ANSOM Project

The goal for the project is to develop a versatile tool for nanometrology. Our original motivation was to build an open source microscope from the ground up that is able to obtain high-resolution optical imaging. We realized during the initial prototyping of the system its potential versatility if it were constructed in a modular fashion.

We have developed the system in LabVIEW with commercially supported, off-the-shelf components and simplified usability, in hopes that the system can be easily recreated. In fact, two similar systems have been developed in-house and, with slight modifications, have been developed into electrochemical scanning and far-field magnetic imaging tools. This versatility was the original motivation for the system design, and should spark interest in the scientific community.

We are in constant development of the ANSOM Project, trying to add new functionality. The ultimate goal of the ANSOM Project is to share the technical details of design that cannot be published in a traditional scientific journal. Due to size limitations in publications, it is difficult to discuss every nut and bolt that encompasses the design of the scientific apparatus. More often than not, it is these design details that make the apparatus and the experiment in general functional. This also helps researchers whose primary focus may interest them in some small detail that will help in their particular discipline, rather than the entire scope of the project. Connections to the project can also help to forge interdisciplinary, collaborative relationships. The stress of finding, creating or maintaining the modular components of the system may also contribute to these collaborative relationships. For researchers who lack the time or resources necessary to build the project themselves, we are currently working on the feasibility of making any or all components commercially available through the university.

• Electrical Design
• Shear Force Microscopy
• Multi-photon Non-Linear Optical Microscopy
• Near-field Optical Microscopy
• Apertureless Near-field Scanning Optical Microscopy
• TENOM Tip-Enhanced Near-field Optical Microscopy
• Project Publications

Electrical Design

The ANSOM Board Collection is the combination of one Controller Board, one Breakout Board, one Phase Detection Board, one Scanhead Board, one Stepper Motor Board, and 12 Mesfet Boards. These are combined to lower the total cost of board production.

Back to top

Shear Force Microscopy

With the invention of the scanning tunneling microscope [1], began a new field of scanning probe microscopy with the use of a physical probe held close to the sample. The piezo-based technology blossomed into the atomic force microscope that placed fewer constraints on the sample [2]. AFMs detect the atomic interaction of the imaging probe and the sample surface. Two common imaging feedback modes for non-contact AFMs are lateral interaction, in which the tip deflection is parallel to sample surface, and tapping mode, in which the deflection is orthogonal.  The lateral interaction feedback mechanism is commonly referred to as shear force microscopy (SFM). SFMs have become popular in the scientific community due to their high degree of customization.  SFMs can be used as near-field microscopes [3-5], in situ liquid imaging systems [6], and ultrasonic imaging [7], just to name a few.  Grober, Karrai and colleagues invented the ability to remove the laser feedback system, replacing it with a tuning fork shear force feedback [8].

Due to the interaction volume of the highly localized field enhancement, the probe must be within a few nanometers of the surface.  To maintain this tip-sample interaction distance, an SFM is used. SFMs have the ability to hold a probe just a few nanometers above the surface of the sample.  High-speed electronics monitor the amplitude and phase of a quartz tuning fork, while a Proportional-Integral-Differential (PID) feedback loop controls the Z-piezo. The role of the feedback loop is to maintain a constant height of the imaging probe above the sample topography while scanning. Using the imaging probes that are mounted to quartz tuning forks, angstrom level resolutions are possible [9]. The sample is moved under the probe so that laser illumination and probe alignment can be maintained during the raster scanning of the sample.

Quartz has natural piezoelectric characteristics. Walter Guyton Cady was the first to develop a quartz crystal oscillator, in 1921 [10]. Dependent on the size and shape of the quartz crystal cuts, the resonant frequency of the crystal can be determined [11]. The high resolution of the crystal oscillator has made it a useful device in the timing of watches and microcontrollers. Crystal oscillators are also used to maintain fixed frequencies for radio (RF) transmissions.

Karrai and Grober were the first to use a quartz tuning fork method for scanning probe microscope (SPM), in particular, for near-field scanning optical microscopy [8]. The sensitivity of oscillation of the crystal oscillator would allow the crystal to be used to detect shear force (friction forces) near a sample with pico-Newton resolution [8].

