Near-field Scanning Optical Microscope (NSOM)

Tuning fork based SNOM probe.

The samples used for study in our laboratory are often structured over few tens of nanometers, which is an order of magnitude smaller than the wavelength of light. In such cases, imaging of the sample is impossible using conventional techniques because of the diffraction limit. A scanning probe method needs to be used in such cases.

A scanning probe microscope uses an ultrasmall probe, the size of which is maintained to be of the desired resolution. The probe interacts with the sample at very close distances (5-10 nm) in such a manner that as it scans the surface, it gives the information of the topography of the surface. The advantage with the near-field scanning optical microscope (NSOM) is that simultaneous to the topography map, it provides a direct image of the light field within the resonator. This measurement is crucial in inferring about the correlations in structure and field distribution.

SEM of a sharp tip, size about 80 nm.

We are in the final stages of constructing a near-field microscope. We have succeeded in fabricating ultra-fine silica tips of dimensions upto 50 nm, by pulling an optical fiber under the heat of a CO2 laser. The neighbouring image shows the scanning electron micrograph of a tip with 80 nm tip size.

Topography scan of a test sample.

Topography measurements are based on the vibrating tuning fork technique. The sharp tip is glued to a quartz tuning fork, and is oscillated in the vicinity of a surface. The amplitude of tip oscillations is sensitive to the shear force subtended by the surface of the tip, and can be calibrated and used to maintain the tip at a fixed distance of a few nanometers from the surface. A raster scan of the surface then creates a virtual image of the topography. The neighbouring image shows the topography of a test sample. Subsequently, the light scattered from the surface is guided by the optical fiber to a photodetector to create an optical image.