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Barcode scanners, eye surgery and the fibre cables that carry the internet all depend on lasers. The laser grew out of an earlier device, the maser, which Charles Townes conceived in 1951 and built with his students. This post tells that story and works through the physics: what stimulated emission is, why Townes had to sort molecules before they would amplify, and what the numbers say.
Who was Charles Townes?
Townes was born in Greenville, South Carolina, on 28 July 1915 and died in Berkeley, California, on 27 January 2015, according to his Nobel Prize facts page. His Nobel biography records that he graduated from Furman University in 1935, at the age of 19, with degrees in both physics and modern languages. He took a master's degree at Duke and a PhD at Caltech in 1939.
At Bell Telephone Laboratories he worked during the Second World War on radar bombing systems. Afterwards he applied wartime microwave techniques to spectroscopy, the study of how molecules absorb and emit radiation. He joined Columbia University in 1948.
What is stimulated emission?
The Nobel facts page explains it simply. When a photon meets an atom or molecule in an excited state, it can make it drop to a lower level and emit a second photon with the same energy. If many particles are excited, one photon can trigger an avalanche of identical ones. The result is a strong, coherent beam.
The catch is that, normally, more molecules sit in the lower state than the upper one, so the material absorbs more than it emits. In his Nobel lecture, Townes describes the condition needed for amplification, sometimes called population inversion or a negative temperature, as a condition of non-equilibrium.
How did the ammonia maser work?
His biography records that in 1951 he conceived the idea and a few months later began building a device with ammonia gas as the active medium. In early 1954 the team obtained the first amplification and generation of electromagnetic waves by stimulated emission. They named it the maser: microwave amplification by stimulated emission of radiation. The result was published by Gordon, Zeiger and Townes in Physical Review in 1954.
The Nobel lecture describes how it worked. A beam of ammonia molecules passed through a "focuser" of non-uniform electric fields, which removed molecules in the lower state and sent excited molecules into a resonant cavity. There they emitted at the ammonia inversion transition, at 23,870 Mc/sec (megacycles per second, the old name for megahertz).
Worked example: why sorting the molecules was necessary
Use h = 6.626 × 10⁻³⁴ J s, c = 3.00 × 10⁸ m/s and k = 1.381 × 10⁻²³ J/K.
Wavelength. f = 23.87 × 10⁹ Hz. λ = c/f = 3.00 × 10⁸ / 2.387 × 10¹⁰ ≈ 0.0126 m, about 1.26 cm. These are microwaves.
Photon energy. E = hf = 6.626 × 10⁻³⁴ × 2.387 × 10¹⁰ ≈ 1.58 × 10⁻²³ J, or about 9.9 × 10⁻⁵ eV.
Thermal energy at room temperature. kT at 300 K ≈ 4.14 × 10⁻²¹ J, or about 0.026 eV. That is more than 250 times the photon energy.
Population ratio. The Boltzmann factor for the upper to the lower level is e^(−E/kT) = e^(−0.0038) ≈ 0.996. The two levels are almost equally full, with slightly more in the lower one.
So a gas of ammonia left to itself would absorb slightly more than it emits. The focuser solved this by physically throwing out the lower-state molecules, creating the inversion that thermal equilibrium never allows.
From maser to laser
In 1958 Townes and his brother-in-law Arthur Schawlow showed theoretically that masers could work at infrared and optical wavelengths, in their paper Infrared and Optical Masers. Townes notes in his Nobel lecture that the first operating laser, using chromium ions in ruby to produce red light, was demonstrated by Theodore Maiman in 1960.
Compare the photon energies. Red light near 700 nm carries hc/λ ≈ 1.77 eV, roughly 18,000 times the energy of an ammonia maser photon.
Townes received half of the 1964 Nobel Prize in Physics "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle". The other half went to Nicolay Basov and Aleksandr Prokhorov.
Common mistakes students make about lasers
"Lasers are just bright lamps." A laser's light comes from stimulated emission, so the photons share energy and phase.
"Any excited gas will lase." Without population inversion, absorption wins.
Unit slips. Convert between joules and electronvolts with 1 eV = 1.602 × 10⁻¹⁹ J, and check whether a question gives frequency or wavelength.
If photon energy and energy levels feel abstract, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing. The laser story continues in our post on Charles Kao and the glass fibres that carry the internet.
What can a student learn from how Townes worked?
Townes took a tool he knew well from wartime radar, microwaves, and pointed it at a new question about molecules. Then he spotted the single obstacle, too few excited molecules, and designed a way around it. Find the one condition your problem needs, then look for a way to create it.
Practice routine:
Find the photon energy in eV for 1.26 cm microwaves, 1.55 μm infrared and 500 nm green light.
Compute the Boltzmann ratio for a 2 eV gap at 300 K and compare with the maser case.
Explain population inversion to a friend in three sentences.
Next step: work through the four-step maser example again with your own calculator and check every power of ten.
