A
recent adaptation of probe microscopy replaces the sharpened probe
tip of STM (a metal wire) or
SFM (a micromachined
Si3N4 cantilever) with a sharpened optical
fibre. The fibre can be formed by either chemical etching or by
micropipette pulling techniques and then coated with a metal sheath
(often Al) to isolate the open tip which can be on the order of a few
nanometers in diameter. Since such an opening is very much smaller
than the wavelength of visible radiation, light normally diffracts
around it rather than going into it. However, when the emitting
source is within a few nanometers of the opening, the electromagnetic
radiation is no longer described by the dipole approximation which
works for the far-field limit. Instead, near-field behaviour
dominates and light works in a different way so that it can couple
efficiently through such a small opening under these conditions.
The probe tip can be brought close to the surface of a sample and used as an illumination source. This is the more typical application of SNOM which irradiates the sample with light coming from the optical fibre. The microscope can operate in either a reflection mode (where the reflected light is collected as efficiently as possible from the surface and then directed to a sensitive detector ) or in transmission (the transmitted light find its way to a detector). The tip is rastered across the surface and an image is formed by the signal intensity at each point. While this experiment provides the same data as optical microscopy, it is not longer limited by the diffraction of light as in traditional microscopy but is rather defined by the tip opening.
Another extension of the technique uses the probe tip to accept light emitted from the surface and couples that light into a spectrometer. This fluorescence process provides the means by which fluorescence spectra can be acquired with spatial resolution defined by the probe tip. A number of researchers have shown how they can image the fluorescence from single molecules. This technique can be developed to provide spectral information in real time and thereby acquire insight to the dynamics of large molecules. We hope to apply this technique to obtain an understanding of the motion of proteins through cell walls. We are currently modifying two commerical SPM's (one from Burleigh Instruments and another from Topometrix) to have fluorscence SNOM capabilities. We hope to be able to study both molecular fluorescence from molecules embedded in phospholipid bilayers and to study the nanoscale fluorescence properties of porous silicon.
An interesting link to other SNOM activity around the world can be found here.
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