Potassium dihydrogen phosphate, KDP

Currently used nonlinear optical crystals are potassium dihydrogen phosphate (KDP) and barium borate (BBO). Compared to KDP, the advantages of BBO are its transparency in the UV and its larger quantum efficiency of up-conversion by a factor of 4—6. For a given position of the crystal, only a narrow band of the fluorescence spectrum is up-converted. Therefore, if the full fluorescence spectrum is of interest, the crystal must be rotated at a series of angles. An example of experimental set-up is presented in Figure 11.2. The fwhm of the response is 210 fs. 

From Eq. (8.40), because the intensity is inversely proportional to (nx— n2)2, the efficient production of SHG radiation in a crystal of appreciable thickness requires the matching of nl and n2. In certain birefringent materials it is possible to choose a direction of propagation such that the refractive indices of the ordinary and extraordinary rays are equal. This is possible in, for example, the crystal potassium dihydrogen phosphate (KDP). 

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]. 

Potassium Dihydrogen Phosphate. KDP crystals are used for second-harmonic generation of laser radiation. 

I.M. Thomas,."Optical and Environmentally Protective Coatings for Potassium Dihydrogen Phosphate (KDP) Harmonic Converter Crystals", SPIE s 1991 International Symposium, San Diego. 

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