31 Near-field Scanning Optical Microscope (NSOM)

Dr. Anchal Srivastava

Contents of this Unit

1.  Introduction

1.1 History

1.2  Concept of far field and near field

1.3  What is Near-field Scanning Optical Microscope (NSOM)?

2. Constructions and Working Principle

3. Diffrents modes of operation inNSOM

4.  Variants of Probes in NSOM

5. Feedback Mechanism in NSOM

6.  Summary

Learning Outcomes

• After studying this module, you shall be able to learn
• The basic principles of NSOM operation The origin of images and their formation
• Know about the different modes and feedback mechanism of NSOM

1.  INTRODUCTION

The ability to view and study samples under high magnification is very important in many disciplines such as the biological sciences and materials research. Traditionally, optical techniques have been the most widely employed for these purposes given their long historical development, noninsensitivity, specificity, ease of use, and relatively low cost. However, the spatial resolution attainable with conventional optical techniques is limited to approximately half the wavelength of the light source used. For visible radiation, this results in a theoretical resolution limit of 200-300 nm which is restrictive for many applications. This limitation motivated the development of higher resolution techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) along with the recent emergence of other scanning probe techniques such as atomic force microscopy (AFM) and scanning tunneling microscopy (STM). The introduction of these and related forms of microscopy have brought about fantastic gains in resolution to the point where it is now possible to image and study single atoms.. 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.

1.1 HISTORY

The history of microscopy starts with the invention of Lenses. Lens is the base element of a conventional microscope. The first written document describing more precisely optical properties of lenses was authored by R.Bacon in 1267. There were several attempts to use lenses in order to make a microscope. It is A. Leeuwenhoeck Who is most often named as the inventor of microscope. In the 17th century he made a very simple instrument, based on a single glass lens, which enabled him to discover bacteria, sperm cells and blood cells. Leeuwenhoek’s skill at grinding lenses enabled him to build microscopes that magnified over 200 times. His contemporaries R. Hooke in England and J. Swammerdam in the Netherlands, started building microscopes using two or more lenses. These are very similar to microscopes in use today. Development of conventional microscopes is still underway today: perfection in the shape of lenses, sophisticated combinations of illumination and collection lenses, use of immersion objectives and confocal microscopes are just a few examples of this improvement.

Edward H. Synge, beginning in 1928, published a series of articles that first conceptualized the idea of an ultra-high resolution optical microscope. Synge’s proposal suggested a new type of optical microscope that would bypass the diffraction limit, but required fabrication of a 10-nanometer aperture (much smaller than the light wavelength) in an opaque screen. A stained 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.

Figure 1: Synge’s representation of NSOM

Extension of Synge’s concepts to the shorter wavelengths in the visible spectrum presented significantly greater technological challenges (in aperture fabrication and positioning), which were not overcome until 1984 when a research group at IBM Corporation’s Zurich laboratory reported optical measurements at a sub diffraction 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 488-nanometer radiation wavelength, utilizing a test specimen consisting of a fine metal line grating.

1.2 CONCEPT OF NEAR FIELD AND FAR FIELD

Figure 2: Near field and Far field view

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. Light in 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 spatial information is filtered out, and the well known diffraction-based Abbe limit on resolution is imposed.

1.3 WHAT IS NEAR FIELD SCANNING OPTICAL MICROSCOPE (NSOM/SNOM)?

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 (NSOM), 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.

Figure 2: Digital image of NSOM

Near-field Scanning Optical Microscopy (NSOM) is a true optical microscopic technique allowing fluorescence, absorption, reflection and polarization contrast with the additional advantage of nanometer lateral resolution, unlimited by diffraction and operation at ambient conditions.

In NSOM, a probe with a sub-wavelength apex is scanned over the sample while maintaining a (probe-sample) distance of a few nanometres (as depicted in Figure 3). Since NSOM utilizes the advantages of both optical microscopy and SPM, it may be envisioned as a combination of both.

