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This paper reports the experimental observation, at optical frequencies, of the electromagnetic local density of states established by nanostructures corresponding to the recently introduced concept of optical corral [G. Colas des Francs et al., Phys. Rev. Lett. 86, 4950 (2001)]. The images obtained by a scanning near-field optical microscope under specific operational conditions are found in agreement with the theoretical maps of the optical local density of states. A clear functionality of detection by the scanning near-field optical microscope is thereby identified since the theoretical maps are computed without including any specific tip model.

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We present a study of the influence of the probe composition on the formation of constant height Photon Scanning Tunneling Microscope (PSTM) images when observing a dielectric sample. Dramatic effects due to the metallization of the tip are presented and discussed in details. We show how the recorded images can look quite different when the probe is dielectric or coated with gold.Comparison with numerical calculations indicate that the experimental signals are of electric or magnetic nature depending on the composition of the tip.For well-defined conditions, gold coated tips provide images of the distribution of the magnetic field intensity associated with the optical near-field. To get more insight into the origin of this intriguing effect, we have performed a careful analysis of the plasmon modes associated with a gold coated dielectric 1 cylinder. We conclude that the detection of the optical magnetic field is related to the conditions of excitation of circular symmetry plasmon, or ring plasmon, in the gold coated tip.

This Letter reports the experimental observation of light confinement effects by near-field optical microscopy. Depolarization effects giving rise to light confinement close to nanoscopic objects have been unambiguously observed in near-field optical images of subwavelength dielectric pads etched on a flat glass substrate. According to the incident polarization, this phenomenon leads to reverse contrasts in the near-field optical image of the same subwavelength objects. [S0031-9007(96)02036-4] PACS numbers: 07.79.Fc, 42.25.Fx, Although the influence of surface roughness on the farfield optical properties such as reflectivity or scattering cross sections was investigated for a long time, the knowledge of the optical field distributions close to small scatterers deposited on surfaces is relatively limited. Today, these optical near fields are observable by scanning nearfield optical microscopy. Their nontrivial distributions often make difficult the interpretation of near-field optical images.The main feature of all near-field optical microscopes is the nanometer-sized tip which is piezoelectrically driven to scan close to the sample surface. According to the experimental setup, the tip may be used as a local photon emitter or as a local probe of the optical near field [1][2][3][4].In this work, in order to study the optical near field around small scatterers, we need only to use the tip as a local probe as in the photon scanning tunneling microscope (PSTM). Tips used as local probes only are mostly bare and sharply elongated optical fibers without any metal coating [5][6][7]. The sample is deposited on a glass prism surface which allows a total internal reflection illumination. The nanometer-sized tip is then scanned above the sample surface, thereby frustrating the total reflection.Theoretical modeling of devices with emitting tips has been rather hindered by the critical dependence of these devices on the exact shape and composition of the tip. A recent review of the theoretical works which is intended to model near-field optical phenomena brought indeed to the fore that the most significant theoretical efforts dealt with the internal illumination geometry [8]. In the case of low dielectric contrast and weak roughness, the local probe used in PSTM can be considered as a passive element in the sense that multiple scattering between the tip and the sample does not contribute significantly to the field inside the detector.Recent theoretical works pointed out that PSTM images could be interpreted as the result of the scattering of quasitwo-dimensional surface waves by the various defects (asperities, kink, etc., . . .) lying on the sample surface [9]. The surface waves generated by total internal reflection are viewed as quasi-two-dimensional because they decay exponentially in the direction normal to the sample surface. The surface defects scatter these waves parallel to the surface and induce some loss due to diffraction which couples light to the external medium. By approaching a local probe very close to the sam...

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