a method of fabricating a color laser , comprising growing a first thin layer of ionic crystal on a substrate . the crystal can comprise many types of ionic crystals , such as sodium chloride or potassium chloride . a second thin layer of a different type of ionic crystal can be deposited above the first ionic crystal layer , such as lithium fluoride or sodium fluoride . an inert metal layer can be deposited between the first and second layers of ionic crystal and above the second layer of ionic crystal . when the first and second ionic crystal layers are radiated with gamma rays , they form color centers at the spots radiated . because of the difference in crystalline properties of the two different ionic crystal centers , their color centers have different wavelengths . each of the ionic crystal layers emit light at different characteristic wavelengths when illuminated at their unique absorption frequencies , and can be made to lase separately .