Scanning aperture probe

B. Hecht, B. Sick, U. P. Wild, V. Deckert, R. Zenobi, O. J. F. Martin and D. W. Pohl, Scanning near-field optical microscopy with aperture probes Fundamentals and applications, J. Chem,. Phys. 112, 7761 (2000). 

Scanning Near-Field Optical Microscopy with Aperture Probes... 

Hecht B, Sick B, Wild UP, Deckert V, Zenobi R, Martin OJF, Dieter DW (2000) Scanning near-field optical microscopy with aperture probes fundamentals and application. J Chem Phys 18 112... 

W Noell, M Abraham, K Mayr, A Ruf, J Barenz, O Hollricher, O Marti, P Guthner. Micromachined aperture probe tip for multifunctional scanning probe microscopy. Appl Phys Lett 70 1236-1238, 1997. 

Lehier C, Frey L, Petersen S, Sulzbach T, Ohlsson O, Dziomba T, Danzebrink HU, Ryssel H. Fabrication of silicon aperture probes for scanning near-field optical microscopy by focused ion beam nano machining. Microelectron Eng 2001 57-8 721-728. 

The principle of the acquisition system is to translate the probe into a tube (including hemispherical drilled holes) step by step, every 0.04 mm, after a forwards and backwards 360 rotation of the tube trigging every 0.2° angular step a 360° electronic scanning of tube with the 160 acoustic apertures. During the electronic scanning the tube is assumed to stay at the same place. The acquisition lasts about 30 minutes for a C-scan acquisition with a 14 kHz recurrence frequency. 

Using a visible light probe NSOM is the eadiest of the probe scopes, at least in conception, and is another apparent exception to the diffraction-liinited resolution rule, in that NSOM illuminates an object with a beam of visible light smaller than the diffraction limit. The resolution then is limited only by the size of that beam. To achieve this, light issuing from a very tiny aperture at the end of a glass capillary scans a very near sample. The tip must be located on the order of X/2 from that surface. Resolution in the range of 10—20 nm has been achieved (31). 

Comprehensive experimental investigations of 2PA processes in fluorene derivatives were performed by Hales et al. [53,56-59] with open aperture Z-scan [26], two-photon induced fluorescence [60] and femtosecond white-light continuum pump-probe methods [61]. For degenerate two-photon excitation, the experimental 2PA spectra of symmetrical and asymmetrical fiuorenes are presented in Figs. 15 and 16. These spectra were obtained with the combination of open aperture Z-scan and two-photon fluorescence methods [57]. For centrosymmetric molecules, two-photon transitions from... 

Near-field Raman imaging with a scanned probe has been reported [18, 19]. However, the technique is painfully slow (5-10 h, even for strong scatterers) and it has found very little use. Acquisition time can be decreased by using a polystyrene bead as a very high numerical aperture immersion lens. Working at 532 nm, Kasim et al. used a 60x/1.2 immersion aperture as an optical tweezer to simultaneously position the bead and operate it as a high NA lens [20]. They obtained a spatial resolution of about 80 nm on doped silicon with a few minutes scan time (Fig. 5.1). However, because of the need for a relatively smooth surface and a very intense scatterer, this technique is not likely to find much application in biomedical or pharmaceutical applications [21, 22],... 

Similar to all scanning microscopies, the resolution in SFM depends on the effective size of the probe and its modifications which arise from sample-probe interactions. Theoretically, the effective size is determined by the probe geometry and the force-distance dependence between the tip and sample. In addition, the aperture increases because of the tip-sample deformation, surface roughness, capillary forces, and various sources of noise. Experimentally, the resolution is limited by the sensitivity of the force detection system, the image noise, and the scanner precision. 

Fig. 3. Typical setup of a scanning near-field optical microscope. Excitation light is coupled into a single-mode fiber with a metal coated taper at its far end. The light emitted by the aperture illuminates a region of the samples whose size is determined by the aperture diameter and the distance between probe and sample. Light from the interaction region is collect using a conventional optical microscope.

While the experimental details involved in implementing NSOM can be found elsewhere [7,8], it is instructive to briefly discuss the two main obstacles that must be overcome in order to conduct NSOM measurements. These revolve around aperture formation and implementing a feedback system for tip-sample distance control. For the former, as in all scanning probe techniques, the quality of the measurements is in large part dictated by the quality of the probe. For the latter, as the schematic in Fig. 1 suggests, high resolution requires that the NSOM probe be maintained within nanometers of the sample surface. 

Fig. 2. (Left) Scanning electron micrograph of an Al-coated near-field probe, fabricated from a tapered optical fiber. (Right) Optical micrograph of a typical NSOM probe with light exiting the aperture at the end.

The principle of the scanning transmission electron microscope (STEM) is, at first glance, very different from that of the transmission electron microscope the electrons are focused on a probe scanned on a sample and the transmitted electrons are detected on a scintillator via a collection aperture. There is, however, a so-called reciprocity relationship between transmission electron microscopy and the STEM that can be used to describe image formation using the same formalism and facilitates the understanding of contrast. 

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