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Probing polarization and dielectric function of molecules with higher order harmonics in scattering–near-field scanning optical microscopy
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Image of FIG. 1.
FIG. 1.

Schematics of the frequency modulated s-NSOM instrument, with the heterodyne detection of the scattered light.

Image of FIG. 2.
FIG. 2.

Intensity of the -polarized light (left) and -polarized light (right) scattered from the HOPG/porphyrin–tip junction. The images displayed were acquired, respectively, at 150, 300, 450, and 600 kHz (image ). The line cross sections (left and right top plots) were taken as the average of the areas outlined by black boxes in the s-NSOM images. The porphyrin island on HOPG is outlined in green in these images. The sample topography is presented at the bottom of the figure.

Image of FIG. 3.
FIG. 3.

Topography of the porphyrin film on a HOPG substrate (top left image), obtained with low nc-AFM. Step I corresponds to the molecule where the porphyrin ring is perpendicular to the sample surface (phase 1) while step II corresponds to the molecule with the porphyrin ring parallel to the sample surface (phase 2). The top right image displays a representative height profile of the porphyrin island, showing that step I has an height, when step II has an height. The molecular ordering within step II was measured (bottom left image) and is highlighted in the bottom right image.

Image of FIG. 4.
FIG. 4.

(a) Schematic of the s-NSOM experiment in which the tip is illuminated by red light from the side. (b) Schematic that describes the data analysis model, which treats the probe tip within the context of a point-dipole approximation, and incoming light as a plane wave. (c) Diagrammatic representation of the physical model used in computations of scattered light intensity as a function of tip-substrate distance.

Image of FIG. 5.
FIG. 5.

Dependence of the difference between the scattering intensity from the HOPG and HOPG/porphyrin surfaces, normalized by the scattering intensity of the HOPG (ratio proportional to the experimentally measured signal in the s-NSOM experiment) as a function of the film dielectric constant for (a) incoming -polarized light and (b) incoming -polarized light. The results obtained for the first harmonic are presented in black, for the second in red, for the third in blue, and for the fourth in green. (c) The comparison of the s-NSOM contrast calculated (black) and measured (red) for different harmonics between the HOPG and the porphyrin film on the HOPG for a film dielectric constant equal to 3.

Image of FIG. 6.
FIG. 6.

Intensity maps of the s-NSOM contrast for the HOPG/porphyrin structure on the HOPG system illuminated with -polarized light. Contrast changes are displayed as a function of harmonic number (1–4).


Generic image for table
Table I.

Dielectric constants for the materials of interest for .


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Probing polarization and dielectric function of molecules with higher order harmonics in scattering–near-field scanning optical microscopy