Near-field scanning optical microscope

An optical Rasternahfeldmikroskop ( scanning near field optical microscope SNOM, the United States also called NSOM ) bypasses the resolution limit of the optical microscope, by evaluating only light that is exchanged between a very small (100 nm or less) near-field probe and the sample examined. With the optical Rasternahfeldmikroskop a spatial resolution of about 30 nm and less can be achieved.

The method was introduced in 1981 by Dieter W. Pohl from the IBM laboratory in Rüschlikon.

Principle

The tip is brought into the near field of the sample and kept at a constant distance by means of a control loop. For this distance regulation, there are several methods:

  • Measurement and control of the tunneling current (see scanning tunneling microscope, only conductive samples)
  • Principle of the atomic force microscope
  • Shear force ( engl. shear force, resonance modification of an oscillator, which supports the tip)

Typical distances between the tip and sample are 1-10 nm The tracking of the tip provides a topographic image of the surface, but in addition to win the Rasternahfeldmikroskop also a visual indication of the surface structure. The optical resolution depends on the delicacy of lace and exceeds that of imaging microscopes many times.

There are two types of lace used:

  • Tips with aperture ( hole in the metallization on a tapered fiber end )
  • Apertureless tips ( metallic tip without light control function )

Photoconductive tips with aperture can be used as a light source or a light collector. In the first case and apertureless peaks only the portion of the sample is excited to emit light, which is located just under the tip. The sample is moved in raster fashion over and under the top, and this is the distance signal and the optical signal is recorded at each position.

The advantage of an optical Rasternahfeldmikroskops against the non-optical scanning probe techniques is that in the conventional optical microscopy known contrast mechanisms may be used, the sample is examined without destruction and chemical information about the sample can be obtained, for example, Raman-effect signals at the tip -enhanced Raman spectroscopy ( TERS ).

Disadvantages of SNOM are

  • The high cost, since in addition the scanning probe principle must be applied
  • Difficulties in analyzing the obtained data (occurrence of artifacts )
  • Still existing theoretical problems of describing the contrast formation

Aperture tips

Aperturspitzen are usually made ​​of glass or silicon fibers that are pointed forward by drawing or etching. In the conical region fibers are vapor-coated with aluminum or silver, since otherwise light would leak. At the top a small opening is not vapor-deposited ( either vapor-deposited from the rear, the front cut off later, a part with an ion beam or drilled hole). Typically, the apertures diameter around 100 nm have also 20 nm have been achieved.

The light is coupled into the fiber, and so, only the part of the illuminated sample located in the near field just below the aperture ( illumination mode). As the aperture is much smaller than the wavelength, the intensity is very low. The light from the sample is by a conventional optical system ( see confocal microscope ) is collected and analyzed usually by a photomultiplier.

The inverse procedure is commonly used: In this case, a larger portion of the sample is illuminated and the aperture of the fiber and the light collecting locally a ( collection mode).

Apertureless tips

Apertureless tips are usually made ​​entirely of metal (silver or gold) or of a harder material (glass, silicon, tungsten) and then vapor-coated with silver or gold. If this tip is brought into a focus of a laser beam, plasmons are excited to oscillate in the tip. The product resulting from this vibration field is at the top of the highest. This field can be used to excite molecules, or other structures on the specimen, and to stimulate the emission of light. The resolution depends on the size of the tip, which can be 20 nm and smaller.

The excitation of the tip can be done by evanescent waves ( see picture above), in which case only transparent samples can be examined. For samples on non-transparent media ( eg silicon or graphite), the light is focused by a lens from the side or by means of a parabolic mirror on the top.

The advantage over the Aperturspitzen are the higher intensities on the sample.

Compression artifact Aluminium Physikalisch-Technische Bundesanstalt
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