Room-temperature solid-state masers
Introduction
What is a maser?
MASER is an acronym for Microwave Amplification by Stimulated Emission of Radiation. Masers work in exactly the same way as lasers and in fact predated them by 6 years. Originally, MASER stood for Molecular Amplification by Stimulated Emission of Radiation until ‘optical masers’ were invented and then renamed LASERS (Light Amplification etc). Stimulated emission is a quantum mechanical process whereby a photon (particle of electromagnetic radiation) induces an electron in a higher energy state to drop down into a lower energy state whilst emitting another photon with the energy difference equal to the incoming photon energy and, importantly, the same phase - i.e. coherent. This process is different to spontaneous emission where an electron in a higher energy state will randomly drop down into a lower energy ground state, emitting a photon in the process. Both these process were theoretically discovered by Einstein in 1916 [1].
Sustained stimulated emission produces an exponential growth in the number of photons over time if there is a large number of emitters - electrons in an excited state (higher energy). The exponential growth can be explained simply: a single photon stimulates an electron to drop down to the ground state, emitting a coherent photon. There are now two photons, these stimulate another two photons to be emitted, now we have four. These four stimulate another four, etc, until there is a large number of photons. Within a real device, photons are sometimes lost or absorbed - if the rate of production of photons by stimulated emission exceeds the rate of loss of photons, then the signal will still increase. The condition where the rates of production and loss are equal is known as the threshold, so if the rate of photon production is greater than the rate of loss, then the device is operating above threshold.
The ability to amplify a single photon into a huge number with the same phase (coherence) is why masers can be used as low-noise amplifiers. They have low intrinsic noise because the rate of spontaneous emission is very low at microwave frequencies. The superior low-noise amplification performance of masers led to them being used in radio astronomy and deep-space exploration. The fact that stimulated emission is coherent in lasers has been used for many applications such as interferometry, LiDAR (light version of radar) and many, many more.
Here’s a video we commissioned for our exhibit at the 2017 Royal Society Summer exhibition that explains what masers can do and how they work.