Near-field scanning optical microscopy (NSOM/SNOM) is a microscopy technique for nanostructure investigation that breaks the far field resolution limit by. AN EXAMPLE OF NEAR-FIELD OPTICAL MICROSCOPY Let us investigate an example of a practical nanometer- resolution scanning near- field optical. Evanescent Near Field Optical Lithography (ENFOL) is a low-cost high resolution Scanning Near-Field Optical Microscopy (SNOM or NSOM).
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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 NSOMalso frequently termed scanning near-field optical microscopy SNOMhas 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.
Near-field scanning optical microscopy is classified among a much broader instrumental group referred to generally as scanning probe microscopes SPMs. The theoretical resolution limit of conventional optical imaging methodology to nanometers for visible light was the primary factor motivating the development of recent higher-resolution scanning probe techniques, such as STM and atomic force microscopy AFMand previously, transmission electron microscopy TEM and scanning electron microscopy SEM.
These and related techniques have enabled phenomenal gains in resolution, even to the level of visualizing individual atoms.
However, prior to the development of near-field scanning optical methods, the superior resolution capabilities have come at the expense of the wide variety of contrast-enhancing mechanisms available to optical microscopy. Furthermore, the extreme specimen preparation requirements for most of the high-resolution methods have limited their application in many areas of study, particularly in biological investigations involving dynamic or in vitro measurements.
The method of near-field scanning optical microscopy combines the extremely high topographic resolution of techniques such as AFM with the significant temporal resolution, polarization characteristics, spectroscopic capabilities, sensitivity, and flexibility inherent in many forms of optical microscopy.
The interaction of light with an object, such as a microscope specimen, results in the generation of both near-field and far-field light components. The far-field light propagates through space in an unconfined manner and is the “normal” light utilized in conventional microscopy. The near-field or evanescent light consists of a nonpropagating field that exists near the surface of an object at distances less than a single wavelength of light.
Near-field scanning optical microscope – Wikipedia
Light fileetype the near-field carries more high-frequency information and has its greatest amplitude in the region within the first few tens of nanometers of the specimen surface. Because the near-field light decays exponentially within a distance less than the wavelength of the light, it usually goes undetected. In effect, as the light propagates away from the surface into the far-field region, the highest-frequency fjeld information is filtered out, and the well known diffraction-based Abbe limit on resolution is imposed.
By detecting and utilizing the near-field light before it undergoes diffraction, the NSOM makes available the full gamut of far-field optical contrast-enhancing mechanisms at much higher spatial resolution.
In addition to non-diffraction-limited high-resolution optical imaging, near-field optical techniques can be applied to chemical and structural characterization through spectroscopic analysis at filwtype beneath nanometers.
The most recent commercial NSOM instruments combine the scanning force techniques of an AFM with the optical detection capabilities of conventional optical microscopy. The overall NSOM design can vary significantly, depending upon the requirements nano,ithography the particular research project. One of the most common configurations is to incorporate the NSOM into an inverted fluorescence microscope.
By basing the NSOM on a conventional optical instrument, many of the microscopist’s familiar optical imaging modes are available in combination with near-field high-resolution capabilities. In addition to the nxnolithography information, NSOM can generate topographical or force data nqnolithography the specimen in the same manner as the atomic force microscope.
The two separate data sets optical and topographical can then be compared to determine the correlation between the physical structures and the optical contrast.
The real power of the NSOM technique may rest in this unique capability of combining a topographical data set with a variety of corresponding optical data at resolutions far better than is possible under the diffraction limitations of focused light.
Presented in Figure 1 is a near-field scanning instrument configured optixal a modern inverted optical microscope. Such an arrangement conveniently allows the NSOM head, incorporating the probe and its positioning mechanism, to be mounted at the specimen stage location, with the objective positioned beneath the stage.
The system illustrated in the figure includes an external laser to provide illumination, dield photomultiplier detector for optical signal collection, and a computer and electronic control unit for management of specimen and probe positioning and image acquisition.
Although the scanning probe microscope family encompasses a vast array of specialized and highly varied instruments, their common operational motif is the employment of a local probe in close interaction with the specimen. A typical SPM local probe is equipped with a nanometer-sized tip whose tip-to-specimen interactions can be sensed and recorded by a variety of mechanisms.
Each different type optucal SPM is characterized by specific properties of the local probe and the nature of its interaction with the specimen surface. A representation of the typical NSOM imaging scheme is presented in Figure 2, in which an illuminating probe aperture having a diameter less than the wavelength of light is maintained in the near field of the specimen surface.
Because close proximity or contact between the specimen and probe separation less than the wavelength is a general requirement for non-diffraction-limited resolution, the vast majority of all SPMs require a feedback system that precisely controls the physical separation of the probe and specimen.
