3.2 Introduction
3.2.1 Near-Field Scanning Optical Microscopy Overview
Modern science relies strongly on spectroscopic and microscopic techniques for material characterization. Optical microscopy has long been an important and powerful characterization tool in the area of scientific research, as it provides a considerable amount of information about structure and dynamics as a result of the interaction of light with matter. The combination of spectroscopy and microscopy is especially useful in material characterization when spectral features can be spatially resolved. Resolution of an optical system is determined by how close two objects can reside and still be
distinguished as two separate objects.
The diffraction limit in conventional optical microscopy arises from the size of the spot to which a light beam can be focused with normal lens elements. Ernst Abbe1, in
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1873, determined that the diffraction limit sets the maximum achievable lateral resolution of objects in a light microscope to:
d = 0.61 ( λ0 / n sin θ ) (1) where, d is the distance between two resolvable objects, λ0 is the vacuum wavelength of the incident radiation, n is the refractive index of the medium the light travels in and θ is the light convergence angle for the focusing element. The denominator in Equation 1, n sin θ, is also referred to as the numerical aperture (NA) of the objective. Numerical apertures of 1.3-1.4 are possible with high quality objective lenses and working in mediums of water or oil (immersion lenses), simplifying the right side of Equation 1 to λ/2. Diffraction therefore limits the resolution of an ordinary optical system to the characteristics of the light source and the photo-detector along with the optical elements in between. For conventional optical microscopy with a visible light source, resolution is diffraction limited to 200-300nm. However, in an apparatus-limited measurement the resolution is improved considerably when a sub-wavelength structure is utilized, enabling microscopic electromagnetic interactions to couple with the incident and outgoing light waves. Near field optics and photonics are classified as apparatus-limited measurements, and interactions occurring on a sub-wavelength scale are relevant to the optical properties of interest.
Near-Field imaging occurs when light is transmitted through an optical probe with a sub-wavelength aperture that is positioned a short distance from the sample surface.
The near-field is defined as the region that is less than one wavelength of the incident light above the sample surface. Within this short distance above the sample surface, the evanescent light is not diffraction limited resulting in nanometer scale spatial resolution.
The near-field phenomenon allows for non-diffraction limited optical microscopy and spectroscopy with resolutions that are not possible with conventional optical imaging techniques.
Near-Field Scanning Optical Microscopy (NSOM) is a relatively new analytical technique capable of providing optical information of materials with a resolution better than 50 nanometers. The chief use of NSOM is to generate high lateral resolution images
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of optical transmission, fluorescence emission, and birefringence from transparent samples. A sharp optical probe tip with nanometer dimensions generates optical information, in the near-field, by sending, collecting, or diffracting light at the sample surface. Laser light coupled into a single mode optical fiber passes through a sub- wavelength aperture at the end of the fiber. Transmitted light or laser induced
fluorescence emission is collected by an objective and imaged onto a detector. An image is formed by raster scanning the sample and recording the collected light intensity as a function of scan position.
The idea of near-field optical microscopy is not a new one, as the theory first appeared in a 1928 paper by Edward H. Synge.2 Given a certain excitation wavelength and an aperture of sub-wavelength dimension, the optical near-field was defined by Synge as the region of space less than one wavelength from the light source within which diffraction does not occur. Synge proposed the use of a scanned aperture to construct a microscope whose resolution was a function of only the size of the light source and the distance from the sample providing resolution far beyond the diffraction limit. When the aperture becomes considerably smaller than the wavelength of light, the transmitted electric field is localized close to the opening and the intensity decreases exponentially, (evanescent waves) away from the opening.3 Synge’s ideas were ahead of the technology available at the time and experimental demonstrations of his idea had to wait for
technology able to fabricate a sub-wavelength size probe tip, and control the tip with high precision, as well as computer power capable of digital image processing.
The first experimental demonstration of near-field scanning microscopy was carried out nearly half a century after the initial near-field paper by Synge. Ash and Nichols4 used 3 cm wavelength microwaves to achieve λ/60 resolution in the first experimental demonstration of NSOM in 1972. The first modern NSOM experiments using visible light were credited to the simultaneous work of several groups in the mid 1980’s. D.W.
Pohl,5 et al. in 1984 recorded a sub-wavelength resolution optical image by moving an extremelynarrow aperture along a test object equipped with fine-line structures. Detail, 25 nm in size, was recognized using 488-nm radiation.The result indicated a resolving power of at least λ/20which was compared with the values of λ/2obtainable in
conventional optical microscopy. Also in 1984 Lewis et al.,6 describe a scanning optical
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microscope based on near-field imaging with 300Å diameter aperture capable of resolution as small as one-tenth the wavelength of incident light. Lewis et al.,6 also
demonstrated the use of a near-field microscope in 1984 capable of sub-micron resolution.
Although the technique is finding many applications in the area of glass and photonics research, the overall success of the technique is limited by problems associated with producing high quality, reproducible, and robust optical fiber probe tips, and
continues to be largely a technique in progress today. Betzig and Trautman7 introduced the first aperture probe in 1992 which allowed for sub-diffraction fluorescence imaging and further stimulated interests in material and biological sciences.
The quality and properties of the tip is one of the most important aspects of a scanning probe microscopy technique. The geometry of the tip end and the interactions of the tip and the sample determines the instrument’s overall performance and ultimately limits the resolution. Near field scanning optical microscopy (NSOM), even more so than other scanning probe techniques, requires very well characterized optical probes for light emission and collection. Optical information with resolution as good as 10
nanometers has been achieved with NSOM, however consistency and quality of the optical fiber probes have limited the overall success of the technique. The ideal NSOM fiber probe is characterized by high transmission of light through an aperture with a diameter of tens of nanometers. Desirable properties of NSOM probes are high brightness, a well defined circular aperture, no loss of light through the sides of the tapered region, and a high damage threshold.
The transmission coefficient of an aperture fiber probe at a given wavelength is defined as the ratio of the light power emitted by the aperture to the power of the light coupled into the taper region. Transmission coefficients as high as 10-4 have been reported, however typical transmission coefficients of glass fiber probes are in the range of 10-5 to 10-6.8 This means that only one photon in approximately one million is
available for measurement. A second problem with the current state of optical fiber NSOM probes is poor damage thresholds. The optical fiber probes are coated with metal to absorb stray light at the tapered fiber end, where light is no longer isolated to the fiber core by total internal reflection. The metal/glass interface is a weak link in the integrity of the fiber, and spalling of the metal occurs at a certain power level, known as the
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damage threshold. More light demanding applications, such as NSOM Raman and NSOM laser ablation, require much higher transmission coefficients and higher damage thresholds for brighter sample illumination.8
The following sections will briefly review the modes of NSOM operation,
describe some promising applications and results of the technique, and review the current state of NSOM probes.