The tuning fork can be modeled simply as a driven harmonic oscillator and electrically by the Butterworth-Van-Dyke equivalent circuit. Starting with the equation of motion for a simple harmonic oscillator,

 

where m is the mass of the system and k is the spring constant.  Building upon this equation and factoring in the driving force for the oscillator and a damping force acting on the oscillator gives a more accurate equation for the oscillation of a tuning fork as a driven oscillator with damping,

 

where b is the viscous damping factor and F_osin(wt) is the sinusoidal driving force for the tuning fork.  The tuning fork can also be modeled by a similar 2^nd order differential equation. The electrical equivalent is the Butterworth-Van-Dyke equation, given by,

 

 

where L is inductance, R is resistance, C is capacitance, and U the potential added to the oscillator.  This equation can also be obtained using Kirchhoff’s law of conservation of energy.  The steady state amplitude solution for a driven – damped harmonic oscillator is,

 

 

where the phase is given as, 

 

 

The resonance condition for free oscillation frequency including damping is,

 

 

The quality factor of the tuning fork oscillator can be defined by the constants in the system, both mechanical and electrical.

 

 

 

 

 

 

 

 

A qualitative measure of the full-width at half-maximum (FWHM) of the oscillator as seen at resonance can also be used to calculate the quality factor. The above Figure demonstrates the qualitative measure of the FWHM of a resonator. The relationships between these parameters are given by,

 

 

Combining the quality factor into the steady state solution will allow the effect of quality factor on the response time of the oscillator to be examined. We include Q by substituting for b, including the resonance condition for the free oscillation damping.  The amplitude is given by,

 

 

 

with the phase defined as,

 

 

If the amplitude x_o is changed by a perturbation, or in the case of an SPM, the distance between the tip and the changes due to topographical changes, a change in frequency will result [12].  The amplitude and phase become terms in the general solution,

 

The introduction of an instantaneous frequency change gives a solution with a steady state and transient term:

 

The time constant for the transient term gives the settling time for the amplitude, and ultimately, the response time of the quartz oscillator.  The time constant in terms of quality factor and frequency is given by,

 

 

A typical tuning fork for the TENOM system presented here has a quality factor ofQ>4000 with a system resonating at 2¹⁵ Hz, and the response time for this tuning fork configuration would be 245 ms. This makes the use of amplitude measurements for a feedback system undesirable. Instead, the phase measurement of the tuning fork should be used, which will be almost instantaneous in response.

This, of course, is idealistic.  A phase measurement would need a reference of at least two periodic cycles (62 ms for 2¹⁵Hz), which is then multiplied and low pass filtered.  The low pass filter for the TENOM system is typically set at 1.7 kHz (is variable), which would place the response time at roughly a half of a millisecond.

