Stable, room-temperature maser finally achieved, 65 years after invention
Image credit: Jonathan Breeze, Imperial College London
Researchers based at Imperial College London have achieved the world’s first stable, room-temperature maser, paving the way for a range of applications.
Maser (microwave amplification by stimulated emission of radiation) was first demonstrated in 1953, years before laser, which uses the same principle at different wavelengths. In the decades since, lasers have been exploited to use in communications, surgery, construction, weapons, measurement instruments, computers, fibre optics, DNA sequencing and countless other applications, while masers are comparatively unknown.
Masers are not widely used; they can be used for timekeeping in ultra-precise atomic clocks and in some applications in radio astronomy and deep space communications.
“Lasers operate at room temperature, whereas until now masers have required cryogenic cooling to a few degrees [above absolute zero] and also high vacuum,” Dr Jonathan Breeze, the Imperial College physicist who led the study, told E&T.
“This made masers impractical so they only found applications in a few niche areas where their exceptional low-noise amplification performance justified the additional bulky equipment overhead.”
Masers require this extreme cooling thanks to a phenomenon called spin-lattice relaxation, which does not occur for the visible light used by lasers. During spin-lattice relaxation, vibrations in the crystal disrupt the spin-polarisation – the alignment of the particles’ internal angular momentum – preventing stimulated emission.
In 2012, a different team of scientists demonstrated that a maser could operate at room temperature using an organic molecule called pentacene, although they were only able to produce very short bursts of radiation. If the maser had operated continuously, the crystal would likely have melted.
“Being organic, it is volatile, has a low melting point and very low thermal conductivity – it really doesn’t like being zapped with a powerful laser,” said Breeze. “After realising this, we thought about alternative materials, most likely inorganic ones.”
Breeze and his colleagues returned to the phenomenon which renders cooling necessary in order to generate maser radiation: spin-lattice relaxation. Using a material with a far slower spin-lattice relaxation could allow for emission to be sustained. They found that the spin-lattice relaxation in a particular type of diamond grown in a nitrogen-rich atmosphere was far, far slower than that of the rubies normally used in solid-state masers. A bonus, Breeze says, is that diamond is mechanically robust and with the highest known thermal conductivity of a bulk material.
Using this synthetic diamond, Breeze and his colleagues at Imperial College and University College London were able to achieve the first stable, room-temperature maser.
During stimulated emission of the diamond, carbon atoms are knocked from the diamond using a high energy electron beam, creating ‘vacancies’ in the diamond. When the diamond is heated, nitrogen atoms pair up with these vacancies and form photoluminescent defects. Placing the diamond inside a ring of sapphire allowed the researchers to concentrate microwave energy, such that when it was illuminated with a green laser light, they could generate stable maser radiation at room temperature.
The demonstration of the first stable maser radiation under practical conditions is a significant step forwards for the technology, although a number of challenges remain, such as replacing the bulky electromagnetic with an array of small, permanent magnets. Breeze and his colleagues plan to continue experimenting, using different types of diamond to optimise the performance of the maser.
If the maser – after more than half a century of experimentation – finds itself becoming widely adopted, it could prove invaluable in security, imaging, remote bomb detection and other applications.
“The most immediate application could be as a frequency standard,” Breeze told E&T. “Atomic clocks like hydrogen masers have fixed frequencies, low power and are complex and very expensive. The diamond maser’s frequency can be tuned using the external magnetic field, and could be miniaturised into a compact device.”
“Further down the line will come low-noise amplifiers and quantum devices. Maser amplifiers have exceptionally low noise, so could be used as sensitive detectors of microwave radiation in ambient conditions,” he said.