First-Hand:The First Continuous Visible Laser

   It is difficult to recreate these days the sense of excitement that accompanied the early days of the laser in 1960-62.    Gordon, Zeiger and Townes demonstration of gain at microwave frequencies had shown inverted populations were a potent new source of coherent electromagnetic energy and Townes and Schalow had pointed the way to extend this result to optical frequencies.   But the sticking point was finding the right medium to use.   Numerous suggestions for suitable media appeared in the literature.   Often, these were contingent on one or more difficult-to-measure parameters being known with some precision.   Ruby, caesium, xxxx were all proposed and some rejected.   Picking the right medium to investigate was important; no one wanted to waste time studying a medium unless there was some hint that inversion was possible.  Inversion seemed to be a rare phenomena presumably because nature favored thermal equilibrium everywhere.   In retrospect, this seems a quaint idea.   Now we know if one dumps enough energy into a system with distinct energy levels, one or more pairs of them will usually invert.   

One of the poorer candidates later turned out to be the first visible laser to operate successfully;  Theodore Maiman was the first to discover the advantages of using a giant pulse of energy to invert a pair of energy levels.  One of the better candidates, a gas discharge through a mixture of neon and helium, produced inversion in a pair of neon levels by excitation transfer from excited helium atoms.  Gas discharges are quite complicated mixtures of relatively simple electronic  excitation processes whose light producing properties have been studied for a long time.  One of the earliest  publications was by  who reported the enhancement of certain neon spectral lines in a HeNe dischage