1. Binnig G., Rohrer H., Gerber Ch., and Weibel E., “Surface Studies by Scanning Tunneling Microscopy,” Phys. Rev. Lett. 49, 57 (1982).
2. Binnig G., Quate C. F., and Gerber Ch., “Atomic Force Microscope,” Phys. Rev. Lett. 56, 930 (1986).
3. Pohl D. W., Denk W., and Lanz M., “Optical stethoscopy: Image recording with resolution λ/20,” Appl. Phys. Lett. 44, 651 (1984).
4. Lewis A., Isaacson M., Harootunian A. and Murray A., “Development of a 500 Å spatial resolution light microscope. I. Light is efficiently transmitted through λ/16 diameter apertures,” Ultramicroscopy 13, 227 (1984).
5. Betzig E., Trautman J. K., Harris T. D., Weiner J. S., and Kostelar R. L., “Breaking the diffraction barrier: optical microscopy on a nanometric scale,” Science 251, 1468– 1470, (1991).
6. Rensen W. H. J., Van Hulst N. F., and Kämmer S. B., “Imaging Soft Samples in Liquid with Tuning Fork Based Shear Force Microscopy,” Appl. Phys. Lett. 77, 1557 (2002).
7. La Rosa A., Cui X., McCollum J., Li N., and Nordstrom R., “The ultrasonic/shear-force microscope: Integrating ultrasonic sensing into a near-field scanning optical microscope,” Rev. Sci. Inst. 76, 093707 (2005).
8. Karrai, K., and Grober, R. D., “Piezoelectric Tip-Sample Distance Control for Near Field Optical Microscopes,” Appl. Phys. Lett. 66, 14 (1995).
9. Grober R. D., Acimovic J., Schuck J., Hessman D., Kindlemann P. J., Hespanha J., Morse A. S., Karrai K., Tiemann I., and Manus S., “Fundamental Limits to Force Detection Using Quartz Tuning Forks,” Rev. Sci. Inst. 71, 2776 (2000).
10. Marrison W. A., “The Evolution of the Quartz Crystal Clock,” Bell System Technical Journal (AT&T) 27, 510 (1948).
11. Kahan A., “Cut Angles for Quartz Crystal Resonators,” U.S. Patent No. 4499395 (12 February, 1985).
12. Albrecht T. R., Grutter P., Horne D., and Rugar D., “Frequency modulation detection using high-q cantilevers for enhanced force microscope sensitivity,” J. Appl. Phys 69, 668 (1991).
13. D. B. Nowak, “The Design of a Novel Tip Enhanced Near-field Scanning Probe Microscope for Ultra-High Resolution Optical Imaging” Ph.D dissertation, Portland State University, UMI 3419910, (2010).

Back to top

Multi-photon Non-Linear Optical Microscopy

The term nonlinear describes an effect that does not follow a linear response function to an external perturbation. Nonlinear systems violate the superposition principle because of the lack of these linear relationships. Our primary focus here will be two-photon excitation (TPE) for molecular excitation as a primer.

Using two or more photons to stimulate a single fluorophore absorption in a single quantized event is the basis for multi-photon excitation. An incoming photon of half the required excitation energy excites the fluorophore to a virtual state when the second photon, also of half the required excitation energy, arrives and adds to the virtual state, completing the excitation transition.  In 1931, Maria Göppert-Mayer published the theoretical bases for TPE transitions and emission processes in atomic quantum states [1]. Her Nobel Prize winning research becomes the basis for two-photon nonlinear imaging techniques.

Traditionally we find research labs using Ti:Sapphire based pulsed laser systems to create TPE events.  Stephen Hell and associates have shown TPE imaging with continuous wave (CW) laser sources [2].  CW laser sources offer a simplification of the optical system design and a substantial reduction in cost.  Analyzing the power levels required for two-photon emission (TPE), one could demonstrate the ability for a CW laser to work as well as a mode-locked pulsed Ti:sapphire laser. This requires the average power to be roughly 100 times that of a typical, mode-locked Ti:sapphire.

We further extended the use of CW laser sources to TENOM or ANSOM techniques. [3]
CW laser sources offer a simplification of the optical system design and a substantial reduction in cost.

1. Goeppert-Mayer M., “Über Elementarakte mit zwei Quantensprüngen,” Ann Phys 9, 273–95 (1931).
2. Hell S. W., Booth M., Wilms S., Schnetter C. M., Kirsch A. K., Arndt-Jovin D. J., and Jovin T. M., “Two-photon Near- and Far-field Fluorescence Microscopy with Continuous-Wave Excitation,” Opt. Lett. 23, 1238 (1998).
3. D. B. Nowak, A. J. Lawrence, E. J. Sánchez, “Apertureless near-field/far-field CW two-photon microscope for biological and material imaging and spectroscopic applications,” Applied Optics 49, 35 6771, (2010).

Back to top

Near-field Optical Microscopy

Near-field is defined as the area that is “very close” to the emission of a signal, normally within one wavelength of the source at the frequency of interest.  Any distance that is greater than one wavelength away is considered to be in the far-field.  Near-field optical imaging is simply the ability to generate optical responses in the near-field that are controlled at known spatial locations in order to generate a mapping of the specimen. These responses are generated from fluorophores, molecular bonds, or any conformational information inherent to the sample that was excited within one wavelength of the incident radiation.  The near-field response of the sample is normally detected in the far-field by a detector that is sensitive to the emitted radiation.  To give a general understanding of near-field excitation and near-field imaging, a short theoretical context is given followed by experimental realization methods.

