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Updated on: July 15, 2010  
  Atomic Force Microscope (AFM)  

Atomic force microscope (AFM) image of an optical bragg grating made out of a glass nanocrystal composite.

Composites are synthesized by combining two or more classes of materials with very different properties so as to exploit the beneficial properties of both of them. For example if one wishes to exploit the strength of a ceramic material and the ductility of a metallic material, a metal-ceramic composite if made would have better strength than the metallic component and better ductility than the ceramic component. The image in the figure is a composite made of crystalline nanoparticles and an amorphous or glassy matrix.

The atomic force microscope (AFM) used to image the surface of this composite has of late become a very common tool to study the surfaces of materials at the nanoscale along with the scanning electron microscope. Many variations of surface scanning microscopy are now available and they are not only used to image surface topography but also used to probe thermal, mechanical, electronic, magnetic and dielectric properties of materials at the nanoscale. In its simplest form such a microscope uses a very sharp tip (radius of curvature of about 20 nm or less) that is moved with or without contact over the surface to be studied using suitable mechanisms. The application of thermal, electrical or magnetic fields through the tip using suitable contacts helps probe matter for the corresponding properties at the nanoscale.


Going Deeper:
The Bragg grating shown in the image is a structure that has a periodically varying refractive index. This structure selectively reflects a very narrow range of wavelengths while transmitting others. As light waves interact with the variations in the refractive index, interference effects occur which cause certain wavelengths to reflect more than others. Bragg gratings are typically created by "burning" or "writing" the periodic variation of refractive index using an intense ultraviolet (UV) source such as a UV laser and a photomask (having the intended grating features) or by exposing to an interference pattern. The pictures shown are Bragg gratings, alternate amorphous and crystalline "strips", of nano dimensions, made spontaneously on glasses by Prof. K.B.R. Varma's group. The term spontaneous is used, because unlike in other gratings no photomask or external interference pattern was involved in making them. Rather they were formed just by irradiating the nanocomposite surface using a 308 nm excimer laser. Interference between the incident light field and the electric field of the bulk electron plasma wave generates a periodic modulation of electron plasma concentration and results in the observed grating with periodic structure.

For more information on Prof. Varma's research visit his faculty page.

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