NSOM System Comparisons

NSOM Probes

Since the tip is a crucial component in any near-field scanning probe optical microscope, we will begin with a discussion of the Nanonics near-field element and its advantage over other solutions.

Nanonics is the only company that is able to provide cantilevered NSOM optical fiber probes.

NSOM system suppliers other than Nanonics can be divided into two groups:

1.Those that provide straight optical fibers that depend on shear force feedback.
2.Those that provide a metal aperture on a silicon cantilever substrate.

STRAIGHT FIBERS

1. No ability to use normal force microscopy
In systems that use straight near-field optical fiber elements and shear force feedback, the entire world of normal force microscopy is excluded. There is a very good reason why most people use normal force, and not shear force, imaging. Normal force feedback is based on well-defined interactions between the tip and the sample that allow the tip to approach the sample with ease and without crashing.

2. Associated shear force interactions and artifacts
The interactions that underlie shear force imaging are far from understood. Therefore, systems which use shear force have a more difficult time using the feedback data to construct images of the sample.
Furthermore, it has been shown that many near field optical images obtained with shear force techniques contain serious artifacts that are connected with the coupling of shear force feedback to the material characteristics of the surface. Thus, as you change from one material surface to another, you cannot discern whether the signal change is due to a change in the frictional force of the material or to an actual topography change. Normal force microscopy does not suffer from such artifacts.
The only reason that the use of shear force persists is that no one other than Nanonics can supply cantilevered near-field optical elements.

3. No ability to work in contact mode
Straight fibers cannot be used to perform contact-mode atomic force microscopy, which is a critical technique for many measurements. For example, cantilevered probes are the only tips that can perform simultaneous NSOM reflection and electrical resistance measurements, or Kelvin probe measurements as a function of illumination.

4. Difficulty of use in solutions
Because straight fibers use a tuning fork as their feedback mechanism, it is extremely difficult for them to operate in a liquid environment. Nanonics cantilevered fiber probes, on the other hand, can obtain high quality images in solution.

5. Transmission
The claim that the transmission of the cantilevered tip is lower than that of the straight tip is a spurious argument. This claim is clearly wrong since the primary losses for all near-field optical elements occur in the region of the subwavelength tip, which is common to both straight and cantilevered tips. In fact, not only are the throughputs of cantilevered and straight tips similar, but the polarization properties of the transmitted light are also preserved to the same extent in both these near-field optical elements.

SILICON CANTILEVERS
Silicon cantilever solutions are not new, with several groups publishing papers on their use. One of the first was published in 1993 [Appl. Phys. Lett. 62, 461 (1993)].

1. Throughput and Index-of-Refraction
The throughput of silicon cantilevered tips is generally worse than or very close to the throughput of optical fiber probes. That the throughput of silicon cantilevered tips is not better is understandable, since most of the loss occurs at the subwavelength aperture.
Silicon cantilevers are illuminated from above with a lens. The light strikes the aperture at a normal incidence, and the illumination of the aperture is through air, not through glass as in an optical fiber. There is thus a relatively large index of refraction difference between the aperture and the surrounding region through which the illuminating light is transmitted.
In Nanonics optical fiber cantilevers, there is no index of refraction difference between the subwavelength aperture and the preceding glass region in which the light is concentrated by the tapered fiber.

2. Heating
In silicon-based subwavelength apertures, the light is focused directly on the coating and heats it. This is unlike optical fiber probes, in which the light approaches the aperture at an angle. There is, therefore, much more heating of the aperture coating in silicon-based subwavelength apertures than in Nanonics optical fibers.

3. Noise
In silicon probes, light scattered at the subwavelength aperture escapes to the surrounding environment. In optical fiber cantilevers any scattered light is contained within the fiber. The detector is therefore exposed to much more noise in the case of silicon probes.

4. Tip movement
Silicon probe-based systems are unable to perform scanning of the tip. Such movement of the tip is essential for the investigation of the light emission from waveguides. In these measurements, the light must be collected by the probe aperture without altering the position of the waveguide relative to its input of light. For similar reasons, tip movement is very important for the investigation of lasers with electrical connections.
Tip movement is also essential in biological research that combines electrophysiological measurements with NSOM. In such experiments, any movement of the sample would disrupt the electrophysiological measurements.
Only Nanonics cantilevered optical fiber probes provide the flexibility that is needed to perform NSOM in both tip-scanning and sample-scanning mode.