Theoretical Context of “Small Holes”

In 1944, Bethe, and in 1950, Bouwkamp theoretically investigated the propagation of electromagnetic radiation impinging on an infinite conducting plane containing a small aperture hole that was smaller in radius than the wavelength of the incident radiation [1,2].  Bethe’s work looked at microwave wavelengths and the effect of a small hole in the cavity of microwave radiation.  His results showed the power dissipation in the near-field was proportional to 1/r6 in comparison to the far-field dissipation that is proportional to 1/r2.  The importance of this finding is that it demonstrates that electromagnetic radiation can be confined to a region that is smaller than the diffraction limit.  Moving from the microwave scale to the nanometer scale of optical light should follow the same basic relationship.

A monochromatic, polarized light source with an incident field that is polarized in the x direction.  Once the light passes the small aperture, the field develops components in both the y and z directions, in addition to x. Bethe’s calculation showed that the incident field polarization maintains the highest magnitude, while the z polarization is an order of magnitude weaker and the y polarization a couple orders of magnitude weaker. The patterns for the field distributions in the near-field differ for the three different polarizations.

Near-Field Optical Imaging with Aperture Probes

The fundamental goal of near-field microscopy is to create a sub-diffraction limited field distribution in a location favorable for use in a scanning probe microscope.  The original idea for a near-field microscope came about before Bethe and Bouwkamp’s theoretical calculations.  In 1928, Synge postulated an instrument composed of a thin metal film and an aperture much smaller than the wavelength of illumination [3].  Synge further pointed to the use of piezo electrical devices to move the imaging source [4].  Ultimately, he would not be able to make a functional microscope with the available technologies.  In 1956, John O’Keefe published a theoretical construct for the development of a near-field microscope, concluding that movement of the pinhole would be too technologically challenging [5].  The first demonstration of sub-diffraction limited imaging would be in 1972, when the diffraction limit was broken using microwave radiation [6].

The experimental reality of using optical light for near-field scanning optical microscopy NSOM/SNOM became successful in the 1980’s.  Lewis and Pohl published some of the first papers showing applications of SNOM [7, 8].  The first creation of a NSOM system in the United States was by Eric Betzig, using a coated micropipette [9, 10]. Although this method was successful, it wasn’t the optimal waveguide for power throughput.  Later, this design was improved with the use of an optical fiber [11].  This design uses a single mode optical fiber as a waveguide, which is pulled down to a sub λ/2 end diameter via simultaneous heating and pulling (micropipette puller).  The fiber probe is then coated with a light confining material, usually a good conductor.  When coated properly, it is left with a small aperture at the end of the probe. Other groups have made improvements and modifications to this basic design, although the fundamental concepts are still employed [12-16].  Many groups use a modified pipette puller with a built-in CO2 laser for the heat source.  Others have used a chemical etching technique in a hydrofluoric (HF) solution to shape the fiber [17-19].  The HF method allows for high taper angle fiber probes, which have much higher light throughput [20].  Some have implemented focused ion beam milling to create aperture probes [21].

The application of the aperture probe can be used in two distinct modes, either as an excitation probe or as a combination of excitation and detection. For excitation mode, a light is guided down the fiber in a propagating TEM00 mode.  This involves a neutral density (N.D.) filter, half and quarter wave plate, and fiber coupler lens system.  The quarter wave plate is added for removal of ellipticity induced by the fiber.  The half wave plate is used to control the linear polarization orientation.  The probe is attached to a scanning probe microscope, which is then applied carefully to a sample surface.

