Taking Microwaves for a Spin

Most lasers operate at infrared and visible wavelengths, but the first laser, developed in 1954, was actually a maser--it used microwaves. In an upcoming issue of PRL, researchers in the Netherlands propose a new design for a maser that would work by harnessing the spins of electrons.

Watts and his Groningen colleague Bart van Wees propose that a maser could be created with a three-layer sandwich, with a ferromagnetic material like iron on top, a paramagnetic material like aluminum on the bottom, and a thin layer of electrical insulator separating the two. Applying a magnetic field pointing downward would create two electron energy states: a ground state for spins pointing down and an excited state for spins pointing up. Electrons could be excited to the higher-energy, spin-up state by sending in microwave radiation.

But the incoming microwaves would not generate enough excited electrons to get the population inversion needed for a maser. So the researchers propose supplementing the excited electron population with a "spin current" from the ferromagnet into the paramagnet--that is, a current whose electrons have a preferred spin-up orientation. The team calculates that with this boost, the paramagnet would contain enough excited electrons for maser action to start spontaneously, and the sandwich would emit amplified, coherent, microwave radiation.

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