The essence of NSOM/FTIR is that it allows the detection of non-propagating evanescent waves in the near-field (less than one wavelength from the sample), thus yielding high spatial resolution. Depending on the detection modes of these non-propagating evanescent waves, two NSOM/FTIR instrumentations are available: apertureless NSOM/FTIR and aperture-based NSOM/FTIR.
;Aperture-based NSOM/FTIR
In aperture-based NSOM/FTIR, the probe is a waveguide with a tapered tip with a very small, sub-wavelength size aperture. When the aperture is brought into the near-field, it collects the non-propagating light and guides it to the detector. In general, there are two modes when the aperture is scanned over the sample: illumination mode and collection mode (Figure 7).
The high-quality infrared fiber tip is very important in realizing NSOM/FTIR technique. There are several types of fibers, such as sapphire, chalcogenide glass, fluoride glass and hollow silica guides. Chalcogenide glasses are widely used because of their high transmittance in the broad IR range of 2–12 μm. The fluoride fibers also exhibit low transmitting losses beyond 3.0 μm.
;Apertureless NSOM/FTIR
The probe is a sharp metal tip ending with a single or a few atoms. The sample is illuminated from far-field and the radiation is focused at the contact area between probe and sample. When this tip approaches the sample, usually within 10 nm, the incident electromagnetic field is enhanced due to the resonant surface plasma excitation as well as due to hot-spots in the sharp tip. The dipole interaction between the tip and sample change the non-propagating waves into propagating waves by scattering, and a detector collects the signal in the far-field. An apertureless NSOM/FTIR usually has better resolution (~5–30 nm) compared with aperture-based NSOM/FTIR (~50–150 nm). One main challenge in apertureless NSOM/FTIR is a strong background signal because the scattering is obtained from both near-field and remote area of the probe. Thus, the small near-field contribution to the signal has to be extracted from the background. One solution is to use a very flat sample with only optical spatial fluctuation. Another solution is to apply constant-height mode scanning or pseudo-constant-height mode scanning.
;Experimental scheme of aperture-based NSOM/FTIR
Figure 8 shows the experimental setup used in NSOM/FTIR in the external reflection mode. FEL source is focused on the sample from the far-field using a mirror. The distance between the probe and a sample is kept at a few nanometers during scanning.
Figure 9 is the cross-section of a NSOM/FTIR instrument. As shown below, sample is placed on a piezo-electric tube scanner, in which the x-y tube has four parts, namely x+, x-, y+ and y-. Lateral (x-y plane) oscillation of the fiber tip is induced by applying an AC voltage to a dither piezo-scanner. Also, the fiber tip is fixed to a bimorph piezo-scanner so that the amplitude of the oscillation of the tip can be monitored through the scanner.