The sample is illuminated in the near-field and the fluorescence emission from the sample is collected through an objective lens and miscellaneous optics and then inevitably reaches n-blocking filters.  Typically, a single photon avalanche photodiode (SPAPD) is used for collection, due to its high quantum efficiency (typically >80%) and very low dark counts.  It is different in its operation from a basic APD in that the detector has single digit background counts due to its design and fabrication.  This low background allows for very accurate time-correlated single-photon counting (TCSPC) measurements involving fast instrument responses since these detectors also tend to possess low jitter from pulse to pulse.  Although the SPAPD is the preferred detector, there exist the dangers of high sensitivity to damage from high-count rates of photons.  Typically, going higher than 15 Mhz can damage the SPAPD due to the high voltages and currents present during longer exposure times to these high counts.  In the latest developments of optical detection, photomultiplier tubes (PMTs) have shown great promise as more robust single photon counters with increasing quantum efficiencies (QEs).  Although the normal collection mode for photons is through the sample and collection objective below, a popular method employed for non-transmissive samples involves collecting light back through the excitation fiber.  In fiber collection mode, fluorescence emission is collected back through the fiber, then diverted by a 45˚ splitter to an SPAPD.  The advantages of this method include the ability to image non-transmissive substrates as well as simple collection optics, although the collection efficiency back through the fiber is typically lower than collection efficiency of a objective lens below the sample.

Although these techniques have the ability to image with spatial resolutions less than λ/2, serious issues exists with this technique.  One issue is that the angle of the fiber pull is less than ideal and the tips often will form very high aspect ratio geometries. Although great for topographic imaging, they have extremely low throughput, typically on the order of nanowatts.  HF chemical etching has allowed the researchers to make better aspect ratio tips.  However, the use of HF is quite hazardous and requires special hoods during the etching process.  Whether the user makes etched or pulled methods, making tips the same from run to run is a large problem.  Making fibers reproducible at the level of sub 100 nm has not demonstrated a very high reproducibility.  Coatings also suffer from a lack of reproducibility.  Coatings should be made at vacuum pressures below 5.0 x 10-7 torr to obtain coatings with minimal contamination.  Oftentimes, nucleation issues of thermal evaporation techniques will result in residual metal on the end apertures.  This nucleation can create problems for throughput.  Another coating issue involves miniature pinholes where light can escape the confinement of the fiber.  Topographically, the tip is usually very wide (> 350 nm), leading to poor surface imaging features.  For a more complete review, an informative primer written by Robert Dunn on SNOM/NSOM is available [22].

