Dieter W. Pohl

Optical Stethoscopy : Image Recording with Resolution λ/20

Subwave length‐resolution optical image recording is demonstrated by moving an extremely narrow aperture along a test object equipped with fine‐line structures. Details of 25‐nm size can be recognized using 488‐nm radiation. The result indicates a resolving power of at least λ/20 which is to be compared with the values of λ/2.3 obtainable in conventional optical microscopy.

Near‐field optical‐scanning microscopy

Near‐field optical‐scanning (NFOS) microscopy or ‘‘optical stethoscopy’’ provides images with resolution in the 20‐nm range, i.e., a very small fraction of an optical wavelength. Scan images of metal films with fine structures presented in this paper convincingly demonstrate this resolution capability. Design of an NFOS microscope with tunnel distance regulation, its theoretical background, application potential, and limitations are discussed.

Scanning near-field optical microscopy with aperture probes: Fundamentals and applications

In this review we describe fundamentals of scanning near-field optical microscopy with aperture probes. After the discussion of instrumentation and probe fabrication, aspects of light propagation in metal-coated, tapered optical fibers are considered. This includes transmission properties and field distributions in the vicinity of subwavelength apertures. Furthermore, the near-field optical image formation mechanism is analyzed with special emphasis on potential sources of artifacts. To underline the prospects of the technique, selected applications including amplitude and phase contrast imaging, fluorescence imaging, and Raman spectroscopy, as well as near-field optical desorption, are presented. These examples demonstrate that scanning near-field optical microscopy is no longer an exotic method but has matured into a valuable tool.

Facts and artifacts in near-field optical microscopy

Near-field optical (NFO) microscopes with an auxiliary gap width regulation (shear force, tunneling) may produce images that represent the path of the probe rather than optical properties of the sample. Experimental and theoretical evidence leads us to the conclusion that many NFO results reported in the past might have been affected or even dominated by the resulting artifact. The specifications derived from such results for the different types of NFO microscopes used therefore warrant reexamination. We show that the resolving power of aperture NFO microscopes, 30–50 nm, is of genuine NFO origin but can be heavily obscured by the artifact.

Dynamic piezoelectric translation devices

The principle of inertial sliding of a platform on a periodically accelerated support is exploited for the design of a piezoelectric fine‐positioning device. The device provides step sizes of 0.04–0.2 μ, speeds of up to 0.2 mm/s, and practically unlimited translation range. It is powered by a sawtooth electric waveform of 60–300‐V amplitude and useable for loads of up to 1 kg and probably even more. Mechanical parts and driver electronics are extremely simple, reliable, and easy to operate.

Scanning near-field optical microscopy

Scanning Near-field Optical Microscopy (SNOM) allows the investigation of optical properties on subwavelength scales. During the past few years, more and more attention has been given to this technique that shows enormous potential for imaging, sensing and modification at near-molecular resolution. This article describes the technique and reviews recent progress in the field.

Scanning near-field optical probe with ultrasmall spot size

A novel light-emitting probe for scanning near-field optical microscopy is investigated theoretically. The three-dimensional vectorial Helmholtz equation is solved for the new probe geometry by using the multiple multipole method. The novel probe consists of a dielectric tip that is entirely metal coated. It provides a single near-field spot that can be smaller than 20 nm (FWHM). The dependence on tip radius, taper angle, and metal thickness in front of the tip is investigated for the power transmission through the probe as well as for the spot size.

Scanning tunneling potentiometry

In certain problems of electrical transport through condensed matter, it is important to know the potential distribution with microscopic resolution, e.g., at interfaces (Schottky barriers) or pn junctions. Scanning tunneling potentiometry, a new application of scanning tunneling microscopy, is capable of providing this information. The tunnel current is used for simultaneously sensing probe-to-sample distance and local potential. The method was tested with a gold-island metal-insulator-metal structure.

Near‐field optics: Microscopy with nanometer‐size fields

The electromagnetic fields that can build up around metallic or dielectric pointed tips are of increasing interest in context with the new scanning probe microscopies (tunneling, near‐field optics, Coulomb and van der Waals forces etc.). The paper presents exact solutions of Laplace’s equations for the tip/sample geometry. For suitable media, plasmons are found whose electric fields are highly localized in the gap region. We believe that the field enhancement associated with such tip plasmons is instrumental for inelastic tunneling and light emission during scanning tunneling microscopy.

Near‐field optical scanning microscopy in reflection

The resolution of near‐field optical scanning microscopy (NFOS) is determined by the dimensions of the microscopic light source rather than the diffraction limit. To demonstrate NFOS in reflection, intensity changes in the (backward) scattering from a 70–100 nm diam hole in a metal film were recorded while the sample was scanned in close proximity to this aperture. Raster‐scan images of a planar metal test pattern yield a resolution comparable to the size of the aperture.