5. System geometry/samples in liquids
The lack of a waveguide in apertured silicon probe systems severely limits the geometry of microscope construction.
The flexibility of Nanonics cantilevered probes allows them to be used with any microscope geometry, making our system ideal for imaging samples in solution.

6. Reflection imaging
Due their restricted geometry and lack of a waveguide, it is virtually impossible to perform reflection NSOM imaging with silicon cantilevered tips. The cantilevered geometry of Nanonics probes leaves the optical axis unobstructed and permits reflection NSOM imaging to be performed with ease.

7. Independent channels of illumination
In apertured silicon probe systems, the lack of a waveguide also precludes using the subwavelength aperture and the lens as independent channels of illumination. Such dual channel illumination, which can be performed using Nanonics cantilevered probes, is crucial to fully integrating online NSOM and AFM measurements with the far-field image. This is becoming more prevalent in advanced NSOM imaging (see literature on 3D deconvolution and cell biology).

NANONICS CANTILEVERED OPTICAL FIBERS

1. Provides for normal force sensing techniques
By using standard normal force sensing techniques, the Nanonics instrument reliability is now at the level of atomic force microscopy, and optical images can be directly correlated with conventional AFM measurements.

2. Contact mode imaging permitted
Contact mode imaging can be performed only by cantilevered elements and is critical in many measurements. As noted above, contact mode is necessary to perform simultaneous NSOM reflection and electrical resistance measurements, or Kelvin probe measurements as a function of illumination (see below). Nanonics cantilevered probes can provide such measurements. 


Contact mode also allows you to collect light through the optical fiber probe and thus correlate the light distribution in semiconductor laser telecommunication devices with the laser's surface topography. In addition, contact mode enables you to measure the carrier distribution through spreading resistance imaging. The capacitive coupling in cantilevered optical fibers is far lower than in conventional silicon cantilevers, making them the ideal probes for such electrical measurements.

3. Ease of use for samples in liquids
Nanonics cantilevered probes allow for a simple liquid cell to be constructed for measurements of biological cellular systems. Seen below are measurements of the topography (left image), optical transmission (center image), and fluorescence (right image) in yeast cells labeled with GFP. 

 
Measurements of fluorescence in solution on cellular systems have not been reported with any commercial system using straight near-field optical elements.

4. Cantilever fiber response time
Nanonics probes allow for dynamic measurements of surface movements introduced by pulses of light. Such measurements can only be performed in contact mode and using cantilevered optical fibers [see Perspectives in Science 277, 637 (1997)] that have response times on the order of microseconds.
Straight near-field elements working in a shear force mode cannot be used for such measurements, and the fastest response time of silicon cantilevers is in the millisecond range, three orders of magnitude slower than Nanonics cantilevered tips.

5. Tip exposure and reflection imaging
Unlike in silicon cantilevers used in conventional AFM or in subwavelength apertures in AFM silicon substrates, the angle used in Nanonics optical fiber cantilevers leaves the tip exposed to the optical axis. Thus, Nanonics probes can be effectively placed under an optical microscope lens to obtain exceptional reflection NSOM images.

6. Dual channel imaging
The unique geometry of the Nanonics cantileved optical fiber probes also permit dual channel imaging: an optical channel through the lens and another channel through the tip can be accessed simultaneously.
Such a dual channel mode of imaging can be seen below. The tips injects light into a waveguide while the lens monitors the position of the tip in a reflection geometry. 

Injecting light into waveguide

Such reflection measurements are extremely difficult to perform with straight tips and are impossible to perform with subwavelength apertures on a silicon cantilever substrate.

7. Unique range of probes

A. Multi-channel probes
The advanced glass-pulling technology capabilities of Nanonics, rivaled by no other supplier, gives us an unusual advantage in helping our customers design probes that no other technology can provide. An example, in which two metal wires have been tapered without shorting them, is shown below. 

