Why was synthetic ruby used in the first working laser?

Theodore H. Maiman chose synthetic ruby for the first working laser because he had carefully studied how the material absorbed and emitted light and calculated that it would function effectively as a laser medium.1

Specifically, the ruby crystal contains chromium atoms that can be excited by bright pulses from a flash lamp. This process creates a population inversion within a three-level laser system, where energy is trapped in a long-lived metastable state before being stimulated to emit coherent red light. Maiman’s successful operation of this device in 1960 proved his theoretical calculations correct, marking the first time a laser produced visible light.123