Nonlinear Optical Glasses

Abstract

There is a great need for non-linear optical materials that can be used with low intensity light sources for applications in phase-conjugation, image processing and optical switching. Such nonlinear materials may be prepared by incorporating organic dyes, metal and semiconductor nanoparticles into glass bulks. Organic materials in sol-gel glasses for nonlinear optics have been of great interest in recent years. The origin of the large nonlinear susceptibility of organic molecules at room temperature which have good singlet-triplet transfer, and incorporated in a glass matrix is as follows. Optical excitation from the ground state of the molecule S0 to first excited state S1 is followed by an intersystem crossing transfer to the lowest lying triplet state T1. Provided that the energy difference ΔE between the first excited singlet S1 and the triplet state T1 is of appropriate value, delayed fluorescence from the excited single state occurs after a back intersystem crossing that returns the population to the excited singlet state from vibrational excited states of the lowest lying triplet manifold. Many of the relaxation mechanisms that can quench the triplet state do not exist when the dye is held rigidly in a solid sol-gel matrix, and hence the lowest-lying triplet state has a very long lifetime, resulting in significant population trapped in that state even by weak optical pumping. The saturation intensity I1 is therefore quite small, less than 100 mWcm-2, and since the non-linear susceptibility Xeff varies inversely with Is, the nonlinear Xeff susceptibility is extremely large, about 0.1 esu.