Near-field Scanning Optical Microscopy (NSOM)

SNOM offers the use of a very small light source as the imaging mechanism. By using a quasipoint light source with a diameter much smaller than the wavelength of light, one can achieve resolutions better than the diffraction limit. The probe, however, must be very close to the surface; much closer than the wavelength of the light. This region is the "Near-Field" and hence the name of the technique.

Typically, laser light is fed to the aperture via an optical fiber. The aperture can be a tapered fiber coated with a metal (such as Al), a microfabricated hollow AFM probe, or a tapered pipette. Normally, the size of the point light source determines the resolution obtainable.

There are two types of feedback typically used to maintain the proper working distance of the probe to the sample. One method is quite similar to how feedback works with an AFM - by using a cantilevered probe, the normal force is monitored, typically by using a beam-deflection setup as in most AFMs. The second method uses a tuning fork. By attaching the fiber to a tuning fork, which oscillates at its resonant frequency, can can monitor changes in the amplitude as the tip moves over the surface. The tip is moved laterally, and this techniques is normally referred to as "shear-force" feedback.

Depending upon the sample being imaged, there are multiple modes of operation for NSOM.

Transmission: Lightsource travels through the probe aperture, and transmits through sample. Requires a transparent sample.

Reflection: Lightsource travels through the probe aperture, and reflects from the surface. Lower light intensity, and tip-dependent, but allows for opaque samples.

Collection: Sample is illuminated from large outside light source, and the probe collects the reflected light.

Illumination/Collection: The probe both illuminates the sample and collects the reflected light.

Detection of the signal can be handled a number of different ways: Spectrometer, APD (Avalanche Photo Diode), Photomultiplier Tube, or CCD

Data Display: several contrast mechanisms in NSOM can be used, including polarization, topography, birefringence, index of refraction, fluorescence, wavelength dependence, and reflectivity.