Near-field optical microscopy with uncoated tips: Calibration, chemical contrast on organic crystals, and photolithography

Abstract

A low-temperature scanning confocal and near-field optical microscope Rev.Reflection scanning near-field optical microscopy with uncoated fiber tips: How good is the resolution really?Reflection-back-through-the-fiber scanning near-field optical microscopy ͑SNOM͒ measurements with uncoated tapered fiber tips, that remain cold, were performed under shear-force distance control for opaque and transparent rough samples. The feedback dynamics of the commercial ͑DME͒ RASTERSCOPE SNOM were tested with a 3000/500 nm gold grating at very high scan rates. Slow rates give rise to thermal broadening due to heating of the gold surface by partial light absorption. Enhancement of the light intensity at the base by reflection from steep walls on four sides is reported, the steepness contrast being less important. Doped gallium arsenide substrates give materials contrast. Extremely rough porous silicon has been probed. Organic chemical contrast is seen for the first time with SNOM on partly oxidized anthracene at large dislocations. Anthracene, anthraquinone, and their charge-transfer complex are clearly discriminated by different reflectivities. Irradiated ␣-cinnamic acid exhibits a chemically uniform surface and, thus, no SNOM contrast despite the anisotropic m high features that are observed. Photolithography on light-absorbing 2,5-dibenzylidene-cyclopentanone crystals with the tip illuminated at an Ϸ10 nm distance provides an easy, inexpensive, and rapid performance test of single tips. Very long local irradiation creates features that widen and reach heights of 300 nm.

The Enhancement Effect at Local Reflectance and Emission Back to Apertureless SNOM Tips in the Shear-Force Gap

“…They indicate the generality of the effect with very different types of materials and shear-force mechanisms. The slope values cover two magnitudes and that is the reason for the exceptionally strong optical contrast that is achieved with this type of apertureless SNOM even at small chemical differences [1,22,23,30]. Similar slope values were obtained when using 632.8 nm light [8].…”

“…It is therefore timely to remove severe misunderstandings and prejudices, inasmuch as our technique is outstanding for all of the important applications of SNOM, which are listed in the Introduction. There is no cross-talk between our SNOM and AFM, because we profit from the enhancement effect [1,2,[22][23][24], and if there is remaining doubt one can use a filter for removal of any presumed light from the control laser. This will be exhaustively demonstrated below.…”

“…Thus, optical images of chemically uniform gold gratings are not chemical contrast but rely on far-field light concentration by mirror effects [22]. True chemical contrast [22,23] must occur precisely at the site of the chemical species for being credible (the "golden rule"). This is also the basis for local spectroscopy if enhanced fluorescence or Raman emission is detected.…”

“…1) is less complicated, provides optical resolution equal to AFM resolution (or better), and avoids problems even with highly rough surfaces. It uses reflection back-to-the-uncoated-sharp-tip and was the first technique of apertureless SNOM [2,[22][23][24]. These tips are used both as emitter and enhanced reflection acceptor in the shear-force gap with undisturbed fast optical shear-force damping control.…”

Artifacts in scanning near-field optical microscopy (SNOM) due to deficient tips

“…29,32,34 Of course, topography on chemically and physically uniform surfaces must not give an optical contrast if sharp uncoated tips with apex angles of`10°are used (steepness contrast occurs at slopes that approach or exceed 80°) [28][29][30]35 and if local far-field light concentration is excluded. 34 Thus, topography itself does not give rise to optical contrast in properly done reflection SNOM using the variations in the near-field enhanced reflected light [ Fig. 1(c,d) is another example].…”