Near-Field Scanning Optical Microscopy
For ultra-high optical resolution, near-field scanning optical microscopy (NSOM) is currently the photonic instrument of choice. Near-field imaging occurs when a sub-micron optical probe is positioned a very short distance from the sample and light is transmitted through a small aperture at the tip of this probe. The near-field is defined as the region above a surface with dimensions less than a single wavelength of the light incident on the surface. Within the near-field region evanescent light is not diffraction limited and nanometer spatial resolution is possible. This phenomenon enables non-diffraction limited imaging and spectroscopy of a sample that is simply not possible with conventional optical imaging techniques.
Introduction - A fundamental principle in diffraction-limited optical microscopy requires that the spatial resolution of an image is limited by the wavelength of the incident light and by the numerical apertures of the condenser and objective lens systems. The development of near-field scanning optical microscopy, also frequently termed scanning near-field optical microscopy (SNOM), has been driven by the need for an imaging technique that retains the various contrast mechanisms afforded by optical microscopy methods while attaining spatial resolution beyond the classical optical diffraction limit.
NSOM probes - The optical probe is the most critical part of the near-field microscope for achieving high resolution images. There are many different ways to fabricate and characterize near-field optical probes. This section introduces a few of the methods that are commonly used in near-field microscopy.
Interactive Java Tutorials
Near-field Scanning Optical Microscope Simulation - In order to create a near-field image, the NSOM probe tip is scanned over the specimen with data collection occurring at defined intervals during scanning. This interactive tutorial explores the difference between scanning with the probe in feedback mode, in which the tip height varies in response to specimen topography, and scanning without feedback engaged.
First Experimental NSOM - In 1972, E. A. Ash and G. Nicholls, from the University College in London, demonstrated the near-field resolution of a subwavelength aperture scanning microscope operating in the microwave region of the electromagnetic spectrum. Utilizing microwaves, with a wavelength of 3 centimeters, passing through a probe-forming aperture of 1.5 millimeters, the probe was scanned over a metal grating having periodic line features. Both the 0.5-millimeter lines and 0.5-millimeter gaps in the grating were easily resolvable, demonstrating sub-wavelength resolution having approximately one-sixtieth (0.017) the period of the imaging wavelength. This interactive tutorial explores the Ash and Nicholls experiment.
Mechanical Oscillators - When monitored by an oscillatory feedback method, the NSOM probe is typically driven at its resonance frequency. A probe's frequency response is dependent upon the values of the spring constant, mass, and damping coefficient. The mechanical system examined in this tutorial represents the interaction of these parameters for both the tuning fork oscillator and the bent optical probe NSOM configurations. In practice, the probe is driven through a range of frequencies to generate a frequency spectrum such as that created interactively in the tutorial. The feedback resonance frequency is then set to a value corresponding to the peak in the probe frequency response curve.
Van der Waal Forces - In the near-field scanning microscopy configuration, several forces exist between the probe tip and the specimen, the most important of which are the van der Waals forces. The potential energy between the tip and the specimen can be expressed as a function of the distance between the lowest atom on the tip and the nearest specimen surface atom. The van der Waals contribution to the total tip-specimen force varies with the electronic properties of the particular atoms involved. This tutorial explores the dependence of these forces on the distance between the NSOM probe tip and the specimen, for a variety of atom pairs.
NSOM Probe Aperture Throughput - Although minimizing the size of the NSOM probe tip aperture is a primary factor in achieving high image resolution, a sufficient diameter to provide the desired optical signal output level must be maintained. The aperture diameter can be controlled by modifying the tip physical characteristics, for example, by changing the heating and pulling parameters, or the etching variables, or by varying the angle of the fiber during metal evaporation.
Unique Reflection NSOM Mode - Imaging opaque specimens by the NSOM method requires the application of a reflection configuration, such as the oblique collection or oblique illumination mode. This tutorial presents a unique (and more difficult) configuration for NSOM imaging of opaque specimens, which can produce exceptional results. The technique requires passing an optical fiber probe through holes drilled in the center of the objective lens elements, for delivery of near-field illumination.
Thermal Effects on NSOM Probes - Thermal heating of the NSOM probe occurs in the taper region due to the absorption of light by the metallic coating engulfing the exterior of the probe. In illumination mode, light is coupled into the core of the fiber, and upon reaching the tapered region of the probe, begins to propagate through the cladding to the metal coating. Because the metal has a non-unity reflection coefficient, some photon absorption occurs in the metal layer, and heat is generated. The optical signal transmitted through the probe is limited by the ability of the tapered region to physically tolerate and to dissipate this heat.
Literature References and Web Resources
Selected Literature References - A number of books and review articles covering important topics in near-field scanning optical microscopy have been published by leading researchers in the field. This section contains periodical location information about these books and articles, as well as providing a listing of selected original research reports from this cutting-edge field of research.
Near-field Scanning Optical Microscopy Web Resources - Near-field scanning optical microscopy provides a technique for examining specimens with ultra-high spatial optical resolution that greatly expands the analytical toolbox of the microscopist. Listed below are links to resources on the web for near-field scanning optical microscopy, including university and government laboratories, technical tutorials, and microscope manufacturers.
Jeremy R. Cummings, Matthew J. Parry-Hill, Robert T. Sutter, Thomas J. Fellers, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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