LaserFest 2010

This year, the laser will turn 50! On May 16, 1960, at the Hughes Research Laboratories in Malibu, California, Theodore Maiman realized for the first time "Light Amplification by Stimulated Emission of Radiation", using a tiny ruby crystal.

Actually, Maiman and his small group of coworkers was back then just one of several teams, all at industrial laboratories, intensely searching for ways to create laser beams. At the end of the year, the ruby laser was replicated and improved, and lasing was realized using other crystals, and helium-neon gas mixtures. So, it's just fair that the American Physical Society, the Optical Society, SPIE, and the IEEE Photonics Society have decided to organize a yearlong celebration of the 50th anniversary of the laser - that's LaserFest.

But in fact, the path to the laser had begun much earlier.

Berlin, 1916

In the summer of 1916, Albert Einstein took a break from general relativity and cosmology and tried to make sense, once more, of the riddle of the quantum. Specifically, he thought about ways to combine the recent ideas of Bohr on discrete energy levels in atoms with the Planck spectrum of blackbody radiation.

Atoms in thermal equilibrium with radiation can absorb radiation, thereby transiting to a state of higher energy, and they can drop from an excited state to a state with lower energy spontaneously, thereby emitting radiation. Could it be, so Einstein's idea, that atoms also will transit from an excited to a lower-energy state when they are hit by radiation with suitable energy?

Indeed, assuming a thermal Boltzmann distribution for the states of the atoms interacting with radiation, and equal rates for absorption on the one hand and spontaneous and stimulated emission – as the newly stipulated process came to be called – on the other hand, as one would expect for a thermal equilibrium between the atoms and radiation, Einstein could reproduce the Planck formula for the spectrum of blackbody radiation. "A splendid light has dawned on me about the absorption and emission of radiation," he wrote in a letter to his friend Michele Besso on August 11, 1916.

Einstein's "splendid light" of stimulated emission of radiation: An atom in a state with energy E₂ is hit by a photon with energy hν = E₂−E₁. This can trigger a transition of the atom to the lower energy level E₁, accompanied with the emission of a photon with energy hν, in phase with the initial photon. After this so-called stimulated emission, there are two photons instead of one, both in the same state – a nice manifestation of the "bunching" Bose character of photons.

It was recognized in the 1920s that theoretically the process of stimulated emission could result in "negative absorption", that is, amplification, of radiation, but nobody had a good idea how to demonstrate this effect in practice.

New York, 1954

To achieve amplification of radiation via stimulated emission, it is necessary to have more atoms in the high-energy state than in the low-energy state. Otherwise, a photon hitting an atom will more likely just be absorbed than trigger stimulated emission, and there is no gain in radiation. This requirement for amplification is called "population inversion".

In 1951, Charles Townes had an idea how to create "population inversion" in an ensemble of ammonia molecules. The ammonia molecule comes with two states which are separated by an energy corresponding to microwave frequencies. A beam of ammonia molecules can be split into two in an inhomogeneous electric field, separating molecules in the higher and the lower energy states, respectively, with an arrangement similar to a Stern-Gerlach apparatus.

In April 1954, Townes and his students Jim Gordon and Herbert Zeiger at Columbia University piped a beam of ammonia molecules in the higher-energy state into a microwave cavity resonating at the frequency of the energy difference between the two states, and obtained "microwave amplification by stimulated emission of radiation" - this was the birth of the maser.

Townes soon started to think about ways how to extend the maser principle to infrared or optical frequencies. With graduate student Gordon Gould, he discussed arrangements of mirrors around the medium in which population inversion is created, replacing the microwave cavity. These mirrors make sure that a beam of light is going back and forth through the medium many times, thus being able to "collect" ever more photons every time it crosses the medium.

Gould realized that such an arrangement, for which he coined the term "laser", could create sharply focussed light beams of extreme intensity, which could be used for communication, as a tool, or as a weapon.

As soon as the concept of the "optical maser", as Townes continued to call it, was explained in detail in a paper written together with Arthur Schawlow, many groups embarked on a race to be the first to actually construct such a device.

Malibu, 1960

Theodore Maiman had received his doctorate in Physics from Stanford University in 1955 to take a job at the Hughes Research Laboratories, which moved to Malibu in 1960. At Hughes, Maiman had constructed masers using ruby crystals, and when he learned of the possibility of the laser, he convinced himself that it should be possible to build a laser using ruby as the "lasing" medium.

Ruby is, chemically speaking, a crystal of aluminum oxide doted with chromium ions. The chromium ions have several energy levels which can be excited by irradiation with light, two of which are metastable and can be used as the upper level of a lasing medium. The energy of the transition to the ground state corresponds to red light with a wavelength of 694 nm.

Maiman's idea was to take a rod of ruby with parallel faces, to coat these faces with silver to realize the mirrors, and to put the rod inside a helical flashlight tube. The flashlight then excites the chromium atoms and creates population inversion, and the spontaneous emission of one photon can trigger an avalanche of photons by stimulated emission.

On the afternoon of May 16, 1960, Maiman and his assistant Irnee D’Haenens saw for the first time directed beams of intense red light emerging from the ruby - they had realized the first laser.

Theodore Maiman holding the first laser. It consists of a small ruby crystal and a helical flashlight which serves to stimulate the chromium ions of the ruby, thus creating the population inversion necessary for laser action. The ends of the ruby rod have been coated with silver to mirror back and forth the light stemming from stimulated emission, thus producing sufficient gain. The whole device is placed in the small white casing. (Source)


Maiman is reported to have said that “A laser is a solution seeking a problem”, Gould's visions notwithstanding. I have no specific idea how fast the laser was used for commercial or industrial purposes, but it immediately grasped public imagination.

When the movie Goldfinger is released in 1964, James Bond has to face a huge laser, looking similar to a scaled-up version of Maiman's first tiny ruby device, and replacing the buzz saw of Ian Flemings original 1959 novel. As Auric Goldfinger explains:

l, too, have a new toy, but considerably more practical. You are looking at an industrial laser, which emits an extraordinary light, unknown in nature. It can project a spot on the moon. Or, at closer range, cut through solid metal. I will show you.

---

At the LaserFest website, you can find a nice description of the mechanism of the ruby laser, and a video with explanations by Theodore Maiman himself. Moroever, there is a long interview with Charles Townes on the history of the maser and the laser.

If you want to know more about the history of the laser, there are two books I can recommend:

• The history of the laser, by Mario Bertolotti, actually tells much more than just the story of the laser: It starts back at the beginning of the 20th century with the early atom models and the puzzle of blackbody radiation, and traces the path to the laser via spectroscopy, magnetic resonance, and the maser.


• Beam: the race to make the laser, by Jeff Hecht, focusses on the developments of the late 1950s and 1960, beginning with just two brief chapters on the early history of stimulated emission and the maser. If you get lost in between all the names, there is a list of dramatis personae at the end of the book which I, unfortunately, discovered only after reading the text.

If you have Feynman's lectures at hand, there is a discussion of Einstein's derivation of the blackbody spectrum using stimulated emission and the Einstein coefficients in Section 42-5 of Volume I, and the whole Chapter 9 of Volume III is devoted to explain the principle of the ammonia maser.

---