We map in real space and by purely optical means near-field optical information of localized surface plasmon polariton (LSPP) resonances excited in nanoscopic particles. We demonstrate that careful polarization control enables apertureless scanning near-field optical microscopy (aSNOM) to image dipolar and quadrupolar LSPPs of the bare sample with high fidelity in both amplitude and phase. This establishes a routine method for in situ optical microscopy of plasmonic and other resonant structures under ambient conditions.

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Direct near-field optical imaging of higher order plasmonic resonances.

(Received 23 August 2002; revised manuscript received 7 November 2002; published 14 February 2003) We develop a theory to study apertureless scanning near-field optical microscopy which takes into account retardation, higher multipoles of the tip, and the multiple scattering between the tip and the surface. We focus on metallic systems and discuss the implication of the formation of tip-induced surface plasmon modes in the tip-surface system. We discuss the effects associated with the shift in energy of those modes as a function of the tip-surface distance. Both the local field and the scattering cross section are enhanced when the tip approaches the surface, but there is no general correspondence between the two enhancements.

Resonance shift effects in apertureless scanning near-field optical microscopy

In coherent homodyne apertureless scanning near-field optical microscopy (ASNOM) the background field cannot be fully suppressed because of the interference between the different collected fields, making the images difficult to interpret. We show that implementing the heterodyne version of ASNOM allows one to overcome this issue. We present a comparison between homodyne and heterodyne ASNOM through near-field analysis of gold nanowells, integrated waveguides, and a single evanescent wave generated by total internal reflection. The heterodyne approach allows for the control of the interferometric effect with the background light. In particular, the undesirable background is shown to be replaced by a controlled reference field. As a result, near-field information undetectable by a homodyne ASNOM is extracted by use of the heterodyne approach. Additionally, it is shown that field amplitude and field phase can be detected separately.

Apertureless scanning near-field optical microscopy: a comparison between homodyne and heterodyne approaches

Scattering-type scanning near-field optical microscopy (s-SNOM) allows for nanoscale-resolved Infrared (IR) and Terahertz (THz) imaging, and thus has manifold applications ranging from materials to biosciences. However, a quantitatively accurate understanding of image contrast formation at materials boundaries, and thus spatial resolution is a surprisingly unexplored terrain. Here we introduce the read/write head of a commercial hard disk drive (HDD) as a most suitable test sample for fundamental studies, given its well-defined sharp material boundaries perpendicular to its ultrasmooth surface. We obtain unprecedented and unexpected insights into the s-SNOM image formation process, free of topography-induced contrasts that often mask and artificially modify the pure near-field optical contrast. Across metal-dielectric boundaries, we observe non-point-symmetric line profiles for both IR and THz illumination, which are fully corroborated by numerical simulations. We explain our findings by a sample-dependen...

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Understanding the Image Contrast of Material Boundaries in IR Nanoscopy Reaching 5 nm Spatial Resolution

We study the tip-substrate interaction in apertureless scanning near-field optical microscopy (aSNOM) with a three-dimensional model of both near and far fields. Starting from the analytically well understood two-sphere model, we obtain a realistic tip-substrate model by continuous deformation of the upper sphere into an elongated conical tip several wavelengths long with a hemispherical apex radius of just a few nanometers. The other sphere is enlarged to represent a locally flat substrate surface below the tip apex. This adiabatic transition toward the final system helps us to understand the origin of the different contributions. We find that dielectric tips, in spite of the absence of plasmonic resonances, are quite suitable for aSNOM. This is supported by experimental results. Working with the sharpest commercially available Si tips holds promise for pushing the routinely achievable lateral resolution down to the sub-10-nm range. At $1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ tip-substrate distance, we obtain local-field enhancements of many tens. The instantaneous, elastically backscattered intensity is calculated for variable tip-substrate distances to study demodulation at higher harmonics of an oscillating probe. We confirm that the local optical information is discriminated even against strong background signals, provided the vibration amplitude is kept small. However, this discrimination scheme is rather sensitive to crosstalk from anharmonic motion and care must be exercised in choosing appropriate experimental parameters to achieve reliable optical contrast.

Tip-substrate interaction in optical near-field microscopy

Modulation of the probe height in a scanning near-field optical microscope (SNOM) is a technique that is commonly used for both distance control and separation of the near-field signal from a background. Detection of higher harmonic modulated signals has also been used to obtain an improvement in resolution, the elimination of background, or artifacts in the signal. This article presents a theoretical model for the effects induced in SNOM images by modulation of the probe. It is shown that probe modulation introduces a spatial filter into the image, generally suppressing propagating field components and enhancing the strength of evanescent field components. A simple example of detection of a single evanescent field above a prism is studied in some detail, and a complicated dependence on modulation parameters and waveform is shown. Some aspects of the application of this theory in a general experimental situation are discussed. Simulated images are displayed to explicitly show the effects of varying modulation amplitude with first and second harmonic detection. Finally, we discuss the suppression of background artifacts due to propagating fields through the use of higher harmonic detection.

J. N. Walford

Papers

Influence of tip modulation on image formation in scanning near-field optical microscopy