Biomolecules and nanostructures
The Optical Sciences group studies the interaction of light and matter at the nanoscale.
We do this by exploring ways to shape light and its environment. It's what we call
active and passive control. Our current focus is on the interaction of light with
biomolecules and nanostructures. We are part of Twente
University's Department of Science and Technology and member of the
Combined near field optical and atomic force microscope
This instrumentation oriented FOM-BR project started in 1993 and will be concluded with a PhD thesis on 19th June 1997.
An integrated NSOM/AFM probe, which exists of an 200µm long flexible cantilever with at its end a 5*m high conical tip, has been realised in the clean room facility of the MESA research institute using standard micro-mechanical techniques like wet and dry etching, photolithography and CVD methods on a 3" Si-wafer. Furthermore a combined NSOM/AFM set-up (both transmission and reflection) has been built based on a static light path and probe and a scanning sample stage. The shape of the probe (cantilever flexibility and tip shape) is suitable for use in AFM applications. Most effort is put in the deposition of an opaque layer surrounding the tip in order to get a sub-micron aperture at the tip end. Aluminium has been chosen as coating material because of its high extinction coefficient and well known depositing characteristics. Because of tip shape and oxidation properties of the Aluminium other coating materials have been tried also. The optical resolution found so far, ~ 300 nm, is not yet competitive with optimised aperture type fibre probes.
Shear force detection using a quartz crystal tuning fork has been implemented to provide higher topographic sensitivity, lower force interaction, reduced optical background signal and liquid operation. This shear force feedback system is used as distance control mechanism for a near-field scanning optical microscope, in which a tapered optical fibre attached to the tuning fork is scanned over the sample surface. The dynamics of a tuning fork shear-force feedback system have been investigated in detail. Experiments were performed measuring amplitude and phase of the tuning fork oscillation as a function of driving frequency and tip-sample distance. These experiments reveal that the resonance frequency of the tuning fork changes upon approaching the sample. Either amplitude or phase of the tuning fork can be used as distance control parameter in the feedback system. Using amplitude a second-order behaviour is observed while with phase only a first-order behaviour is observed. Numerical calculations confirm these observations. This first-order behaviour results in an improved stability of our feedback system. As an example a sample consisting of DNA strands on mica was imaged which showed a height of the DNA of 1.4 nm.