Figure 3: A representation of the typical NSOM imaging scheme

As already mentioned, NSOM operates in the near field zone, that is, in a region, where probe-sample distance remains below the excitation wavelength. Figure schematically represents the different zones together with the optical wave’s Characteristics. In the near field zone, the evanescent waves associated with the small details (Numerical Aperture size (d) << λ) of the investigated sample, become prevalent. However, these evanescent waves decay exponentially with the distance to the surface, explaining the need to control and maintain the probe-sample distance in sub-wavelength domain. To control the probe sample distance, the main implementations are shear (lateral) force, normal force (tapping) similar to AFM modes, or tunneling current feedback. Once this distance control is established, the probe is scanned over the sample plane and the topographic and optical signals are simultaneously recorded point-by-point to reconstruct an image. In addition, an x-y-z scanner (usually piezoelectric) is utilized to control the movement of the probe over the specimen. 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 collected and recorded by the computer point-by-point during the raster movement. The computer then renders this data into two-dimensional data sets (lines).

2.  CONSRTRUCTION AND WORKING PRINCIPLE OF NSOM

Figure 4: Near-field optical microscope used with the microfabricated probes

In order to achieve an optical resolution greater than the diffraction limit (the resolution limit of conventional optical microscopy), the 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.

In order to test the tips a near-field optical microscope system was built in which these probes could be mounted. The apparatus is shown in Fig. 4. The laser is a 8 mW HeNe laser focused onto a pinhole, which is subsequently imaged on the base of the tip using a Nikon extra long working distance (ELWD) objective with NA 0.5. The ELWD objective is necessary to provide space for the beam deflection system. The transmitted light through tip and sample is collected using a similar objective. Background light is further reduced using a pinhole in front of a photomultiplier tube. The resulting setup effectively consists of a confocal microscope with the probe inserted in its focus.

The sample is scanned using a three-axis piezo-electric scanner, consisting of a Photon Control x-y piezo flexure stage with a 200*200 µm2 range combined with a home built 10 mm piezo stage for the z direction. The scanner was designed to support microscope object slides. For detection of the bending of the cantilever the optical beam deflection technique is used. A laser diode at 780 nm wavelength is focused on the backside of the cantilever and its reflection is imaged onto a quadrant detector. The deflection signal, obtained by subtracting the signals from the vertically aligned quadrants, can be used as feedback

4.  DIFFERENT TYPES OF MODES OF OPREATION IN NSOM

Figure 5: Different modes of operation in NSOM

Transmission mode imaging: The sample is illuminated through the probe, and the light passing through the sample is collected and detected.

Reflection mode imaging: The sample is illuminated through the probe, and the light reflected from the sample surface is collected and detected.

Collection mode imaging. The sample is illuminated with a macroscopic light source from the top or bottom, and the probe is used to collect the light from the sample surface.

Illumination/collection mode imaging: The probe is used for both the illumination of the sample and for the collection of the reflected signal.

3 VARIENTS OF NSOM PROBES

As we mentioned above, the NSOM probes can be divided into two kinds: the aperture and apertureless types. However, here we briefly describe and distinguish NSOM probes via the constituting material and the fabrication process.

Optical fibers:

The most widespread support for NSOM is pulled optical fibers that allow making apertureless and aperture probes. Starting from a pulling process that can be chemical or mechanical, the apex of optical fibre is narrowed down to few tens of a nanometer. Thereafter, this pulled fiber can be either used directly or metal coated by vacuum deposition. One can lastly mill the apex of the covered pulled fiber, using Focused Ion Beam (FIB), to fashion a well-defined aperture probe. For these latter, one usually utilizes aluminum coating to create good optical guidance toward the tip apex whereas for apertureless probe gold is preferred (gold being optically interesting and inert).

Metallic tips

As in Scanning Tunneling Microscope (STM), metallic tips are easy to make in a controlled way and with a large panel of materials using electrochemical techniques. They also benefit from the large knowledge from STM probes fabrication.

Microcantilevers

They have been developed for commercial AFM, and the NSOM technique benefited from it. They are reasonably cheap and can be reproducibly mass-produced. As for the optical fibers they can be designed as aperture or apertureless probes. For these reasons, they are mainly used in commercialized NSOM systems.

