Generation of color centers in alkali halide single crystals using ultrafast laser pulses

The evolution of color centers generated by 400-nm femtosecond laser radiation in three single-crystal alkali halides (sodium chloride, potassium chloride and potassium bromide) were probed by focusing appropriately chosen probe lasers through the femtosecond laser spot. The evolution of coloration after the termination of femtosecond laser radiation, and subsequently after the resumption of femtosecond radiation, was also probed. While all three materials color gradually during initial femtosecond laser radiation, rapid changes are often observed when femtosecond radiation is terminated or resumed.