Potassium dihydrogen phosphate KH2PO4 (KDP) is a remarkable electro-optic and nonlinear material which large single crystal can be grown [1, 2]. KDP crystal has been widely used in laser physics, such as frequency doublers and triplers in large laser systems. In practical applications, the major issue is that the laser induced damage threshold is an order of magnitude below the expected limits [3, 4]. Many former studies have been performed to understanding this phenomenon and further more expecting contribution of improvement of the damage threshold.

KDP crystals were grown with the traditional temperature-lowering method from the aqueous solution with different sulfate. Transmission spectrum measurement was performed by HITACHI U-3500 spectrometer at room temperature. Sample with high sulfate concentration density exhibits heavy absorption property in the ultraviolet region, while only tiny difference was observed in the rest of the spectrum. This crystal is also studied by the first-principles simulation method and the density of its states was calculated. We found that sulfate can reduce the band gap of KDP crystal to 3.90eV (318nm), which is consistent with the experimental work and indicates that sulfate is an important source of the low damage threshold.

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