KEYWORDS: Near field, Microscopy, Near field scanning optical microscopy, Near field optics, Microscopes, Spectroscopy, Scattering, Solids, Optical microscopy, Objectives
Far-field microscopy techniques are routinely used for the visualization of biological systems, but are limited according
to Abbe`s criteria to about λ/2. The objective of this work is to integrate a solid immersion lens (SIL) as a near-field
probe into a standard microscope spectrophotometer in order to perform polychromatic illumination near-field
microscopy as well as near-field spectroscopy. The SIL concept can achieve a higher resolution than expected by the
increase of the numerical aperture. Even with a tip diameter of 700nm and a tip point angle of 130° the lateral resolution
is in the range of about 30 nm, therefore overcoming the tradeoff between the resolution and intensity restrictions in
aperture limited SNOM probes. In this paper the optical setup of the system is described and some images of biological
samples on a nanoscale with high contrast are presented.
To achieve high lateral resolution in microscopy, we exploit the localized electromagnetic field of a Solid Immersion
Lens (SIL). The lens is mounted on a cantilever of a Scanning Probe Microscope (SPM) to allow a dynamic scan with
constant tip-sample force. This unit can be integrated into a micro fluorescence (Zeiss UMSP) or Raman spectrometer
(Renishaw) to allow spectroscopy and spectrally resolved imaging in the near field. Three methods can be applied with a
lateral resolution of less than 30 nm: 1) Reflectance-SNOM: the sample is imaged by illuminating the surface through
the SIL and detecting the reflected near-field. 2) Photon-tunneling-SNOM: the contrast is generated by the ability of
the photons to tunnel through the energy barrier into the substrate. 3) Fluorescence- SNOM: the chromophore is excited
and the fluorescence light is collected by the SIL. The collection efficiency for the fluorescence is increased by a factor
of 10 due to the high refractive index of the SIL compared to conventional methods.
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