main axis, govern the position of the plasmon resonances. The light transmission spectrum for several probes where a single elliptical gold particle has be...

Improved tip performance for scanning near-field optical microscopy by the attachment of a single gold nanoparticle

Nanometer-size optical probes are gaining increasing interest in near-field optical microscopy. Optimization of the probe shape is still a challenging research and development issue. Here, we propose to improve the optical properties of a fiber-based probe by attachment at the tip apex of one single gold particle of 60 nm diameter. This probe produces an enhancement of the light throughput, both in the near and the far fields, a homogenization of the diffracted light polarization, and a higher accuracy of the topographic sensitivity. In this letter, the chemical procedure for the fixation of one single gold particle on the apex of a standard tip for scanning near-field microscopy is described. Far-field as well as near-field measurements with this probe are performed, showing improvement of the light distribution in excellent agreement with the theory.

Confining the sampling volume for Fluorescence Correlation Spectroscopy using a sub-wavelength sized aperture

For the observation of single molecule dynamics with fluorescence fluctuation spectroscopy (FFS) very low fluorophore concentrations are necessary. For in vitro measurements, this requirement is easy to fulfill. In biology however, micromolar concentrations are often encountered and may pose a real challenge to conventional FFS methods based on confocal instrumentation. We show a higher confinement of the sampling volume in the near-field of sub-wavelength sized apertures in a thin gold film. The gold apertures have been measured and characterized with fluorescence correlation spectroscopy (FCS), indicating light confinement beyond the far-field diffraction limit. We measured a reduction of the effective sampling volume by an order of magnitude compared to confocal instrumentation.

Density determination of focused-electron-beam-induced deposits with simple cantilever-based method

Freestanding deposits are grown on a silicon cantilever from a precursor gas by an electron induced process. Deposit mass determination is performed with an atomic force microscopy setup, where the cantilever resonance frequency shift, resulting from mechanical removal of the deposit, is measured. Deposits from hexafluoroacetylacetonato–Cu(I)–vinyltrimethylsilane show densities ranging from 2.05±0.45 to 3.75±0.55g∕cm3. Deposits from tetramethoxysilane have a constant density of (1.9±0.3)g∕cm3. Densities of deposits from Co2(CO)8 and [RhCl(PF3)2]2 are linearly related to their composition. The ratio of impinging electrons per deposited atom, beam heating, and thermal stability of the precursor molecule determine the density and composition in focused-electron-beam-induced deposits.

Mass sensor for in situ monitoring of focused ion and electron beam induced processes

A cantilever-based mass sensor for in situ monitoring of deposition and milling using focused ion and electron beams is presented. Carefully designed experiments allowed for mass measurements with a noise level of ±10fg by tracking the resonance frequency of a temperature stabilized piezoresistive cantilever using phase locking. The authors report on measurements of precursor surface coverage, residence time, mass deposition rates, yields, and deposit density using the (CH3)3PtCpCH3 precursor.

Laser coupling with a multimode water-jet waveguide

The coupling of a laser focused into a water microjet is studied. Using a high-power laser, the light guided in the jet is used to process various materials. To explain the observed ablation patterns, the propagation of a low-power and highly coherent laser beam coupled into a laminar water jet is studied. The light of a He-Ne laser (5 mW) is focused into the water jet, which behaves as a multimode waveguide. The distribution of the light intensity in the jet impinging on a glass plate cutting the jet perpendicularly to its propagation direction is recorded for various laser coupling conditions. The influence of the jet diameter, as well as the influence of the depth of focus of the incident beam and its position with respect to the center of the jet is studied. A nearly homogeneous grain size was observed over the whole jet cross section. The characteristic grain size was then compared with predictions from standard multimode fiber theory. Finally, it is confirmed that the structures resulting from material ablation using the laser-microjet technology when coupling Q-switched Nd:YAG (=1064 and 355 nm) are closely related to the predicted light intensity distributions. Furthermore, recommendations are made concerning the coupling conditions for optimizing the laser processing applications.

Determination of local refractive index variations in thin films by heterodyne interferometric scanning near-field optical microscopy

We report on a heterodyne interferometric scanning near-field optical microscope developed for characterizing, at the nanometric scale, refractive index variations in thin films. An optical lateral resolution of 80 nm (lambda/19) and a precision smaller than 10(-4) on the refractive index difference have been achieved. This setup is suitable for a wide set of thin films, ranging from periodic to heterogeneous samples, and turns out to be a very promising tool for determining the optical homogeneity of thin films developed for nanophotonics applications.

Tailoring of Optical Properties of Alumina films deposited by High Vacuum CVD (HV-CVD)

The deposition of amorphous alumina (Al2O3) films on four inch wafers with high deposition rate (up to 50 nm/min) by an advanced HV-CVD technique is discussed. Amorphous nature of the films is confirmed by X-ray diffraction. Refractive index of the deposited alumina can be adjusted in the large range (1.3-1.62@633 nm) by varying the substrate temperate during the deposition. The films are highly transparent. The density and stoichiometry of the deposits are also influenced by deposition conditions. Presence of -OH groups is observed in some cases.

Two-dimensional electrostatic force field measurements with simultaneous topography measurement on embedded interdigitated nanoelectrodes using a force distance curve based method

Accurate simultaneous measurements on the topography and electrostatic force field of 500nm pitch interdigitated electrodes embedded in a thin SiO2 layer in a plane perpendicular to the orientation of the electrodes are shown for the first time. A static force distance curve (FDC) based method has been developed, which allows a lateral and vertical resolution of 25 and 2nm, respectively. The measured force field distribution remains stable as result of the well controlled fabrication procedure of Pt cantilever tips that allows thousands of FDC measurements. A numerical model is established as well which demonstrates good agreement with the experimental results.

Combinatorial Chemical Vapor Deposition of Lithium Niobate Thin Films

Combinatorial lithium niobate deposition on 150 mm naturally oxidized silicon (100) wafers in a high-vacuum chemical vapor deposition reactor using Li(OBut) and Nb(OEt)4(dmae) is presented. The novel precursor supply system allows individual spatial control of precursors impinging rates on the substrate. This results in variations of the film properties in a single experiment at a certain substrate temperature due to the influence of different precursors flow rates and ratios. It efficiently leads to deposition conditions to achieve highly oriented polycrystalline lithium niobate films.