1. Bethe, H. A., “Theory of Diffraction by Small Holes,” Phys. Rev. 66, 163 (1944).
2. Bouwkamp, C. J., “On Bethe’s Theory of diffraction by Small Holes,” Phillips. Res. Rep. 5, 401, (1950).
3. Synge E. H., “A suggested method for extending the microscopic resolution into the ultramicroscopic region.” Phil. Mag. 6, 356 (1928).
4. Synge E. H., “An application of piezoelectricity to microscopy,” Phil. Mag. 13, 297 (1932).
5. O’Keefe J. A., “Resolving power of visible light,” J. of the Opt. Soc. of Am. 46, 359 (1956).
6. Ash E. A. and Nichols G., “Super-resolution aperture scanning microscope,” Nature 237, 510 (1972).
7. Lewis A., Isaacson M., Harootunian A. and Murray A., “Development of a 500 Å spatial resolution light microscope. I. Light is efficiently transmitted through λ/16 diameter apertures,” Ultramicroscopy 13, 227 (1984).
8. Pohl D. W., Denk W., and Lanz M., “Optical stethoscopy: Image recording with resolution λ/20,” Appl. Phys. Lett. 44, 651 (1984).
9. Betzig E., Trautman J. K., Harris T. D., Weiner J. S., and Kostelar R. L., “Breaking the diffraction barrier: optical microscopy on a nanometric scale,” Science 251, 1468– 1470, (1991).
10. Betzig E., Finn P. L., and Weiner S. J., “Combined shear force and near-field scanning optical microscopy,” Appl. Phys. Lett. 60, 2484–2486, (1992).
11. Betzig E., Grubb S. G., Chichester R. J., DiGiovanni D. J., and Weiner J. S., “Fiber Laser Probe for Near-Field Scanning Optical Microscopy,” Appl. Phys. Lett. 63, 3550 (1993).
12. Garcia-Parajo, M. F., Cambril, E., and Chen Y., “Simultaneous scanning tunneling microscope and collection mode scanning near-field optical microscope using gold coated optical fiber probes,” Appl. Phys. Lett. 65, 1498 (1994).
13. Ambrose, W. P., Goodwin, P. M., Martin, J. C., and Keller, R. A., “Alterations of Single Molecule Fluorescence Lifetimes in Near-Field Optical Microscopy,” Science 265, 364 (1994).
14. Bielefeldt, H., Hörsch I., Krausch G., Lux-Steiner M., Mlynek J. and Marti O., “Reflection-scanning near-field optical microscopy and spectroscopy of opaque samples,” Appl. Phys. Lett. 59, 103 (1994).
15. Valaskovic, G. A., Holton, M., and Morrison, G. H., “Parameter control, characterization, and optimization in the fabrication of optical fiber near-field probes,” Applied Optics 34, 1215 (1995).
16. Yakobson, B. I., Moyer, P. J., and Paesler, M. A., “Kinetic limits for sensing tip morphology in near-field scanning optical microscopes,” J Appl. Phys. 73, 7984 (1994).
17. Schoch, B., Jones, B. E., and Franks, A., “A simple technique for the manufacture of optical probes for scanning near-field optical microscopes,” Meas. Sci. Tech. 5, 663 (1994).
18. Courjon, D., Banier, C., and Spajer, M., “Imaging of submicrion index variations by scanning optical tunneling,” J. Vac. Sci. Tech. B 10, 2436 (1992).
19. Marchman, H. M., Griffith, J. E., and Filas, R. W., “Fabrication of optical fiber probes for nanometer-scale dimensional metrology,” Rev. Sci. Instrum. 65, 2538-2541 (1994).
20. Novotny L., Pohl D. W. and Hecht B., “Light Confinement in Scanning Near-field Optical Microscopy,” Ultramicroscopy 61, 1-9 (1995).
21. Veerman, J. A., Otter, A. M., Kuipers, L., and Van Hulst, N. F., “High definition aperture probes for near-field optical microscopy fabricated by focused ion beam milling,” Appl. Phys. Lett. 72, 3115-3117 (1998).
22. Dunn, R. R., “Near-field scanning optical microscopy,” Chem. Rev. 99, 2891-2927 (1999).

Back to top

Apertureless Near-field Scanning Optical Microscopy (ANSOM)

In order to obtain sub 20 nm resolutions reproducibly in the near-field, a technique known as apertureless near-field scanning optical microscopy (ANSOM) can be used. John Wessel first proposed the idea of apertureless scanning optical microscopy in 1985 [1]. ANSOM gains resolution over traditional NSOM by decreasing the size of the near-field interaction volume. A two-fold benefit results one is improved topographic imaging over the aperture method, and two, the apertureless probes are easier to manufacture.  Initial ANSOM probe designs showed the ability to scatter incident light, which generates images in the far-field of the light’s interaction with the surface [2, 3].

1. Wessel J., “Surface-Enhanced Optical Microscopy,” J. Opt. Soc. Am. B 2, 1538 (1985).
2. Inouye Y. and  Kawata S., “Near-field Scanning Optical Microsocpe with a Metallic Probe Tip,” Opt. Lett.19, 159 (1994).
3. Zenhausern F., Martin Y., and Wickramsinghe H. K., “Scanning Interferometric Apertureless Microscopy: Optical Imaging at 10 Angstrom Resolution,” Science 269, 1083 (1995).

Back to top

TENOM Tip Enhanced Near-field Optical Microscopy

A subset of the more general ANSOM technique is tip enhanced near-field optical microscopy (TENOM) [1, 2].  The principle of TENOM is to use excitation light in the proper polarization to induce a strong localized enhanced field at the end of the tip [3].  The enhanced field consists mainly of non-propagating (evanescent) components and is, thus, strongly confined to the end of the metal tip.  The enhanced fields locally interact with the sample surface and generate a spectroscopic response that can be detected in the far-field.  Furthermore, the tips can be illuminated by a two-photon excitation source in order to improve contrast, due to a quadratic emission profile.  This experimental arrangement can be used for the excitation of molecular florescence, surface enhanced Raman scattering, and other forms of linear and non-linear imaging [1, 4, 5].  Field enhancements of TENOM probes are several orders of magnitude greater than the background excitation light. With this increased field, the time required for imaging of the chromophores with a good signal-to-noise ratio is reduced.  The ultimate resolution obtainable by TENOM is purely dependent on the geometry of the tip [6, 7].

