Maser core consisting of a pink crystal of pentacene-doped p-terphenyl surrounded by a clear sapphire ring.
Masers, the microwave ancestor of the laser, have nowhere near the modern-day utility and ubiquity of the laser, in part because they require cryogenic cooling. Now, nearly 60 years after the first working maser, three British scientists have devised an optically pumped solid-state maser that operates at room temperature (Nature 488, 353).
The maser’s gain medium is an organic semiconductor known as pentacene, which is doped into another hydrocarbon named p-terphenyl, according to lead author Mark Oxborrow of the U.K. National Physical Laboratory. A pulsed dye laser designed for dermatological applications provides the yellow pump light.
Traditional masers, developed in the 1950s, contain inorganic gain media whose spin-lattice relaxation rates soar with increasing temperature. Oxborrow and his collaborators, Neil Alford and Jonathan Breeze of Imperial College (London, England), learned of recent experiments using C60 “buckyballs” and porphyrins to accelerate coherent electron beams. Pentacene, one of the polycyclic aromatic hydrocarbons, happens to absorb light most strongly at 585 nm.
Once excited by the yellow photons, the pentacene molecules transition to a triplet state with a population inversion. The drop to the lowest sublevel of the triplet produces the maser emission at 1.45 GHz. The room-temperature maser emission is about 100 million times more powerful than that of a hydrogen atomic maser at 1.42 GHz.
Although pentacene is the actual masing medium, the p-terphenyl optimizes the splitting ratios, according to Oxborrow. Breeze and his collaborators surrounded the gain medium with a sapphire ring to concentrate the magnetic flux within the maser cavity and optimized the design of the microwave cavity to boost its Q factor.
Next, Oxborrow would like to find a continuous source of yellow light, possibly a bright LED. (Dye lasers work well only as pulsed sources.) Researchers also may find other organic molecules that show strong maser action for green or blue wavelengths of pump light. Oxborrow and Alford envision that their maser could assist wherever a low-noise amplifier is needed to pick up extremely weak signals, from telecommunications to trace-chemical detection to national security applications.