Since the 1960s, astronomers have spotted numerous sources of intense optical and microwave light at specific frequencies. These sources first puzzled astronomers but it soon became clear that the light was being generated by naturally occurring lasers (or masers for microwaves).
It turns out that in certain circumstances the very atmospheres of stars and planets can lase, generating light in the same way as the lasers we use inside CD players and laser pointers.
These naturally-occurring lasers have puzzled astronomers for many years because they appear to occur in dilute gases made up of random atoms. Today, Robin Kaiser and pals at the Institut Non Linéaire de Nice in Southern France say they have created lasers that work in the same for the first time on Earth.
Physicists make conventional lasers using atoms that emit light at a specific frequency. The idea is to excite these atoms by adding energy. In this state, the atoms then release this energy in the form of coherent light when photons pass nearby–so-called stimulated emission.
By bouncing light back and forth past the atoms, physicist can trigger this release and amplify the light. Hence the name: light amplification by the stimulated emission of radiation and its acronym, laser.
The medium that contains these atoms is important. In conventional lasers, the atoms are usually confined in a crystal. That’s useful because physicists can easily place a mirror at each end of the crystal to make light bounce back and forth within it. Other lasers rely on gases confined inside an optical cavity with mirrors at each end.
But the gas in a stellar or planetary atmosphere is clearly not confined in a cavity so an important question is how light can be confined in a way that triggers stimulated emission.
In recent years, physicists have found an answer in the form of random lasers. These consist of some kind of disordered medium, such as semiconductor powder. The light that stimulates emission is not confined by mirrors but by the disordered state of the powder–the light simply bounces around inside it at random.
Natural space lasers work in the same way, except that the random medium is the gas in a stellar or planetary atmosphere. But while physicists have made random lasers work using powder or liquid dyes, nobody has succeeded in making one using a gas.
Until now, that is. “We report the experimental observation of random lasing in a controlled, cold atomic vapour,” say Kaiser and co.
These guys have built their laser out of a small cloud of rubidium atoms confined in a magneto-optical trap. They excite the atoms and then zap them with a laser tuned close to the expected emission frequency of rubidium. This bounces around at random inside the cloud triggering the stimulated emission of light.
Sure enough, the team’s measurements of the light emitted from the cloud show that it is indeed lasing.
That’s interesting because the ability to reproduce the lasing mechanism on Earth will allow physicists to study the processes that lead to natural space-based lasing for the first time.
It might also have other applications. The ability to make piles of powder or clouds of cold atoms produce laser light might lead to new sources of artificial light.
So naturally-occurring space lasers might one day be the inspiration for a new generation of lights on Earth. Cool!
Ref: arxiv.org/abs/1301.0522: A Cold-Atom Random Laser