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SNOM sketch and pictures |
- SNOM (Scanning Near-field Optical Microscope),
also referred to as NSOM, is a Scanning Probe Microscopy (SPM)
technique able to overcome diffraction that limits
resolution of conventional optical microscopy to about l/2.
Emission-mode SNOM is based on a sharp metal coated tip fabricated at the end of an optical fiber, coupled to a laser beam. At the end of the tip the coating has a small aperture (50- 100 nm) from which the light comes out.
The tip acts like a nanoscopic light emitter. It is scanned across the sample by means of xyz piezoactuators.
A feedback loop acting on the z direction prevents the tip getting in contact with the sample, avoiding its damage. It also provides stabilization for tip-sample distance (maintained on the order of a few nanometers) in any environment (air, liquid, vacuum). Tip/sample distance is detected by measuring the so-called shear-force .
While the tip scans the sample, topographic and optical data can be acquired simultaneously.
Lateral resolution, is typically about 50 nm, although the latest developments are increasing resolution towards about 1 nm.
Topographic and optical data can be used for investigation and characterization of materials, also underneath the sample surface, like in the case of buried layers not directly accessible. SNOM technique is not destructive.
Description
The SNOM apparatus developed at INFM
Research Unit at the Physics Department
of Pisa is a homemade instrument. The measurement head is based on a common
body and two interchangeable heads, namely, the AFM and SNOM heads.
The microscope is driven by a low cost PC via an analog controller.
The maximum scan area is 80x80 square microns with the employed 3 inch
long piezo tubes.
The flexibility of SNOM permits three configurations:
a) Reflection Mode: light scattered backwards
by the sample, at an angle of 45°, is collected by a microscope objective
and detected by a photomultiplier.
b) Transmission Mode: if the sample is transparent
at the laser wavelength, it is possible to collect forward scattered light
crossing the sample.
c) Stereo Mode: both near-field reflected and transmitted signal are
collected.
Possible Applications
Measurement of thin film deposition (~ 5 Angstrom or less).
Nanolithography for linewidths under 100 nm.
Fluorescence spectroscopy.
Single molecule analysis.
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