Research on near-field distribution of nanometric apertures, fiber-optic probes and influence of parameters of metal-coated probes

In this paper near-field distributions of nanometric apertures, the uncoated and metal-coated fiber-optic probes used in near-field scanning optical microscope are characterized by the method of finite-difference time-domain. Moreover, to give a clear view of the influence of parameters, say, the taper angle (a), the aperture diameter (d), the refractive index of the fiber-optic (n) and the sample (ns), the tip-sample separation (z), on the throughput of metal-coated fiber-optic probes, we investigated it in detail. The result demonstrates that the polarized incident wave becomes depolarized on emitting out of the aperture as well as two kinds of probes. it implies that two electric-field components with the polarization direction perpendicular to that of the incident wave are produced. We discussed and analyzed the relationship between the near-field distribution and depolarization in detail. In addition, the impact of aperture shape on the near-field distribution is also considered. The field distributions of the two kinds of probes reveal that both the near-field distribution and the field distributed inside the probe are quite different from one another because of the distinction of the wave-guide structures between the two kinds of probes. What is more, we analyze quantitatively the effects of the metal-coated probes' parameters variation on the probe throughput. The results are that the throughput increases faster than exponentially with the increase of a, d and n. In addition, the sample existing near to the probe will change the near-field distribution of the probe. Namely, the smaller the tip-sample separation is the more strong the interaction of the tip and the sample is. Finally, the increase of the refractive index of the sample will enhance the tip-sample interaction. Accordingly, we propose that the probe throughput will be improved if the probe is immersed into a kind of high refractive-index liquid.