Laser
During World War II, South Carolina–born Charles Townes worked on nascent microwave technology and designed radar-based bombing systems for Bell Labs. After hostilities ended, he accepted a position at Columbia University. One spring morning in 1951 he experienced a eureka moment when he realized he could generate microwaves with molecules instead of free electrons. He and colleague Arthur Schawlow used that insight to invent the maser, short for “microwave amplification by stimulated emission of radiation.” This invention would lead eventually to the development of the laser or “light amplification by stimulated emission of radiation.” Today few aspects of daily life don’t involve lasers, whether it’s talking on the phone, surfing the Internet, buying groceries, or even playing with a cat.
Townes built upon his maser work, as did others, including Nicolaas Bloembergen of Harvard University and Robert Dicke of Princeton University. Similar work was also being done in the Soviet Union by Nikolay Basov and Aleksandr Prokhorov. Masers soon found applications as highly accurate atomic clocks and as low-noise amplifiers for radio astronomy and deep-space communication with satellites and interplanetary probes.
Scientists realized they could apply the principles of the maser to visible light. In 1958 Townes and Schawlow published a paper in Physical Review setting out the concept of an “optical maser,” and the race was on to build one. The critical question centered on what material would serve as the medium into which energy would be “pumped” to set off the lasing action. Townes’s original 1954 maser used gaseous ammonia, and since then others had built masers using solid materials. But what would lase at the much higher frequencies and shorter wavelengths of optical light?
A year after publication of their paper, almost everyone in the field attended the first conference in quantum electronics, organized by Townes at Shawanga Lodge in the Catskills. Among them was Gordon Gould, a 37-year-old Columbia physics graduate student and inventor who presented ideas about building an optical maser, for which he coined the word “laser.”
The breakthrough came about six months later from Hughes Research Laboratories in California, where Theodore Maiman created the world’s first working laser. He used a ruby crystal and a helical xenon flash-photography lamp. Maiman liked ruby because the crystals could lase at room temperature, not the cryogenic temperatures that masers required, and they would emit visible light. He also used a pulsed mode instead of a continuous mode for his device.
For the intensely bright light source needed to pump the required energy into the ruby, Maiman selected the intensely bright General Electric FT506 bulb. He focused the light by simply placing the cylindrical crystal inside the helical bulb, thus surrounding it with light, and put the entire assembly inside a highly polished aluminum cylinder to reflect everything back onto the ruby. He coated each end of the crystal with silver to turn it into an optical cavity and left a small hole in the coating at one end so that he could measure what was going on inside.
On May 16, 1960, Maiman assembled his device and fired up the xenon bulb. Inside the crystal, the sudden flood of photons excited some of the ruby’s chromium atoms to higher energy levels, causing them to emit photons, which were reflected back into the optical cavity by the crystal’s silvered ends to strike and excite still more chromium atoms. Soon more chromium atoms were in a higher energy state than in the normal ground state—a “population inversion” had been achieved. The cascade of high-energy photons bounced back and forth inside the optical cavity, but some of the photons found their way out of the tiny hole, through which Maiman could detect and measure a stream of these emerging in a coherent beam—the first laser light produced on Earth.
Maiman’s accomplishment galvanized the research community. Almost immediately, ruby laser experiments were repeated everywhere, including by Townes and Schawlow at Bell Labs. In December 1961 a ruby laser destroyed a retinal tumor at Columbia-Presbyterian Hospital in New York. As researchers continued to build ever more varied types, whose diverse materials created beams of different wavelengths and power, additional scientific, industrial, and even artistic applications emerged.
