  An atomic force microscope~(AFM) tip, with a few nm-thick noble metal
coating, gives rise to strong electric-field at the near-field of tip apex,
i.e. hot spot, when illuminated with a beam of light linearly polarized in the
axial direction. This strong near-field enables resolving molecular landscape
or nano-scale defects on crystal surfaces in apertureless scanning near field
optical microscopy or tip enhanced Raman spectroscopy applications. However,
strong near fields appear only at certain illumination wavelengths at which
material and geometry dependent plasmon resonances take place. Once the metal
coated tip is manufactured, optimal operation wavelength remains fixed since
the material and geometry of the tip apex remains fixed. Here, we show for the
first time a method which renders an AFM tip useful at wavelengths off-resonant
to its plasmon resonances. The technique relies on decoration of the tip with
appropriate auxiliary molecules. For instance, a tip originally bearing a
plasmon resonance at $\lambda_{\rm p}=581$ nm can be effectively operated
off-resonantly at $\lambda_{\rm exc}=532$ nm, when it is decorated by an
appropriate auxiliary molecule. Furthermore, the near-field is found to be
strongest just below the auxiliary molecule which enables a
single-molecule-size ultra-high spatial resolution imaging. We demonstrate the
phenomenon with exact solutions of 3D Maxwell equations. We also show why such
an enhancement takes place.