In addition, an x-y-z scanner usually piezoelectric is utilized to control the movement of the probe over the specimen. The NSOM configuration illustrated in Figure 2 positions the objective in the far field, in the conventional manner, for collection of the image-forming optical signal.
Depending upon the design of the particular instrument, the x-y-z scanner can either be attached to the specimen or to the local probe. If the scanner and specimen are coupled, then the specimen moves under the fixed probe tip in a raster pattern to generate an image from the signal produced by the tip-specimen interaction. The size of the area imaged is dependent only on the maximum displacement that the scanner can produce.
A computer simultaneously evaluates the probe position, incorporating data obtained from the feedback system, and controls the scanning of the tip or specimen and the separation of the tip and specimen surface. The information generated as a result of sensing the interaction between the probe and specimen is nanolithoography and recorded by the computer point-by-point during the raster movement.
The computer then renders this data into two-dimensional data sets lines.
Two-dimensional data sets gathered by the NSOM instrument are subsequently compiled and displayed as a three-dimensional reconstruction on a computer monitor. The typical size scale of features measured with a scanning probe microscope ranges from the atomic level less than 1 nanometer to more than micrometers. The scanning probe microscopy family includes modalities based on magnetic force, electrical force, microscoly interactions, mechanical interactions, capacitance, ion conductance, Hall coefficient, thermal properties, and optical properties NSOM, for example.
NSOM images are typically generated by scanning a sub-wavelength aperture over the specimen in a two-dimensional raster pattern and collecting the emitted radiation in imcroscopy optical far-field, point-by-point.
Explore the difference between near-field scanning with the probe in feedback mode, in which the tip height varies in response to specimen topography, and scanning without feedback engaged. Previously developed high-resolution techniques, such as scanning electron microscopy, transmission electron microscopy, scanning tunneling microscopy, and atomic force microscopy, do not benefit from the wide array of contrast mechanisms available to optical microscopy, and in most cases, are limited to the study of specimen surfaces only.
Aside from the available contrast-enhancing techniques of staining, fluorescence, polarization, phase contrast, and differential interference contrast, optical methods have inherent spectroscopic and temporal resolution capabilities.
The high-resolution afforded by electron microscopy techniques nnanolithography achieved at the cost of greater limitations on acceptable specimen types viletype increased demands of sample preparation, including vacuum fieltype requirements, preparation of thin sections for transmission microscopy, and generally, the application of a conductive coating for non-conducting specimens STM also has this requirement.
For biological materials, specimen preparation is especially demanding, as complete dehydration is generally required prior to carrying out sectioning or coating. Although atomic force microscopy is free nanolithogdaphy many of these specimen preparation considerations, and can be applied to study specimens optival the atomic level in ambient conditions, the method does not readily provide spectroscopic information from the specimen.
An additional limitation is that AFM is not able to take advantage of the wide array of fielx dyes available to fluorescence microscopy. The NSOM method is particularly useful to nano-technologists nanolithograpjy, materials scientists, chemists, and biologists who require ultra-high resolution spatial information from the broad range of materials encountered in their varied disciplines.
Although newer near-field instrumentation techniques are being developed to image three-dimensional volume sets, NSOM has typically been limited to specimens that are accessible by a local probe, which is physically attached to a macroscopic scan head.
A schematic illustrating the control and information flow of an inverted optical microscope-based NSOM system is presented in Figure 3. The laser excitation source is coupled into a fiber optic probe for specimen illumination, with the probe tip movement being monitored through an optical feedback loop incorporating a second laser focused on the tip.
The motion of the probe tip, translational stage movement, and acquisition and display of optical and topographic or other force images is controlled through additional electronics and the system computer. Synge, beginning inpublished a series of articles that first conceptualized the idea of an ultra-high resolution optical microscope. Synge’s proposal suggested fielv new type of optical microscope that would bypass the diffraction limit, but required fabrication of a nanometer aperture much smaller than the light wavelength in an opaque screen.
A nwnolithography and embedded specimen would be ground optically flat and scanned in close proximity to the aperture.
While scanning, light illuminating one side of the screen and passing through the aperture would be confined by the dimensions of the aperture, and could be used to illuminate the specimen before undergoing diffraction. As long as the specimen remained within a distance less than the aperture diameter, an image with a resolution of 10 nanometers could be generated. In addition, Synge accurately outlined a number of the technical difficulties that building a near-field microscope would present.
Included in these were the challenges of fabricating the minute aperture, achieving a sufficiently intense light source, specimen positioning at the nanometer scale, and maintaining the aperture in close proximity to the specimen. The proposal, although visionary and simple in concept, was far beyond the technical capabilities of the time.
Experimental verification of the feasibility of Synge’s proposals had to wait until when E. Nicholls demonstrated the near-field resolution of a sub-wavelength aperture scanning microscope operating in the microwave region of the electromagnetic spectrum illustrated in Figure 4.
