In 1917, through the vision of Albert Einstein, the possibility of the stimulated emission of radiation was first conceived.<sup>18 Einstein A. Zur quantentheorie der strahlung Physiol Z 1917 ;  18 : 121-128

Cliccare qui per andare alla sezione Riferimenti</sup> The theory was that a photon of electromagnetic energy could stimulate an excited atom with a corresponding transition energy, to emit another photon with the same energy. This hypothesis was realized in a practical experiment 10 years later, but the world had to wait until 1960 for the first laser (light amplification by stimulated emission of radiation) to be built.<sup>31 Maiman T.H. Stimulated optical radiation in ruby Nature 1960 ;  187 : 493-494  [cross-ref]

Cliccare qui per andare alla sezione Riferimenti</sup> This was a construction of synthetic ruby surrounded by a helical flashlamp, and the discharge of this instrument heralded the start of the laser revolution.

Some of the lasers in common use today, including the neodymium, CO₂, organic dye, and argon lasers, appeared in the following several years. Their availability opened up a wide range of applications that were developed subsequently. Communications, defense, and the music industry have all benefitted. The expansion of lasers into medicine began in the early 1960s with research on the skin and eyes because of their accessibility. Further medical applications were discovered when the argon and CO₂ lasers were introduced in the early 1970s, and since then there has been a plethora of new devices that provide novel ways of treating difficult conditions. This expansion in available hardware spawned a greater understanding of laser–tissue interactions.