Review: Metal nanoclusters in glasses as non-linear photonic materials

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Although electronics technologies have made great advances in device speed, optical devices can function in the time domain inaccessible to electronics. In the time domain less than 1 ps, optical devices have no competition. Photonic or optical devices are designed to switch and process light signals without converting them to electronic form. The major advantages that these devices offer are speed and preservation of bandwidth. The switching is accomplished through changes in refractive index of the material that are proportional to the light intensity. The third-order optical susceptibility, χ(3), known as the optical Kerr susceptibility which is related to the non-linear portion of the total refractive index, is the non-linearity which provides this particular feature. Future opportunities in photonic switching and information processing will depend critically on the development of improved photonic materials with enhanced Kerr susceptibilities, as these materials are still in a relatively early stage of development. Optically isotropic materials, e.g. glasses that have inversion symmetry, inherently possess some third-order optical non-linearities. Although this is quite small for silica-glasses at λ = 1.06 μm, the absorption coefficient is extremely low, thereby allowing all-optical switching between two waveguides, embedded in a silica fibre, simply by controlling the optical pulse intensity. Different glass systems are now under investigation to increase their non-linearity by introducing a variety of modifiers into the glass-network. The incorporation of semiconductor microcrystallites enhances the third-order optical response. Metal colloids or nanoclusters, embedded in glasses, have also been found to introduce desired third-order optical non-linearities in the composite at wavelengths very close to that of the characteristic surface-plasmon resonance of the metal clusters. Ion implantation is nowadays an attractive method for inducing colloid formation at a high local concentration unattainable by the melt-glass fabrication process and for confining the non-linearities to specific patterned regions in a variety of host matrices. Recent works on metal-ion implanted colloid generation in bulk silica glasses have shown that these nanocluster–glass composites under favourable circumstances have significant enhancement of χ(3) with picosecond temporal responses. The remarkable achievements in developing such novel photonic materials seem to open the way for advances in all-optical switching devices, e.g. in inducing metal-colloids into coupled waveguides acting as a directional coupler. The present paper addresses the phenomena of optical non-linearities in metal nanocluster–glass composites that are synthesized by ion implantation, and the potential uses of these novel composite materials in photonics.

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