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Abstract

We study apertureless field enhancing optical probes beyond the
spherical approximation in a smooth transition towards up to 3 mu m
long conical silicon tips. Such tips are used in apertureless scanning
near field optical microscopy, which holds the promise of sub 10 nm
lateral resolution. A fully three-dimensional numerical solution to the
Maxwell equations is obtained with the multiple multipole method giving
simultaneously both near fields and scattered far fields. The
significance of focused beam excitation for work with long tips is
illustrated and the relative influence of relevant length scales such
as tip length, excitation wavelength, and beam waist radius is
discussed. In the limit of vanishing tip apex radius, the near field
grows without bounds, whereas the far field remains finite. We verify
that for small apex radii the near field confinement at the tip apex,
which is related to the achievable lateral resolution, scales with the
inverse of the radius. We find, however, that long tips exhibit a
markedly lower confinement than spherical or very short tips. Relevant
for experimental studies, we demonstrate how scanning the excitation
field with long conical tips can be a useful technique for mapping the
focal volume. We show that the normalized near field at the tip apex is
robustly tolerant against small misalignments or misorientations of
illumination focus and tip apex.