Susanne Quabis

On the experimental investigation of the electric and magnetic response of a single nano-structure

We demonstrate an experimental method to separately test the optical response of a single sub-wavelength nano-structure to tailored electric and magnetic field distributions in the optical domain. For this purpose a highly focused y-polarized TEM10-mode is used which exhibits spatially separated longitudinal magnetic and transverse electric field patterns. By displacing a single sub-wavelength nano-structure, namely a single split-ring resonator (SRR), in the focal plane, different coupling scenarios can be achieved. It is shown experimentally that the single split-ring resonator tested here responds dominantly as an electric dipole. A much smaller but yet statistically significant magnetic dipole contribution is also measured by investigating the interaction of a single SRR with a magnetic field component perpendicular to the SRR plane (which is equivalent to the curl of the electric field) as well as by analyzing the intensity and polarization distribution of the scattered light with high spatial resolution. The developed experimental setup as well as the measurement techniques presented in this paper are a versatile tool to investigate the optical properties of single sub-wavelength nano-structures.

Extraordinary transmission through a single coaxial aperture in a thin metal film

We investigate experimentally the transmission properties of single sub-wavelength coaxial apertures in thin metal films (t = 110 nm). Enhanced transmission through a single sub-wavelength coaxial aperture illuminated with a strongly focused radially polarized light beam is reported. In our experiments we achieved up to four times enhanced transmission through a single coaxial aperture as compared to a (hollow) circular aperture with the same outer diameter.We attribute this enhancement of transmission to the excitation of a TEM-mode for illumination with radially polarized light inside the single coaxial aperture. A strong polarization contrast is observed between the transmission for radially and azimuthally polarized illumination. Furthermore, the observed transmission through a single coaxial aperture can be strongly reduced if surface plasmons are excited. The experimental results are in good agreement with finite difference time domain (FDTD) simulations.

Using a quantum well heterostructure to study the longitudinal and transverse electric field components of a strongly focused laser beam

We report a method to measure the electric energy density of longitudinal and transverse electric field components of strongly focused polarized laser beams. We used a quantum well photodetector and exploited the polarization dependent optical transitions of light holes and heavy holes to probe the electric field distribution in the focal region. A comparison of the measured photocurrent spectra for radially and azimuthally polarized beams at the light and heavy hole absorption peaks provides a measure of the amount of the longitudinal electric field component.

Interaction of highly focused vector beams with a metal knife-edge

We investigate the interaction of highly focused linearly polarized optical beams with a metal knife-edge both theoretically and experimentally. A high numerical aperture objective focuses beams of various wavelengths onto samples of different sub-wavelength thicknesses made of several opaque and pure materials. The standard evaluation of the experimental data shows material and sample dependent spatial shifts of the reconstructed intensity distribution, where the orientation of the electric field with respect to the edge plays an important role. A deeper understanding of the interaction between the knife-edge and the incoming highly focused beam is gained in our theoretical model by considering eigenmodes of the metal-insulator-metal structure. We achieve good qualitative agreement of our numerical simulations with the experimental findings.

Reduction of the spot size by using a radially polarized laser beam

We present numerical calculations on the field distribution in the focus of an optical system with high numerical aperture. It is shown that a radially polarized doughnut mode fits the symmetry of the optical system and focuses down to spot sizes significantly smaller as compared to the case of linear polarization. An experimental setup to generate a radially polarized beam is presented along with a setup which is based on the knife edge method and tomographic reconstruction to measure the 3D intensity distribution in the focal region.

High numerical aperture imaging with different polarization patterns

The modulation transfer function (MTF) is calculated for imaging with linearly, circularly and radially polarized light as well as for different numerical apertures and aperture shapes. Special detectors are only sensitive to one component of the electric energy density, e.g. the longitudinal component. For certain parameters this has advantages concerning the resolution when comparing to polarization insensitive detectors. It is also shown that in the latter case zeros of the MTF may appear which are purely due to polarization effects and which depend on the aperture angle. Finally some ideas are presented how to use these results for improving the resolution in lithography.

Tailored polarization patterns for performance optimization of optical devices

In the case of strong focusing the smallest possible focal spot can be reached, provided one uses a specially designed polarization pattern. Other optical set-ups also employing high numerical aperture imaging are likewise expected to improve performance under polarization optimization. We propose a strategy for calculating the polarization pattern required for system optimization. A single element modulating only the polarization and not amplitude and phase may lead to satisfactory performance in some cases.

Radial polarization minimizes focal spot size

We experimentally demonstrate that a radially polarized beam can be focused to a spot size significantly smaller than for linear polarization and that the longitudinal field component arising at focus best coupled to small particles

Polarization-dependent effects on phase singularities in the vicinity of sub-lambda structures

We present theoretical and experimental studies of the reflected field in the vicinity of sub λ structures. Rigorous numerical calculations and measurements were performed to get high-precision information of certain object parameters which go beyond the limits of classical microscopy. We show that the polarization of the illumination plays a key role for the field distribution, which is reflected from the examined objects. Furthermore, we present a simplified model, which is able to qualitatively predict the behavior of the phase singularities correctly.

Untersuchung von Phasensingularitäten in der Umgebung kleiner Strukturen mit einem Linnik-Interferenzmikroskop (Investigation of Phase Singularities in the Vicinity of Small Structures Using a Linnik Interference Microscope)

Wir präsentieren Untersuchungen zur Feldverteilung in der Umgebung von Sub-λ-Strukturen. Das Auftreten von Singularitäten in der Phasenverteilung des reflektierten Feldes wird theoretisch und experimentell in Hinsicht auf die Möglichkeit der lateralen Überauflösung untersucht. Ein theoretisches Modell für die Berechnungen des elektrischen Feldes, welches auf dem Greenfunktion-Formalismus basiert, wird diskutiert. Messungen an zweidimensionalen Objekten wurden mit Hilfe eines Linnik-Interferenz-Mikroskopes durchgeführt.