Dual-wire nanotweezer/electrode

When there is no voltage difference between the two wires, they remain separate and this unique probe can be used as an electrode. When the voltage difference between them is raised to 70 V, then two the wires clamp together. By altering the voltage difference between the wires from 0-70 V, this device can be used as a nanotweezer. 

0 V

70 V

Using similar technology, multi-channel probes can be produced with one channel consisting of an optical fiber for optical imaging while a second channel is a wire for thermal measurements. Shown below is an example of using such a probe to measure the distribution of light emission from a waveguide while simultaneously monitoring the heat distribution on its surface.

Similar, dual-channel cantilevered optical elements with a metallic channel can also be used for simultaneous electrochemical and optical probing.
No other technology today can produce such multi-channeled probes for near-field optics.

B. Nanopipettes
In addition to the above fiber optic probes, Nanonics has also extensive experience in cantilevered nanopipette probes. These probes can be used for two purposes.
First, the nanovessel tip of the cantilevered nanopipette can be filled with an ion-sensing fluorophore that is sensitive to ionic concentrations of protons, calcium, potassium, etc. [S. Shalom et al., "An Optical Submicrometer Calcium Sensor with Conductance Sensing Capability," Analytical Biochem. 244, 256 (1997)].
These tips, which are very important for biological applications, are excited with external illumination methodologies using patents that are part of the Nanonics portfolio (see United States Patent Number 5,264,698- 1993 and Diagram below).

  
 
An alternate use of such cantilevered nanopipettes is for holding a single gold or silver nanoparticle that can enhance surface Raman or non-linear optical signals by many orders of magnitude.

Such proprietary cantilevered glass tips have been produced by Nanonics and have been fully characterized. An EDX spectrum for such a silver-tipped nanopipette is shown below.  

Nanonics is the only producer and supplier of such cantilevered nanopipette probes.

8. Overall advanced architecture and capabilities
In summary, the cantilevered optical fiber is the element of choice for all general applications of near-field optics, and Nanonics is the world expert in this technology.
Furthermore, in the last five years there has been a revolution in fiber optical element production that has been driven by the optical communications industry. Nanonics has implemented its knowledge in this area to increase automation in their production and to fine-tune the reliability and geometry of optical fiber tip apertures.

THE NANONICS MULTIVIEW 1000 NSOM/SPM SYSTEM MULTIPLE PROBE CAPABILITY
In spite of the Nanonics preference for using cantilevered optical fiber probes, most existing forms of NSOM and AFM probes can be used with a Nanonics NSOM system. These include:

• Glass pulling technology as applied to NSOM (pioneered by Nanonics)
• Cantilevered optical fiber probes (pioneered by Nanonics)
• Silicon apertured cantilevered NSOM probes
• Standard cantilevered AFM probes
• Straight NSOM fibers
• Cantilevered nanopipettes tipped with fluorescent chemical sensors or gold or silver nanoparticles (pioneered by Nanonics)
• Dual channel NSOM/electrochemical and NSOM/thermal probes (pioneered by Nanonics)

Thus, in addition to producing high quality images using cantilevered optical fiber probes (see example below), the Nanonics system also has an ideal architecture for both straight optical fibers, apertureless geometries, and apertures on silicon substrates.

AFM image of gold balls with a diameter of 30 nm.

NSOM image of gold balls with a diameter of 30 nm.

The arrow in the NSOM image above, which was taken using a Nanonics cantilevered optical fiber probe, indicates the ultra high-resolution NSOM measurement relative to the AFM image, whose resolution is dominated by the dimension of the end of the NSOM tip.
 
Advantages of Nanonics MultiView 1000 NSOM/SPM Confocal System™

In addition to being the only company that uses cantilevered optical fibers, Nanonics has also designed its near-field microscopy system to be flexible and modular, giving it many additional advantages over its competitors.

1. Integration with All Optical Microscopes
The Nanonics MultiView 1000 NSOM/SPM System™ is a compact, award-winning near-field microscope system that is unlike any other system available -- not only in its full integration of near-field optics with conventional AFM methodology, but also in its unique capability to be placed on any host optical microscope, including ones supplied by Raman instrument manufacturers, without detracting from its online capabilities.

