Masers, Forgotten Cousins of Lasers, Come Out of The Cold

Masers, Forgotten Cousins of Lasers, Come Out of The Cold

Scientists from the National Physical Laboratory (NPL) and Imperial College London demonstrate, for the first time, a solid-state ‘MASER’ capable of operating at room temperature, paving the way for its widespread adoption – as reported in the journal Nature.

MASER stands for Microwave Amplification by Stimulated Emission of Radiation. Devices based on this process and known by the same acronym were developed by scientists more than 50 years ago, before the first LASERs were invented. Instead of creating intense beams of light, as in the case of LASERs, MASERs deliver a concentrated beam of microwaves.

Conventional MASER technology works by amplifying microwaves using crystals such as ruby – this process is known as ‘masing’. However, the MASER has had little technological impact compared to the LASER because getting it to work has always required extreme conditions that are difficult to produce; either extremely low pressures, supplied by special vacuum chambers and pumps, or freezing conditions at temperatures close to absolute zero (-273.15 °C), supplied by special refrigerators. To make matters worse, the application of strong magnetic fields has often also been necessary, requiring large magnets.

Now, the team from NPL and Imperial have demonstrated masing in a solid-state device working in air at room temperature with no applied magnetic field. This breakthrough means that the cost to manufacture and operate MASERs could be dramatically reduced, which could lead to them becoming as widely used as LASER technology.

The researchers suggest that room-temperature MASERs could be used to make more sensitive medical instruments for scanning patients, improved chemical sensors for remotely detecting explosives; lower-noise read-out mechanisms for quantum computers and better radio telescopes for potentially detecting life on other planets.

Dr Mark Oxborrow, co-author of the study at NPL, says:

“For half a century the MASER has been the forgotten, inconvenient cousin of the LASER. Our design breakthrough will enable MASERs to be used by industry and consumers.”

Professor Neil Alford, co-author and Head of the Department of Materials at Imperial College London, adds:

“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.”

Conventional MASER technology works by amplifying microwaves using hard inorganic crystals such as ruby. However, masing only works when the ruby is kept at a very low temperature. The team in this new study have discovered that a completely different type of crystal, namely p-terphenyl doped with pentacene, can replace ruby and replicate the same masing process at room temperature. As a curious twist, the pentacene dopant turns the otherwise colourless p-terphenyl crystal an intense reddish pink – making it look just like ruby!

Our design breakthrough will enable MASERs to be used by industry and consumers

Dr Mark Oxborrow, co-author of the study

The twin challenges the team currently face are getting the MASER to work continuously, as their first device only works in pulsed mode for fractions of a second at a time. They also aim to get it to operate over a range of microwave frequencies, instead of its current narrow bandwidth, which would make the technology more useful.

In the long term, the team have a range of other goals including the identification of different materials that can mase at room temperature while consuming less power than pentacene-doped p-terphenyl. They will also focus on creating new designs that could make the MASER smaller and more portable.

The research was funded by the Engineering and Physical Sciences Research Council and, at NPL, through the UK’s National Measurement Office.

The full paper, ‘Room-temperature solid-state maser’, was published in Nature on 16 August 2012.

Dr Mark Oxborrow introduces the paper and discusses the maser in the following video.

NPL’s Quantum Detection Group has also recently had research published in Nature Nanotechnology and Nature Communications.

The invention of the laser has resulted in many innovations, and the device has become ubiquitous. However, the maser, which amplifies microwave radiation rather than visible light, has not had as large an impact, despite being instrumental in the laser’s birth1, 2. The maser’s relative obscurity has mainly been due to the inconvenience of the operating conditions needed for its various realizations: atomic3 and free-electron4 masers require vacuum chambers and pumping; and solid-state masers5, although they excel as low-noise amplifiers6 and are occasionally incorporated in ultrastable oscillators7, 8, typically require cryogenic refrigeration. Most realizations of masers also require strong magnets, magnetic shielding or both. Overcoming these various obstacles would pave the way for improvements such as more-sensitive chemical assays, more-precise determinations of biomolecular structure and function, and more-accurate medical diagnostics (including tomography) based on enhanced magnetic resonance spectrometers9 incorporating maser amplifiers and oscillators. Here we report the experimental demonstration of a solid-state maser operating at room temperature in pulsed mode. It works on a laboratory bench, in air, in the terrestrial magnetic field and amplifies at around 1.45 gigahertz. In contrast to the cryogenic ruby maser6, in our maser the gain medium is an organic mixed molecular crystal, p-terphenyl doped with pentacene, the latter being photo-excited by yellow light. The maser’s pumping mechanism exploits spin-selective molecular intersystem crossing10 into pentacene’s triplet ground state11, 12. When configured as an oscillator, the solid-state maser’s measured output power of around -10 decibel milliwatts is approximately 100 million times greater than that of an atomic hydrogen maser3, which oscillates at a similar frequency (about 1.42 gigahertz). By exploiting the high levels of spin polarization readily generated by intersystem crossing in photo-excited pentacene and other aromatic molecules, this new type of maser seems to be capable of amplifying with a residual noise temperature far below room temperature.

The invention of the laser has resulted in many innovations, and the device has become ubiquitous. However, the maser, which amplifies microwave radiation rather than visible light, has not had as large an impact, despite being instrumental in the laser’s birth1, 2. The maser’s relative obscurity has mainly been due to the inconvenience of the operating conditions needed for its various realizations: atomic3 and free-electron4 masers require vacuum chambers and pumping; and solid-state masers5, although they excel as low-noise amplifiers6 and are occasionally incorporated in ultrastable oscillators7, 8, typically require cryogenic refrigeration. Most realizations of masers also require strong magnets, magnetic shielding or both. Overcoming these various obstacles would pave the way for improvements such as more-sensitive chemical assays, more-precise determinations of biomolecular structure and function, and more-accurate medical diagnostics (including tomography) based on enhanced magnetic resonance spectrometers9 incorporating maser amplifiers and oscillators. Here we report the experimental demonstration of a solid-state maser operating at room temperature in pulsed mode. It works on a laboratory bench, in air, in the terrestrial magnetic field and amplifies at around 1.45 gigahertz. In contrast to the cryogenic ruby maser6, in our maser the gain medium is an organic mixed molecular crystal, p-terphenyl doped with pentacene, the latter being photo-excited by yellow light. The maser’s pumping mechanism exploits spin-selective molecular intersystem crossing10 into pentacene’s triplet ground state11, 12. When configured as an oscillator, the solid-state maser’s measured output power of around -10 decibel milliwatts is approximately 100 million times greater than that of an atomic hydrogen maser3, which oscillates at a similar frequency (about 1.42 gigahertz). By exploiting the high levels of spin polarization readily generated by intersystem crossing in photo-excited pentacene and other aromatic molecules, this new type of maser seems to be capable of amplifying with a residual noise temperature far below room temperature.

Source : http://www.npl.co.uk/news/maser-power-comes-out-of-the-cold

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