Near-Field Scanning Optical Microscopy (NSOM)

Diffraction limits the spatial resolution of conventional far-field optical techniques to dimensions on the order of l/(2NA) where NA is the numerical aperture of the system.   For visible wavelengths, this places a practical limit on the order of 0.5um on the spatial resolution that can be achieved in optical measurements.  In near-field microscopy a sub-wavelength optical aperture is placed very close to the surface of interest.  The separation is typically fractions of a wavelength.  By working in the near-field, spatial resolutions comparable to the aperture diameter are achieved.

While near-field techniques are not new, the recent interest in NSOM has been stimulated by the development of near-field apertures which are suitable for visible to UV wavelengths. The apertures are formed by tapering an optical fiber and are typically 100nm or less in diameter.  The fiber tip is scanned across the sample surface while maintaining a fixed tip-sample separation using shear force feedback. The separation is much less than the wavelength of light and, hence, in the near-field.  Light emitted or collected from the fiber is used to record optical images such as transmittance, reflectance, photovoltage or photoluminescence with lateral resolutions much better than 100nm.  Images of surface topography are obtained simultaneously with optical images.

The most critical component in the microscope is the fiber optic tip.  These are generally prepared in a micro pipette puller. In this approach a CO2 laser pulse heats the fiber while the ends are pulled tapering the heated section down to a point.  Since light begins to leak out of the fiber when the core diameter shrinks below cutoff, a metal coating is applied to the outside of the fiber tip leaving the very end, the aperture, uncoated.   We have also been preparing tips by chemically etching the fiber to form the taper.

As mentioned above, shear force feedback is commonly used to maintain a constant tip sample separation.  In one approach the tip is attached to a piezoelectric tranducer which is used to dither the tip parallel to the sample surface.  Often the dither frequency is chosen to be a mechanical resonant frequency of the tip.  For tip sample separations less than about 20nm, the dither amplitude is found to decrease linearly with decreasing separation.    As the tip is raster scanned over the sample surface, a feedback loop adjusts the z-motion of the tip to maintain a constant dither amplitude and hence tip sample separation.  This allows a topographic image of  the sample surface to be built up simultaneously with optical images. Non-optical approaches to dithering the tip have also been developed including attaching the tip to a tiny quartz tuning fork. The fork is driven at resonance, and the voltage or current generated by the tuning fork gives a measure of the amplitude of motion of the tip. Both optical and tuning force based shear force measurements are used in our lab.

The remainder of the microscope is composed of optical excitation sources, scanning, optical collection, and data acquisition systems.

As an example of the function of our microscope, this figure shows NSOM topographic and photocurrent images of an electroplated Au contact on a GaAs pn junction.  The Au has delaminated at the edge of the junction. This is visible as a step in the red region of the topographic image.  In the photocurrent image obtained simultaneously, we observe an increase in photocurrent (red is high in topography but low in photocurrent).  In addition small features in topography in the delaminated region are also visible as reductions in photocurrent indicating small regions of Au still remain in these spots.

Last Update July 26, 2002