2. Simultaneous Viewing Through Microscope Lens
The Nanonics MultiView 1000 NSOM/SPM System™ is the first scanned-probe system, NSOM or otherwise, that allows viewing of the tip position from directly above the sample during the scan, regardless of the type of optical microscope upon which it is placed. The tip of cantilevered optical fibers is exposed to the lens of the microscope and can be readily placed over opaque and odd geometry samples. Simultaneous viewing is not possible when using standard silicon cantilevers because they require illumination from directly above the cantilever.
Thus, one can inject light at specific points into a waveguide while viewing the positioning of the probe with an upright optical microscope (see Nanonics Cantilevered Optical Fibers, number 6). One can also position the probe to collect light from the edge of a waveguide (see picture below). The Nanonics system's ability to operate in contact-mode ensures good coupling between the probe and the waveguide and is critical in such applications.

A waveguide structure is held in a perpendicular geometry below the optical microscope stage

 
In the picture above, the Nanonics MultiView 1000™ is positioned between the optical microscope objective and the optical microscope stage. Straight near-field optical elements and standard scanned probe microscope systems without the unique properties of the Nanonics 3D Flat Scanner would not permit such sample geometries, which are crucial in the measurement of all types of waveguide structures.

3. Nanonics Patented Flat Scanning Stage
Because of the Nanonics patented flat scanning stage, our system can fit under an objective in the upright microscope with a numerical aperture (N.A.) as high as 0.73. The geometry of the flat scanner also permits the light that is transmitted through the fiber and reflected off the sample to be collected with the efficiency of a high N.A. lens in the conventional upright microscope geometry. This latter characteristic makes reflection NSOM imaging routine. Inverted microscopes are also fully compatible with the Nanonics MultiView 1000 NSOM/SPM System™ , and such microscopes permit the use of oil immersion objectives with an N.A. of 1.4.
Unlike in all other NSOM systems that employ upright cylindrical piezo tubes, our flat scanning stage permits the microscope lenses to be rotated into place without having to move the Nanonics MultiView 1000 NSOM/SPM system™. In addition, the condenser column can remain in place when using the Nanonics MultiView 1000 NSOM/SPM™ with an inverted microscope. This is not the case in any other commercial AFM that uses standard piezo scanning or straight probes.

4. Unique Microscope System Combinations
In addition, Nanonics can supply a unique dual microscope. With this option, the customer receives two separate Zeiss Axiotech Varios which can be used for simultaneous viewing in transmission and reflection modes of operation. The dual microscope option provides the customer with a flexible and modular integrated solution.

5. Large Z-Range
As the only system thats permits z-motion of the sample by up to 70 microns, the NSOM/SPM actually enhances the capabilities of its host optical microscopes. This large z-range permits optical sectioning combined with confocal microscopy imaging. No other AFM piezo system has a z-range as large as the Nanonics Flat Scanner. The largest such range in any other system is 10 microns, which is insufficient for confocal z-sectioning.

6. Use of a Wide Variety of Probes
In addition to our cantilevered optical fibers, the Nanonics MultiView 1000 NSOM/SPM™ is compatible with standard and apertured silicon cantilever probes, thus allowing our users a great deal of flexibility.

7. Flexible Movement of the Platform from One Microscope to Another
The user can move the system from one optical microscope to another in just a few minutes. No NSOM system that uses straight near-field optical elements or apertured silicon cantilevers has such flexibility.

8. Bridging and Integrating All Forms of Microscopy
The Nanonics instrument is the only system that bridges all forms of optical microscopy and scanned probe microscopy without detracting from any of the functions of its host microscope.

9. The First System to Provide Tip or Sample Scanning Online
Nanonics has recently introduced the NSOM/SPM 2000 series. With this new system, the customer can, for the first time, perform both tip or sample scanning online.

10. Tied to an Internationally Recognized Research Laboratory
Finally, Nanonics is the only NSOM manufacturer that is exclusively tied to a world-class research and development laboratory where most of the innovative methods have been developed and commercialized. Thus, the customer is provided with state-of-the-art technological innovations that are based on cutting edge research in near-field optics and in process development and control.