Optical Sciences

Biomolecules and nanostructures

The Optical Sciences group studies the interaction of light and matter. Our current focus is on detection and sensing/imaging with an emphasis on the development of integrated photonics. We are part of Twente University's Department of Science and Technology and member of the MESA+ institute.


An Evanescent-Field Optical Microscope

Journal of Microscopy-Oxford
vol. 163 p.117-130 Aug 1991

The classic diffraction limit of resolution in optical microscopy (approximately lambda/2) can be overcome by detecting the diffracted field of a submicrometre-size probe in its near field. The present stage of this so-called scanning near-field optical microscopy (SNOM) is reviewed.

An evanescent-field optical microscope (EFOM) is presented in which the near-field regime is provided by the exponentially decaying evanescent field caused by total internal reflection at a refractive-index transition. A sample placed in this field causes a spatial variation of the evanescent field which is characteristic for the dielectric and topographic properties of the sample. The evanescent field is frustrated by a dielectric probe and thus converted into a radiative field.

In our case the probe consists either of an etched optical fibre or of a highly sharpened diamond tip. The probe is scanned over the sample surface with nanometre precision using a piezo-electric positioner. The distance between probe and sample is controlled by a feedback on the detected optical signal. The resolution of the microscope is determined by both the gradient of the evanescent field and the sharpness of the tip. Details of the experimental set-up are discussed.

The coupling of the evanescent field to the submicrometre probe as a function of probe-sample distance, angle of incidence and polarization has been characterized quantitatively. The observed coupling is generally in agreement with presented theoretical calculations.

Microscopy has been performed on a regular latex sphere structure, which clearly demonstrates the capacity of the evanescent-field optical microscope for nanometre-scale optical imaging. Resolution is typically 100 nm laterally and 10 nm vertically.

The technique is promising for biological applications, especially if combined with optical spectroscopy.
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