TENOM’s dependency on proper polarization to maximize the enhanced field has resulted in multiple experimental configurations.  Side illumination of the tip is one of these configurations [8].  A sharp metal probe absorbs the incident field, which has a polarization along the tip length to maximize the localized field enhancement [9].  This configuration allows for imaging of opaque samples lending itself to tip-enhanced Raman spectroscopy.  Imaging usually suffers from poor signal to noise ratios and collection efficiencies (low NA lens).  Side illuminated tips are relatively easy to manufacture, usually requiring a basic etching circuit.  A drawback is that high resolutions need sharp tips on the order of 30 nm end diameters; the basic chemical etching methods lack reproducibility at this size.  More complex etching methods have been developed to improve this reproducibility, including modifications to the probe itself [10, 11].  Moreover, the small cross sectional area present at the end of the tip creates difficulty with side illumination, resulting in poor field enhancements.

Ideally, epi-illumination would give the highest collection efficiency for a TENOM system, but some difficulties have to be overcome.  Epi-illumination would rule out opaque samples.  While this would be acceptable for biological samples, bulk material samples would not be possible.  It would still be possible to analyze nanoparticle objects such as carbon nanotubes, quantum dots, and nanocrystalline structures from below, assuming care was taken in the preparation of the samples on glass substrates. The benefits of epi-illumination include much higher collection efficiency due the use of high N.A. lenses and existing optical microscopes to provide optical pathways.  The biggest drawback to the epi-illumination is that the polarization will be rotated 90 degrees, making it useless with high aspect ration sharp metal probes.   A new probe design will be needed to take advantage of epi-illumination.

The use of nanoparticles attached to the end of an imaging probe has demonstrated successful apertureless near-field imaging in an epi-illumination configuration [12, 13].  The overall field enhancement is typically limited to the finite number of electrons available in a confined particle.  If the size and shape of the geometry can be tailored to generate a resonance condition with the incident radiation, the enhancement can be further increased [14].  The use of bow-tie geometries has also demonstrated feasibility [15, 16].

The detection of the near-field response in TENOM imaging will use similar far-field detectors as well as aperture probes, SPAPDs, PMTs, and intensified CCDs for spectral imaging.  Two-photon excitation can be used to minimize the background contribution from the far-field signal that will be mixed with the near-field signal.

Scientific Motivation for the Development of a TENOM System

There are two main scientific motivations for the development of a TENOM system: subdiffraction limited instantaneous measurements with the ability to measure molecular time dynamics and the integration of CW laser sources.  From a scientific standpoint, a TENOM system has the ability to image spectroscopic unknowns at high-resolutions with minimal sample interaction, maintaining comparably high-speed imaging. TENOM’s reach into Raman spectroscopy is another real benefit of the technique, commonly known as TERS, (Tip Enhanced Raman Scattering).

A demonstration of the use of a CW laser system in contrast to a mode-locked pulsed laser system for TENOM imaging will greatly simplify optical pathways for TENOM.  Of course, the damage threshold of both the sample and the imaging probe will have to be investigated, due to the higher power levels required with the CW excitation.  Using two-photon excitation, a single set of excitation and emission filters can be used for a wide range of fluorescence molecules with the benefit of a diminished background signal.  This simplification will help lower the overall cost of a TENOM system, placing it with in reach of many academic labs.

