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The infrared transmission spectra of KTiOPO4 (KTP) have been closely inspected in the region between 4000 and 1000 cm−1 (2.5–10 μm) using single-crystal samples of various thicknesses. The spectral information obtained reveals that the band edge at 4.5 μm is due to the first overtone of the ν3 tetrahedral phosphate fundamental vibration. A heretofore questionable absorption centered at 3.3 μm has been shown to be the second overtone of this same fundamental vibration.

A quantitative assessment of the bulk-darkening phenomenon for flux-grown KTiOPO(4) is reported. Measurements were made for KTiOPQ(4) use as a component for second-harmonic generation of 1064-nm Nd:YAG radiation operating in a pulsed mode. The results show that the bulk-darkening severity increases with fluence and exposure time. Irreversible bulk darkening has been observed at laser energies ranging from 8.8 to 18 J/cm(2) depending on the shot count. Fluences exceeding 15 J/cm(2) can cause catastrophic damage. Data taken at elevated temperatures indicate an improved reliability. The onset of slight bulk darkening revealed no adverse effects on conversion efficiency.