Four years after he had received the Nobel Prize in Physics with NikolayBasov and Charles Townes for their fundamental contributions to the invention of the maser and the laser technology, Aleksandr Prokhorov was the first of this Prize-winning trio to lecture in Lindau. Charles Townes was also present at this meeting, but did not give a talk, Nikolay Basov appeared for the first time in Lindau eleven years later. The field of quantum electronics to which Prokhorov refers in the title of his talk, „started to exist at the end of 1954 - beginning of 1955“ with two independent publications by the future laureates from the USSR and the US, as Prokhorov had explained in his Nobel lecture: „Just by that time theoretical grounds had been created, and the first device, - a molecular oscillator - had been designed, and constructed“.[1]

Today, the term quantum electronics is merely a historical reminiscence. Respective research activities have been absorbed by other sub-disciplines of physics such as optoelectronics or solid-state physics. Until the 1970s, however, the term described a highly flourishing field in its own right. Its studies on the interactions between electrons and photons yielded amazing results. They were based on the introduction of first microwave and then light amplification by stimulated emission of radiation (maser and laser). Theoretically, Einstein had predicted the existence of stimulated emission already in 1916. Practically, however, the efforts of scientists like Basov, Townes and Prokhorov were required, who had been primarily interested in increasing the resolution of radio spectroscopy, and while following this objective succeeded in turning those predictions into reality.

Prokhorov begins this lecture by briefly touching upon the power intensity potentially associated with a laser beam. The field strength in the focus of a laser, he explains, may considerably exceed interatomic fields, so that “in the future we can succeed in producing electron-positron pairs in the focus of such an enormous power laser”. In the remaining time, Prokhorov exclusively focuses on just one aspect and amazing result of current quantum electronics, namely the phenomenon of the self-focusing of light and electromagnetic waves in general. Under normal conditions, a light beam is divergent. Its cross section grows with the distance from its source. Yet beyond a certain threshold of intensity a very powerful beam of light, such as a laser, increases the refractive index of the medium through which it propagates. Consequently, the light is focused by the medium it crosses – it generates its own collective lens while propagating. Gurgen Askaryan (1928-1997), Prokhorov’s colleague at the Lebedev Institute for Physics in Moscow, had discovered and first measured this process in 1962. Prokhorov discusses the implications of this non-linear effect of quantum electronics in much technical detail. You can hear him writing many equations onto the blackboard, which in this audio file naturally and unfortunately remain invisible. “A great contribution in solving self-focusing problems was made by Professor Townes who is here” Prokhorov praises his co-laureate. This was an honest appreciation and a clear sign of how science could transcend the divide of the Cold War. Basov, Prokhorov and Townes had first met at a conference in Cambridge in England in 1955. They soon established a friendship, which “endured much in the following years, as efforts began to secure recognition for the invention of the maser, and lasted up until the end of Prokhorov’s life”[2] in 2002.

Joachim Pietzsch

[1] http://www.nobelprize.org/nobel_prizes/physics/laureates/1964/prokhorov-lecture.pdf

[2] cf. Benjamin Johnson. Research Profile A.M. Prokhorov http://www.mediatheque.lindau-nobel.org/research-profile/laureate-prochorow