1. Sánchez E. J., Novotny L., and Xie X. S., “Near-field Fluorescence Microscopy Based on Two-Photon Excitation With Metal Tips,” Phys. Rev. Lett. 82, 20 (1999).
2. Sánchez, E. J., Ph.D. dissertation, “A Novel Scheme for High Resolution Near-Field Fluorescence Microscopy”, Portland State University, 1999 (UMI No. 3018652).
3. Novotny L., Sánchez E. J., and Xie X. S., “Near-field Optical Imaging Using Metal Tips Illuminated by Higher-Order Hermite-Gaussian Beams,” Ultramicroscopy 71, 21 (1998).
4. Hartschuh A., Sánchez, E. J., Xie, X. S., and Novotny L., “Near-field Raman Spectroscopy of Single-Walled Carbon Nanotubes,” Phys. Rev. Lett. 90, 9 (2003).
5. Ichimura T., Hayazawa N., Hashimoto M., Inouye Y. and Kawata S. et. al. “Tip Enhanced Coherent Anti-Stokes Raman Scattering for Vibrational Nanoimaging”, Phys. Rev. Lett. 92, 220801 (2004).
6. Krug, J. T., Sánchez, E. J., and Xie, X. S., “Design of near-field optical probes with optimal field enhancement by finite difference time domain electromagnetic simulation,” J. Chem. Phys. 116, 10895 (2002).
7. Nowak D. B., Krug J. T., Xie X. S., and Sánchez E. J., “Pushing the Limits of Near-field Microscopy,” World Multi-conference on Systemics, Cybernetics and Informatics, WMSCII Proceedings, Orlando, Florida, 2005.
8. Pettinger B., Ren B., Picardi G., Schuster R., and Ertl G., “Nanoscale Probing of Absorbed Species by Tip-Enhanced Raman Spectroscopy,” Phy. Rev. Lett. 92, 096101 (2004).
9. Martin O. J. F., and Girard C., “Controlling and Tuning Strong Optical Field Gradients at a Local Probe Microscope Tip Apex,” Appl. Phys. Lett. 70, 705 (1997).
10. Ren B., Picardi G., and Pettinger B., “Preparation of Gold Tips Suitable for Tip-Enhanced Raman Spectroscopy and Light Emission by Electrochemical Etching,” Rev. Sci. Inst. 75, 837 (2004).
11. Neacsu C. C., Berweger S., Olmon R. L., Saraf L. V., Ropers C. and Raschke M. B., “Near-Field Localization in Plasmonic Superfocusing: A Nanoemitter on a Tip,” Nano Lett, 10, 592 (2010).
12. Kalkbrenner T., Ramstein M., Mlynek J., and Sandoghdar V., “A Single Gold Particle as a Probe for Apertureless Scanning Near-Field Optical Microscopy,” J. Microscopy 202, 72-76 (2001).
13. Palomba S., Danckwerts M., and Novotny L., “Nonlinear Plasmonics with Gold Nanoparticle Antennas,” J. Opt. A.: Pure Appl. Opt. 11, 114030 (2009).
14. Jensen T. R., Malinsky M. D., Haynes C. L., and Van Duyne R. P., “Nanosphere Lithography: Tunable Localized Surface Plasmon Resonance Spectra of Silver Nanoparticles,” J. Phys. Chem. B 104, 10549 (2000).
15. Grober R. D., Schoelkopf R. J., and Prober D. E., “Optical Antenna: Towards a Unity Efficiency Near-Field Optical Probe,” Appl. Phys. Lett. 70, 1354 (1997).
16. Farahani, J. N., Pohl D. W., Eisler, H. J., and Hecht B., “Single quantum dot coupled to a scanning optical antenna: A tunable super emitter,” Phys. Rev. Lett. 95, 017402 (2005).

Back to top

Project Publications 

1. D. B. Nowak, “The Design of a Novel Tip Enhanced Near-field Scanning Probe Microscope for Ultra-High Resolution Optical Imaging” Ph.D dissertation, Portland State University, UMI 3419910, (2010).
2. D. B. Nowak, A. J. Lawrence, E. J. Sánchez, “Apertureless near-field/far-field CW two-photon microscope for biological and material imaging and spectroscopic applications,” Applied Optics 49, 35 6771, (2010).
3. D. B. Nowak, A. J. Lawrence, Z. K. Dzegede, J. C. Hiester, C. Kim, and E. J. Sánchez, “FPGA Based Reconfigurable Scanning Probe / Optical Microscope”, Rev. Sci. Instrum. 82, 103701, (2011).

Back to top