Special-Purpose Optical Microscope

Omicron Scanning Near Field Optical Microscope (TwinSNOM)


A typical optical microscope cannot resolve images smaller than the wavelength of light used to illuminate the specimen. The near-field microscope is an advanced optical microscope that is able to resolve details slightly smaller than the wavelength of visible light. This high resolution is achieved by passing a light beam through a tiny hole (aperture) at a distance from the specimen of only about half the diameter of the hole and by detecting the reflected (or transmitted) light for image formation. The resolution of the SNOM image is defined by the size of the aperture, typically 50 - 100 nm, i.e. smaller than half the wavelength of visible light. Light cannot pass through such an aperture, however an evanescent field, the optical near-field, protrudes from it. The optical near-field decays exponentially with distance, and is thus only detectable in the immediate vicinity of the tip.

Among SNOM applications is a surface topography. SNOM can measure sample surface topographical properties without touching the surface at around 100 nm scale. The images measured by SNOM are optical images that are different from images measured by AFM. This advantage can sometimes be important. For instance, the sample is composed of two identical components in shape and size, but optical properties are distinct. SNOM is capable of distinguishing the distribution of these two components in the sample. Another advantage of SNOM over AFM is measuring inner structure of samples.


Principle of SNOM


Reflection mode
Transmission mode
Raman spectroscopy



Far field optical microscopy
Resolution = 1/2 µ, about 300nm

µ : wavelength of light


Near field optical microscopy

Resolution = the size of the aperture
Diameter of aperture is 50 - 100 nm, << 1/2 µ



Images taken in Laboratory

Shear Force Image

Optical Image

Aluminum triangle islands on glass slides ( 2.5 µm * 2.5 µm ).