NSOM

NSOM Near-Held scanning optical microscopy [103a] Light from a sharp tip scatters off sample Surface structure to 3 nm... 

Of the methods described in this section, scaiming near-field optical microscopy (SNOM or NSOM) is tlie closest to being able to provide useful infonuation that is unobtainable by other means. Indeed, this teclmique has already been made available as a conunercial instmment. A detailed review of SNOM has been written by Pohl 11931. 

Near-field scanning optical microscope (NSOM)... 

Nc.ar-Fi ld Scanning Optical Microscope.. The near-field scanning optical microscope (NSOM) should, strictiy speaking, be NSLM for near-field scanning light microscopy because "optical" includes electron optical as well as light optical and NSOM is a light microscope. 

Using a visible light probe NSOM is the eadiest of the probe scopes, at least in conception, and is another apparent exception to the diffraction-liinited resolution rule, in that NSOM illuminates an object with a beam of visible light smaller than the diffraction limit. The resolution then is limited only by the size of that beam. To achieve this, light issuing from a very tiny aperture at the end of a glass capillary scans a very near sample. The tip must be located on the order of X/2 from that surface. Resolution in the range of 10—20 nm has been achieved (31). 

Although the idea on which NSOM is based goes back more than 50 years (32), D. W. Pohl first beheved it could be achieved with visible light and brought the concept to do it to fmition in 1984 (33). There is considerable interest in NSOM, and two commercial instmments have already been aimounced. A recent appHcation involves using NSOM for localized absorption spectroscopy and fluorescence imaging of living cells (33). 

NSOM Near-field scanning optical PES Photoelectron spectroscopy... 

Fig. 2. Schematic diagrams of alternative force feedback modes. (A) Illustrates noncontact atomic force microscope feedback. (B) Shear-force feedback frequently used in NSOM imaging applications. From Paesler and Moyer (41), with permission.

Near-field scanning optical microscope (NSOM), 16 501-503 Near-infrared absorbing dyes, 9 341 Near-infrared spectroscopy (nir),... 

For samples thicker than the depth of field, the images are blurred by out-of-focus fluorescence. Corrections using a computer are possible, but other techniques are generally preferred such as confocal microscopy and two-photon excitation microscopy. It is possible to overcome the optical diffraction limit in near-field scanning optical microscopy (NSOM). 

The NSOM tip is obtained by heating and pulling a single-mode optical fiber down to a fine point. A reflective metal coating (aluminum, silver or gold) is deposited by vacuum evaporative techniques in order to prevent light from escaping. 

Fig. 11.6. Schematic of an NSOM instrument built around an inverted fluorescence microscope and operating according to the illumination mode.

Discrimination of species in complex samples can be made via lifetime measurements using the single-photon timing method coupled to NSOM. 

Two-photon excitation in NSOM has been shown to be possible with uncoated fiber tips in shared aperture arrangement. This represents an interesting extension of the technique to applications requiring UV light through two-photon excitation. 

Near-field scanning optical microscopy (NSOM) allows an extension of optical microscopy to near that of electron microscopy. The central feature is the optical element that is similar. 

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