Near-field
Scanning Optical Microscopy (NSOM)
Summary:
The objective of this work, which was carried out at the National Institute of Standards and Technology in Gaithersburg, MD, was to extend the measurements and standards
infrastructure for the nanoscale optical characterization
of thin films and interfaces. We developed near-field
scanning optical microscopy (NSOM) for quantitative
evaluation of surfaces, with a particular emphasis on
understanding organic multicomponent films. The facilities at NIST included a metrological NSOM (Fig. 1)
built on a linearized flexure stage, a wet-cell NSOM
suitable for investigating biological or biomimetic
films (see Fig. 2), and
a near-field probe preparation and evaluation facility.
FIGURE 1: The Metrological
NSOM
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FIGURE 2: The Wet
Cell NSOM
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Recent
years have seen explosive growth in the use of organic
materials, composite materials, or organic materials
bound to an inorganic substrate, to solve engineering
problems that traditionally were approached only with
inorganics. Fields such as tissue engineering are burgeoning
in light of new advances in organic materials science.
The remarkable speed with which these new technologies
are appearing is contrasted by the remarkable scarcity
of non-destructive, in-vitro, and in-vivo
techniques for characterizing the biophysical, chemical,
and mechanical properties of these often delicate and
nano-structured materials.
FIGURE
3: Schematic of near-field optical microscopy.
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Near-field Scanning Optical Microscopy:
Near-field scanning optical microscopy (NSOM) is a type
of microscopy where a sub-wavelength light source is
used as a scanning probe. The probe is scanned over
a surface at a height above the surface of a few nanometers
(see Fig. 3). We use as
a probe a small aperture on the end of a tapered and
aluminum-coated optical fiber (see Fig. 4).
By illuminating a sample with the "near-field" of a
small light source, we can construct optical images
with resolution well beyond the usual "diffraction limit",
and typically about 50 nm. We currently have two
near-field microscopes in our program, a metrological
instrument and a microscope designed specifically for
doing research on wet samples (see Fig. 1
and Fig. 2).
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FIGURE 4: Optical microscope image
of an NSOM tip.
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We used NSOM to investigate polymer blends
and composites, and developing near-field techniques
to enable quantitative evaluation of these films. Simultaneous
fluorescence, transmission, and topography measurements
have been used in conjunction with modeling to study
the phase separation of polymer blends [in collaboration
with Alamgir
Karim and Connie Gettinger (now at 3M)] (see Fig. 5
and Ref. 3 below).
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FIGURE 5: Shear force (topography), transmission
NSOM, and fluorescence NSOM images of a phase
separated polymer blend sample [enlarged 2 MB
(800×600 pixels)]. |
Our most recent work in NSOM involved the construction of a
near-field polarimeter to enable detailed investigation
of the strain, defect, and domain structure of thin
films. Applications so far include studies of block
copolymer morphologies (Ref. 4)
and polymer crystallite formation in thin polystyrene
films (see Fig. 6 and Ref. 6).
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FIGURE
6: Shear force (topography), transmission
NSOM, and fluorescence NSOM images of a phase
separated polymer blend sample [enlarged 2 MB
(2084×1292 pixels)].
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Modeling:
An understanding of the nature and details of the tip-sample
interaction is imperative for quantitative evaluation
of near-field images. In conjunction with Garnett Bryant
in the Atomic
Physics Division, we implemented complete models
for some of the systems we have studied, including a
2-dimensional photonic crystal (Ref. 2),
and for the first time, completely modeled near-field
data. We investigated in detail the index-of-refraction
and thickness dependence of the near-field signal and showed that this modeling is crucial for correct interpretation of our polymer
film results. Near-field modeling is closely related
to the models for light
scattering from sub-wavelength particles used and
developed by Thomas
Germer at NIST; near-field studies provide
a complimentary test of these models.
Resources at NIST included:
NSOM measurements in the visible; NSOM tip characterization;
theoretical modeling of probe-surface interactions and
optical contrast mechanisms; and access to complementary
scanning microscopies such as SEM and AFM.
Representative Publications:
- McDaniel, E.B., Hsu, J.W.P., Goldner, L.S., Tonucci,
R.J., Shirley, E.L., and Bryant, G.W.,
"Local characterization of a two-dimensional photonic
crystal,"
Phys. Rev. B 55, 10878 (1998).
- Bryant, G.W., Shirley, E.L., Goldner, L.S., McDaniel,
E.B., Hsu, J.W.P., and Tonucci, R.J.,
"Theory of probing a photonic crystal with transmission
near-field optical microscopy,"
Phys. Rev. B, 58, 2131 (1998).
- Hwang, J., Goldner, L.S., Karim, A., and Gettinger,
C.,
"Imaging phase-separated domains in conducting polymer
blend films with near-field scanning optical microscopy,"
Appl. Opt. 40(22) 3737-3745 (2001).
- Fasolka, M.J., Goldner, L.S., Hwang, J., Urbas,
A.M., DeRege, P., Swager, T., and Thomas, E.L.
"Measuring Local Optical
Properties: Near-Field Polarimetry of Photonic Block
Copolymer Morphology"
(228 kB) 
Phys. Rev. Lett. 90, 016017 (2003).
- Goldner, L.S., Fasolka, M.J., Nougier, S., Hguyen,
H.-P., Bryant, G.W., Hwang, J., Weston, K.D.,
Beers, K.L., Urbas, A., and Thomas, E.L.
"Fourier Analysis Near-Field
Polarimeter for Measurement of Local Optical Properties
of Thin Films," (2.13 MB)
Appl. Opt. 42, 3864-3881 (2003).
- Goldner, L.S., Goldie, S.N., Fasolka, M.J., Renaldo,
F., Hwang, J., and Douglas, J.F.,
"Near-Field
Polarimetric Characterization of Polymer Crystallites,"
(594 kB)
Appl. Phys. Lett. 85, 1338 (2004).
Related Publications:
Synge,
E.H.,
Phil. Mag. 6, 356 (1928).
Betzig, E. and Trautman, J.K.,
Science 257, 189 (1992).
Pohl, D.W.,
"Scanning near-field optical microscopy"
in Advances in Optical and Electron Microscopy
12, ed. by C.J.R. Sheppard and T. Mulvey (Academic
Press, London, 1990).
Page last updated Dec 11, 2008
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