The growth and selected properties of single crystals of KCl doped with ReO₄^- is described. The crystals have been used as saturable absorbers to modulate and control CO₂ laser radiation. ReO₄^- ion concentrations in excess of 10¹⁷ cm^-3 were achieved in KCl with good optical quality. The room temperature absorption of the ReO₄^- ion in KCl was centered at 936.8 cm^-1 with 1.5 cm^-1 linewidth and with absorption cross-section determined to be (0.46 + 0.02) × 10^-16 cm². The addition of Li⁺ as a second dopant resulted in a splitting of the ReO₄^- resonance into two components at 957.5 cm^-1 and 900.7 cm^-1.

The characteristically sharp resonance of ReO₄^- was not detected in NaCl grown with NaReO₄ additions to the melt. Only with addition of Ca⁺⁺ as a co-dopant was the ReO₄^- resonance observed. The absorption, detected as a very weak resonance through a 92 mm path length, was centered at 946.0 cm^-1with linewidth of 6 cm^-1, over-lapping the P(20) transition in the 10 micron CO₂ band.