The Optics Laboratory

Surface Enhancement in Near-Field Raman Spectroscopy

The NSOM is a good tool for studying surface enhancement, since the probe can be moved toward and away from the surface with very high precision. Force feedback can be used to stabilize the position if the probe is close enough to the surface. We expect that Raman spectra, and most other optical signals, will be enhanced as the NSOM probe approaches the surface, since there are many evanescent modes of the electric field near the tip than can begin to interact with the sample.

A good starting point is the Bethe-Bouwkamp model for electric fields near an aperture. To calculate the distance dependence, we use this model to calculate the electric field above the surface, then average the relevant quantity for all points in each plane above the surface, giving us one averaged value per distance,

Independent calculation of the components is meaningful, since the vibration is usually along a bond, and only one component of the field will couple to the bond when the bond happens to line up with that axis. The squares of the projected fields appear in the Raman intensity, so they are shown. The calculation assumes light polarized along the x-direction incident on an aperture in the xy-plane. The plot with the derivative of Ez is what is expected in gradient field Raman, for comparison.

Below is a nano-Raman spectrum taken with our cooled CCD apparatus.

We take spectra similar to this at several distances from the surface, and subtract those taken closer from the average of several taken with the probe retracted from the surface – in the far-field. The difference spectra below represent the change as the probe enters the near field.

The spectra are shifted by 20 counts between each, and the spectra closer to the surface are towards the top of the figure. There are enhancements as one approaches the surface. At first one might think that these are simply enhancements of the existing Raman lines. They are not, since they are at different energies. Nor are they shifted by the same amount from the existing lines. They are at the energies of vibrations in KTP that are different than the vibrations observed in the far-field spectra. Two questions arise:

The answer to the latter question is gradient field Raman, GFR, described on an accompanying web page. The answer to the former has to do with coupling of light from the tip region to the spectrometer. A theoretical map of the field profile shows the field in the x-direction (symmetric, large magnitude), and y-direction (4-fold symmetric, small magnitude) concentrated under the aperture:

while the z-component of the electric field is concentrated under the metal region around the aperture

Combine this with the constraint that only perpendicular electric fields (light) are allowed near a metal surface,

to realize that the x- and y-polarized light cannot get around the probe tip to propagate to the spectrometer in reflection mode. These modes are enhanced, but their light is not detected in our experiment. The signal from the x and y modes that we do see probably originates from deeper within the sample.

To quantify the enhancement of the new peaks, we integrate the area under the peaks in the difference spectra, and subtract the background, as measured from averages on either side of the peaks. We obtain the following plots for the two main peaks in the difference spectra:

The first change of the difference spectra from zero (the curves should continue to the right at zero), is a decrease at ~120 nm, before being enhanced at smaller z. This continues until the tip makes contact with the sample for the last two points. When contact is made, the vibration energies are modified by the stress of the probe tip, and the spectra change. We will ignore these data points in the following analysis. The two peaks, once scaled, are enhanced in almost exactly the same manner. This is because they are due to the same mechanism – GFR. A comparison with the predictions from the figure at the top of the page,

support this conclusion. The standard Raman mechanism predicts an Ez^2 dependence, which is too low at small z and too high at large z. The GFR derivative prediction does a good job with the qualitative trends, but misses the decrease at ~120 nm and increase ~70-90 nm. The origins of these oscillations are under investigation.