Apertureless near-field optical microscopy of metallic nanoparticles.
Image formation in apertureless near-field optical microscopes employing evanescent-wave excitation is studied quantitatively as a function of the polarisation and the wavelength of the excitation. Aggregate Mie theory is used to describe the probe-sample interactions self-consistently, including retardation. Only p-polarised excitation yields images, which closely reproduce the sample, and the contrast is much higher in this case than for s-polarised waves. Particular attention is paid to the case of imaging of metallic nanoparticles, for which local and nonlocal versions of aggregate Mie theory are compared. Nonlocality arises from the excitation of longitudinal bulk plasmons at the particle surface. It is shown that this effect is essential in the imaging of such particles and implies comparatively rapid convergence, in contrast to the local theory. The converged images calculated within the nonlocal theory resemble the results of the local theory, when, arbitrarily, within the latter only dipole-dipole interactions are taken into account. Significant qualitative and quantitative differences, however, are shown to exist. Signal and contrast enhancements due to resonant excitation of surface plasmon polaritons are studied quantitatively using the results of the converged nonlocal theory.