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A Room Temperature Maser

August 22, 2012

In the history of electromagnetic amplifiers, the maser preceded the laser by quite a few years. The maser, a microwave amplifier, was demonstrated in 1954 (or, late 1953, in some accounts) by Charles Townes and his students at Columbia University using ammonia gas as the amplification medium.[1-3]

It was later realized that the maser principle should work at optical frequencies, as well. Townes and Art Schawlow theoretically proved that an optical maser was possible in 1958, but it wasn't until 1960 that Ted Maiman demonstrated laser action at 694 nm in an optically-pumped synthetic ruby (chromium-doped alumina) crystal.

Townes shared the 1964 Nobel Prize in Physics for his maser work, and Schawlow shared the 1981 Nobel Prize in Physics for his laser work. Maiman received many accolades, but never the Nobel Prize. Nor was the Nobel Prize awarded to Gordon Gould, but that's another, sad story. Gould is remembered mainly for patent litigation, rather than his physics.

Charles Townes in June, 2007.Charles Townes in June, 2007.

In his book, "How the Laser Happened: Adventures of a Scientist," Townes writes that Niels Bohr, John von Neumann, Isidor Rabi, Polykarp Kusch, and Llewellyn Thomas were fairly certain that a maser was impossible.[3]

(US National Institutes of Health photograph (cropped), via Wikimedia Commons).

The Townes ammonia maser amplified microwave signals at about 24 gigahertz. Working at those frequencies was quite an accomplishment in 1954. Masers using other gases, particularly hydrogen, were useful for amplifying signals used by interplanetary probes. Since there were limited applications for masers, and the technical requirements for maser operation are rather extreme, they drifted into the background as their sibling lasers took over the world.

Masers exist in several varieties. Atomic and free electron masers require a high vacuum. A photograph of Townes standing behind the original maser shows the huge diffusion pumps that were required to attain a vacuum.[1] Solid state masers will only work at very low temperatures. Most masers require a strong magnetic field to align spins, or considerable magnetic shielding to keep stray fields away.[4,6]

An advance in maser technology has just been made by a team comprised of a physicist at the National Physical Laboratory (Teddington, UK) and two materials scientists at Imperial College London.[4,6] They've demonstrated a room temperature solid state maser that works near 1.45 GHz. The gain medium is an organic crystal, pentacene-doped p-terphenyl. The maser is pumped with yellow laser light, and it operates, pulse-mode only at this point, at room temperature, in air, in Earth's magnetic field.[4,6]

The idea came about when Mark Oxborrow, an NPL physicist, was inspired by an old publication by some Japanese scientists who speculated that the electrons in pentacene could be excited by a laser to produce a maser. He borrowed some pentacene and crystallized it with p-terphenyl into a pink crystal several centimeters long.[5]

The laser source was a medical laser purchased on eBay and transported from a North London warehouse. Oxborrow admits that there's much room for material improvement. He slightly decomposed the organic solution while heating, but the imperfect crystal still worked.[5]

Room-temperature solid-state maserRoom-temperature solid-state maser being held by Mark Oxborrow.

(Still from a YouTube video).[7]

The performance of this maser far outstrips its predecessors. Early masers could only produce a few nanowatts of power when operated as oscillators. The pentacene maser has an output power of -10 dBm (0.1 milliwatt), which is 100,000 times more powerful.[4] Neil Alford, Head of the Department of Materials at Imperial College London and a co-author of the Nature paper announcing this discovery, says
"When LASERs were invented no one quite knew exactly how they would be used and yet, the technology flourished to the point that LASERs have now become ubiquitous in our everyday lives. We've still got a long way to go before the MASER reaches that level, but our breakthrough does mean that this technology can literally come out of the cold and start becoming more useful."[6]
At present, this maser works only in pulsed mode, for just a few fractions of a second for each pulse. The goal is to produce a continuous maser, perhaps consuming less power, and operable over a range of frequencies.[6] The maser team will also investigate other materials that might work in their maser device.[6] One application would be as an amplifier for radio astronomy. This maser research was funded by the Engineering and Physical Sciences Research Council and the UK's National Measurement Office.[6]

References:

  1. Invention of the Maser and Laser, Phys. Rev. Focus, vol. 15, no. 4 (January 27, 2005)
  2. J. P. Gordon, H. J. Zeiger and C. H. Townes, "Molecular Microwave Oscillator and New Hyperfine Structure in the Microwave Spectrum of NH3," Phys. Rev., vol. 95, no. 1 (July 1, 1954).
  3. Charles H. Townes, "How the Laser Happened: Adventures of a Scientist," Oxford University Press, April 8, 1999, p. 69.
  4. Mark Oxborrow, Jonathan D. Breeze and Neil M. Alford, "Room-temperature solid-state maser," Nature, vol. 488, no. 7411 (August 16, 2012), pp. 353-356.
  5. Geoff Brumfiel, "Microwave laser fulfills 60 years of promise - Physicists build first practical maser," Nature (August 15, 2012).
  6. MASER power comes out of the cold, National Physical Laboratory Press Release, August 16, 2012.
  7. Room-temperature solid-state maser, YouTube video, August 16, 2012.