Barium spectral frequencies

The material composition of the FPAs determines the detectable IR-spectral frequency range. Many types of detectors are available, ranging from the commonly used indium antimonide (InSb) for near IR and mercury cadmium telluride (HgCdTe, MCT) for the mid IR to the more exotic silicon arsenide (Si As) [10] and uncooled barium strontium titanium (BST) [11]. Mid-IR imaging using MCT FPAs [12] has been the most popular in terms of the number of studies performed, due to its ability to provide access to the molecular-fingerprint region. 

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]. 

Fig. 8.16. Experimental spectrum of doubly-excited resonances in the barium spectrum obtained by three-photon spectroscopy. The horizontal arrows in the figure indicate lines which are not spectral features but frequency markers. Because of the mode of excitation, the lines tend to be more symmetrical than in some of the other spectra, but nevertheless exhibit a clear q reversal as the main feature is traversed. A theoretical fit by MQDT is also shown (after F. Gounand et al. [421]).

By closely studying the work of Kirchoff and Bunsen, Hinrichs found that some of the spectral hne frequencies, those referred to as dark hnes, could be related to the chemistry of the elements through their atomic weights, as well as to their postulated atomic dimensions. The difference between the spectral line frequencies seemed to be inversely proportional to the atomic weights of the elements in question. Hinrichs quoted the values of calcium, where the frequency difference is 4.8 units, and barium, which is chemically similar but has a higher atomic weight and shows a frequency difference of 4.4 units." ... 

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