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Dr. C. Vijayan, Professor, |
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Dept. of Physics, IIT Madras |
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Links to other introductory articles: nanostructures , photonics, photoacoustics Problems and Prospects of Photonic Materials Perspective of Photonic Materials Progress in Photonics over the last decade has indeed been fascinating. However, there exists a serious limitation in exploiting the potential of Photonic processes to their fullest extent. This is known as the `materials bottleneck . It turns out that most of the available materials do not meet the simultaneous requirements of large as well as fast nonlinearity required for the practical devices. Several inorganic materials such as KDP, ADP, barium titanate etc, have been used from the earliest days. Organic materials and semiconductors also have been found to be excellent materials. Special molecules such as fullerenes and porphyrins are also studied as promising materials. Examples of Molecular Engineering Some specific examples here may help to illustrate the potential of an active role for of the Materials scientist in present day scenario of Photonics. The first case is that of molecular engineering in organic polymers. The structures of certain polymers such as polyphenyl acetylene (PPA) have well-spread electron clouds resulting in large values of linear as well as nonlinear susceptibility. Structure of such polymers can be modified easily by doping with other materials and/or attaching appropriate side groups to the main chain. Some of these variations are known to enhance the nonlinearity of the basic polymer by way of extending the spatial spread of electrons and hence enlarging the linear as well as nonlinear susceptibilities. A second example is that of structure variation studies in metalloporphyrins which also has yielded interesting results by way of enhancement of optical nonlinearity and low laser power applications. Porphyrins allow for incorporation of various ligands and different core metal ions, which alter the susceptibilities dramatically. Incorporation of porphyrins into solid polymer membranes is found to enhance nonlinearity, besides stabilizing the materials against photodegradation and providing a convenient solid matrix. Examples of Band Gap Engineering One of the frontier area of Photonic materials development today is that of `Quantum Engineering' of semiconductor nanostructures. Quantum wells are made of thin alternate layers of semiconductors of different band gaps, thus confining the electron to virtually a plane. These are now being used in several devices. Electrons can be further confined to one dimension by embedding a semiconductor wire or a conducting polymer chain in a material of larger band gap. Research on quantum wires and nanotubes is being pursued actively these days. Quantum dots can be formed by embedding tiny crystalline islands of semiconductors or a metals (called nanoclusters) in glass or polymer matrices. These are practical realizations of `particle in a box' whose optical properties depend on the particle size. Some of the leading techniques of obtaining such monosize clusters have been perfected by Materials Scientists. These include ion-exchange strategies and control of cluster formation in chemical reactions by capping with surfactants. Other techniques are ion-implantation, monolayer building and molecular beam epitaxy, which also are being used widely to produce strongly confined quantum dots. Quantum confinement alters the electronic structure of the semiconductor and hence influences the linear and nonlinear optical properties. The band gap is modified by the increased oscillator strength of exciton transitions and hence there is an effective blueshift of the absorption edge with decreasing cluster size. This results in an alteration of the effective band gap, which is also known as `Bandgap Engineering'. Recent studies indicate that strong quantum confinement results in considerable increase in the nonlinear susceptibilities along with a decrease in response times. Collaborative attempts by Materials Scientists and physicists along these directions are sure to be productive in designing novel types of photonic materials ideally suited for exploiting the fascinating possibilities of Photonics to their fullest extent.
Links to other introductory articles: nanostructures , photonic materials, photoacoustics |