Near-Field Scanning Optical Microscopy (NSOM)

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Near-field Scanning Optical Microscopy (NSOM)

Summary:

Our objective is to extend the measurements and standards infrastructure for the nanoscale optical characterization of thin films and interfaces. We are developing near-field scanning optical microscopy (NSOM) for quantitative evaluation of surfaces, with a particular emphasis on understanding organic multicomponent films. Current facilities include 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. Our program was recently extended to include the use of single molecules as a probe of their local environment (see single molecule microspectroscopy).

Fig. 1 The Metrological NSOM (enlarged) 27 kB (800×600 pixels)

FIGURE 1: The Metrological NSOM

Fig. 2 The Wet Cell NSOM (enlarged) 30 kB (800×600 pixels)

FIGURE 2: The Wet Cell NSOM

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.

Fig. 3 Schematic of near-field optical microscopy (enlarged) 33 kB (800×502 pixels)

FIGURE 3: Schematic of near-field optical microscopy.

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).

FIGURE 4: Optical microscope image of an NSOM tip.

We have been using 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).

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 involves 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).

FIGURE 6: Shear force (topography), transmission NSOM, and fluorescence NSOM images of a phase separated polymer blend sample [enlarged 2 MB (2084×1292 pixels)].

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 have 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. Currently we are investigating in detail the index-of-refraction and thickness dependence of the near-field signal; modeling is underway that impacts the 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 in this group; near-field studies provide a complimentary test of these models.

Resources:

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).

For technical information or questions, call:

Lori S. Goldner Phone: (301) 975-3792 Fax: (301) 840-8551 Email: lori.goldner@nist.gov		Jeeseong Hwang Phone: (301) 975-4580 Fax: (301) 869-5700 Email: jeeseong.hwang@nist.gov

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Online: September 1997 - Last updated: January 2006