Utilizing microwaves, with a wavelength of 3 centimeters, passing through a probe-forming aperture of 1. Extension of Synge’s concepts to the shorter wavelengths in the visible spectrum presented significantly greater technological challenges in aperture fabrication and positioningwhich were not overcome until when a research group at IBM Corporation’s Zurich laboratory reported optical measurements at a subdiffraction resolution level.
An independent group working at Cornell University took a somewhat different approach to overcome the technological barriers of near-field imaging at visible wavelengths, and the two groups’ results began the development that has led to the current NSOM instruments. The IBM researchers employed a metal-coated quartz crystal probe on which an aperture was fabricated at the tip, and designated the technique scanning near-field optical microscopy SNOM. The Cornell group used electron-beam lithography to create apertures, smaller than 50 nanometers, in silicon and metal.
The IBM team was able to claim the highest optical resolution to date of 25 nanometers, or one-twentieth of the nanometer radiation wavelength, utilizing a test specimen consisting of a fine metal line grating. Although the achievement of non-diffraction-limited imaging at visible light wavelengths had demonstrated the technical feasibility of the near-field aperture scanning approach, it was not until after that the NSOM began to evolve as a scientifically useful instrument.
The mode of light propagation is primarily evanescent and parallel to the specimen surface when the radius of the illuminating source is less than one-third of the imaging light wavelength. In order to achieve an optical resolution greater than the diffraction limit the resolution limit of conventional optical microscopythe probe tip must be brought within this near-field region.
For NSOM, the separation distance between probe and specimen surface is typically on the order of a few nanometers. Radiation near the source is highly collimated within the near-field region, but after propagation of a few wavelengths distance from the specimen, the radiation experiences significant diffraction, and enters the far-field regime.
This tutorial illustrates a near-field scanning experiment utilizing a microwave resonator source, with a metal-on-glass specimen being scanned beneath an illuminating aperture in an opaque metal screen. There are two fundamental differences between near-field and far-field conventional optical microscopy: In conventional far-field optical microscopy, the distance between the light source and the specimen is typically much greater than the wavelength of the incident light, whereas in NSOM, a necessary condition of the technique is that the illumination source is closer to the specimen than the wavelength of the illuminating radiation.
At the heart of all scanning probe microscopy techniques is the scanning system.
Near-field scanning optical microscope
Its design and function are primary determinants of the attainable scan resolution. The scanner must have low noise small position fluctuations and precision positioning nanolihhography typically less than 1 nanometer.
The required precision of the probe positioning usually necessitates that the entire instrument rest on a vibration isolation table, or be suspended by some other means, to eliminate the transfer of mechanical vibrations from the building to the instrument. Low-noise electronics and high-voltage amplifiers having large dynamic range are necessary to drive the mkcroscopy actuators of the probe and specimen positioning systems.
For most NSOM applications, it is necessary to maintain the probe in constant feedback nicroscopy the specimen surface being imaged. Precise control of the probe is required because it must be maintained within the narrow near-field regime, but prevented from actually contacting the surface. The stringent requirement of maintaining a constant gap between the probe and the specimen is best met by employing a real-time feedback control system.
The advantages of this type of position control are numerous. Perhaps the most important consideration is damage to the probe tip or the specimen, which is likely if the two come into contact. Furthermore, it is possible for the tip to accumulate debris from the specimen fileype being scanned if contact is made. Although much less likely, this artifact can occur even with the tip under feedback control, especially if the feedback set point is not correctly chosen.
A further benefit of operating the probe scanning system with feedback control is to obtain accurate optical signal levels, eliminating the dramatic variations caused by the exponential dependence of these signals on the tip-to-specimen separation. The exponential variation of signal level with changing tip-to-specimen separation can produce artifacts in the image pptical do not accurately represent optical information related to the specimen.
A critical requirement of the near-field techniques is that the probe tip must be positioned and held within a few nanometers of tiletype surface in order to obtain high-resolution and artifact-free optical images, and this is not readily achieved without utilizing some form of feedback mechanism.
Several different techniques have been employed to monitor the z-position of the probe tip, and its instantaneous separation from the specimen surface. To date, the two fiiletype commonly employed mechanisms of tip positioning have been optical methods that monitor the tip vibration amplitude usually interferometricand a non-optical tuning fork technique.
Both of these are versions of the nanolighography feedback method and are described in more detail in a following section. In order to improve signal-to-noise ratios for the feedback signal, the NSOM tip is almost always oscillated at the resonance frequency of nexr probe. This allows lock-in detection techniques basically a bandpass filter with the fileyype frequency set at the reference oscillation frequency to be utilized, which eliminates positional detection problems associated with low-frequency noise and drift.