First-Hand:The First Continuous Visible Laser
It is difficult to recreate these days the sense of excitement and expectency that accompanied the early days of the laser in 1959-62. Gordon, Zeiger and Towne's demonstration of gain and oscillation 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 picking 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 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 seemingly poorer candidates, ruby, 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 (proposed by Javan), an electrical discharge in a mixture of neon and helium gases, 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 Duffenback who reported the enhancement of certain neon spectral lines in a HeNe dischage through collisions of the second kind. It was this enhancement that caught Javan's attention. Indeed, Javan's intuition was correct; measurements by Bennett confirmed the possibility of inversion between the neon 2s2 and 2p4 levels of neon resulting from enhanced transfer of excitation energy to the 2s2 level from the metastable helium level by resonance transfer. The transition wavelength was in the near infra-red at 1.15 microns. The experimental confirmation of inversion and oscillation was a tour de force. Don Herriott's optical expertise was crucial in the experiment. Curved mirror optical cavities with their large angle tolerance where not well understood those days, so Herriott had to struggle with the problem of aligning plane mirrors enclosed within a vacuum tube to within a fraction of an arc-second to find a cavity resonance. Patience paid off in the end and oscillation at 1.15 microns was achieved in Dec 1960. It wasn't visible light, but from the Bell Labs point of view it was coherent and continuous, both telephone company imperatives.
