Inorganic semiconductors, nonlinear

Table II. Nonlinear coefficients of some inorganic semiconductors (reference 13)...

For devices the important classes of nonlinear materials are (i) molecular and polymeric materials, (ii) multiple quantum well semi-conductors, (hi) bulk semiconductors and (iv) semiconductor doped glasses. Of these classes, organic polymeric structures have shown the largest non-resonant x - Important nonlinearities observed in inorganic semiconductor materials are in a narrow wavelength region determined by excitonic resonances. Therefore, these nonlinearities are resonant, slower in response and produce thermal effects. 

The key material property for making a modulator is that a change in an electrical parameter must alter an optical property of the material. Other material properties that are also important are the optical loss and the stability, both thermal and optical. There are three classes of materials from which a modulator can be made inorganics, semiconductors, and nonlinear polymers. 

One dimensional conjugated carbon polymers can occur in many configurations as depicted in Figure 2 where also we included some chains with nitrogen and sulfur for later reference. Also included there are inorganic one dimensional semiconductors, like SbSI and SbSBr for later comparison. Besides the depicted one-dimensional system others like TCNQ- and KCP-salts could be included here as well but rough measurements of their nonlinear coefficients gave deceptively small values which combined with their ill-characterisation make them poor candidates for nonlinear optical devices. 

In contrast, the nonlinearities in bulk materials are due to the response of electrons not associated with individual sites, as it occurs in metals or semiconductors. In these materials, the nonlinear response is caused by effects of band structure or other mechanisms that are determined by the electronic response of the bulk medium. The first nonlinear materials that were applied successfully in the fabrication of passive and active photonic devices were in fact ferroelectric inorganic crystals, such as the potassium dihydrogen phosphate (KDP) crystal or the lithium niobate (LiNbO,) [20-22]. In the present, potassium dihydrogen phosphate crystal is broadly used as a laser frequency doubler, while the lithium niobate is the main material for optical electrooptic modulators that operate in the near-infrared spectral range. Another ferroelectric inorganic crystal, barium titanate (BaTiOj), is currently used in phase-conjugation applications [23]. 

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