Near-field microscopy

Near-field scanning optical microscopy (SNOM) is a technique that enables non-diffraction limited imaging and spectroscopy of a sample that is simply not possible with conventional optical imaging. In this techniques a sub-micron optical probe is positioned a very short distance from the sample and light is transmitted through a small aperture at the tip of this probe. The near-field is defined as the region above a surface with dimensions less than a single wavelength of the light incident on the surface. Within the near-field region evanescent light is not diffraction limited and nanometre spatial resolution is possible.
To extend the spatial resolution beyond the diffraction limit several experimental configurations are common in literature [1]. The above figure shows the main operational configurations of the near-field microscope. In (a) is presented the illumination configuration. In this case the an aperture probe illuminates a small area of a sample surface (reddish area under the SNOM probe). The resulting signal fromthe interaction with the sample is then collected in the far-field both in transmission on in reflection. This configuration can be also used to excite a photoluminescent signal of a sample in the near-field regime and then collect the luminescence in far-filed. In (b) the collection configuration is shown. In this case far-field light illuminates the sample (or it is coupled inside it) and the near-field signal is then collected by the probe. This particular technique is commonly used for the direct detection of localized field on the sample as in the previous configuration, can be exploited for photoluminescence measurements. In (c) is the illumination/collection configuration is presented. In this case the probe is used both to illuminate and collect the near-field signal. While in the first two configurations the near-field probe consists in a metal coated optical fiberin the illumination/collection case it is convenient  to use uncoated probes. Since this configuration is the mainly used in this thesis, it is convenient to highlight that it allows a huge sensitivity as compared with the other two cases or photoluminescent experiments. The price to be payed is that the resolution, in such configuration, will be lower with respect to the case of using coated near-field probe, but the resolution is always below the diffraction limit. Finally, in (d) the apertureless techniques is shown. In this configuration a strongly confined optical field is created at the apex of a sharply pointed probe tip by external far-field illumination. The relevant near-field enhanced signal therefore often has to be extracted from a huge background of far-field scattered radiation. Resolutions ranging from 1-20 nm have been reported in laboratory experiments. The general applicability of the technique to a wider range of samples is currently being investigated.