A combination of both the fiber-based and cantilever-based probes

These probes are made of bent optical fibers used as a microcantilever. Although, these NSOM probes offer some advantages (i.e., AFM electronics), they are not extensively used as the optical losses are large due to the bent and the fabrication process is complicated and not well-developed.

4.    FEEDBACK MECHANISM IN NSOM

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. A critical requirement of the near-field techniques is that the probe tip must be positioned and held within a few nanometers of the 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.

In order to improve signal-to-noise ratios for the feedback signal, the NSOM tip is almost always oscillated at the resonance frequency of the probe. This allows lock-in detection techniques (basically a bandpass filter with the center frequency set at the reference oscillation frequency) to be utilized, which eliminates positional detection problems associated with low-frequency noise and drift. As the oscillating tip approaches the specimen, forces between the tip and specimen damp the amplitude of the tip oscillation.

A measure of the mechanical (or electrical) oscillator quality is given by a dimensionless parameter called the quality factor, or Q-factor, or simply Q. The quality factor is defined as the oscillator’s resonance frequency divided by its resonance width. It is generally beneficial to maximize the Q of the probe oscillation in order to achieve greater stability and more sensitive tip height regulation. The lower the Q of the oscillating probe, the lower the signal-to-noise ratio, which results in correspondingly lower quality topographic information being obtained from the oscillatory feedback mechanism. Typically, both the peak resonance and the Q-factor are found to change upon approach of the probe tip to the specimen surface. The tip oscillation amplitude and frequency can be monitored by several different techniques, which generally can be categorized within two groups. The shear-force mode utilizes lateral oscillation shear forces generated between the tip and specimen (parallel to the surface) to control the tip-specimen gap during imaging. In contrast, the tapping mode relies on atomic forces occurring during oscillation of the tip perpendicular to the specimen surface (as in AFM) to generate the feedback signal for tip control. Each oscillatory mode has several advantages and disadvantages.

The shear-force feedback method laterally dithers the probe tip at a mechanical resonance frequency in proximity to the specimen surface. The dither amplitude is usually kept low (less than 10 nanometers) to prevent adversely affecting the optical resolution. For optimum image quality, shear-force feedback techniques are usually restricted to use with specimens that have relatively low surface relief, and longer scan times are required compared to operation in tapping mode. However, the straight probes typically employed in shear-force feedback techniques are easier to fabricate and have a lower cost per probe than their bent probe counterparts.

With respect to light throughput, the straight probe has a decided advantage over the bent probe, exhibiting much lower loss in propagation intensity. Shear-force imaging with a straight probe, however, is usually very difficult to perform in a liquid medium because the additional viscous damping of the fluid causes a dramatic decrease of the probe oscillation amplitude. In typical operation, as the oscillating probe approaches the specimen surface, the amplitude, phase, and frequency of oscillation each change, due to dissipative and adiabatic forces present at the tip of the probe. The probe oscillation damping due to tip-specimen interaction increases nonlinearly with decreasing tip-specimen separation.

The nature of the shear forces that are responsible for damping the probe tip oscillations during near-field specimen approach is the subject of much research interest. One group of investigators used electron tunneling current measurements between a metalized NSOM probe and specimen, in shear-force feedback mode, to conclude that the probe actually contacts the surface during the approach cycle of the oscillation. Measurements of the tunneling current, made as the tip approaches the specimen, indicate that the tip touches the specimen initially as the probe goes into feedback and continues to lightly touch the surface once per oscillatory cycle. From this information it is clear that the most beneficial approach is to make the feedback set-point as high as possible (for example, approximately 99.9 percent of the original undamped signal) in order to reduce the physical interactions between the probe and the specimen. In practice, the upper limit on the feedback set-point is determined by the signal-to-noise ratio of the feedback signal.

Optical feedback methods of monitoring the tip vibration amplitude were the most commonly employed during early development of shear-force techniques in NSOM, and can also be applied in the tapping mode. In this approach, for either the straight or bent probe types, a laser is tightly focused as close to the end of the NSOM probe as possible. With the straight probe variation, when under laser illumination, a shadow is cast by the probe onto a split photodiode. In the case of the bent probe method, the laser is reflected from the top surface of the probe to the split photodiode (similar to the optical feedback techniques employed in the AFM). With the laser feedback established, the probe is then vibrated in either tapping mode or shear-force mode, at a known frequency, utilizing a dither piezo (Figure 7). The split photodiode collects the laser light, and the difference between the signals from each side of the detector is determined. A higher signal-to-noise ratio can be obtained by using a lock-in amplifier to select a portion of the signal that is at the same frequency as the dither piezo drive signal.

Figure 6: Representation of Tuning-fork technique

The main problem associated with this type of feedback mechanism is that the light source (for example, a laser), which is used to detect the tip vibration frequency, phase, and amplitude, becomes a potential source of stray photons that can interfere with the detection of the NSOM signal. One mechanism for dealing with this effective increase in background signal is to provide a feedback light source that has a different wavelength (usually longer) than the near-field source. This scheme requires additional filtration in front of the detector to selectively block the unwanted photons originating within the feedback system. In most cases, the added filters also block a small percentage of the near-field photons, resulting in reduced signal levels. A non-optical feedback method is not subject to problems of this nature, and is a primary reason that methods such as the tuning-fork technique (described above) have become increasingly popular.

Tapping-mode feedback is another popular method for tip-to-specimen distance control, and is implemented using several different probe types. A useful design consists of a modified AFM cantilever and transparent tip, usually fabricated from silicon nitride and coated with metal on the bottom of the probe tip. The most commonly employed probe for tapping-mode methods is the conventional fiber optic probe having a near 90-degree bend close to the tip aperture. A representation of a bent optical fiber NSOM probe is presented in Figure 7

The resolution of the tapping-mode near-field image is defined not only by the radius of the tip but also by the amplitude of the oscillation occurring perpendicular to the specimen surface. This is due to the acute sensitivity of the optical signal to the tip-to-specimen separation. In order to maintain high near-field resolution, it is necessary to either maintain small oscillation amplitude relative to the tip aperture, or to compensate for larger oscillations. A mechanism that has been demonstrated to improve resolution is to synchronize the collected NSOM signal with the cycle of tip oscillation. Modulating the light coupled into the probe, and adjusting the phase such that the specimen is only illuminated when the tip is at its closest approach point, allows maintaining high resolution imaging at fairly large tip oscillation amplitudes.

Figure 7: A representation of a bent optical fiber NSOM probe

In the bent probe feedback mode, the probe is oscillated perpendicular to the specimen surface similar to tapping-mode AFM. The amplitude of oscillation can be monitored either mechanically, with a piezo-electric device such as a quartz tuning fork, or optically by reflecting a laser from the top surface of the tip cantilever. The probe is excited to oscillate in one of its eigenmodes and the tip-specimen distance is recorded and used dynamically as the feedback signal. The tip of the probe is prevented from adhering to the specimen due to the oscillation, which provides both a short contact time and a reverse driving force due to the cantilever bending. The success of this feedback method is dependent on the increased sensitivity of the tip-specimen distance control resulting from the resonance enhancement of the tip vibration. The sharpness and sensitivity of the tip vibration is characterized by the Q of the cantilever (similar to the measured Q in the shear-force oscillation). The Q value of the probe is substantially reduced by the viscosity of a liquid environment, and this is usually accompanied by a large shift in the resonance frequency.

5.    SUMMARY

In this module you study:

NSOM has developed into a viable technique for elucidating new insights into sample properties at the nanometric scale.

One of the most important parts of the near-field scanning optical microscope is the optical probe. This can be either a sub-wavelength light source, a detector or a scatter source. The advantage of using a light source as a probe is that only a small area of the sample is illuminated and that all light (scattered/emitted) from this area is collected and selectively detected, maximizing the detection efficiency of all optical processes occurring at the sample

Combining atomic force measurements and near-field scanning optical microscopy has proven to be an extremely powerful approach in certain areas of research, providing new information about a variety of specimen types that is simply not attainable with far-field microscopy.

Typical resolutions for most NSOM instruments range around 50 nanometers, which is only 5 or 6 times better than that achieved by scanning confocal microscopy. This moderate increase in resolution comes at a considerable cost in time required to set up the NSOM instrument for proper imaging, and in the complexity of operation.